TY - JOUR
AU - Prlic,, Ivica
AB - Abstract Radiation protection and radiation dosimetry strongly rely on measurements performed by dosimetry instrumentation. Two categories of dosimetry instrumentation prevail: personal dosemeters and survey meters. Passive dosemeters were for many years the base of personal and area dosimetry (environmental, including workplace). Survey meters have been long-established between area meters due to their dose rate measurement capability, but just over a decade ago, debates over possibility that electronic personal dosemeters (EPDs) could replace passive personal dosemeters as legal monitoring devices have started. These debates have now branched into the use of EPDs, but also survey meters in various exposure scenarios, where some concerns have been reported. These concerns were mostly related to the response in pulsed X-ray fields and poor energy response. This article summarizes recent literature related to electronic dosemeters for strongly penetrating photon radiation and covers technologies used in contemporary EPDs and survey meters, their performance and future perspectives. INTRODUCTION The quality of measurements in radiation protection and radiation dosimetry relies on dosimetry instrumentation. Two categories of those instruments, or dosemeters, prevail: personal dosemeters and area meters. Passive dosemeters were for many years the base of personal dosimetry. They were and still are used for area dosimetry (environmental, including workplace). According to the results of a large EURADOS survey published in 2014(1), more than 80% of the issued personal dosemeters in Europe were passive thermoluminescent dosemeters (TLDs) (40%) or film dosemeters (42%). The remainder employed other passive methods such as optically stimulated luminescence and radio-photoluminescence and only a small portion were electronic personal dosemeters (EPDs). In passive dosemeters, an operational quantity is measured for monitored personnel (Hp(10), Hp(0.07), Hp(3)) or area of interest (H*(10), H′(d,Ω))(2–5). After a defined time (measuring period), most commonly a month or longer, dosemeters are sent to a dosimetric service for a readout. This is a demanding process and the received dose is known only after a significant delay of several weeks to several months. In the case of an accident, there is no way for a passive dosemeter to signal a warning. More than a decade ago, debates over the possibility that EPDs could replace passive dosemeters as legal monitoring devices began(6). Now debates have branched into various exposure scenarios. Since that time, the use of EPDs as additional dosemeters in medicine and industry has increased, mainly as alarm devices as well as for radiation protection teaching purposes(7). The advantage of EPDs over passive dosemeters includes higher sensitivity, instantaneous alarm function after exceeding a certain pre-set dose rate or dose, real time readings and the dose information in time domain. Unlike EPDs, area survey meters (commonly referred to as just survey meters) are long-established among area meters. They usually offer H*(10) and/or H′(d,Ω) measurements of both the dose and the dose rate. Due to lower constraints in price and size compared to EPDs, survey meters commonly employ different technologies. Their ability to measure the dose rate has given us a practical and intuitive method to evaluate and compare different exposure scenarios. Recent performance investigations of EPDs and survey meters has revealed some disadvantages. They mainly relate to the unsatisfactory performance of EPDs and survey meter performance in pulsed fields, poor energy response of EPDs and saturation when exposed to higher dose rates(8, 9). The performance of both EPDs and survey meters depends mainly on its detector and supporting electronics. Therefore, the aim of this article was to present a technical and performance overview of technologies present in current dosemeters for strongly penetrating photon radiation (>15 keV) with an overview of current research and future perspectives. DETECTORS USED IN DOSEMETERS The main part of every radiation measurement instrument is a detector. An ideal detector used in an EPD or area meter needs to have a linear response over a wide range of dose rates and energies, no dead time, no angle dependence, low energy consumption and compact dimensions(10, 11). Requirements for low energy consumption and compact dimensions are especially important for EPDs because they have to be wearable and their cost per instrument unit must be lower than for survey meters. Detectors of ionizing radiation commonly in use today can be divided into three groups: gas-filled, scintillation and semiconductor detectors. Gas-Filled Detectors All gas-filled detectors operate in a similar way. Gas is enclosed with two electrodes between which a voltage potential is applied. Incident radiation causes gas ionization and charge carriers move toward the electrodes due to an electric field between them. The behavior of the detector is determined by the strength of the electric field, and depending on its strength, few useful regions can be defined: ionization region, proportional region and the Geiger Muller (GM) region. Ionization chambers operate within the ionization region. Various designs of chamber geometry are available. Some geometries that are more common are cylinder, sphere and parallel plate. Ionization chambers are fundamentally the simplest among gas detectors because