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Potential use of wallboard (drywall) for EPR retrospective dosimetry
Radiation Measurements 44 (2009) 243–248
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Potential use of wallboard (drywall) for EPR retrospective dosimetry Jeroen W. Thompson a, *, Ibrahim Abu Atiya a, W. Jack Rink b, Doug Boreham a a b
Department of Medical Physics & Applied Radiation Sciences, McMaster University, 1280 Main St. West, Hamilton ON L8S 4K1, Canada School of Geography and Earth Sciences, McMaster University, 1280 Main St. West, Hamilton ON L8S 4K1, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 July 2008 Received in revised form 19 November 2008 Accepted 15 March 2009
Concern regarding the possibility of criminal or terrorist use of nuclear materials has led to an interest in developing the capability to measure radiation dose in a variety of natural and manufactured materials. Electron paramagnetic resonance (EPR) measurements of radiation dose following a radiological incident may aid in screening affected populations (triage) and in reconstruction of doses following accidents. One such EPR dosimeter is wallboard (drywall), a common construction material composed largely of gypsum (calcium sulphate dihydrate). We have identified the CO 3 and SO3 dose-sensitive lines in drywall and developed a measurement protocol using the intensity of CO3 line. Proper background subtraction is a major difficulty, and we demonstrate a procedure based on alignment of a contaminant Mn2þ line. As a proof-of-concept, a wallboard panel was irradiated with a 60Co source, and a twodimensional map of the absorbed dose was measured. While most aliquots yielded reasonably accurate doses, a spatially contiguous region of apparent dose-insensitivity in one panel was identified. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Electron paramagnetic resonance EPR Electron spin resonance ESR Dosimetry Retrospective dosimetry Forensic dosimetry Wallboard Drywall Gypsum Calcium sulphate dihydrate
1. Introduction Electron paramagnetic resonance (EPR, or ESR: electron spin resonance), discovered in 1944 by E. K. Zavoisky, is used for the fundamental study of free radicals and points defects, as well as the measurement of radiation dose (Weil et al., 1994). Radiationinduced EPR signals were first studied in the 1950s (Gordy et al., 1955). Later, the amino acid alanine was shown to be an EPR dosimeter (Bradshaw et al., 1962) and has been used as a transfer dosimeter for radiation therapy since the 1980s (Regulla and Deffner, 1982). EPR dosimetry has been applied to natural materials for the purposes of geochronology, first beginning with a study of dose measured in natural calcite (Ikeya, 1975); many other geological and biological materials have been studied for purposes of applying EPR dosimetry as well (Blackwell, 1995; Rink, 1997). There is growing interest in the use of EPR dosimetry following a radiological incident (such as an industrial accident or, less likely, a terrorist attack). It may be necessary in such cases to screen the
* Corresponding author. E-mail addresses: [email protected] (J.W. Thompson), abuatii@ mcmaster.ca (I.A. Atiya), [email protected] (W.J. Rink), [email protected] (D. Boreham). 1350-4487/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2009.03.027
public in order to distinguish the exposed from the non-irradiated and the ‘‘worried well’’ (Alexander et al., 2007). EPR dosimetry may also be used to reconstruct doses well after the exposure has taken place (Ishii et al., 1990; Romanyukha et al., 1994; Skvortsov et al., 2000; International Atomic Energy Agency, 2002; Stepanenko et al., 2007). A suitable biophysical dosimeter would be most appropriate for triage purposes, assuming that the absorbed dose to the target dosimeter represents the dose to critical organs. However, other common natural or manufactured materials (so-called ‘‘fortuitous’’ dosimeters) may serve as useful EPR dosimeters. Sugar (sucrose), for example, is known to be a very sensitive EPR dosimeter, with a minimum detectable dose of approximately 100 mGy (Nakajima, 1995; Fattibene et al., 1996; Da Costa et al., 2005). Other proposed dosimeters include bone (Desrosiers, 1993; Ikeya, 1993), hair (Trivedi and Greenstock, 1993; Trompier et al., 2007), foodstuffs containing collagen, hydroxyapatite, or calcite (Ikeya et al., 1984; Ikeya, 1993; Blackwell, 1995; Engin et al., 2006; Cutrubinis et al., 2007; Maghraby, 2007; Duliu et al., 2007; Parlato et al., 2007; Yordanov and Aleksieva, 2007; Da Costa et al., 2007), window glass (Engin et al., 2006), chalk (Wieser et al., 1994), and resin and bark (Serzhant et al., 1996). Some textiles have been shown to be suitable as fortuitous EPR dosimeters. For example, cotton and untreated polypropylene fabrics have a minimum detectable dose of 1 Gy and
