TY - JOUR
AU - Maltar-Strmečki,, Nadica
AB - Abstract In this work, we report some preliminary results regarding the analysis of electron spin resonance (ESR) response of soda-lime samples used for retrospective dosimetry. Six different soda-lime glass batches were evaluated after irradiation. We compared several dose reconstruction techniques: saturation method, subtraction method and g-effective, geff, approach. The differences were observed and discussed. ESR signal responses of soda-lime glass samples to different radiation doses for the triage application were investigated. Results confirmed that geff approach has potential for the identification and dosimetry of irradiated soda-lime glass samples using either additive dose method or only calibration curve. INTRODUCTION The electron spin resonance (ESR) detection of dosimetric signals originating from radicals induced under the influence of ionising radiation allows to determine the dose absorbed by the casualties of radiation accidents. The study material can be worn by the victims (clothing, fabrics, plastics, glass, sugar, touch screens of mobile phones) or solid biological tissues (tooth enamel, nails, hair, bone). Each of these materials has its limitations and advantages. The use of ESR on detection of ionising radiation-induced radicals in different types of glasses is a well-established technique in radiation dosimetry and has been extensively reviewed in the literature(1–8). As a retrospective dosemeter, the commercial soda-lime glass has proven to possess many required qualities due to the specific radiation-induced ESR signal with a linear dose response (generally ascribed to an oxygen hole centre) and the detection limit (DL) around 1–2 Gy. Additionally, ESR signal decay is about 10 up to 35% at room temperature during the first 24–48 h after irradiation, after which the signal remains stable. A major effort in recent years has been devoted to improving the lower detection limit for retrospective dosimetry purposes because the useful low dose radiation-induced signal (RIS) is superimposed over the native background signal, BKS. Recently, we have shown that Δg-shift value determination and temperature dependence can be useful for improvement of dose reconstruction in the low-level dose range(9). In the present study, dose reconstruction from the ESR signal of gamma-irradiated soda-lime samples in the range of high, low and medium triage dose levels has been investigated. The triage dose levels are differently defined in the literature and for purpose of this study triage levels defined in MULTIBIODOSE project have been used(10, 11). Low triage dose level is less than 1 Gy when it is unlikely to develop symptoms of acute radiation syndrome (ARS) and no immediate care is required. The medium level implies doses 1-2 Gy when victim may experience mild or delayed ARS symptoms and follow-up care may be necessary and high for the doses higher than 2 Gy when moderate to urgent care may be required(10, 11). Regarding these dose levels, several methods for correct determination of dosimetric signal magnitude in retrospective ESR dosimetry have been proposed in the literature. Deconvolution method is based on mathematical simulation of BKS and RIS that approximates measured spectra(12). The approach based on the best fit of the experimental spectrum with a set of Gaussian lines, spectrum-simulated lines, and experimental reference spectra is used in intercomparison organised among participants of European Radiation Dosimetry Group (EURADOS)(7). For the retrospective dosimetry purposes, the widely used method is ESR spectrum subtraction approach based on background ESR signal subtraction from total ESR signal of irradiated samples(1, 3–6, 13–16). Also, there are other techniques like selective saturation method(17, 18) and second derivative method(19). For the purpose of accuracy improvement and dose reconstruction in the range 0–10 Gy, we have investigated the ESR parameters of two components (RIS and BKS) of soda-lime glass ESR spectrum irradiated in the Co-60 beam. After establishing