there is no charge multiplication present. They are generally characterized by relatively low efficiency for photon radiation due to the low density of the gas, but have good dose rate linearity. The lack of charge multiplication makes them more difficult to implement compared to other gas detectors. Ionization chambers are mostly used in the current mode, where current values typically span from atto and femto to nano amperes. Such small currents demand very sensitive, complex and thus expensive measurement instruments—electrometers, while the chamber on its own sometimes requires pressure and temperature compensation. Ionization chamber wall material plays an important role in signal generation due to the generation of secondary particles inside the wall. Air-walled ionization chambers are often used for dose determination in reference conditions, where the air kerma can be measured directly. A popular example of such systems are Physikalisch-Technische Werkstätten (PTB) Unidos reference class dosemeters/electrometers(12), which can be used with, but not limited to, PTW ionization chambers. Higher surface density walls are chosen when a tissue-equivalent response is needed. Material choice can vary from poly(methyl methacrylate) (PMMA) to resin-bonded paper and others. Such chambers are commonly used for measurement of operational quantities for area monitoring. Examples of corresponding instruments are Step OD-02(13), Thermo Scientific SmartION(14) and Fluke 451B(15). Due to complex sensitive electronics required for ionization chamber readout, they are generally not used in EPDs with the exception of direct ion storage (DIS) dosemeters. DIS is a proprietary technology, where voltage between two electrodes is held by their own capacitance. The first electrode is a chamber wall, while the second electrode is an exposed floating gate of a modified metal-oxide-semiconductor field-effect-transistor (MOSFET), which acts as a non-volatile memory. A DIS dosemeter is readout simply by passing a current through the source and drain of the MOSFET and determining equivalent resistance, which corresponds to the absorbed dose. A dosemeter reading is performed only on demand, which is one of the reasons why they are considered passive type(16). When the voltage becomes too low, it is not possible to charge and reuse the chamber because the chamber is charged by placing a charge on the floating gate by injecting electrons using a tunneling process through the oxide(17). In a DIS dosemeter, multiple chambers are used to enable measurements in various dose ranges and for different measurement quantities(17). In proportional counters, the electric field is strong enough to enable secondary ionizations, which results in charge multiplication and a signal that is easier to measure. This process is known as charge multiplication and the output of the proportional detector is proportional to the absorbed energy of incident radiation, thus the origin of the name. This enables counting and energy resolving of the incident radiation. There are many designs of proportional counters but in the scope of this work, one should be given special attention. Tissue-equivalent proportional counter plays an important role in photon and neutron dosimetry(18). It is composed of chamber walls and filled with gas, both of which are specially engineered to mimic the absorption in biological tissue when exposed to ionizing radiation. Proportional counters are rarely used in personal dosemeters, but can sometimes be found among survey instruments such as Thermo Scientific FH 40 G-L10(19) and Berthold LB 1236-H10(20). GM detectors operate in such high electric fields that a single ionization causes an avalanche breakdown. Due to the avalanche nature of the GM detector, information about incident energy is lost and quenching is required which altogether causes a dead time. The dead time determines the saturation point of the GM detector and for modern GM detectors its value is in range ~1 μs. Because of their relatively small size, low price and simple supporting electronics, GM detectors are commonly found in both survey meters and EPDs. In real conditions, dosimetry instruments are rarely exposed to mono-energetic radiation. Therefore, efficiency of the detector as a function of incident radiation energy should coincide with the measurement quantity. When that is the case, it is often said that the detector has a flat energy response. While the energy response of ionization chambers and proportional counters is determined mainly by the chamber walls, the response of GM tubes is compensated with energy compensation filter that will attenuate part of the incident spectrum, where detector efficiency is too large relative to the other energies, and thus achieve a flattened energy response. A common filter used with GM tubes is thin layer of lead and/or tin with cut-outs that attenuate, but do not extinguish the lower energy spectrum(21). Examples of commercial survey meters employing GM detectors are Thermo Scientific RadEye G Series(22) and Automess 6150 AD(23). Examples of EPDs are Polimaster PM1621A(24), Mirion DMC 3000(25) and Tracerco PED series(26). Scintillation Detectors Scintillation detectors are present in survey meters, instruments for in situ spectrometry and for radioisotope identification due to their energy discrimination capability. They make use of the fact that certain materials when struck by incident radiation emit a small flash of light, i.e. a