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0.5 Gy, respectively (Barthe et al., 1989), although it is unclear whether this applies textile fabrics generally or just the particular textile samples studied. In addition, quartz grains extracted from a granite pillar at Hiroshima were used to show that the EPR intensity profile of the germanium paramagnetic centre matched the expected intensity profile of atomic bomb radiation, suggesting that quartz-bearing construction materials may also be useful as EPR dosimeters (Ikeya, 1993). In this study, we examine the suitability of wallboard as a fortuitous EPR dosimeter. The near-ubiquity of wallboard in new construction in North America (and its increasing use in developing countries) suggest that EPR dosimetry of wallboard may be important in future radiological incidents. Although wallboard is not a biophysical dosimeterdand hence, the dose to the wallboard does not directly reflect the dose to an individualdit may still prove useful in triage, provided that the dose to the individual may be estimated through multiplication by an appropriate occupancy factor. An initial study of wallboard was performed by Haskell et al. (1996), in which a dose-response curve for wallboard was demonstrated. The goals of this study were to identify the radiosensitive EPR lines in wallboard, to develop the necessary protocols to measure dose in wallboard, and to measure a two-dimensional ‘‘map’’ of dose in irradiated wallboard panels. 1.1. Wallboard Wallboard is composed primarily of gypsum (CaSO4$2H2O), a natural monoclinic, evaporitic mineral, which may also be a component of plaster and stucco, and is an additive to cement and may also be used in soil mitigation (Panagapko, 2007). Gypsum is pulverized and calcined. The resulting hemihydrate is combined with water and hardens into the desired shape: wallboard is gypsum held between two sheets of paper. The United States are the world’s largest producer of natural (mined) gypsum (17.5 Mt in 2005), followed by Iran (11 Mt) and Canada, Thailand, and China (about 8 Mt each) (Panagapko, 2007). Synthetic gypsum also forms a significant percentage of the gypsum used to manufacture wallboard. Synthetic gypsum is largely produced at coal-fired power plants through a process called flue-gas desulphurization (FGD), but may also a by-product of titanium production (Panagapko, 2007).
dose measured was greater than was geologically reasonable, which may indicate problems with determination of the natural dose rate. 2. Methods 2.1. EPR measurements and dose sensitivity All first-harmonic EPR spectra were collected with a JEOL JESFA100 X-band (9.4 GHz) spectrometer, with a TE011 cylindrical cavity and with modulation frequency of 100 kHz. Aliquots were placed inside a 5 mm (OD) Suprasil quartz EPR sample tube; aliquot masses were approximately 100 mg. The EPR spectrometer was tuned as needed using the JEOL control software. In the text, EPR scan parameters are given as follows: power, centre field halfwidth, modulation amplitude, (# of sweeps) (sweep time), time constant, gain. All irradiations were performed at the hot cell in the McMaster Nuclear Reactor. A winch was used to raise and lower a 60Co source (1.17 and 1.33 MeV gamma-rays), which was placed vertically in a holder on a tabletop. Before samples were placed in the hot cell, the dose rate at various locations was measured with a Farmer dosimeter. Exposure is converted to dose through a calibration factor (0.87). The desired dose was delivered to the samples by irradiating for the desired length of time; the dose delivered while raising and lowering the source is measured directly with the dosimeter. Uncertainties in the doses are assumed to be 5%. 2.2. Samples Half-inch wallboard panels were purchased from a local (Ancaster, Ontario, Canada) retailer. The wallboard was manufactured at the Canadian Gypsum Company (a subsidiary of United States Gypsum) manufacturing plant in Hagersville, Ontario on 27 March, 2008. According to publicly available information (see http://www.usg.com/), wallboard from this plant contains approximately 24% FGD synthetic gypsum (averaged over one year). The paper backing was peeled away, and a sample of wallboard was extracted, lightly ground with an agate mortar and pestle, and sieved to a particle size of less than 250 mm. Individual aliquots were irradiated in borosilicate culture tubes. The chemical composition of the wallboard was determined through powder X-ray diffraction (XRD).