the exact g-values of the BKS and total ESR signal at the position of maximum amplitude, we have investigated the change of g-effective, geff, combination of the two reference signals g-values, value of ESR signal of irradiated samples and observed regular change of geff value of spectral lines after increasing the gamma irradiation dose. EXPERIMENTAL METHODS In this study, the soda-lime samples were chosen from six different batches. Chemical composition of the samples is same and defined and certified according to European standards(20–22). The certified composition of soda-lime glass identifies following chemical compounds, silicon dioxide, SiO2, 69–74%, sodium oxide, Na2O, 10–16%, calcium oxide, CaO, 5–14%, magnesium oxide, MgO, 0–6%, aluminium oxide, Al2O, 0–3% and other, less important chemical compounds, 0–5%, varies within few percentages for each component. The samples were cut in plates of size about 7 mm × 3.5 mm × 1.6 mm and average mass around 100 mg as described before(9). The irradiations were performed at a calibration teletherapy unit Co-60 Alcyon (CIS Bio International). The measurements were performed 24 hours after irradiation. All samples were stored in the same conditions, in dark place at room temperature after irradiation process. ESR spectra were recorded using Varian E-9 spectrometer equipped with Bruker ER 041 XG microwave bridge working at X-band, i.e. microwave frequency 9.5 GHz. The spectrometry settings were: magnetic field modulation frequency 100 kHz, microwave power 7.9 mW and modulation amplitude 0.2 mT. A manganese standard reference, Mn2+ in MgO, was used to calibrate the magnetic field of the spectrometer and for normalisation of the ESR intensity. The same sample holder, i.e. specially made quartz rod, shown on the scheme 1 was used for all measurements of the given samples. Sample mass correction and ESR measurements in six different orientations were performed to avoid imprecision on determination of signal intensity or amplitude due to angular dependence of the ESR signal. Scheme 1. Open in new tabDownload slide Scheme of the quartz sample holder. Scheme 1. Open in new tabDownload slide Scheme of the quartz sample holder. RESULTS AND DISCUSSION All the samples were measured at the same microwave power. The choice of 7.9 mW was decided because we have fairly strong enhancement of the ESR signal and yet we are still in the linear regime for both, irradiated and non-irradiated samples. As presented in Figure 1, we see that the total ESR signal. i.e. combined RIS and BKS, saturates faster at 2 Gy than BKS independently. Besides, it can be seen that saturation behaviour of total ESR signal is dose dependent. This indicates that selective saturation method is not suitable for soda-lime glas(17, 18). Figure 1. Open in new tabDownload slide Variation of peak-to-peak amplitude of non-irradiated and samples after 2 and 20 Gy irradiation as a function of square root of the microwave power. Figure 1. Open in new tabDownload slide Variation of peak-to-peak amplitude of non-irradiated and samples after 2 and 20 Gy irradiation as a function of square root of the microwave power. Dose dependent calibration curve obtained by analysis of peak-to-peak ESR amplitude, Ipp during the application of the subtraction method, didn’t provide us the accurate answer in the dose range below 1-2 Gy. This finding is in agreement with results already published(2). In Figure 2, ESR spectra of six samples used as reference samples are presented. The BKS and the total ESR signal after irradiation of the samples with the dose of 10 Gy are presented in Figure 2a and b, respectively. The total ESR signal at 10 Gy is composed of BKS and RIS. As it can be seen from the Figure 2a and b, the variation of shapes of the spectra and intensities is quite low. Despite that in this study, we did not standardise single BKS and use it for all the samples, but we measured BKS for each sample separately. Figure 2. Open in new tabDownload slide Variation of shapes of the spectra and intensities of (a) BKS and (b) total ESR signal at 10 Gy for