scintillation. The number of these light photons is proportional to the absorbed radiation energy. When coupled to an optical photon readout device, these scintillations can be converted into electrical pulses, which can then be analyzed and counted electronically to give information concerning the incident radiation(27). Scintillation materials are normally used together with photomultiplier tubes for readout but they can also be coupled to a semiconductor detector such as silicon photomultipliers (SiPM), which are rapidly developing. Scintillation materials are roughly grouped into two categories: inorganic and organic. The inorganic ones tend to have the best light output and linearity, but most of them have a slow decay constant. Organic scintillators are generally faster but yield less light. In radiation-dosimetry instrumentation, scintillator detectors are mostly used as counting devices calibrated to measure an operational quantity such as H*(10) and commonly utilizing plastic scintillators because of their tissue equivalence. Plastic scintillators belong to the organic group and due to fast scintillation decay, the probability for pulse pileup is lower, which results in a better performance in high dose rate exposure scenarios. On the other side, their low efficiency for gamma photons and low light yield will produce smaller number of lower amplitude pulses in a readout detector, which, especially for lower energy incident photons, can be hard to distinguish from noise in low dose rate situations. When higher light yield and efficiency is required, a common choice of crystals is NaI(Tl) and CsI(Tl) from the inorganic group. They are one of the brightest scintillators available but come with the downside of an energy response that deviates from the tissue-equivalent response. Examples of commercial systems are: Automess 6150 AD-b/E(28) utilizing plastic scintillator, Thermo Scientific FHZ 672 E-10(29) probe for FH 40G-L10(19), which uses two scintillators; organic and NaI(Tl)(29) and Thermo Scientific FHZ 502 probe for FH 40G-L10(19) utilizing a 2″ × 2″ NaI(Tl) scintillator. Semiconductor Detectors A semiconductor detector is a radiation detector based on a semiconductor, such as silicon, to measure the effect of incident ionizing particles. Semiconductor detectors are very similar in operation to photovoltaic plates that generate an electric current. Their low band gap energy (1.12 eV for silicon) allow for large charge carrier creation and excellent energy resolution. Charges in semiconductors move relatively fast thus enabling good timing characteristics. Semiconductor detectors can be divided into two classes: direct conversion and indirect conversion. Direct conversion detectors contain a photoconductive material which converts high energy photons directly into electrical charges. In contrast, indirect conversion detectors are covered with an energy compensation filter that converts incident high-energy photons or particles into electrons, or the detector is coupled to a scintillation material which outputs optical photons. These electrons and photons are then converted into electrical pulses using photodetectors, such as SiPMs. In EPDs, due to their small size, low energy consumption and low price requirements, the most common detector in use is the PIN diode (commonly referred to just as silicon diode if silicon-based)(7). It is formed by sandwiching a thick high ohmic intrinsic (undoped or depletion) layer between highly doped P- and N-type semiconductors that are, respectively, electron deficient or electron rich. When incident radiation strikes the detector, if radiation energy is higher than the material’s band gap, an electron-hole pair will be created(30). Electron-hole pairs can be generated by optical photons (light), ionizing particles, gamma radiation and X-rays. The depleted layer should be considered sensitive volume for detection, as generated electron-hole pairs are quickly swept to P- and N-type layers by a high electric field between them, thus generating a current that can be detected. By applying a reverse bias to the system, the potential difference across the PIN junction is increased, which in turn increases the size of the depletion region and hence the detection volume. If the second event takes place before the charge from the first is collected, the charge carriers produced by the second event would be added to the pulse of the first event thus leading to a pile-up. If the detector is read-out by pulse counting, this would result in a system dead time, but when operating in current/charge mode, there would be no dead time present. The small size of semiconductor and low atomic number of silicon means that the quantum efficiency for most practical X-ray and gamma energies is very small. In fact, only low energy photons (1–20 keV), which primarily interact via the photoelectric effect, are detected directly within a silicon semiconductor(31). Efficiency for a typical diode falls off by several orders of magnitude as the incident energy increases from keV to MeV range(18, 31, 32). In order to weaken that effect and achieve a flatter energy response, metallic absorbers such as Cu, Sn and Br can be used to tailor system efficiency across the same energy range(32). These metallic absorbers (or energy compensation filters) attenuate lower energy photons and boost the response at high energies by means of photoelectric effect, emitting secondary electrons that could be detected. This method has