TY - JOUR
AU1 - McKeever, S W S
AU2 - Sholom, S
AU3 - Chandler, J R
AB - Abstract Proposed physical dosimetry methods for emergency dosimetry in radiological, mass-casualty incidents include both thermoluminescence (TL) and optically stimulated luminescence (OSL). Potential materials that could feasibly be used for TL and OSL dosimetry include clothing, shoes and personal accessories. However, the most popular target of study has been personal electronics, especially different components from smartphones. Smartphones have been a focus because they are widely available and, in principle, may be viewed as surrogates for commercial TL or OSL dosimeters. The components of smartphones that have been studied include surface mount devices (such as resistors, capacitors and inductors) and glass materials, including front protective glass, display glass and (with more modern devices) back protective glass. This paper reviews the most recent developments in the use of TL and OSL with these materials and guides the way to future, and urgently needed, research. INTRODUCTION The International Commission on Radiation Units and Measurements (ICRU) Report 68, Retrospective Assessment of Exposures to Ionizing Radiation(1), summarized the retrospective dose estimation methods to be used for studying the relationship between the dose and long-term health effects in individuals exposed to radiation. In the report, ‘retrospective dosimetry’ was defined as the retrospective assessment of the dose absorbed by humans who were exposed to a radiological source, or sources, when conventional dosimetry was not available. The specific focus of Report 68 was the assessment of dose at times up to several decades after the exposure in order to provide data for epidemiology studies and to support judgments concerning the induction of stochastic injury. More recently, the ICRU published Report 94, Methods for Initial-Phase Assessment of Individual Doses following Acute Radiation Exposure to Ionizing Radiation(2). The focus of Report 94 is the determination of doses to individuals a short time after the event, hence the emphasis on ‘acute’ exposure and not long-term, chronic exposure due, say, to contamination of the environment. Example exposure events range from small-scale incidents, such as accidental overexposure of patients or staff in medical procedures, to large-scale incidents such as nuclear power plant accidents. Also considered in Report 94 were potential nuclear terrorism incidents using improvised nuclear devices (INDs), radiation dispersal devices (RDDs) or radiation exposure devices (REDs). In each event of this kind, multiple hundreds or thousands of people have the potential to be exposed. In order to evaluate the risks of both short-term deterministic and long-term stochastic adverse health risks, assessment of dose to each individual is required. Active dosimetry methods are of little value in such assessments and reliance on passive dosimetry is paramount. All passive dosimetry is retrospective; that is, the dose assessment occurs after the exposure event. However, as used in both ICRU 68 and ICRU 94, the term ‘retrospective dosimetry’ specifically concerns those situations in which conventional dosimeters were unavailable, or inadequate, at the time of the exposure. ICRU 94 considered passive dosimetry methods including biodosimetry and physical dosimetry. The latter includes use of electron paramagnetic resonance (EPR), thermoluminescence (TL) and optically stimulated luminescence (OSL). This paper concentrates on the luminescence methods of TL and OSL. Such methods are potentially very attractive since the basic physics is well understood and methods for monitoring TL and OSL are well established. TL and OSL dosimetry have been used in health physics for decades and are accepted radiation dose assessment methods worldwide. Disadvantages include the need to extract the dosimetry material from the target object (clothing, smartphone, etc.) and this often involves destruction of that object. As a result, current research efforts are aimed toward the development of in situ methods in which the target object is not damaged. Several reviews are available of the application of TL and OSL in these applications, including ICRU Report 94(2) and those by Woda et al.(3), Ainsbury et al.(4), Trompier et al.(5), Bailiff et al.(6) and McKeever and Sholom(7). These reviews describe the TL and OSL developments and the applications of several potential TL and OSL dosimetry materials in emergency applications. The materials include clothing, shoes and personal accessories, but the most popular target of study has been personal electronics, especially different components from modern smartphones. In principle, these may be viewed as surrogates for commercial TL or OSL dosimeters (TLDs or OSLDs). The components of smartphones that have been studied include surface mount devices (SMDs, such as resistors, capacitors and inductors) and glass materials, including front protective glass, display glass and (with more modern devices) back protective glass. Attention in this paper is given to the most recent developments in the use of TL and OSL with smartphone materials, and it points the way to future research. The paper includes new data, previously unpublished, on surface mount inductors as well as a review of already-published data on other smartphone components. GENERAL MATERIALS OF INTEREST Electronic components The principle of using TL and/or OSL in radiation dosimetry is that the absorption of ionizing radiation by a wide-band-gap insulator produces free electrons and holes which may subsequently be captured (‘trapped’) at physical defects within the materials, such as impurities, vacancies, interstitial atoms and clusters of these. The energies of the trapped electrons and holes lie within the energy band-gap of the material and may be stable at these energies for anywhere from seconds to thousands of years. Dosimetry relies upon the existence of a stable trapped charge population on the time scale of the irradiation period, the recovery time of the sample to be used and the duration of sample preparation for the readout procedure. Once stimulated after irradiation, luminescence is emitted in proportion (in an ideal case) to the concentration of the initial trapped charge. The intensity of the emitted luminescence is a surrogate for the energy absorbed from the radiation field, and therefore, it may be used as a measure of the radiation dose. Calibration of the luminescence intensity against known absorbed doses allows an unknown dose to be estimated by measuring the luminescence intensity due to the unknown dose and comparing it with the calibration. If the luminescence signal is stimulated by heat, the emitted luminescence is TL; if the stimulation is by light, the luminescence is OSL. The search for suitable materials of potential use in emergency dosimetry focusses on ubiquitous materials likely to be carried by the individual. The goal is to determine the dose absorbed by the material and thereby infer the dose to the individual, in a similar fashion to the use of conventional TLDs or OSLDs. Once a potential material is identified, tests of the various parameters required for a useful dosimeter are performed. These include: determination of the sensitivity (TL/OSL signal strength per absorbed dose); examination of the response of the signal to absorbed dose (determination of the linearity of the response, or lack of it); estimation of the minimum detectable dose (MDD, related to sensitivity); examination of any non-radiation-induced luminescence signals, and/or the existence of any radiation-induced background signals (BGS) not related to the exposure of interest; stability of the signal (to characterize any fading of the signal that may have occurred between irradiation and signal measurement) and tests for variations in the TL/OSL signal with the energy or incident angle of the exposing radiation field. In the published research literature to date, several materials have emerged as potentially useful in TL or OSL emergency dosimetry. These include items of clothing (natural or synthetic fibers), shoes (polymeric materials), accessories (purses, backpacks, etc.) and personal items (including paper money, plastic credit/identification (ID) cards and more). However, perhaps the personal item that has received the most attention is a smartphone. Such personal electronic devices are very common, being carried by most individuals in pockets, purses, attached to belts, etc. A typical smartphone is full of different materials, many of which have the potential for use in dosimetry. The first suggested use of OSL from smartphone components for emergency dosimetry was by Inrig et al.(8,9) who examined the OSL properties of the alumina substrates of surface-mount resistors, extracted from the circuit board of early-model smartphones. The response to dose was found to be linear with enough sensitivity to enable the devices to be used in dosimetry. The chief disadvantage was from fading of the embryonic OSL signal following initial irradiation. The fading appears to follow a t−1 law (t = time since irradiation). Fading has been the subject of extensive work in the years since the observations of Inrig et al., and both thermal and athermal fading processes have been proposed(10–14). Later research included other surface-mount devices, including inductors, resonators and capacitors(10,15–20), but resistors seem to be the most commonly studied, leading to several laboratory intercomparisons (e.g. Bassinet et al.(21)). Published uncertainties in the estimation of the dose using OSL from surface-mount resistors reveals an expected uncertainty of ∼±20%, consistent with the uncertainties from other suggested emergency dosimetry methods(22). Integrated circuits (ICs) have also been examined as emergency OSL or TL dosimeters. These are attractive since they are among the largest components in personal electronics and are easier to extract than the surface-mounts devices (which have become smaller and smaller with later generations of phones). The luminescence signal from ICs is not from the semiconductor material but from the resin encapsulation. Although colored black, the encapsulation provides sufficient luminescence sensitivity for use in emergency dosimetry. Sensitivity, dose–response and fading characteristics have all been the object of study(10,20,23,24). Perhaps the biggest disadvantage of using internal electronic components as dosimeters is the fact that the electronic device must be destroyed for the absorbed dose to be evaluated. As a result, significant effort has been devoted to using glass from phones as a potential dosimetry material. Some of the glass from smartphones (from the electronic displays) also requires destruction of the device, but others (front protective glass and back protective glass) may only require some dismantling of the phone. The protective glasses are easily replaced. Efforts have included studies of TL, phototransferred TL (PTTL) and OSL from the various glass types(25–31). Although the sensitivity is sufficient for emergency dosimetry, issues relating to fading, and a large background (non-radiation-induced) signal, especially with TL, have been the focus of much research effort (e.g.