1.2. Gypsum 2.3. Dose-response curve Dose-sensitive EPR signals in gypsum were originally studied by Wigen and Cowen (1960) and Albuquerque and Isotani (1982). Three primary paramagnetic centres have been identified in powder samples of natural and synthetic gypsum (Kasuya et al., 1991; Ikeda and Ikeya, 1992; Ikeya, 1993): G1, G2, and G3. G1 is a narrow, isotropic line at g ¼ 2.004, corresponding to the sulfite (SO 3 ) radical (electron centre). G2 is a nearly axial EPR line with gx ¼ 2.0084, gy ¼ 2.0088, and gz ¼ 2.0192, likely the carbonate (CO 3) radical (hole centre). G3 is an orthorhombic EPR line with gx ¼ 2.0029, gy ¼ 2.0027 and gz ¼ 1.9973, possibly corresponding to the CO 2 radical. EPR dosimetry has been applied to natural gypsum precipitates in several studies (Nambi, 1982; Yijian et al., 1989; Ikeda and Ikeya, 1992; Mathew et al., 2004; Ulusoy, 2004); the accumulated dose, or equivalent dose (the laboratory dose assumed to be equivalent to the actual dose), is used to determine the age of the deposit (Rink, 1997). For example, the G2 centre was used to date gypsum deposits at Mammoth Cave, Kentucky (Ikeya, 1993). The same centre was used to date gypsum precipitates from a fault surface in central California (Ikeda and Ikeya, 1992); however, the equivalent
100 mg aliquots of crushed and sieved wallboard (untreated) were irradiated with doses of 1, 5, 10, and 19 Gy. The EPR scan parameters used for the dose-response curve (DRC) are: 9.0 mW, 334.0 4.0 mT, 0.1 mT, 1 120 s, 0.1 s 1000. The 0 Gy spectrum was subtracted from the irradiated spectra in order to enhance the radiosensitive signal. The Mn2þ line was used to align the background and signal spectra; the mean-square deviation was minimized over the region 330.0–331.0 mT. 2.4. Dose measurement with irradiated panels For irradiation of whole wallboard panels, two 20 40 wallboard panels were placed on a table, perpendicular to the 60Co source. Aliquots were extracted from the panel by peeling off the paper backing and scraping the wallboard with a metal spatula. Grinding was not necessary; the aliquots were individually sieved to particle size of less than 250 mm. Unknown doses were determined by comparison with the DRC. For the panel closest to the source, the EPR scan parameters were
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the same as used to measure the DRC; for aliquots extracted from the panel farthest from the source, the same EPR parameters were used, except the number of sweeps was increased to 4. The zerodose spectrum used to construct the DRC was subtracted as the background from all spectra obtained from the wallboard panels (a comparable spectrum was obtained with 4 sweeps to use as the background for the panel farthest from the source). Due to the difficulty in obtaining useful dose measurements over the entire panel (the manipulators could not be used to move the dosimeter into the proper location), the EPR measurements were made using approximately three-quarters of each panel. Irradiation and EPR measurements were made within 10 weeks. 3. Results and discussion 3.1. EPR measurements and dose sensitivity XRD analysis determined that the wallboard was composed of 90% gypsum (CaSO4$2H2O), 4% dolomite (CaMg(CO3)2), 1% anhydrite (CaSO4), and 0.5% each of plaster of paris (CaSO4$0.5H2O), calcite (CaCO3), crystalline quartz (SiO2), and fibrous glass (SiO2), in addition to other amorphous components (2.5%); proportions are approximate and were estimated from the relative intensities of the diffraction peaks. These results compare well with the manufacturer’s Material Safety Data Sheet (MSDS), although dolomite was not listed. Initial EPR measurements revealed the presence of a narrow EPR line in the irradiated wallboard, near B ¼ 336 mT (see Fig. 1). In order to allow accurate measurements of the EPR intensity (peakto-peak height in the first-harmonic difference spectrum), it was necessary to subtract the interfering background spectrum. The previously noted line is quite clear in the difference spectrum, but an additional line appears at a slightly lower field strength, near B ¼ 335.5 mT (Fig. 1). On the basis of its uncalibrated g-value (g ¼ 2.010), the low-field line was tentatively identified as the CO 3 radical, the so-called G2 line (Ikeda and Ikeya, 1992). The CO 3 radical should give an
Fig. 1. Difference between irradiated (top: 18.8 Gy) and unirradiated wallboard EPR spectra (not mass-normalized). Uncalibrated g-values: 2.010 (CO 3 ) and 2.007 (SO3 ). EPR scan parameters: 0.5 mW, 336.2 1.0 mT, 0.01 mT, 1 30 s, 1000 .