given us a practical way of measuring photon pulses over a larger range of energies, but at the expense of the energy information of the incident photons themselves(18, 31). Figure 1 represents the energy response of a typical diode to X- and gamma-ray photons with and without various energy compensation filters. Although these filters help in flattening the energy response somewhat, there is still a large deviation from uniform sensitivity over the wider energy range. Figure 1 Open in new tabDownload slide Calculated sensitivities (in counts per mR) of a typical PIN diode to X- or gamma-ray photons as a function of photon energy. Curves are shown for diode without and with filters, where filters are given in the legend(32). Figure 1 Open in new tabDownload slide Calculated sensitivities (in counts per mR) of a typical PIN diode to X- or gamma-ray photons as a function of photon energy. Curves are shown for diode without and with filters, where filters are given in the legend(32). Therefore, if the detector is to be employed in circumstances in which the incident spectrum is variable, substantial errors would occur as it is used far from the calibration energy. Semiconductor detectors in general are not commonly found among survey meters, but PIN diodes are dominant among EPDs. Some examples are Thermo Scientific EPD Mk2+(33), RaySafe i2(34) and Atomtex AT3509C(35). OBTAINING DOSE AND DOSE RATE INFORMATION Most radiation detectors have some kind of current or voltage output per detected event. How that output is further processed depends on the detector type and decision of the instrument’s designer, but generally speaking there are two approaches: analyzing current/charge or pulses. For applications relevant to this article, ionization chambers are operated in current/charge mode as direct current devices. The dose rate is directly proportional to the current, which is measured by an electrometer and sampled with low frequency and high precision. The dose, on the other hand, can be obtained by calculating integral of measured current, or better yet, with analog integration by means of charging a capacitor, which ensures that the detector system is invariant to incident radiation pulse duration. Current integral (charge) is then proportional to the dose. The same principle is valid for DIS detectors where the current is effectively integrated by chamber/MOSFET capacitance to obtain the dose and for all other different kinds of detectors operating in the current/charge regime. Although most EPD and survey meter manufacturers are not willing to disclose the working principle of their devices, the general understanding is that the vast majority of them (with the exception of the ionization chamber) use a pulse counting method(36–38). Using this method, the measured dose is obtained with the following model function: $$\begin{equation} {G}_{\mathrm{dose}}=K\;{G}_{\mathrm{count}}{n}_{\mathrm{count}}{k}_{\mathrm{dead},\operatorname{int}} \end{equation}$$ (1) where Gdose is the indicated dose, Gcount is the dose per pulse, ncount is number of detected pulses, kdead,int is the correction factor for the dead time and K is the product of all other correction factors internal to the dosemeter(36). Dose rate measurements provide more direct means of indicating the rate at which the dose is accumulated. The International Commission on Radiation Units and Measurements (ICRU) has defined the dose equivalent rate as a time derivation of the dose equivalent(2). Let us consider a hypothetical detector with no dead time that outputs infinitesimally short pulses and neglect detector efficiency effects. Because events are quantized, if we blindly follow the ICRU definition of a dose rate, the dose rate would also look like a series of infinitesimally short pulses which is not particularly useful. To obtain a useful result, measurements are averaged with a time constant. This time constant is chosen to be large enough to obtain a statistically significant result and for contemporary instruments and its value is in order of magnitude of several seconds. If measuring pulsed radiation, where pulse width is comparable or shorter than the time constant of the instrument, significant underestimation can occur. This behavior is further analyzed in the next section “Performance of contemporary dosimetry instrumentation.” For a counting instrument, in its most common form, the dose rate meter can be represented by the diode pump circuit showed in Figure 2, whose equivalents are used in commercial instruments. Pulses from the detector are conditioned in such a way so they are all identical and each pulse represents one count. In Figure 2, input pulses are represented with a signal generator symbol and the serial input impedance Rf. Each pulse deposits a small amount of charge on the capacitor Ct. This capacitor is continuously discharged over a resistor R. If the frequency of incoming pulses is constant, an equilibrium, in which charge deposition on the capacitor equals its discharge, will be established. This will result in the voltage on the capacitor that is proportional to the dose rate. If a sudden change is introduced, the equilibrium is reached after several time constants, where the time constant is the product of R and Ct. This process is illustrated in Figure 3. Figure 2 Open in new tabDownload slide Dose rate diode pump circuit(18). Figure 2 Open in new tabDownload slide Dose rate diode pump circuit(18). Figure 3 Open in new tabDownload slide Input