(31)). Other potential emergency dosimetry materials It is briefly mentioned here that although personal electronic devices can be expected to be carried by a large proportion of the population, there is one source of potential dosimetry material that is carried by everyone—namely, clothing. Clothing fabrics are made from a variety of natural and synthetic long-chain polymer materials, including cotton (cellulose), polyester (polyethylene terephthalate), polyvinyl chloride (PVC), polyurethane and ethylene vinyl acetate. There have been few studies to date regarding the potential of these materials in luminescence dosimetry, but initial studies have been encouraging(32,33). OSL is the preferred method in order to avoid the burning or melting of the material during heating. A non-radiation-induced BGS and signal fading present potential barriers to the use of these materials, although methods have been proposed to separate the background from the radiation-induced signals (RIS), and protocols have been proposed to account for fading. Accessories such as shoes, bags and backpacks have also been examined(34,35). The OSL signal originates either from the material itself(34) or from mineral particles embedded in the fabrics(35). Other materials examined have included paper money, coins, paper cards, plastic credit cards and ID cards(32–33,36). This paper This paper describes in detail the progress made in the use of TL and OSL using the components of smartphones and emphasizes the most recent technological advances. State-of-the-art research is described, and knowledge gaps and needed future research are highlighted. TL AND OSL FROM SMARTPHONE ELECTRONIC COMPONENTS OSL from SMRs Currently, the list of SMDs that could be found inside a smartphone and used for emergency dose reconstruction with either TL or OSL includes surface mount resistors (SMRs), inductors and capacitors. The material that is responsible for RIS in these devices is their substrate, usually consisting of alumina, Al2O3(8), but may be also manufactured from compounds containing silica, barium and tantalum(10,37). General characteristics of OSL from SMRs SMRs are the most popular component of the mobile phone regarding to the possible application in emergency dosimetry. One of main benefits of applying OSL to SMRs is absence of the zero-dose signal (i.e. a signal from a non-irradiated sample, or BGS). After irradiation, the OSL is observed when stimulated with blue light (so-called blue-OSL, or BSL) and emission is recorded in the ultraviolet (UV) wavelength region. (Experimentally, the BSL is usually recorded through a Hoya U-340 filter, which transmits the light between 290 and 390 nm.) The shape of the OSL decay curve recorded immediately after irradiation can be fitted by a generalized hyperbolic decay function plus a constant, thus: $$\begin{equation} {I}_{\mathrm{OSL}}(t)=a{\left(1+ bt\right)}^{-1/c}+d, \end{equation}$$(1) where a, b, c and d are empirical constants(38). Stimulation with laser diodes emitting violet light (~405 nm; so-called violet-OSL, or VSL) was also tested(37). Comparison of the OSL curve shapes for the two stimulation wavelengths is shown in Figure 1. The VSL response to 1 Gy was about two times higher than that of BSL, but in non-irradiated samples, the VSL response increased with stimulation time up to 50 s. This was interpreted as a light-induced OSL signal and is currently the main drawback of VSL dosimetry with SMRs. As a compromise, the 0–2 s portion of a second VSL curve recorded after the first VSL curve was used as the corresponding BGS (37). This requires further verification. Figure 1 Open in new tabDownload slide VSL and BSL signals (on a log–log scale) measured from an aliquot of 10 resistors (NOKIA 2730c). (Reproduced from Bassinet et al.(37).) Dose–response characteristics Most papers report a linear dose–response relationship for stimulation with both BSL(10,12,13,17,39) or VSL(37) in a dose range from 0 to 30 Gy. A sublinear response above 1 Gy has been observed when the dosimetric signal (DS) used to construct the dose–response curve was calculated as DS = OSL(0–40 s) − OSL(130–150 s) (17). This was explained by signal build-up with repeated irradiation. Geber-Bergstrand et al.(39) concluded that a single irradiation with a dose in the range of 0.5–5.0 Gy can be used for OSL signal calibration; currently, this is a commonly accepted approach, and it has been used in several intercomparisons with SMRs (e.g. Bassinet et al.(21)). MDD The MDD depends on many factors, including the size and number of resistors measured as one sample (i.e. together), the time between irradiation and OSL readout, wavelength of the stimulation light, intra- and inter-phone variability in resistors’ radiation sensitivity, etc. Reported values of MDD are usually in the range from few mGy for the OSL readout taken immediately after irradiation(8,39) and up to 0.43 Gy if measured 144 h after exposure(40). An estimation of a possible MDD for even longer storage times showed that a dose of 2 Gy could be detected even 9 y after exposure(39). Fading of OSL Fading of OSL signals in SMRs with time after irradiation was first observed by Inrig et al.(8) and was believed to include both thermal and so-called athermal or anomalous components. The latter is related to the recombination of trapped charges through a tunneling mechanism(41). After preheating the sample to 120°C, fading was described to follow a log-dependence: $$\begin{equation} I={I}_c\left[1\hbox{--} g/100\ {\log}_{10}\left(t/{t}_c\right)\right], \end{equation}$$(2) where Ic is the luminescence intensity at some time tc following irradiation, and g (fading rate) is the percent decrease in intensity per decade of time. The value of g depends on the choice of tc. For tc = 8.3 h, g = 23.7%(8). The g-value depends on several factors, including the preheat (PH) temperature. The fading rate at 160°C was reduced to 10.6% per decade, but this benefit was accompanied by the significant decrease of the OSL signal. The fading rate is also variable among samples from different phones, and fading rate values of 26.08% and 20% have been obtained for the same tc time as above (8.3 h)(17,42). It should be noted that fading in samples without a PH does not follow the log-dependence and some authors used a three-exponential decay function with half-life values of 0.8 ± 0.1 d, 22.3 ± 4.5 d and 790 ± 210 d for rapid, intermediate and slow fading components, respectively(39), or even a five-exponential decay function(14). It should be noted that since the substrates of SMRs is amorphous alumina, a distribution of optical trap depths might be expected, in which case, a simple exponential decay might not be expected. Fading demonstrated both intra- and inter-brand variability. Sholom and McKeever(43) showed that SMRs of the same size (0.5 × 1.0 × 0.4 mm3) but obtained from eight different manufacturers showed an array of different fading rates, as shown in Figure 2 for a fading time of 1 d. The variability for both intra-brand (for samples of the same brand) and inter-brand (from samples from different brands) are shown in this figure. The intra-band variability was in the range of 0.4–7.0% (one standard deviation) with an average value 2.6%, while the inter-brand variability was as high as 8.4% (one standard deviation). The same study also showed that OSL fading in SMRs is dose dependent(43). The fading increases from ~21 to ~ 39% as the dose increases from 0.17 to 8.5 Gy, with the most variable dependence observed for doses below 3 Gy. For higher doses, the dependence was approximately constant. Figure 2 Open in new tabDownload slide Variability of the fading of OSL signals from SMRs of different brands tested with the ‘full’ mode (see text). Each datum point is an average of measurements of three samples of the same brand. Also shown are the lines for the average and standard deviations for all brands. The inter-brand variability is as high as 8.4% (one standard deviation). (Reproduced from Sholom and McKeever(43).) Energy–response characteristics and angle dependence Only a few papers discuss the topics of energy–response characteristics and angle dependence. An increase in sensitivity of the order of 4 times (comparing to a 60Co reference) has been observed for 48 keV photons(16), 6.3 times for 45 keV photons(14) and 2.6 times at about 50 keV(12). The large variation in the reported numbers is mainly related to the differences in the beam spectra used by different research teams. For example, the maximum increase (6.3 times) was observed using ISO Wide Series of X-ray beams(14), and this series includes a greater proportion of lower-energy X-rays than the Narrow Series, which increases the response. The surroundings of the resistors (i.e. the shielding factors) during irradiation were also different in the above-cited works, which also affects the measured energy–response relationship of these dosimeters. The angle dependence of the SMRs’ response was studied using a Monte Carlo method(44). Phones were simulated mounted on a human cyber phantom in four typical locations (namely, ‘chest’, ‘leg’, ‘back’ and ‘hip’) and irradiated in seven different geometries (i.e. anterior–posterior, posterior–anterior left-lateral, right-lateral isotropic, floor and rotational. The last one was taken as the mean of the first four geometries’ data). Three different photon beams (137Cs, 60Co and 192Ir) were also simulated. The simulated OSL doses from the phones’ resistors were in reasonable agreement with the corresponding whole-body doses only for isotropic, floor and rotational geometries. In other geometries, the OSL doses depended on the phone location relative to the body, which includes the angle between an incident beam and the phone. Variations due to resistors’ position inside a phone were found to contribute just a few percent to the total uncertainties. In an experimental study with phones irradiated on a phantom in different geometries(45), two phones were exposed in the anterior–posterior geometry and two other phones were exposed in the rotational geometry. The distance between the source and the center of the phones was kept the same during the rotational exposure. OSL doses reconstructed 2 weeks after exposure and corrected on fading were compared with the corresponding reference value (i.e. air kerma at 1368 mm from the source). For the anterior–posterior geometry, the difference between the OSL and reference doses were within 10%, while for the rotational geometry, the difference was much higher (15–21%). This was assumed to be due to an angle dependency. Dose reconstruction tests Several dose recovery tests have been conducted on the samples exposed to doses in the range of 0.3–16 Gy and have been measured at different times after irradiation(10,11,12,21,17,45,46). Usually, fading-corrected OSL doses were found to be within 25% deviation from the corresponding nominal values, in agreement with earlier estimates(22). The better coincidence was observed if sample-specific fading correction was used (in comparison with correction using a universal fading curve)(12). Improper fading correction (i.e. with the universal fading curve) was named as one of main reasons for the discrepancy between the reconstructed and nominal doses obtained by some participants of recent intercomparison of OSL dosimetry techniques with SMRs(21). It is appropriate to mention that reconstructed doses may be systematically underestimated (by about 10%) if the phone was connected to the cellular network after irradiation(42). This effect is related to the elevated temperature inside the phone, which in turn increases the fading rate. TL from SMRs General characteristics of TL glow curves in SMRs SMRs have also been tested as potential emergency dosimeters using TL. A variable TL BGS has been reported as the possible main issue for the TL technique with SMRs. This is illustrated in Figure 3a, in which the glow curves of resistors from seven unexposed mobile phones are shown. For reference, a typical TL radiation-induced glow curve is shown in Figure 3b. All curves were obtained using the filter pack HA-3 plus Schott BG12, which transmits in the wavelength range of 300–530 nm, with a heating rate of 2°C/s and after sample PH at 120°C for 10 s. Figure 3 Open in new tabDownload slide (a) TL signal of resistors of seven different mobile phones extracted in the laboratory white light. (b) Typical radiation-induced glow curve from SMRs with the dosimetric peak at approximately 170°C. (Reproduced from Ademola and Woda(47).) As seen in Figure 3, a pronounced zero-dose signal is observed at temperatures above 320°C for five out of seven tested samples, but a less-intense signal is also present at lower temperatures in all samples. As a result, the region between 100 and 200°C was chosen as a detection window for integration of the RIS. Doses from 0 Gy to several hundreds of mGy were measured in unexposed samples. Zero-dose signals (BGS) in the range of 100–200 °C were reduced to an average value 32 ± 23 mGy when the resistors were extracted under subdued red light. Quite similar TL glow curves have been reported in other publications. For example, TL peaks were observed at 80, 185 and 330°C(12), at 89 and 190°C(8) and at 80, 180 and 345°C(46). A heating rate of 5°C/s was used in above papers, and it should be noted that no preheating was used; this is the reason for the appearance of the low-temperature peak. Dose–response characteristics The TL dose–response relationship is reported to be linear (usually after correction using the response to a test dose) in the ranges of 0–4 Gy(47), 0–30 Gy(12) and 0–10 Gy(48). In the last paper, the TL RIS was integrated over 100–260 °C. Irreversible sensitivity change has been observed after the first TL readout, which resulted in a systematic dose overestimation by a factor of 1.39 ± 0.11(47). The sensitivity change effect is a significant potential drawback of TL dosimetry with SMRs, which requires further investigation. MDD The MDD obtained with the TL is comparable with values reported for the OSL, and is about 10 mGy(49). Fading of TL signals The data concerning TL fading in SMRs are contradictory to some degree. The same fading was observed for TL and OSL when the TL signal is integrated over the range of 100–150 °C, but the TL fading rate was observed to be lower for the region from 100 to 200 °C(47). A slightly different dependence is reported when the TL signal was integrated over the range of 120–400°C(12) (see Figure 4.) In other studies, practically no decay of the TL signal was observed up to 10 d after irradiation(49). It should be noted that this latter study tested quite unusual resistors of size 2 × 1.25 × 0.55 mm3 obtained directly from a vendor, and these may have different properties from those used in phones. More studies are required to better characterize the fading of the TL in SMRs from phones. Figure 4 Open in new tabDownload slide Fading of TL and OSL signals. The signals were normalized to the first measurement conducted at 100 s after irradiation. (Reproduced from Ekendahl and Judas(12).) For the samples with fast fading, the rate may be reduced if the radiation-induced TL signals from the deep traps are measured. This was realized using PTTL for which a sample is first heated to some enhanced temperature (say, 400°C) to remove charges from shallow and intermediate traps. After subsequent exposure to either blue or UV light, the shallow and intermediate traps are repopulated by transferring charges from the deep traps unaffected by the 400°C heating. Finally, a TL glow curve is recorded, which is the PTTL glow curve. The PTTL technique was tested with SMRs by Bossin et al.