six batches of soda-lime glass. Two satellite lines on the left and right side are third and fourth lines of the Mn2+/MgO internal standard. Figure 2. Open in new tabDownload slide Variation of shapes of the spectra and intensities of (a) BKS and (b) total ESR signal at 10 Gy for six batches of soda-lime glass. Two satellite lines on the left and right side are third and fourth lines of the Mn2+/MgO internal standard. The subtraction method gave the calibration curve presented in Figure 3. The dose refers to the dose delivered to the soda-lime sample in terms of air kerma at the energy of 1.25 MeV. The calculated ratio of mass energy absorption coefficients in soda-lime glass to air(23, 24) varies in the range 99.4-99.7% which is needed to represent the assessed dose to the ISO 14 003. Variations are due to the composition of the soda-lime samples. As we can see in Figure 3, in the dose range below 2.5 Gy the ESR Ipp amplitudes are dispersed and it is not unambiguous to say what is the dose absorbed by the sample. This method is sufficiently precise only when the background signal, sample tube and other natural contributions are negligible compared to the ESR RIS.(19) Figure 3. Open in new tabDownload slide Peak-to-peak amplitudes dependences of six samples from different batches (○, □,▼,◊,▲,●) on dose. Figure 3. Open in new tabDownload slide Peak-to-peak amplitudes dependences of six samples from different batches (○, □,▼,◊,▲,●) on dose. The dose range with which we are dealing induces ESR signal RIS that are comparable to background signal, BKS. Our method for improvement of the low dose range reconstruction is based on the behaviour of g-effective value, geff (D), i.e. the g-values of two recognised signals: ESR signal at the maximum irradiation dose, gMAX and BKS, g(BKSMAX). The BKS was evaluated after alignment. The spectra of a sample irradiated with 10 Gy were used showing the maximal level of RIS signal in the monitored low-level dose range as shown in Figure 4. When delivering additional dose to the sample, the RIS is more prominent than the BKS component and the geff (D) value is exhibiting the behaviour of exponential growth to the gMAX. We have tested the exponential behaviour of geff values with six samples of different batches and all the samples confirmed the predicted behaviour (Figure 4). Figure 4. Open in new tabDownload slide ESR spectra of irradiated (10 Gy) and non-irradiated (0 Gy) soda-lime glasses. The marks and abbreviations are explained in the text. Figure 4. Open in new tabDownload slide ESR spectra of irradiated (10 Gy) and non-irradiated (0 Gy) soda-lime glasses. The marks and abbreviations are explained in the text. The six samples from different batches indicated that geff(D) values follow following equation: geff(D)=y0+a(1−e−bD) (1) D=x+∑idi (2) where geff(10 Gy) presents dosimetric characteristic signal, gMAX, and geff(0 Gy) presents unirradiated background characteristic signal, g(BKSMAX), x stands for unknown dose and di for added doses. All of the samples satisfied equation (1). Therefore, we fitted measured data to the curve: f(D)=y0+a(1−e−bD) (3) where y0 is parameter presenting fit value of the g(BKSMAX), a is parameter representing the difference between gMAX and g(BKSMAX), and b inverse value of the dose at which curve saturates to 63% (b = 1/D37%)(25). The proposed way to use the observed phenomenon follows. After reading out the sample with unknown dose, the g-effective value of the sample should be noted. Additional dose irradiation method must be used. If the geff values after certain added dose reach the constant value this means, according to our investigated samples, that geff approached dosimetric gMAX and that the background component is overridden. The variation of the measured geff (RISMAX) values for each sample batch presented on the Figure 5 is due to the variable composition of soda-lime samples and also due to the components influencing uncertainty of the measurements. For more precise results, the samples were measured in six orientations, but the performance of the spectrometer