pulses voltage waveform (left), output voltage from the diode pump rate meter for periodic pulses and random pulses (right)(18). Figure 3 Open in new tabDownload slide Input pulses voltage waveform (left), output voltage from the diode pump rate meter for periodic pulses and random pulses (right)(18). PERFORMANCE OF CONTEMPORARY DOSIMETRY INSTRUMENTATION Research on the performance of dosimetry instrumentation, especially of EPDs and survey meters is trending. In 2007, Ginjaurme et al. published an overview of active personal dosemeters in the European Union(9). It was observed that only 12 out of 31 dosemeters measured photons at energies lower than 50 keV. This parameter is crucial for medical exposures where contribution to the dose in the lowest energy range from 10 to 60 keV is dominant. In 2009, Ankerhold et al. published a study(8) about the deficiencies of active electronic dosemeters in pulsed fields. In the scope of that research, measurements were made in two pulsed photon fields with several personal dosemeters and survey meters. None of the tested instruments yielded satisfactory results within a reasonable limit in both fields. This put the performance of electronic dosemeters in pulsed fields in focus. Later, within the scope of the ORAMED project(39) eight different electronic dosemeters were tested for the use in interventional radiology (IR) and interventional cardiology (IC). The majority of dosemeters were silicon diode-based with the exception of Polimaster PM1621A, which uses a GM tube and Rados DIS-100 which uses a DIS detector. Investigation was carried out in continuous and pulsed X-ray beams. Tests performed in continuous fields showed that most of the dosemeters had an adequate angle and dose equivalent rate response. The energy response of the tested EPDs (shown in Figure 4) was within the IEC 61526(10) required interval (0.71–1.67) from S-Co energy down to ISO N-30 for all EPDs except PEDD30 and DoseAware. EDD30 had an energy response within the interval between N-80 and N-20, while DoseAware had a response N-120 and N-40. Figure 4 Open in new tabDownload slide Energy response for eight different dosemeters tested within the ORAMED project(39). IEC 61526 upper and lower limits are marked with dash-dotted lines. Figure 4 Open in new tabDownload slide Energy response for eight different dosemeters tested within the ORAMED project(39). IEC 61526 upper and lower limits are marked with dash-dotted lines. PM1621A equipped with GM tube and Atomex AT3509C with a silicon diode detector were the only dosemeters that had a satisfying response even at N-15 beam quality, but the GM dosemeter had significant divergence in the dose equivalent rate response. Series of tests were performed in pulsed field with constant frequency of 10 Hz and pulse width of 20 ms. All the tested dosemeters gave similar results to those in the continuous fields with a slight underestimation at dose rates above several Sv/h. The exception was a GM detector that did not give any signal in pulsed fields. In 2017, Krzanovic et al.(40) tested 10 active personal dosemeters in different beams. They concluded that state-of-the-art EPDs have reached a level of reliability comparable with, or even better than passive dosemeters for most radiological applications for energies >80 keV. For some EPDs that can measure at energies lower than 80 keV, they strongly recommended appropriate type testing. Additionally, they stated that neutron EPDs need further development to reach a similar high standard as that for photon dosemeters, mainly regarding robustness and electromagnetic immunity. In 2018, Friedrich and Hupe published an article(41), where the performance of five different survey meters in pulsed fields was investigated. Tests were performed by varying the dose per pulse and the pulse width with the capability to generate short pulses down to 0.4 μs in duration. They showed that all of the tested instruments could be used for dose measurements in pulsed fields in principle, but the parameters of radiation field and instrument specifications had to be considered. Second, they stated that if the instrument did not have a manual measurement range selection, and only auto-ranging was available, the instrument would not measure correctly in pulsed fields due to the time needed to switch to a correct range. It should be emphasized that if the instrument could measure the dose in pulsed fields, one should not assume that the dose rate could also be measured correctly. In the same study by Friedrich and Hupe, not a single one of the tested instruments measured the dose rate during the pulse correctly. Every instrument has a response time, and when considering a counting instrument, dose rate electronics can be represented with a diode pump circuit (or equivalent) showed in Figure 2 that introduces a time constant over which the counts are averaged. The minimum value of that time constant is directly correlated to detector sensitivity, since enough counts must be collected to obtain a statistically relevant measurement. The response times of dose rate meters are often in the range from seconds or tens of seconds. If exposures shorter than the response time of an instrument were measured, the dose rate measurement would result in a serious underestimation of the dose rate up to several orders of magnitude and therefore only the dose measurement would be preferred. In 2019, Hupe et al.