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additional weak line at a lower field (roughly 333.8 mT in these studies); this line has not yet been observed in our wallboard samples. The high-field line (uncalibrated g ¼ 2.007) was tentatively identified as the SO 3 radical, or G1 (Ikeda and Ikeya, 1992). The intensity of each line (after background subtraction) was measured as a function of microwave power (Fig. 2) and modulation amplitude (not shown). The intensity of the CO 3 line increases with power, and saturation has not been reached by 9 mW. The SO 3 intensity saturated at a moderate power of approximately 1 mW. These results confirm identification of the CO 3 and SO3 radicals (Kasuya et al., 1991). The largest signal intensities were found with the CO 3 radical, using a microwave power of 9.0 mW and modulation amplitude of 0.1 mT. Although it may be possible in principle to obtain even greater signal intensities with larger power and modulation amplitudes, the chosen parameters were necessary to avoid clipping of the EPR spectra. 3.2. Dose-response curve The EPR spectra of irradiated wallboard are shown in Fig. 3. At a power of 9.0 mW and modulation amplitude of 0.1 mT, the SO 3 line is not clearly visible, but the CO 3 line grows with dose as a narrow peak on a large, interfering background. A large signal from Mn2þ is also visible in Fig. 3; the Mn2þ probably originates in the dolomite phase, as well as a possible contamination in the gypsum phase. One of the major challenges in obtaining reliable signal intensities is in properly performing background subtraction. If the background spectrum is not properly aligned with the signal spectrum, the signal intensities will not be accurate, and spurious peaks may arise. Shifts in the position of the resonance field are possible due to small changes in the microwave frequency. The microwave frequency, even after tuning, will not necessarily be identical between samples that have slightly different masses, or packing densities.
Fig. 2. Mass-normalized EPR intensity versus power for irradiated wallboard (18.8 0.9 Gy). EPR scan parameters: (varies), 336.2 1.0 mT, 0.01 mT, 32 30 s, 0.1 s, 1000 . Corresponding background (unirradiated) spectra were obtained and subtracted: (varies), 336.2 1.0 mT, 0.01 mT, 36 30 s, 0.1 s, 1000 .
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Fig. 3. Stacked EPR spectra of crushed, irradiated wallboard. Doses given are (bottom to top): 0.0, 1.1, 5.1, 9.9, 18.8 Gy. EPR scan parameters: 9.0 mW, 334.0 4.0 mT, 0.1 mT, 1 120 s, 0.1 s, 1000.