(50), and the fading of the PTTL signals over 200 d was about 30% less than the TL fading measured in the same samples over the same period. Energy–response characteristics and dependence on angle Although there are no published TL-specific data regarding these effects in SMRs, they are expected to be essentially the same as those studied by OSL(44). Dose reconstruction tests There are few papers dealing with dose recovery using TL from SMRs. Several phones have been exposed to doses of 200 and 500 mGy and were measured shortly after irradiation and 9 d after irradiation(47). TL doses without correction for sensitivity changes demonstrated significant overestimation due the irreversible sensitivity changes already noted, but were within 6% of the corresponding nominal values after proper correction. An extensive dose reconstruction test was conducted by Hayes and O’Mara(49). Samples of new, large SMRs were exposed to laboratory doses from 5 to 500 mGy and then measured with the TL using a single-aliquot, regeneration (SAR) protocol. Good correspondence with the known applied doses was obtained, with an uncertainty of ~10 mGy (1σ) for doses below 250 mGy, and around ±50 mGy for a dose of 0.5 Gy. Again, results were obtained with the unusual, large format resistors that did not demonstrate any fading over 10 d following irradiation. An interesting approach was tested by Lee et al.(46), where samples of both resistors and inductors were exposed to three different doses (304.5, 1695.5 and 3297.3 mGy) and then measured with the OSL followed by remeasuring with the TL. Two doses, one from OSL and one from TL, were then obtained for each sample consecutively. Both OSL and TL doses of resistors were within 24% of the corresponding nominal values. OSL and TL dosimetry with capacitors Little information is available about the dosimetric properties of this electronic component of mobile phones, which is probably related to the relatively low sensitivity of capacitors. According to Bassinet et al.(10), the RIS OSL signal was detected only in tantalum capacitors. Other publications that deal with this component do not specify the type of capacitor tested(11,20,51). The sensitivity of capacitors, as determined by Trompier et al.(51), is about seven times lower than that of resistors. Sensitization was about 14% per 10 cycles of repeated 9.4 Gy irradiation and OSL readout(10) but was absent for several cycles of 0.5 or 2 Gy doses(11,20). The dose–OSL response relationship has been found to be linear(10,11) and variable (linear, exponentially saturated and exponentially associated)(20) for different tested capacitors. The fading behaviors reported by different research teams are also quite different. The same fading was observed by Trompier et al.(51) for capacitors, resistors and inductors, which resulted in a 50% signal loss after 10 d with respect to the signal recorded 0.5 h after irradiation. Much faster fading was observed by Beerten et al.(11) when only 20% of the signal remained in the samples after 10 d. The fading dependence in this paper was fitted by a three-exponential decay function. Finally, the fading observed by Bassinet et al.(10) followed a logarithmic law but also depended on the temperature of sample PH. All the aforesaid suggest that the properties of capacitors used in mobile phones are quite variable, probably due to a large variability in the materials used during manufacture. More research is required to categorize these components and systematize their properties. Nevertheless, the first dose-recovery tests showed the potential of capacitors for emergency dosimetry(10,11). The samples were exposed to doses of either 15(10) or 1 Gy(11) and were measured with the OSL technique. A fading correction was applied to the OSL doses by Beerten et al.(11), while no correction was required in the work of Bassinet et al.(10) because the measurements were undertaken immediately after irradiation. The reconstructed doses were within 10–20% of the corresponding nominal values, which is a quite promising result. OSL dosimetry with inductors Inductors are probably the most underestimated electronic components of the mobile phones regarding to their possible application in emergency luminescence dosimetry. They usually have an alumina substrate similar to that of SMRs but of much higher sensitivity. Below are summarized the data available from the literature concerning this material along with some new results. General characteristics of OSL decay curves in inductors Typical OSL decay curves in inductors are shown in Figure 5. They were obtained from inductors extracted from a Samsung Galaxy S2 smartphone; five inductors of the size 1 × 0.5 × 0.5 mm3 were measured together at room temperature (RT). The decay rate of the OSL is much faster than that for SMRs, and the sensitivity is a higher by a factor of 5 compared to SMRs(19). No zero-dose signals were detected in inductors tested with OSL. Figure 5 Open in new tabDownload slide Continuous wave OSL (CW-OSL) decay curves of a typical smartphone surface-mounted inductor. (Reproduced from Lee et al.(19).) Dose–response characteristics In the literature, the dose–response of inductors is reported for absolute OSL intensities and is linear for stimulation with either blue(19) or violet(37) light. So, by analogy with SMRs, only a single irradiation to the laboratory dose in the range of 0.5–5 Gy will be enough for the OSL signal calibration for inductors. It should be noted that three different types of inductors—green/black, white/black and an unspecified one—were tested by Bassinet et al.(37), and Lee et al.(19), all demonstrating high radiation sensitivity and a linear dose–response function. MDDs MDD values as low as 2.4 and 8 mGy have been reported(19,51). These values correspond to the signals observed soon after irradiation and are expected to be higher in a real dose reconstruction scenario when the OSL is recorded at some time after exposure. Fading of OSL signals According to Trompier et al.(51), fading for 0402 inductors (of size 1 × 0.5 × 0.5 mm3) was the same as for resistors and capacitors; 50% of the signal remained after 10 d with respect to the signal at 30 min after irradiation. Much slower fading was observed for inductors of smaller size (type 0201) for which the signal loss was about 26% for the same time. However, as noted by Trompier et al.(51), this result was obtained for a small subset of 12 samples and requires further verification. Faster fading was observed by Lee et al.(19), with the OSL being reduced by 50% after 36 h. Energy–response characteristics and dependence on angle There is no information about these effects in inductors, but because the composition and sizes of inductors are very similar to those of SMRs, one can expect a similar energy–response characteristic and angle dependence for the two materials. Dose reconstruction tests Lee et al.(46) report the results of dose reconstruction using OSL with inductors. In this paper, as was already mentioned above, the samples of both resistors and inductors were exposed to three different doses and then consecutively measured with the OSL and TL. OSL and TL doses of inductors were within 20% of the corresponding nominal values. TL dosimetry with inductors General characteristics of TL glow curves in inductors Typical TL glow curves for inductors is shown in Figure 6. They were obtained from a set of 10 multilayer, chip inductors (the same as the white/black ones used by Bassinet et al.(37)), which were irradiated to 800 mGy using a heating rate of 2°C/s. Five peaks located at 100, 140, 170, 270 and 340°C are seen on the glow curve recorded without sample PH; two unstable low-temperature peaks are removed using the PH at 120°C for 10 s. Quite similar TL glow curves were observed by others(36,46,48) with some variations in peak positions due to difference in the heating rates and, possibly, inter-sample variability in inductor properties. Figure 6 Open in new tabDownload slide Glow curves of a set of 10 inductors obtained from the same manufacturer (TDK Corporation) and measured with and without preheating. The applied dose was 800 mGy. (Reproduced from Fiedler and Woda(18).) The zero-dose TL signal in inductors is relatively low. According to Lee et al.(48), it was found to be equivalent to a signal after 15 mGy, while Fiedler and Woda(18) found it to be almost zero around the two main TL peaks (170 and 270 °C). Dose–response characteristics The reported dose–response relationship is linear at least up to the 10 Gy if the SAR protocol is utilized(18,19,48). Without the SAR protocol, the response is supralinear due to a strong sensitization effect. At the same time, if the TL was measured after OSL, the response (also for normalized signals) was linear up to 4 Gy and then sublinear(46). MDDs The MDDs determined with TL are slightly higher than the corresponding values observed for the OSL but are still far below the level required for emergency dosimetry. An MDD of about 10 mGy was determined when 10 inductors of the size 0.5 × 1.0 mm were measured together(18). If the TL signal is read out after the OSL, the MDD increases to about 67 mGy(46). It should be noted that the TL sensitivity of inductors depends strongly on the filter installed between the sample and the photomultiplier tube. As was demonstrated by Lee et al.(48), the TL emission from inductors increases by a factor of 85 if it is recorded without the U-340 filter, which is usually used for the OSL readout. This is because the TL emission has two peaks: a main emission peaking at 700 nm and a broad one between 330 and 550 nm. An infrared-sensitive detector may be useful in the TL dosimetry with inductors, if issues due to background black-body radiation from the heater strip can be overcome. Fading of TL signals TL fading from inductors depends on the temperature range used for integration of the signal but is usually much slower than for OSL. Fading as low as 14% over 1 week was observed for the 270°C peak in comparison with 31% for the 170°C peak in this material and 45% for the 170°C peak in resistors (also for over 1 week)(18). Lee et al.