(42) investigated the dose rate response of 10 different EPDs in continuous and pulsed fields. Their measurements showed that the dose rate range specified by the manufacturer was valid only for continuous radiation. For pulsed radiation, the range is lower as the dead time correction is insufficient. They reported that the dose rate in direct beam of hospital X-ray generators can be up to 400 Sv/h and up to 2 Sv/h in scattered field. Some of the tested dosemeters showed acceptable performance for dose rates present in scattered radiation, but not one of the tested instruments was capable for in-beam measurements, which can be very important in case of accidental exposure. NEW DEVELOPMENTS Current trends in the development of EPDs and survey meters are mainly focused on the development of new semiconductor and scintillation detector systems with a goal to overcome poor performance in pulsed fields and for lower energy photons. Due to size and cost constraints, scintillation detectors are not very common among dosemeters and virtually non-existent among EPDs. Classic photomultiplier tubes used for scintillator readout require bias voltage of several kV, they are bulky, fragile and sensitive to magnetic fields. Recent advances in SiPM technology made them comparable and in some aspects better performers then their former counterparts. They are robust, compact, and insensitive to magnetic fields and operate at low voltage. This naturally induced emerging research on employing SiPMs for handheld or compact wearable instruments for radiation dosimetry and radiation protection purposes. Researchers are investigating and developing SiPM-based instrumentation for detecting neutrons and gamma/X-ray radiation. In a study by Foster and Ramsden(43) in 2008, a neutron detector prototype was created with a tiled array of SiPMs coupled to a LiI(Eu) scintillation crystal. Their results were competitive with the established He-3 tubes. Soon after, in 2009 Risigo et al. investigated SiPM technology applied to radiation sensor development with focus on real-time dosimetry in mammography(44). The developed instrument was meant to be used during mammography scan to monitor doses delivered to the patient. In 2015, Yoo et al.(45) have optimized CsI(Tl) crystal geometry for the use in compact devices. They developed a crystal geometry of tapered cylinder with a radius of 5 mm and total length of 5 mm. All of the mentioned research used an amplifier followed by a pulse digitizer for data collection. The data were recorded on a PC for offline analysis. In a recent study done by Buzhan et al. in 2017(46), a successful prototype H*(10) dosemeter which employed SiPM coupled to a CsI(Tl) scintillator was developed. The device was characterized by a dynamic range from 0.1 μSv/h to 10 mSv/h for dose measurements and the energy range from 50 to 3000 keV. The energy response was flattened by defining 11 energy zones where the response in each zone could be shifted using custom electronics. Other groups of researchers have gone in a direction of employing CMOS semiconductor detectors for dosimetry measurements. CMOS stands for complementary metal oxide semiconductor, which is the fabrication process that uses complementary and symmetrical pairs of p- and n-type MOSFETs. In a study by Michel et al.(47), the photon counting pixel detector made in CMOS technology was investigated for low energy dose measurements. The detector was developed by Medipix collaboration(48) and composed out of a matrix of PIN diode pixels organized in 256 rows and 256 columns with a pitch of 55 μm with a high spatial, high contrast resolving pixel read-out chip working in single photon counting mode. Below the detection layer, the detector had an ASIC (application-specific integrated circuit) for signal processing and data acquisition. It was initially developed as a new solution for various X-ray and gamma-ray imaging applications. When radiation is absorbed in the sensor layer and charge carriers are created, that charge is compared to the adjustable discriminator threshold in the ASIC. An event is counted for each pixel in which the discriminator threshold is exceeded. Because the detector is not tissue equivalent and there is only one discriminator per pixel, the detector area is divided into eight different regions with different discriminator thresholds to obtain the energy information. This allows tailoring of the energy response, which is shown in Figure 5. They claim that the detector, being of the photon counting type, enables dosimetry at very low dose rates, and due to small pixel size, can also be used in high dose rate measurements up to 57 Sv/h for Hp(0.07) and 19 Sv/h for Hp(10) for 20 and 15 keV respectively. Figure 5 Open in new tabDownload slide Simulated systematic error of the reconstructed Hp(10) dose for a CMOS detector with energy compensation by means of differentiating incident radiation into eight different energy zones(47). Figure 5 Open in new tabDownload slide Simulated systematic error of the reconstructed Hp(10) dose for a CMOS detector with energy compensation by means of differentiating incident radiation into eight different energy zones(47). A significant push forward from that research was made by Wong et al.