is not constant throughout of experiment. The stability of equipment and the environmental conditions influence the result. Figure 5. Open in new tabDownload slide (a) Dependence of geff on dose; experimental data and (b) curves obtained by equation (3), fitted parameters are shown in Table 1. Figure 5. Open in new tabDownload slide (a) Dependence of geff on dose; experimental data and (b) curves obtained by equation (3), fitted parameters are shown in Table 1. We have irradiated a random sample, without knowing the batch, with a blind dose and we have tried to assess the dose using all the fitted curves. The idea is to standardise the curves in order to assess the doses from the soda-lime samples irradiated in radiological or nuclear accidents. The resulting assessment of blind dose and parameters obtained of different fitted curves (Figure 5b), presented in Table 1, indicate possibility to standardise the curve. Blind dose determination using the described procedure of 0.3 Gy was evaluated within 20% using five different calibration curves with measurement uncertainty presented in Table 1 (see details in ESI), where all the components of the uncertainty budget were included. Table 1. Results of blind dose assessment and parameters of curves fitted to equation (3). Sample batch number . Assessed blind dose (Gy) . Fit parameters . y0 . a . b (Gy−1) . R2 . 1 0.24 ± 0.11 2.0082 ± 0.0002 0.0027 ± 0.0004 0.2260 ± 0.0925 0.9734 2 0.29 ± 0.13 2.0082 ± 0.0001 0.0030 ± 0.0006 0.1612 ± 0.0661 0.9837 3 0.25 ± 0.09 2.0082 ± 0.0001 0.0028 ± 0.0004 0.2058 ± 0.0674 0.9849 4 0.25 ± 0.06 2.0082 ± 0.0001 0.0026 ± 0.0002 0.2202 ± 0.0386 0.9952 5 0.27 ± 0.08 2.0081 ± 0.0002 0.0025 ± 0.0002 0.3724 ± 0.0985 0.9800 Sample batch number . Assessed blind dose (Gy) . Fit parameters . y0 . a . b (Gy−1) . R2 . 1 0.24 ± 0.11 2.0082 ± 0.0002 0.0027 ± 0.0004 0.2260 ± 0.0925 0.9734 2 0.29 ± 0.13 2.0082 ± 0.0001 0.0030 ± 0.0006 0.1612 ± 0.0661 0.9837 3 0.25 ± 0.09 2.0082 ± 0.0001 0.0028 ± 0.0004 0.2058 ± 0.0674 0.9849 4 0.25 ± 0.06 2.0082 ± 0.0001 0.0026 ± 0.0002 0.2202 ± 0.0386 0.9952 5 0.27 ± 0.08 2.0081 ± 0.0002 0.0025 ± 0.0002 0.3724 ± 0.0985 0.9800 Open in new tab Table 1. Results of blind dose assessment and parameters of curves fitted to equation (3). Sample batch number . Assessed blind dose (Gy) . Fit parameters . y0 . a . b (Gy−1) . R2 . 1 0.24 ± 0.11 2.0082 ± 0.0002 0.0027 ± 0.0004 0.2260 ± 0.0925 0.9734 2 0.29 ± 0.13 2.0082 ± 0.0001 0.0030 ± 0.0006 0.1612 ± 0.0661 0.9837 3 0.25 ± 0.09 2.0082 ± 0.0001 0.0028 ± 0.0004 0.2058 ± 0.0674 0.9849 4 0.25 ± 0.06 2.0082 ± 0.0001 0.0026 ± 0.0002 0.2202 ± 0.0386 0.9952 5 0.27 ± 0.08 2.0081 ± 0.0002 0.0025 ± 0.0002 0.3724 ± 0.0985 0.9800 Sample batch number . Assessed blind dose (Gy) . Fit parameters . y0 . a . b (Gy−1) . R2 . 1 0.24 ± 0.11 2.0082 ± 0.0002 0.0027 ± 0.0004 0.2260 ± 0.0925 0.9734 2 0.29 ± 0.13 2.0082 ± 0.0001 0.0030 ± 0.0006 0.1612 ± 0.0661 0.9837 3 0.25 ± 0.09 2.0082 ± 0.0001 0.0028 ± 0.0004 0.2058 ± 0.0674 0.9849 4 0.25 ± 0.06 2.0082 ± 0.0001 0.0026 ± 0.0002 0.2202 ± 0.0386 0.9952 5 0.27 ± 0.08 2.0081 ± 0.0002 0.0025 ± 0.0002 0.3724 ± 0.0985 0.9800 Open in new tab CONCLUSION From obtained results according to the proposed model, it can be concluded that the g-effective value behaviour is useful tool in dose reconstruction. The preliminary results are presented. Furthermore, this method should be tested on a larger number of the soda-lime samples and the geff (D) should be evaluated for more dose values, i.e. shorter intervals in the dose range 0–10 Gy. As all approaches for the dose evaluation, it needs to be tested in interlaboratory intercomparison, ILL, to prove the method on the large number of samples. 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TI - DOSE RECONSTRUCTION FROM ESR SIGNAL OF GAMMA-IRRADIATED SODA-LIME GLASS FOR TRIAGE APPLICATION
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncy290
DA - 2019-12-31
UR - https://www.deepdyve.com/lp/oxford-university-press/dose-reconstruction-from-esr-signal-of-gamma-irradiated-soda-lime-HqIPjydxa6
SP - 88
VL - 186
IS - 1
DP - DeepDyve
ER -