(48) reported similar fading, with a loss of TL over 6.4 d of 50% for the range of 100–210°C and 23% for the 210–340°C range. The corresponding fading of OSL was >80%. In the case when TL is measured after OSL, the TL signal loss over 36 h was found to be ~7% in comparison with more than 80% for the OSL signal(46). Energy–response characteristics and angle dependence As in the case of OSL, there is no information about the energy–response characteristic and the angle dependence effects in inductors measured with the TL, but because composition and sizes of inductors are very similar to those of SMRs, one can expect similar effects for the two materials. Dose reconstruction tests Systematic dose overestimation by 10–40% has observed for a given dose of 500 mGy(18). This effect is explained by a sensitivity change during the first TL readout which is not fully corrected by normalization using a test dose. For inductors extracted from mobile phones, the overestimation was higher than for those obtained directly from the manufacturer, which suggests that glue residues may amplify this effect. Fiedler and Woda(18) concluded that the dose overestimation effect needs to be further investigated using intact mobile phones. TL doses reconstructed from the samples of inductors exposed to three different doses and then consecutively measured with the OSL and TL were within 20% of deviation from the corresponding nominal values(46). Variability of OSL and TL dosimetry properties of inductors; comparison with resistors and capacitors The basic dosimetry properties of OSL and TL signals—namely, responses to dose, MDDs and fading dependences—have been tested for several samples. Some SMRs as well as capacitors were included for a comparison purpose. The list of samples tested is shown in Table 1, and a photograph of the tested samples is shown in Figure 7. Table 1 The list of inductors tested in a variability study. Some resistors and capacitors are also included for comparison. Sample # . Manufacturer/source . Mfr. # . Size (mm3) . Phone small inductors (Ind-small) Samsung S6 N/a 0.60 × 0.30 × 0.33 Phone large inductors (Ind-large) Samsung S6 N/a 1.60 × 0.81 × 0.45 New inductors #1 (Ind1) Murata LQP03TN4N7H02D 0.60 × 0.30 × 0.33 New inductors #2 (Ind2) TE Connectivity 36401E3N6ATDF 1.00 × 0.50 × 0.37 New inductors #3 (Ind3) Murata LQP15MN6N8B02D 1.00 × 0.50 × 0.45 New inductors #4 (Ind4) ABRACON ATFC-0201HQ-2N0B-T 0.60 × 0.30 × 0.28 SMRs CTS Resistor Products 73L1R47J 1.00 × 0.50 × 0.40 Capacitors AVX Corporation 06033K220FBTTR 1.60 × 0.81 × 0.60 Sample # . Manufacturer/source . Mfr. # . Size (mm3) . Phone small inductors (Ind-small) Samsung S6 N/a 0.60 × 0.30 × 0.33 Phone large inductors (Ind-large) Samsung S6 N/a 1.60 × 0.81 × 0.45 New inductors #1 (Ind1) Murata LQP03TN4N7H02D 0.60 × 0.30 × 0.33 New inductors #2 (Ind2) TE Connectivity 36401E3N6ATDF 1.00 × 0.50 × 0.37 New inductors #3 (Ind3) Murata LQP15MN6N8B02D 1.00 × 0.50 × 0.45 New inductors #4 (Ind4) ABRACON ATFC-0201HQ-2N0B-T 0.60 × 0.30 × 0.28 SMRs CTS Resistor Products 73L1R47J 1.00 × 0.50 × 0.40 Capacitors AVX Corporation 06033K220FBTTR 1.60 × 0.81 × 0.60 Open in new tab Table 1 The list of inductors tested in a variability study. Some resistors and capacitors are also included for comparison. Sample # . Manufacturer/source . Mfr. # . Size (mm3) . Phone small inductors (Ind-small) Samsung S6 N/a 0.60 × 0.30 × 0.33 Phone large inductors (Ind-large) Samsung S6 N/a 1.60 × 0.81 × 0.45 New inductors #1 (Ind1) Murata LQP03TN4N7H02D 0.60 × 0.30 × 0.33 New inductors #2 (Ind2) TE Connectivity 36401E3N6ATDF 1.00 × 0.50 × 0.37 New inductors #3 (Ind3) Murata LQP15MN6N8B02D 1.00 × 0.50 × 0.45 New inductors #4 (Ind4) ABRACON ATFC-0201HQ-2N0B-T 0.60 × 0.30 × 0.28 SMRs CTS Resistor Products 73L1R47J 1.00 × 0.50 × 0.40 Capacitors AVX Corporation 06033K220FBTTR 1.60 × 0.81 × 0.60 Sample # . Manufacturer/source . Mfr. # . Size (mm3) . Phone small inductors (Ind-small) Samsung S6 N/a 0.60 × 0.30 × 0.33 Phone large inductors (Ind-large) Samsung S6 N/a 1.60 × 0.81 × 0.45 New inductors #1 (Ind1) Murata LQP03TN4N7H02D 0.60 × 0.30 × 0.33 New inductors #2 (Ind2) TE Connectivity 36401E3N6ATDF 1.00 × 0.50 × 0.37 New inductors #3 (Ind3) Murata LQP15MN6N8B02D 1.00 × 0.50 × 0.45 New inductors #4 (Ind4) ABRACON ATFC-0201HQ-2N0B-T 0.60 × 0.30 × 0.28 SMRs CTS Resistor Products 73L1R47J 1.00 × 0.50 × 0.40 Capacitors AVX Corporation 06033K220FBTTR 1.60 × 0.81 × 0.60 Open in new tab Figure 7 Open in new tabDownload slide Photograph of samples tested in the variability study: 1–4—inductors (new1–new4; see Table 1 for details); 5 and 6—small and medium inductors from Samsung S6 phone; 7—samples of small SMRs from the same phone and 8—samples of new SMRs. OSL properties of the tested inductors OSL decay curves of tested samples are presented in Figure 8. They were recorded immediately after sample exposure to a beta dose of 5 Gy, using the so-called ‘fast’ (Figure 8a and c) and ‘full’ (Figure 8b and d) protocols developed for SMRs(21). The curves are shown for absolute (Figure 8a and b) as well as for normalized intensities (Figure 8c and d). A large variability in both the intensities and the shapes is observed for the different samples; the lowest sensitivity was observed for capacitors, while the highest intensity was detected in inductor Ind3 (i.e. new inductor 3 in Table 1). Figure 8 Open in new tabDownload slide OSL decay curves for several inductors as well as some representative SMRs and capacitors. (See Table 1 for descriptions of the samples.) Plots (a) and (b) are of absolute intensities, while (c) and (d) are normalized intensities. Samples shown in (a) and (c) were recorded using a ‘fast’ protocol (no PH before OSL readout), while those in (b) and (d) utilized the ‘full’ protocol (PH at 120°C before OSL; OSL is recorded at 100°C). Doses are 5 Gy. Different functions have been tested to fit the OSL curves. The best results were obtained with two different types: a hyperbolic decay function |${I}_{\mathrm{OSL}}(t)=a{\big(1+ bt\big)}^{-1/c}$|⁠, with a, b, c and d being empirical constants(38), and an exponential plus a power-law decay function, |${I}_{\mathrm{OSL}}(t)={a}_1\exp \big\{-t/\tau \big\}+{a}_2{t}^{-n}$|⁠, where a1, a2, τ and n are also empirical constants. The latter function is a combination of two terms, first of which is due to ionization, i.e. excitation to the conduction band, while the second component is due to tunneling from an excited state. Examples of fitting the OSL curve to each of the above functions are shown in Figure 9. Figure 9 Open in new tabDownload slide An example of an OSL decay curve fitted with a hyperbolic function and an exponential plus power-law decay function. OSL was obtained from sample Ind-large (see Table 1) irradiated to 5 Gy and recorded at RT without sample PH. It can be seen in this figure that the exponential plus power-law function fits the OSL curve shape better at the beginning part of the OSL curve, while the hyperbolic one provides more accurate fitting for longer stimulation times. The former function is better related to the possible physical processes during OSL stimulation (when some trapped charges may recombine through ionization to the conduction band, while others recombine via a tunneling mechanism). This function was used in further analysis of OSL curves. The effect of PH temperature on the OSL curves is shown in Figure 10 for an example of Ind3 irradiated to a dose of 3 Gy. Figure 10a represents the original OSL curves for PH temperatures in the range of 20–200°C. (An arrow shows the direction of the PH temperature increase.) Figure 10b shows the dependence of the normalized OSL intensity for the cumulative signal (integrated over the entire OSL curve) as well as for its components obtained after fitting with the exponential plus power-law function. It is seen that the relative contribution from the tunneling component increases with PH temperature, but the absolute intensity of the OSL signal is significantly reduced at the same time. It is expected, by analogy with SMRs(9), that the fading rate will be slower for the higher PH temperatures (due to a reduced contribution from the ionizing component), but this possible improvement in the fading will be accompanied by a loss of sensitivity. A PH in the range of 120–160°C could be recommended as a compromise between fading and sensitivity. Figure 10 Open in new tabDownload slide The effect of PH temperature on the OSL in the sample of Ind-new3 irradiated to a dose of 3 Gy. (a) The original OSL curves (an arrow shows the direction of the PH temperature increase from 20 to 200°C, with an increment of 20°C). (b) Dependence of the normalized OSL intensity integrated over the entire OSL curve as well as for its components obtained after exponential plus power-law decay fitting. Dose versus OSL response curves are illustrated in Figure 11 for the Ind-large sample. The dose–response curves are presented for two DSs calculated in the following ways: DS1 = IOSL(0–6 s) − IOSL(6–12 s) and DS2 = IOSL(0–6 s) − IOSL(144–150 s). The responses are shown for absolute intensities (Figure 11a and c) as well as intensities normalized to the response to a test dose (DT) of 0.8 Gy (Figure 11b and d). The OSL curves were recorded at RT for the minimum possible delay time, tdel, between irradiation and readout (about 17 s, required to move the sample inside OSL reader from the ‘irradiation’ position to the ‘OSL readout’ position, Figure 11a and b). Also shown are the OSL curves and for an effective fading time, tfad = 68 s, which was determined as tfad = tdel + tirr/2 + tpause, where tirr is the irradiation time and tpause is the variable pause between irradiation and OSL readout (Figure 11c and d). It is seen in this figure that the dose–response curves for fixed tdel are non-linear (due to the different effective fading times for different irradiation doses) for both the absolute and normalized signals. However, the curves become linear if the OSL curves are measured at the same effective fading time. This behavior of inductors is related to the fading of the OSL during the irradiation, which may be significant even for the fading times of 10 s. The dose–response curves obtained after sample a PH at 120°C were linear in the tested dose range (0–8 Gy) even for the same delay time mainly due to two reasons: (1) The most unstable part of the OSL was removed by the PH and (2) there is a much longer minimum delay time (about 3 min) in this measurement mode, where the slope of the fading curve becomes flatter. Figure 11 Open in new tabDownload slide Typical OSL dose–response curves for inductors (using the Ind-large sample). The DS were calculated in two different ways: (1) DS1 = IOSL(0–6 s) − IOSL(6–12 s) and (2) DS2 = IOSL(0–6 s) − IOSL(144–150 s). The dose–response curves are shown for absolute intensities (a, c) and for intensities normalized to the response to a test dose of 0.8 Gy (DT) (b, d). OSL curves were recorded either for the same delay time tdel between irradiation and readout (a, b) or for the same effective fading time tfad (c, d), see main text for details). Values of the MDD were estimated for the different samples at different PH temperatures and were found to be in the range of 0.9–6.1 mGy for different inductors measured at the RT immediately after exposure and in the range of 3.4–39 mGy after a PH at 120°C. For comparison, the corresponding values for tested SMRs and capacitors were 2.4 and 12.7 mGy at RT and 13.5 and 40.4 mGy at 120°C, respectively. The OSL fading dependencies are shown in Figure 12 for samples tested without any PH (Figure 12a) as well as with a PH at 120°C (Figure 12b). Figure 12 Open in new tabDownload slide Fading of OSL signals in inductors measured without sample PH (a); the average for all tested samples is shown) and with PH at 120°C (b); fading plots are shown separately for Ind-A and Ind-B. Also shown in the (b) is also the fading curve for SMRs. Fading without any PH was quite significant and was monitored only within the first 6 h after exposure; an average (over all tested inductors) is presented in Figure 12a, which demonstrates that the signal dropped by 20% after 1 min and 80% after 6 h following irradiation. According to results of the fading test with a PH, all inductors could be split in two categories: Ind-A, depicted in Figure 12b, consisted of samples Ind-small, Ind1, Ind3 and Ind4; and Ind-B includes the samples Ind-large and Ind2. Shown in the Figure 12b is also the fading curve for SMRs, which is similar that observed elsewhere(21). Several new samples (unused), taken either from a phone or obtained directly from inductors’ manufacturers, were irradiated to a beta dose of 1.44 Gy and were measured with OSL 3 d later to simulate a dose reconstruction test. The results are shown in Table 2 and demonstrate that the differences between the fading-corrected doses and the corresponding nominal values are better than 20%. It should be mentioned that only one inductor of the type Ind-large was available for the dose reconstruction test (it was extracted from a Samsung S6 phone, which has just single inductor of such type). Nevertheless, the OSL from this inductor was strong enough to ensure a reliable dose estimation. TL properties of tested inductors The TL glow curves from the tested inductors irradiated to a dose of 5 Gy are shown in Figure 13. Before irradiation, the samples were annealed at 450°C to remove all possible pre-existing signals. The TL glow curves were recorded using a BG-39 filter in front of the photomultiplier tube at a 5°C/s heating rate and by subtracting the background. Shown are both original curves (Figure 13a) and after normalization to the 180°C TL peak (Figure 13b). Figure 13 Open in new tabDownload slide TL glow curves of various inductors irradiated to 5 Gy (beta). (a) Original TL curves and (b) curves normalized to the 180°C TL peak. As it can be seen in Figure 13, TL glow curves from the different samples have peaks at 80, 140 (only sample Ind3), 180, 280 and 350°C. The positions of the peaks vary slightly due to different thicknesses of samples and overlap with neighboring peaks, but, in general, the observed TL curves are consistent with those reported by others(18,48). The intensities of the different peaks demonstrate significant variability between the different samples. Zero-dose TL signals were measured for some representative samples and are shown in Figure 14 (not all samples from Table 1 were available for this test). A large variability is also seen in this figure with TL BGS observed mainly above 200°C. This may affect the accuracy of dose reconstruction if high-temperature TL signals are used. Figure 14 Open in new tabDownload slide Zero-dose TL glow curves for some representative inductors. The curve of sample Ind3 was multiplied by a factor of 0.01 for clarity. The glow curves in Figure 