(49) who developed an ASIC pixel detector specifically for dosimetry measurements with pixelated semiconductor detectors. The developed ASIC performs channel-level analogue to digital conversion and signal pre-processing. They concentrated toward dosimetry measurements where it is important to gather precise, time-specific dose information and spectral information about the radiation field which can be readout in real time. For example, measurement of occupational exposure in high flux medical pulsed X-ray generators. The energy deposited in detector is measured using time-over-threshold method after which that information is binned into 16 energy windows, which enables tailoring energy response. Conti et al., motivated by the poor performance of EPDs in low energy and pulsed radiation fields, conditions which are commonly present in IR/IC, investigated the performance of another CMOS image sensor for use in EPDs(50). They used active pixel sensor (APS) as a detector which is commonly exploited in medical imaging. In APS, each pixel includes a few control devices (usually MOSFETs) for photodiode buffering, precharge and reset. It has been shown that CMOS image sensors used as X-ray detectors have reasonable efficiency for photon energies up to several 10 of keV, where the energy resolution of 3–4% can be obtained(51, 52). In their experimental setup, PMMA slabs were used to diffuse X-ray photons from an interventional angiography X-ray generator. Measurements were carried out in continuous and pulsed regime with a pulse width of 1.9 ms. A custom two-threshold algorithm has been implemented to obtain a number of detected photons and the sum of reconstructed photon signals. A correlation between these two quantities has been demonstrated with measurement uncertainty below 10% for both quantities. Measurements were compared with TLD dosemeters which showed differences in the response between irradiation in continuous and pulsed fields under 10%. In 2014, Cogliati et al. published a study about CMOS camera sensors from a cell phone for gamma detection and classification(53). Their idea is based on the fact that CCD and CMOS image sensors often found in mobile phones are, beside the visible light, also sensitive to ionizing radiation. The results showed that a smartphone camera could be used as a low sensitivity dose rate meter and limited spectrum information can be obtained if radiation is coming from a known direction. In 2015, Klein et al. submitted a patent(54) for the detection of radiation using a mobile phone camera. They stated that the camera sensor is sensitive to ionizing radiation and emphasized “in particular pulsed high-energy radiation”. Klein also created a smartphone application (“Radioactivity counter”) for Android and iOS mobile devices, which uses the phone camera for radiation detection. The authors are suggesting validation of this technology for measurements in pulsed fields and investigation of energy response. Also in 2015, an article was published about the development of a hybrid pixelated CMOS detector(55) based on ideas of Medipix collaboration and Wong et al.(49). The detector is made from a pixelated matrix that consists of 16 × 16 square pixels with 12 rows of (200 μm)2 and 4 rows of (55 μm)2 sensitive area for the silicon sensor layer and 16 rows of pixels with 220 μm of pixel pitch for CdTe. To each sensitive layer, a second ASIC readout layer was bonded. Besides digital energy integration and photon-counting mode, a novel concept of energy binning is included in the pixel electronics, allowing energy-resolved measurements in 16 energy bins, which in turn enables tailoring of the energy response. Their focus was mainly on developing a kVp meter for medical X-ray quality assurance, but they claimed that the detector was also appropriate for dosimetry measurements. The trend of employing CMOS detectors continues. Rubovic et al.(56) started to develop a dosemeter based on a new generation of hybrid pixelated CMOS sensors where the first layer of a 300 μm thick silicon semiconductor detector is bonded to the ASIC readout chip in the second layer. The detector has the ability to determine the energy deposited in each pixel and identify the type of an interacting ionizing particle through a morphological analysis of the particle track registered by the detector. They successfully demonstrated the detector implementation as X-ray/gamma dosemeter. Their tests were made in several ISO radiation qualities(57) from N-100 to N-300, as well as with 137Cs and 60Co radionuclides in kerma rates of 1–10 mGy/h. They stated that 300 μm thick silicon detector has limitations because it possesses a relatively low Z (Z = 14), photoelectric effect where energy was completely deposited in detector was dominant interaction channel only for photon energies up to ~100 keV. Their further plans include measurements in higher air kerma rates, calibration for personal dosimetry, experiments in pulsed source fields used in medicine and CdTe and GaAs pixel detectors. In 2018, Servoli et al.(58) published findings about their in-house developed prototype EPD based on APS with real-time and wireless capability. Developed EPD has Dosemeter prototype is tested in pulsed fields and has been evaluated as proof of concept on more than 50 clinical procedures. The group reported that the developed dosemeter had a relative precision of measurement and linearity of the response below 10%. Aside from the development of semiconductor and scintillation dosemeters, there is still research into the further development of gas-filled detectors. In 2019, Krzanovic et al.(59) published an article about the development of a survey meter for measurements of the ambient dose equivalent with focus on lowering the cost of the instrument, which would make it available to the widest community, including citizen networks. For that purpose, they employed a commercially available non-compensated detector, which they wrapped in 400 and 600 μm thin lead foils with slits to achieve energy compensation. An energy response measurement was performed in ISO radiation qualities(57) ranging from N-40 to N-200, S-Cs and S-Co, while angle dependence was measured in N-40 and N-100 beam qualities for different filter thicknesses with and without filter slit. These measurements were then used to choose the optimum configuration of filters. Also in 2019, Singh and Kulkarni(60) published their development of open air ionization chamber for measurement of ambient dose equivalent at low and medium photon energies. The developed ionization chamber was composed out of 10 mm PMMA walls enclosing a volume of 225 cm3. The energy response of the chamber was measured in a range of beam qualities(57), where the output current was measured using an electrometer. The chamber showed a near flat energy response for the monitoring of ambient dose equivalent in the energy range 30–200 keV. It also fulfilled the ISO 4037-4 requirements concerning the quality of a secondary standard dosemeter in the energy range from 17.6 to 200 keV. They claimed that if the chamber was to be sealed (to avoid pressure and temperature corrections), it could be used as a detector of the ambient dose equivalent/rate in radiation protection instruments like survey meters. Recently, the ICRU Report Committee 26 proposed a new set of operational quantities. Singh and Kulkarni reported that the developed ionization chamber could also be used to measure these new quantities and would only require slight recalibration. DISCUSSION Nowadays, the majority of dosimetric services are still using passive dosemeters. Such dosemeters need to be sent out to dosimetric service for readouts, usually after one or more months of monitoring. Such a process is arduous and information about the received dose is obtained with significant delay. Compared to passive dosemeters, EPDs offer many advantages, the most important ones being the alarm function and easy implementation of the online dosimetry system. For these reasons, more than 10 y ago, experts started considering replacing passive dosemeters with EPDs. The testing of EPDs showed that dosemeters in that time had problems measuring the dose in pulsed radiation fields and lower energy scattered radiation. Both pulsed and scattered radiation from 10 to 60 keV are dominant dose contributors in the medical sector. New research shows that the situation today has improved. Now there are electronic dosemeters available on the market that can measure dosed well (by IEC 61526 standards), both in stationary and pulsed fields up to a certain dose rate that covers normal but not accidental exposures. Recently a new IEC 63050:2019(61) standard for dosemeters in pulsed fields was published and reports are yet to come how contemporary dosemeters fulfill the requirements given therein. Dose rates produced by pulsed field X-ray generators are usually much higher than those from continuous radiation generators and radioactive sources, which means that in the case of accidental exposure, a worker is much more likely to be exposed to dose rates that exceed the detector saturation point and the measured dose could be incorrect by orders of magnitude. Measurements of lower energy radiation are possible with some EPDs, but they should still be taken with due caution. Information given by manufacturers is commonly misleading and appropriate testing and calibration is strongly suggested. The problem of dose rate measurement in short pulses still remains unsolved. Both EPDs and radiation survey instruments have to be used with caution in pulsed fields and that can be extended to use in continuous fields with short exposure because underestimation up to several orders of magnitude can occur. So far, when the duration of exposure of radiation generator is shorter than the response time of the survey meter, measurement of the dose rate must be avoided and the use of dose measurement with an integrating meter is suggested. Taking into account that diagnostic radiological equipment that uses short exposures is one of the most used X-ray generator technologies and that pulsed radiation fields have become the mainstream technology, there is an urgent need for a novel dose rate meter that works well under all of these conditions. Many emerging technologies try to solve these challenges. Nowadays, research is mainly concentrated on developing a new semiconductor-based detector and detector systems that could be employed in dose and dose rate meters, but the majority of this research is still at its infancy. Currently, there are two main research directions. One is directed toward employing SiPMs with scintillators and the other toward pixelated semiconductor detectors that have their roots either in X-ray imaging or phone cameras. Most of that research requires innovative solutions to achieve a flat energy response but on the other hand offer high sensitivity and good dynamic range. 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TI - A TECHNOLOGY OVERVIEW OF ACTIVE IONIZING RADIATION DOSEMETERS FOR PHOTON FIELDS
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncz294
DA - 2020-06-24
UR - https://www.deepdyve.com/lp/oxford-university-press/a-technology-overview-of-active-ionizing-radiation-dosemeters-for-SUnTXFpmzk
DP - DeepDyve
ER -