13 suggest that a PH may be useful to remove the contribution from the low-temperature TL peaks and thereby reduce possible fading. Since some inductors demonstrated a TL peak at about 140°C, a PH at 160°C was used. This resulted in the TL glow curves and corresponding dose–response dependences presented in Figure 15. Figure 15 Open in new tabDownload slide TL glow curves for the sample Ind3 irradiated to different doses and recorded with a PH of 160°C for 10 s (a), and corresponding dose–response curves obtained for two different temperature regions (b). MDDs were estimated for different samples and were in the ranges of 38–99 mGy and 23–213 mGy for the temperature regions of 140–250°C and 250–350°C, respectively. It should be emphasized that these values were obtained on the samples annealed before irradiation and do not take into account the potential contribution from possible zero-dose signals, which might be as high as a few Gray in the range of 140–250°C and much higher for the 250–350°C interval. Due to the high variability in possible zero-dose signal intensities, TL dosimetry with inductors seems to be questionable. Table 2 Results of dose reconstruction test for inductors using OSL. Sample # . OSL dose (Gy) . Fading-corrected dose (Gy) . Nominal dose (Gy) . Deviation (%) . Ind-large 0.45 1.19 1.44 17 Ind1 0.80 1.38 1.44 4 Ind3 0.73 1.25 1.44 13 Ind4 0.75 1.29 1.44 10 Capac 0.55 1.52 1.44 6 SMRs 0.65 1.30 1.44 10 Sample # . OSL dose (Gy) . Fading-corrected dose (Gy) . Nominal dose (Gy) . Deviation (%) . Ind-large 0.45 1.19 1.44 17 Ind1 0.80 1.38 1.44 4 Ind3 0.73 1.25 1.44 13 Ind4 0.75 1.29 1.44 10 Capac 0.55 1.52 1.44 6 SMRs 0.65 1.30 1.44 10 Open in new tab Table 2 Results of dose reconstruction test for inductors using OSL. Sample # . OSL dose (Gy) . Fading-corrected dose (Gy) . Nominal dose (Gy) . Deviation (%) . Ind-large 0.45 1.19 1.44 17 Ind1 0.80 1.38 1.44 4 Ind3 0.73 1.25 1.44 13 Ind4 0.75 1.29 1.44 10 Capac 0.55 1.52 1.44 6 SMRs 0.65 1.30 1.44 10 Sample # . OSL dose (Gy) . Fading-corrected dose (Gy) . Nominal dose (Gy) . Deviation (%) . Ind-large 0.45 1.19 1.44 17 Ind1 0.80 1.38 1.44 4 Ind3 0.73 1.25 1.44 13 Ind4 0.75 1.29 1.44 10 Capac 0.55 1.52 1.44 6 SMRs 0.65 1.30 1.44 10 Open in new tab OSL and TL dosimetry with integrated circuits ICs seem to be very attractive for possible applications in emergency dosimetry due to several factors, including their wide occurrence (every mobile phone has at least one or two ICs of large (at least 10 × 10 mm2) size and several smaller ones), easy location within a phone (not necessary to use an microscope to locate this component of the phone) and moderate radiation sensitivity. Nevertheless, there are only few papers devoted to possible emergency dosimetry with these electronic components. Early work(23) demonstrated the prospect of using ICs with TL. Values of MDD for this technique were in the range of 40–250 mGy and fading within 1 week was about 20%. These are quite promising parameters for the emergency dosimetry. However, follow-up work(52) on ICs from newer phones revealed significant changes in their TL properties, especially a reduction in radiation sensitivity. It was noted that the TL signals observed after exposure to 1 Gy are quite low for possible dosimetric application. The main attention below is paid to the potential use of OSL. General characteristics of OSL curves in ICs Example OSL decay curves recorded at two different temperatures (i.e. the RT and 100°C) are shown in Figure 16(52). These signals were reproducible within 9% for 10 irradiation/OSL readout cycles, which was similar to the data reported by others(20) but are in contradiction to the sensitization effect observed elsewhere(10). In the latter reference, the OSL response increased by 53% after 10 cycles, but it should be noted that a high dose (9.4 Gy) was used for each cycle. Figure 16 Open in new tabDownload slide OSL decay curves after irradiation of sample iPhone 5S with a dose of 0.98 Gy, measured at RT and at 100°C. (Reproduced from Mrozik et al.(52).) Dose–response characteristics Several linear dose–response relationships have been reported(24,46,52), while, at the same time, a non-linear dose–response curve has also been observed and fitted by a cubic function(20). MDDs ICs are in general less sensitive than SMRs. Bassinet et al.(10) noted that the sensitivity of the ICs studied by them was 1–2 orders of magnitude lower than that of SMRs, while Lee et al.(19) estimated the difference in sensitivity to be ~eight times. The absolute values of the MDD are reported to be 13.7(19)~50 mGy(52) and 130–260 mGy(24). These values are acceptable for possible emergency dosimetry applications. Fading of OSL signals Fading of BSL appears to follow a logarithmic law, but it also depends on the temperature(10,52). Mrozik et al.(52) showed, counter-intuitively, that the fading is slightly faster at over the first 60 min of storage if held at RT compared to storage at 100°C. However, after 1 d, more signal had faded when held at 100°C compared to when held at RT. For both storage temperatures, ~30% of the signal still remained in the sample after 1 month from the irradiation, implying that there is a residual, stable signal. McKeever and Sholom(7) compared the fading rates of OSL from the IC samples stimulated with blue and infrared light (BSL and IRSL, respectively). The corresponding curves are shown in Figure 17 and demonstrate almost no fading if the sample is tested with IRSL. This is a quite promising property of the ICs, which should be tested in more detail. Figure 17 Open in new tabDownload slide (a) Fading of BSL and (b) fading of IRSL from ICs. The BSL fading can be described as the sum of two exponentials, whereas little or no fading is observed with IRSL. (Reproduced from McKeever and Sholom(7).) Dose reconstruction tests Several papers report the results of dose recovery test using ICs with OSL. Bassinet et al.(10) irradiated two IC samples to 15 Gy and measured the BSL immediately after exposure using the SAR protocol. Mrozik et al.(52) irradiated 20 samples to different doses and measured the BSL at different times within the range of 5 h to 14 d after irradiation. Several samples were exposed to 0.8 Gy and measured within 7 d by Sholom and McKeever(24). In all these experiments, the reconstructed doses were within 25% of the corresponding nominal values. TL AND OSL FROM SMARTPHONE GLASSES Glasses have long been known to exhibit the necessary properties for use as retrospective/emergency dosimeters; this holds true of the glasses found in earlier-generation mobile phones as well as in modern-day smartphones. In fact, glasses have a few advantages over other phone components in their use as fortuitous dosimeters. The most obvious being the amount of sample available for measurement; while phones, and therefore glasses, continue to get larger, the internal components continue to decrease in size and number. Another advantage is that the glass removal process is the least time consuming and least destructive; this aspect is closely associated with the repair process that is the most feasible and economical for glasses. There are two distinct categories of glass associated with phones. The first consists of those that are integrated into the phone’s display unit (display glasses) as substrates on which the display technologies are built; these are found in both older mobile phones and modern smartphones. The second category is comprised of glasses that are utilized as an element of the phone’s outer protective layer (protective glasses); these glasses are only found on modern smartphones due to the emergence of the capacitive-style touchscreens. An important fact to consider when dealing with smartphone technology is that it has progressed rapidly over the last decade and will continue to do so in the future. These developments are occurring not only in the components that increase the phone’s performance but also in the materials used in making the body as well as the glue that holds it all together. This means that, where smartphones are concerned, there is an ongoing need to not only investigate alternative solutions but to continue research into those previously studied. A prime example of this perpetual change can be observed in the glass that arrived on the front of smartphones in 2007 (Corning’s Gorilla Glass (GG))(53). Since first appearing on the iPhone, it has undergone numerous overhauls and is now in its seventh generation. This section will review the studies of smartphone (and mobile phone) glasses, focusing on those that analyzed their luminescence properties, i.e. TL, PTTL and OSL, and discuss how the ever-changing smartphone landscape has affected their dosimetry properties. Display glasses Display glasses are associated with the smartphone’s display unit, of which modern smartphones employ both liquid crystal (LCD) and light-emitting diode (OLED) display types. There are technical differences between the two display types, but of most importance is that each traditionally utilize glass as the substrate material in its design. These substrates are usually made from variations of borosilicate and aluminosilicate glass that have been optimized for the application of thin-film transistors (TFTs). It should be noted that both display types have two substrates stacked on top of one another. The substrates are made of the same glass; however, the glass furthest away from the front of the device is the one used in all studies due to having, on average, a lower BGS as well as more protection from light exposure. The first mobile phones used both monochrome and color LCD displays (with OLED appearing in 2008), with some models including resistive-style touchscreens. The first smartphones appeared in 2007 and have since become the technology standard; however, basic feature phones are still important to emergency dosimetry as there is still a large audience for them, especially in less-developed nations. For this section, all non-smartphone devices will be referred to as mobile phones. TL studies Display glasses exhibit several favorable TL properties, e.g. good sensitivity to ionizing radiation, a linear dose–response and good signal reproducibility. However, glasses have been shown to possess disadvantageous properties as well; these include signal fading, susceptibility to light and the presence of a BGS that interferes with the determination of the RIS. These sections will examine the favorable properties and delve into the strategies employed to overcome the glasses’ negative aspects. TL glow curves There have been several studies that have reported the different aspects of the RIS glow curves(25,27,29,54–57). Each group uses different measurement techniques, i.e. heating rate and filter set, and it is impractical to try to directly compare the glow curve shapes, intensities and peak positions between studies. Nevertheless, there are some overarching conclusions that can be made, the main one being that there are four distinct glow curve groups, three of which produce well-formed TL peaks, maxima ranging from approximately 100–300°C (Figure 18)(25,27). The fourth group has low or no sensitivity to ionizing radiation. This has been attributed to devices with old monochrome displays(27). A lack of sensitivity has also been observed in mobile phones with color displays and even in smartphones with model years as late as 2018(57). Two of the radiation-sensitive groups feature either a single symmetric or asymmetric curve, while the third is notable for having two distinct peaks. The glow curve shapes are attributed to the different glass types used in the display. Those having a sensitivity to radiation were characterized as lime aluminosilicate and boron silicate glasses, with the former exhibiting greater sensitivity to ionizing radiation(27). Figure 18 Open in new tabDownload slide Classification of typical TL glow curves of glass samples coming from different mobile phone producers classified according to their TL glow curve shapes (Groups A, B and C). (Reproduced from Mrozik et al.(25).) TL emission spectra Determining the emission wavelength of the display glasses is critical for selecting the proper detection window (filter set) for use in measurement. If the incorrect filter is applied, a large portion of the emission spectra of the display glass may be cut off which can severely limit the intensity of signal detected. This in turn could be an issue when dealing with low-sensitivity glasses or exposure to small doses. It should be noted that application of different filter sets will change the shape and intensity of the resultant TL glow curves. Display glass TL emissions have been observed in the 340–680 nm range with lime aluminosilicate glasses exhibiting two peaks at around 380 and 465 nm, whereas boron silicate glasses had a single peak at around 605 nm(28,56). The results indicate that the selection of the detection window is glass specific. Of note, the same TL emission results seen in older mobile phones were also observed in more modern smartphones, up to the 2015-y models(56). This is indicative that smartphone displays are still using the same basic glasses found in older style mobile phones, which is beneficial for emergency dosimetry applications. BGS All display glasses with sensitivity to ionizing radiation display a BGS, which is removed by the first TL readout. The BGS varies in intensity and shape even among glasses of the same type(25,27,29,55). In comparison to the RIS glow curves, the BGS glow curves are noticeably more narrow with peaks at higher temperatures (>350°C) in all detection windows, as shown in Figure 19. Even with noticeable differences between the glow curves, the BGS and RIS overlap is still considerable; thus, to account for this, the integration window has been restricted to 100–250°C, which is a compromise between thermal stability and minimizing the influence of the BGS(27). Figure 19 Open in new tabDownload slide (a) Zero-dose BGS in comparison with the RIS (after 1 Gy). The heating rate was 2°C/s for each measurement. (b) BGSs of 25 different glass samples (lime aluminosilicate) to illustrate the degree of variability in shape. The glow curves were normalized to the peak maximum. All measurements were in the blue (300–580 nm) wavelength range. (Reproduced from Discher and Woda(27).) It has been surmised that the BGS originates from UV exposure during the TFT application process; however, efforts to reproduce this signal with exposure to 366 and 254 nm UV were unsuccessful(27). For older mobile phones, the intensity of the BGS, in terms of dose, was observed to be equivalent to <200 mGy for the majority of lime aluminosilicate samples, with the exception of a few that were as high as 800 and <600 mGy for the boron silicate glasses with a few over 1000 mGy. A sufficient degree of homogeneity of the BGS was found across the display glass(27). Likewise, as illustrated in Figure 20 for smartphone displays, no particular pattern could be identified for the distribution of the BGS on the glass as a whole; however, it was determined that these displays do exhibit high variation of the BGS in some glasses(56). To account for this variation, it is recommended that as many aliquots as possible should be used to ensure an accurate account. Figure 20 Open in new tabDownload slide Distribution of the zero-dose BGS according to the position on the glass for three mobile phone samples. (Reproduced from Kim et al.(56).) The MDD for both glasses, without any glass treatment, were similar with reported values of 150 mGy for older devices and 190 mGy for smartphones(27,56). These MDD values are more than acceptable for emergency dosimetry, where 1 Gy is acceptable for triage purposes. BGS mitigation Three different techniques have been proposed to mitigate the effects of the BGS on dose reconstruction; these include subtracting an average BGS value from the total RIS, using various filter setups to block the BGS or to remove the BGS from the sample. The first technique, subtraction of an average BGS value from the total calculated dose, was utilized after recognizing that the recorded distribution of the BGS comes from a normally distributed population(27). In a dose recovery test, its application has led to an average reconstructed dose that was ~10% lower than the delivered dose. For phones that do not exhibit a large variation in the BGS from one phone to another or across the display of a single phone, this technique could be used for triage purposes. The second technique, blocking the BGS through selective filter selection, was determined to be impracticable through the analysis of the TL emission spectra where the BGS was found to emit in similar wavelengths as the RIS(28). The third technique is by far the most effective. The premise behind this strategy is that the BGS arises from the surface layers of the glass only and therefore it may be removed by chemical etching or mechanical grinding of the glass surface(54,55). The method was able to reduce the BGS, in some samples, by >95% (in the 100–250°C integration region; see Figure 21). As illustrated in Figure 22, neither removal technique created any adverse signals and, in both cases, greatly reduced the BGS while leaving the weight-normalized RIS intact. Application of this technique results in significantly lowered MDD values, with reductions in pre-bleached samples from 340 to 76 mGy and from 1370 to 210 mGy for lime aluminosilicate glasses and boron silicate glasses, respectively. Figure 21 Open in new tabDownload slide BGSs measured for NOKIA 6021 mobile phone glass. Aliquot 1 was untreated, while the surface of aliquot 2 was ground before measurement. A TL RIS following 2 Gy irradiation is also shown for aliquot 2. (Reproduced from Bassinet et al.(55).) Figure 22 Open in new tabDownload slide Glow curves from a NOKIA 3109c mobile phone glass aliquot. A glow curve was measured after applying a surface grinding process on the irradiated aliquot (2 Gy). The other one was measured on the same aliquot irradiated a second time with the same dose (same time delay between irradiation and TL measurement, no additional grinding process). The mechanical treatment affected neither the intensity nor the structure of the glow curve. The glow curve of an unirradiated aliquot recorded after signal surface process is also shown. No TL signal was created by grinding. (Reproduced from Bassinet et al.(55).) The chemical etching method was further optimized by testing the effects of different acids and etching times on the BGS efficacy(25). In doing so, it was discovered that some glasses are more sensitive to pure HF acid, while others reacted better to an acid mixture. The optimal etching time is dependent on the thickness of the glass, but 4 min appeared to be the best selection for most samples for optimizing the BGS reduction while retaining sufficient sample mass for analysis. Optical stability Several studies have observed that the RIS in display glasses is bleachable through light exposure(25,27,29,55), the degree of which is dependent on the light source intensity and wavelength and the light exposure duration. If a sample is exposed to environmental light and the bleached signal is not taken into consideration, a large underestimation in the reconstructed dose could arise. One study showed that, in comparison with an equivalent sample stored in the dark, a loss in TL signal of up to 55% is incurred in samples that are used normally (exposure to indoor and outdoor light) for 5 d(27). In a test of TL signal bleaching effectiveness, the largest TL loss incurred over 500 s, due to direct sunlight exposure, with approximately 14% of the original signal still intact. The second most effective bleaching was with blue light, leaving about 22% of the signal intact(27). During normal use, the rear display glass is protected from direct sunlight exposure by the extra layers of glass and filters. The light bleaching sources most likely to be encountered, i.e. indoor light, diffuse sunlight and self-illumination, can have their exposures simulated with blue light. The blue light exposure has the effect of isolating a more stable signal by preferentially removing the less stable, low-temperature portion of the glow curve. With 500 s determined to be the optimal duration, applying the pre-bleach treatment results in around 80% signal removal. In a subsequent study(25), several light sources were tested for their use in a proposed pre-bleaching protocol; these included blue light (470 nm) as well as 254, 302 and 365 nm UV light. The results showed that various glasses can be more susceptible to different light sources (Figure 23). In particular, the best bleaching effect was obtained with the blue light for one glass sample, whereas the 302 nm UV was the most effective for another. Figure 23 Open in new tabDownload slide TL signal of glass extracted from mobile phone Nokia 6101 (a) and Sony Ericsson p900i (b), both irradiated with a dose of 2 Gy (90Sr/90Y beta source) and then bleached with different wavelengths: 470, 365, 302 and 254 nm (UV lamp), day light and without exposure to light before the TL readout. The glow curves were normalized to the mass of the samples. (Reproduced from Mrozik et al.(25).) Fading All display glasses have been shown to experience anomalous signal fading with increased storage time, with fading rates specific to each glass type. Where lime aluminosilicate glasses retain approximately 50% of their original signal after 1000 h, boron silicate glasses retain only half that amount(27,28). The reproducibility of the fading signal for the lime aluminosilicate glasses was within 7% for older mobile phones and less than 2% for smartphones. This is in contrast to the boron silicate glasses, shown in Figure 24, which experienced an increased dispersion, up to 27%, for longer fading times(56). Another distinction between the two glass types was that the proposed pre-bleach protocol (mentioned above) improved the fading rate for lime aluminosilicate glasses but had no effect on fading in boron silicate glasses. Figure 24 Open in new tabDownload slide Fading of TL signals up to 5 d for the 22 AMOLED substrate glasses (15 lime aluminosilicate glasses and 7 boron silicate glasses). (Reproduced from Kim et al.(56).) While signal fading is an unfavorable attribute, it can be accounted for by fitting to a logarithmic decay function(28). Assuming the storage time is known, this allows for a fading correction factor to be applied in the final dose evaluation. While not appropriate for long-term retrospective dosimetry, the stability of the RIS in display glass is sufficient for emergency dosimetry purposes, where results would be calculated in a matter of days. Dose–response characteristics and reproducibility The dose–response relationship for both glass types has been reported to be linear in several studies(27,55). This includes the 10–20 Gy range in both the UV and blue detection windows for older mobile phones as well as up to 10 Gy in smartphones. It is also been observed that the glasses that undergo a pre-bleach or BGS removal, through chemical etching or grinding, also remain linear over the tested dose ranges(58). The reproducibility of the signal for repeated cycles of irradiation and measurement has been reported as <10% for 10 cycles of older mobile phones(58), with another group reporting within 3% for the same style phones(27); whereas, smartphones demonstrated a reproducibility within 11% for five cycles(57). These results illustrate that the sensitivity of the sample does not change with repeated exposures to radiation or high temperatures; this is an important detail for dose reconstruction where a change in radiation sensitivity would need to be taken into account, complicating the reconstruction process. Energy–response characteristics and angle dependence The position in which a phone is exposed and the source it was exposed to could play a major role in dose reconstruction. Fortunately, the display glass has been investigated for both its possible angle and energy dependences(55,57). For the photon energy dependence, two studies(55,59) showed that the observed responses were in good agreement with the theoretical calculations, with each showing an over-response varying from 5 to 11% when compared to the response to a reference energy >500 keV. The responses were similar for both the lime aluminosilicate glasses and boron silicate glasses, with no differences seen whether the glass was from an older mobile phone or from a smartphone. For the angle dependence, using free-in-air exposures, a strong shielding effect was observed between 90 and 270° for low energies, with the variation in angle response ranging from 13% for 137Cs to 51% for 118 keV photons. These results illustrate that, especially when exposed with low-energy photons, correction factors will need to be applied to account for shielding to be able to accurately determine the received dose. Dose reconstruction tests Experiments have been performed with the glass samples both intact and previously extracted from the displays(54,58). The results showed good correlation with the given doses; however, for the intact experiment, the results did slightly overestimate the administered doses by 5.8–22.2%(58). In comparison to the alumina resistors, with accuracy within 5% for all but one sample, the method performed worse; however, the display glasses do have the advantage of easier sample preparation and lower fading rates. There has been one interlaboratory comparison that has assessed the applicability of a TL protocol on display glasses(4), which was organized by a joint effort between Running the European Network of Biological and Retrospective Physical Dosimetry (RENEB) and the European Radiation Dosimetry Group (EURADOS). The protocol called for the application of a pre-bleach and a chemical etching treatment. Each group was given three samples irradiated to doses of 0.6, 1.5 and 2.5 Gy. The results showed that the reconstructed doses were all within the 95% confidence interval and no significant dose underestimation was observed. This implies that the prescribed protocol successfully accounted for the signal fading and optical bleaching incurred by the sample. In addition, no systematic differences in the quality of results could be seen between the laboratories that participated in a sample preparation and measurement training exercise and those that received detailed written instructions. PTTL properties PTTL is a luminescence method that seeks to overcome some of the complications associated with the TL method by taking advantage of deeper, more stable radiation-sensitive traps that demonstrate a slower fading time with less sensitivity to light exposure. The glass is first heated to temperatures >350°C, then cooled to RT and exposed to a light source to stimulate the transfer of electrons from the deeper traps into the traps that were just emptied during the initial heating. When the signal is reheated a second time, the emitted signal is now PTTL. To date, there has been only one study of PTTL from display glass(60). The research was performed on two lime aluminosilicate display glasses, one from an older mobile phone and the other from a more modern smartphone. The aim of this study was to analyze various aspects of a potential PTTL protocol, such as selecting the appropriate filter set, optimizing the PH temperature and hold time and determining the most suitable phototransfer source. It was determined that the optimized parameters were a PH at 400°C and a hold time of 10 s. The choice of detection window was based on the ratio of the RIS to the non-radiation-induced signal (nRIS) measured by integrating the signals between 200 and 300°C(60). The results, shown in Figure 25, revealed that the best RIS/nRIS ratio was achieved by a filter set with a detection window centered at a wavelength of 340 nm with a full-width-half-maximum value of 30.2 nm. In a similar fashion, the selected light exposure source was determined by examining the RIS/nRIS ratio produced by each source integrated between 200 and 300°C. The experiment was conducted using 254, 302, 365, 385 and 405 nm light sources. In general, the results indicated that the intensity of both the RIS and nRIS PTTL signal increases with decreasing stimulation wavelength. However, shorter wavelengths produced their own TL signals. Therefore, the optimal choice of light source to use is a compromise between the PTTL intensity and the prevention of an nRIS signal generated by the source itself. The RIS/nRIS ratio suggests that any of the sources with wavelengths >302 nm would be appropriate for use. Figure 25 Open in new tabDownload slide Calculated RIS/nRIS ratio of the radiation-induced and the non-radiation-induced PTTL signals. The results are shown in descending order. (Reproduced from Discher et al.(60).) The reproducibility of the PTTL protocol was tested by performing five cycles of irradiation, PH, light exposure and final readout; the results were observed to be within 10% of the original readout for each cycle. In addition, the PTTL dose–response was found to be linear over the 0.24–11.8 Gy dose range with no discernible differences between the smartphone and mobile phone displays. The PTTL signal was confirmed to exhibit an improved signal stability in comparison to that of a standard TL signal. Over a 254-h storage period, the PTTL signal experienced a signal loss of <10%. Furthermore, the PTTL signal was found to be virtually stable over a 20-h period, whereas the TL signal had lost 20% of its original signal over the same time. These observations demonstrate that, unlike a TL protocol, the application of fading corrections may be unnecessary for PTTL. Protective glasses Protective glasses can be subdivided into those built into the device at the time of manufacture and those added after the phone has been purchased by the consumer. The built-in protective glass is located on both the front and back of the device. On the front (screen side), it is commonly referred to as a touchscreen glass and is responsible for protecting the display underneath while also facilitating the phone’s capacitive touch interactions. It is predominately made of Corning’s GG; however, there are other chemically strengthened glasses in use (e.g. Dragontrail glass manufactured by AGC Inc.). GG is also found on the back of many modern smartphones, and its role here is to protect the phone’s internal components while simultaneously enabling the phone’s wireless charging capability. The second group of protective glasses is known as screen protectors. Commonly, they are made of tempered soda-lime glass, where tempered glass is one that has been strengthened through a heating and rapid cooling process. The screen protectors are purchased aftermarket and placed by the consumer on top of the touchscreen glass as an extra layer of protection against scratching. There are many brands in this competitive market (e.g. Zagg, Tech Armor, Spigen and others). Built-in protective glasses The following sections analyze the TL, PTTL and OSL properties of built-in protective glass. Throughout this section, both styles of built-in protective glasses will be referred to as touchscreen glasses, and all data are specific to the GG brand of chemically strengthened glasses. Protective glasses exhibit the same favorable properties (i.e. high sensitivity to radiation, good signal reproducibility, modest signal fading and a linear response to dose). They also suffer from the equivalent drawbacks (i.e. susceptibility to light and a large BGS that is an obstacle for determining the RIS)(26,31,61). TL glow curves Unlike the display glasses, touchscreen glasses do not fall into characteristic categories. Instead, they all belong to the same group since they are all made of the same alkali-aluminosilicate glass. Although this minimizes the variations between samples, it does not signify a consistent set of properties. In fact, while the same general glow curve shape persists, the peak position, signal intensity and glow curve width all vary with the different generations of GG. As illustrated in Figure 26, the glow curves all reside within the 75–450°C range and, generally, consist of only one dominant peak located between 250 and 350°C. However, some generations do have an additional low-intensity peak at around 75–100°C(26,31). Figure 26 Open in new tabDownload slide TL glow curves for samples of various GG generations from the Samsung Galaxy S-series of devices. (a)–(h) represent the Samsung Galaxy S1 to S8, respectively. In each figure, the glow curves represent: (1) the signal from an as-received sample after a dose of 20 Gy; (2) the RIS obtained after (1) and after an additional dose of 20 Gy and (3) the BGS signal of an as-received sample, but for zero dose. (Reproduced from Chandler et al.(31).) TL emission spectra While the RIS glow curves from the lime aluminosilicate display glasses most resemble that from the touchscreen glass, the TL emission wavelengths correlate more closely to those from the boron silicate display glasses. The emission wavelengths for the touchscreen glasses extend into the 650–750 nm range (Figure 27)(31). The choice of the transmission window should be taken into consideration based on the emission wavelength of the sample so as not to cut off a portion of the spectrum. An appropriate filter choice for touchscreen glasses should extend into the visible region. Figure 27 Open in new tabDownload slide TL emission spectra from two samples of different radiation sensitivity (a, b). The samples were irradiated to a beta dose of 20 kGy and the heating rate was 2°C/s. All spectra were corrected for the system wavelength response. (Reproduced from Sholom et al.(62).) BGS The characteristics of the BGS, like those of the RIS, are observed to fluctuate in shape and intensity with the GG generation. The BGS glow curves peak at temperatures of 25–50°C higher than that of the RIS, begin at approximately 150°C and extend past 450°C(31). The observed overlap of the BGS and RIS is more severe for touchscreen glasses than for display glasses, thus, further restricting the possible integration window to between 100 and 175°C(26). The intensity of the BGS glow curves from the earliest generation of GG is minimal (between 141 and 252 mGy); however, the BGS intensity increased steadily, to the tens of Gy, with the subsequent generations. The BGS is thought to be produced by exposure to UV light during production, and is thought to be a by-product of the UV light exposure used during curing of the adhesive used to affix the touchscreen glass to the display(31). The increase in BGS intensity with later GG generations appears to coincide with the transition from adhesive films to the superior, liquid optically clear adhesives (LOCAs), which require the use of UV light for curing. It was also observed that GG that had not yet been attached to a smartphone did not exhibit a BGS, further supporting the notion that the creation of the BGS occurs during the smartphone manufacturing process(31). For touchscreen glasses made of later generation GGs, the BGSs are not homogenous across the display. In fact, one study observed differences of up to 30% between aliquots of the same sample, whereas an increased BGS for aliquots taken from the edge compared to those taken from the center was encountered in another(31,61). The BGS was able to be reproduced in the laboratory environment after undergoing trial exposures of varying wavelengths for 30 min; the tested light sources included 254, 302 and 365 nm UV light as well as a solar simulator(31). No individual exposure source was able to match the BGS in either strength or peak position, but a combination of the solar simulator and the 302 nm UV source was able to do so, indicating that the BGS arises from exposure to a wide-spectrum source. BGS mitigation The same BGS mitigation techniques employed for display glasses were less successful for touchscreen glasses, mostly because of the very large BGS intensity but also due to the lack of uniformity. It has not been found feasible to use a standard reference protocol for subtraction of the BGS from the RIS without encountering uncertainties that are too large (>1 Gy) for emergency dosimetry applications. For display glasses, the removal of the surface layers through chemical etching or mechanical grinding was a successful technique, which resulted in BGS reductions of up to 95%, because the BGS was not able to penetrate into the glass, i.e. it is a surface effect. However, depending on the thickness of the touchscreen glass prior to treatment, a reduction of only 11–51% could be achieved using the same techniques(31). When considering the magnitude of BGS measured from touchscreen glasses, a 50% reduction in the BGS is not adequate for use in emergency dosimetry. The explanation for the disparity between the methods’ effectiveness lies in the different optical absorption coefficients of each glass type. Specifically, a display glass has an optical absorption coefficient that is 50 times higher than that for touchscreen glasses in the 302 nm region, which results in less penetration into the glass medium compared with touchscreen glasses(31). After calculating the UV penetration profiles of the two glass types, it can be seen in Figure 28 that the BGS is attenuated faster as a function of depth (for 365, 302 and 254 nm UV) in the display glass compared to the touchscreen glass. Therefore, taking into consideration the thickness of touchscreen glasses found on smartphones, it is not possible to sufficiently reduce the influence of the BGS by removing the outer layers of the glass. Figure 28 Open in new tabDownload slide Calculated UV light penetration profiles for: (a) Display glass from an Alcatel Pixi Pulsar and (b) for GG from an LG G4 device. Data for three UV wavelengths are indicated: (1) 365 nm, (2) 302 nm and (3) 254 nm. (Reproduced from Chandler et al.(31).) Even though the BGS cannot be removed, a computational matrix method for TL glow curve deconvolution has been applied with some promising initial results(31). The method relies on the comparison of an unknown combined TL signal to reference RIS and BGS glow curves, the BGS being produced by exposure to a solar simulator and a 302 nm UV light source, as described previously. The method, applied to several GG generations, was used for a 10 Gy exposure with results ranging from 10.5 to 14.2 Gy, which were within a 5–43% deviation from the applied dose. Optical stability The optical stability of touchscreen glasses was examined by exposure to varying durations of blue light(26). The results show a strong reduction in TL intensity with increasing exposure time. For display glasses, that are partially protected from light exposure by layers of glass and filters, the use of a pre-bleaching was able to account for any light exposure that would occur during normal use. However, touchscreen glasses that were exposed to 2 h of direct sunlight and 16 h of indoor light had reconstructed values of 51 and 57% of the given dose for annealed and unannealed samples, respectively(26). The underestimation is a consequence of the higher bleaching efficiency of sunlight (compared to blue light) combined with the lack of protection from it for the touchscreen glasses. Fading, dose–response characteristics, and reproducibility The dose–response characteristics of touchscreen glass are linear over the investigated range of 0.1–5.5 Gy, with a reproducibility within 6% for 10 cycles of irradiation and measurement(31). Fading was studied over a 51-d period, with results varying between the touchscreen glasses from the same smartphone model(26). The samples were observed to fade by approximately 30 and 50% over 1-d and 10-d storage periods, respectively. Fortunately, the results can be approximated by a logarithmic function which allows for corrections to be made to recovered doses. PTTL studies The initial PTTL study of touchscreen glasses(30), utilizing first-generation GGs, observed that the best signal was obtained when integrating between 250 and 350°C to avoid the UV-induced TL signal and using a PH temperature ranging from 350 to 400°C, depending on the sample. The optimal stimulation wavelength was observed to be 365 nm, as this wavelength allowed for adequate signal intensity while also limiting the UV-induced TL. Using these parameters, the PTTL dose–response exhibited a linear dose–response from 1 to 20 Gy. In addition, the RIS was found to have excellent fading characteristics, with minimal signal loss over a 20-d storage period (Figure 29). The method was able to perform comparably well alongside SMRs, ICs and display glasses during dose recovery tests. Figure 29 Open in new tabDownload slide Changes to the glow curves due to fading (4 d storage under ambient conditions) for: (a) the TL signal and (b) the PTTL signal. The PTTL signals were obtained following a 400°C preheat and 365 nm exposure for 20 min. The initial delivered beta dose was 20 Gy. (Data from McKeever et al.(30).) Additional studies of PTTL from touchscreen glasses on later generations of GG were less successful(31) due solely to the extensive BGS mentioned earlier. For less sensitive glasses, the differences between aliquots exposed to 20 Gy and those with only the BGS were barely discernible from one another in the 250–350°C range. Following exposures to doses of interest for emergency dosimetry, the RIS in even higher sensitivity glasses would be substantially obscured. OSL studies The most recent smartphones have a glass backing to enable wire-free charging. The opportunity for utilizing OSL as a dosimetry method with smartphones is due to the majority of people protecting their smartphone by installing it in a case. In doing so, and assuming the case is opaque, the back glass is protected from any exposure to light. The OSL BGS was observed to be exceedingly small, to non-existent, with MDDs between 2 and 100 mGy for the tested samples (Figure 30)(62). The RISs showed significant differences in both shape and intensity between different glasses but were able to be divided into two major groups. The first group consisted of the higher sensitivity samples that demonstrated slower OSL decay, whereas the second group was made up of lower sensitivity samples exhibiting a faster OSL decay. When comparing the two groups, the OSL intensity and decay time of Group 1 samples was about one order of magnitude and two times longer, respectively, than their Group 2 counterpart. The OSL dose–response was observed to be linear up to 10 Gy and lost around 82 and 88% of the original signal over the course of 1 week for Group 1 and Group 2 glasses, respectively(62). Figure 30 Open in new tabDownload slide BGS (a, b) and 0.8 Gy RIS (c, d) OSL signals from the back (a, c) and corresponding front (b, d) glasses, tested using continuous wave OSL. The OSL signals were stimulated with 470 nm light at RT and within 2 min after exposure. (Reproduced from Sholom et al.(62).) A pulsed OSL (POSL) technique was tested with the aim of allowing for fiber-optic, non-destructive measurements(62). The premise behind the technique is that the glass could remain intact on the device and an OSL reader outfitted with a fiber-optic attachment could be placed on the glass when performing measurements. Using this setup, the dose–response was found to be linear for doses <7 Gy with estimated MDD values in the range of 0.5–30 Gy, depending on the sample. Utilization of this POSL technique would have the fastest possible result turnaround time as no sample preparation is necessary; however, more work is required to optimize the method. Screen protectors At the onset, screen protectors have the advantage of a simpler sample preparation when compared to both touchscreen and display glasses as the screen protectors are only lightly adhered to the front of the device and their removal causes no damage. An initial study of screen protectors, recently completed on 20 different models of glass screen protectors, examined several of their dosimetry properties(63). The TL signals from the screen protector glass were seen to vary from sample to sample in shape and intensity; however, they were able to be grouped into two distinct categories while using a wideband blue detection window (Figure 31). The first category’s TL glow curve is comprised of three separate peaks; the first peak appears at approximately 90°C, the second at around 350°C and at least one peak in the 150–250°C range. The second category has its main peak in the 200–300°C range, with another prominent peak located at around 100°C. The RIS intensity was observed to be higher for the Category 2 glasses. Figure 31 Open in new tabDownload slide Radiation-induced TL signals from the screen protector glass. Two categories of glasses are identified: (a) Category 1 and (b) Category 2. The glass was previously annealed by heating as in a normal TL measurement. The TL signals (after a dose of 5 Gy) are normalized to the aliquot mass. (Reproduced from Bassinet et al.(63).) Every screen protector glass possessed a BGS that varies in shape and intensity from one screen protector to another. The BGS, after employing the same etching strategy used for display glasses, was largely able to be eliminated. Due to its interference with the RIS, the integration window must be restricted to between 100 and 150°C. The RIS signal was found to be sensitive to light; however, a pre-bleach with blue light was able to restrict further measurements to the hard-to-bleach components. Using this protocol, the reproducibility for five cycles was observed to be within 4% of the original value. In addition, the dose–response was linear up to 20 Gy. Lastly, a fading study on previously annealed aliquots showed that fading of the RIS was negligible within the first 100 h and experienced only around 20% fading after 12 d. An initial dose recovery test observed good agreement between the reconstructed and administered dose(63). SUMMARY AND FURTHER RESEARCH The smartphone as a personal dosimeter Technique development for the assessment of dose to large numbers of individuals in the event of a small-scale or large-scale radiation incident, in which personal dosimeters are unavailable, has progressed significantly in the last decade. Organizations such as ICRU and EURADOS have been instrumental in pushing research into these important areas. Proposed dosimetry techniques include biological and physical dosimetry methods, but the interest of this paper is in luminescence methods applied to ubiquitous materials. The luminescence methods include both optically stimulated and thermally stimulated techniques (OSL and TL), and the main item of interest for dosimetry is a smartphone. Smartphones are widespread and popular worldwide, and it may be expected that they will be carried by most individuals at the time of the radiological incident. If methods for assessing dose using OSL or TL from the components of a smartphone can be successfully developed, these devices could act as surrogates for the accepted TLDs and/or OSLDs for personal radiation dosimeters. Needed research The need for standard protocols One of the clear issues that emerges from a review of current research is that many of the TL and/or OSL techniques proposed by various laboratories are research programs only, and few, if any, standard protocols have yet to emerge and to be thoroughly tested under a wide variety of conditions. Protocols concerning the use of OSL with SMRs are the closest to being generally accepted, with the ‘fast’ and ‘full’ protocols having been tested in interlaboratory tests. An obvious issue, however, is that none of the methods have been tested in a real accident situation; only laboratory tests have been, and indeed can be, conducted and assessed. Future research must include the goal of arriving at standardized methods so that when or if they are needed in a real situation, accepted methods can be applied. The need for non-destructive dose assessment An aspect of using TL or OSL from the components of smartphones is that many of the components proposed (e.g. SMRs, ICs) require destruction of the device for the dose to be determined. Clearly, this is unsatisfactory in that few people may be willing to sacrifice their phone in order to have their dose determined. In an emergency, access to a personal phone will be a high priority for most individuals. Therefore, the development of techniques that use external, rather than internal, phone components must be a high priority. Such components include protective glass, both front and back (for the more recent phone models), and advancement in the use of in situ testing of the protective glass using OSL could be a particularly fruitful and important line of research. Such developments would remove the need to damage the phone and could be performed rapidly. The phone could then be returned to the owner unscathed. It is evident that smartphone glasses provide a strong opportunity on which to build a viable emergency dosimetry procedure. Both display and protective glasses suffer from a fading signal, the presence of an interfering BGS and an adverse susceptibility to light exposure; however, methods have demonstrated their ability to account for these factors. Another important finding is that advances in technology can directly impact the glasses’ ability to function in emergency dosimetry applications, both positively and negatively. An observed adverse effect can be seen in the dramatic increase in the BGS, whereas the possibility to take advantage of an OSL method was the direct result of these technological advancements. The need for studies using phantoms Another issue that requires further study is the testing of phone dosimetry using human anthropomorphic phantoms. Such studies have only been tentative so far and have mostly concerned computer simulations. The primary question centers on the fact that dosimetry techniques using phones determine the dose to the phone, when what is needed is the dose to the person. Therefore, reliable conversion coefficients are needed that relate the dose to the phone, independent of where it is located on the person, to the dose to critical organs (blood-forming organs, central nervous system, reproductive organs, etc.). Note that the dose units should be in Gy since deterministic effects are the primary concern in an emergency. (For a more detailed discussion of this topic, see ICRU Report 94(2).) Experimental tests with phones and human phantoms, irradiated under a wide variety of conditions and performed in parallel with computer simulations, are essential to provide such information. The need for interlaboratory comparisons and field exercises It is also vital that interlaboratory testing be carried out. In cases of large numbers of potentially exposed individuals, multiple laboratories will be called in to help in the dose-assessment task. This presumes the availability of standardized protocols, but it also assumes the availability of multiple, accredited laboratories, each certified to meet accepted standards and to perform such work. In the early, pre-accreditation stages, however, this must involve interlaboratory collaborations and dose comparisons and well-planned field exercises. It is only through such collaborations that standard methods will be achieved, after which accreditation by cognizant organizations can be contemplated. ACKNOWLEDGEMENTS S. Sholom and S. W. 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TI - DEVELOPMENTS IN THE USE OF THERMOLUMINESCENCE AND OPTICALLY STIMULATED LUMINESCENCE FROM MOBILE PHONES IN EMERGENCY DOSIMETRY
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncaa208
DA - 2020-12-30
UR - https://www.deepdyve.com/lp/oxford-university-press/developments-in-the-use-of-thermoluminescence-and-optically-stimulated-oLH0xbyv8t
SP - 205
EP - 235
VL - 192
IS - 2
DP - DeepDyve
ER -