The simplest approach to proper alignment is to calibrate the spectrum by use of a standard with known g-values. Unfortunately, the MnO standard in use at the McMaster Archaeometry and Geochronology (AGE) Laboratory was completely overwhelmed by the Mn2þ present in the wallboard samples (Fig. 3). Initial studies (optimizing power and modulation amplitude) utilized a Mathematica routine (Wolfram Research, Inc., 2007) that forced the isotropic SO 3 line to be symmetric about the x-axis in the difference spectrum. However, with the larger power and modulation amplitude used from this point forward, the SO 3 line was no longer resolved, and alignment by this method was not possible. The EPR scans were therefore extended to lower fields to include an EPR line from the contaminating Mn2þ. Alignment was performed by minimizing the mean-square deviation of the difference spectrum between 330.0 and 331.0 mT. The background spectrum used for this and all subsequent analysis was the 0 Gy spectrum shown in Fig. 3. The background-corrected spectra are shown in Fig. 4. Alignment by use of the Mn2þ (III) line is not perfect; a residual peak is visible in the spectra near 330.4 mT. Additionally, the SO 3 line is visible, though poorly resolved, to the high-field side of the CO 3 line. The intensity versus dose of the CO 3 line is plotted in Fig. 5. The intensity grows linearly with dose, and the fitted line passes very close to the origin (y-intercept of 2.248 2.069). This doseresponse curve (DRC) may be used to determine the dose in an unknown sample, assuming the unknown sample has both the same radiosensitivity as the aliquots used to determine the DRC and that the unknown has the same zero-dose background.
Fig. 4. Background-corrected EPR spectra for irradiated wallboard. Doses given are (bottom to top): 1.1, 5.1, 9.9, 18.8 Gy. The EPR scan parameters are given in Fig. 3. Arrow indicates the measured peak-to-peak intensities of the CO 3 line (see Fig. 5).
In general, the EPR doses compare well with the known doses measured with a Farmer dosimeter. A large difference is seen at the origin, for which the EPR dose (73.0 Gy) dramatically underestimates the correct dose (112.5 Gy). However, this dose is much larger than the largest dose used to generate the DRC, so the lack of accuracy is not surprising. Additionally, along the bottom and right side of the left wallboard panel, one can see that the doses are underestimated (squares, Fig. 6b): doses of 6.5, 4.4, 1.7, and 3.9 Gy were measured with EPR, which compare to the known doses of 20.6, 12.1, 6.4, and
3.3. Dose measurement with irradiated panels Wallboard panels were irradiated with a 60Co source in such a way as to deliver a wide range of doses across the panels (2– 112 Gy). EPR intensities were converted to dose through comparison with the DRC. The expected and obtained (EPR) doses are shown in Fig. 6.
Fig. 5. EPR intensity versus dose for the CO 3 line in irradiated wallboard, with background subtraction. The EPR scan parameters are given in Fig. 3. The data have been fit with a line (y ¼ mx þ b): m ¼ 1.825 0.189 and b ¼ 2.248 2.069 (R2 ¼ 0.979).
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Fig. 6. (a) Contour plot of measured EPR doses for irradiated wallboard panels. The central column of EPR doses was determined using aliquots taken from the right edge of the left panel. EPR scan parameters are given in Fig. 3. (b) Comparison of EPR and known doses (measured with a Farmer dosimeter). The dashed line is a guide for the eye. EPR doses that were measured to be zero are not included. The data plotted as squares are discussed in the text.
7.2 Gy. It is possible that the wallboard near this edge is less sensitive to radiation, presumably due to a difference in the concentration of parent centres that can be converted to CO 3 radicals through ionizing radiation. At lower doses (farther from the source), the EPR dosesdwhere the EPR dose is not zerodare all within about 1.5 Gy of the known doses. The zero-dose measurements result when a definite peakto-peak measurement cannot be made in the backgroundcorrected spectrum. In order to estimate the accuracy of the EPR dose measurements, we exclude the four points with relatively low dose-insensitivity, which form a contiguous region in the lower portion of the left panel. For the remaining points, we find between 10 and 50 Gy, the EPR dose is accurate to within 10%, between 6 and 10 Gy, to within 30%, and less than 6 Gy, to within 45%. Very roughly, the detection limit appears to be approximately 2.5 Gy. A few important assumptions have been made in this analysis. First, we have assumed that the natural radiosensitivity of any particular aliquot of synthetic gypsum is identical to all other aliquots. If this is not the case, comparison with a ‘‘universal’’ DRC (Fig. 5) is not appropriate. The results shown in Fig. 6 strongly suggest that one particular region of the wallboard panel is less radiosensitive than the aliquots used to determine the DRC, which violates the assumption made here. In the future it may be necessary to determine the radiosensitivity of each aliquot by giving an additional dose, or series of additive doses. Second, we have assumed that the zero-dose background (see Fig. 5) is the same for all aliquots used in this study. Both the radiosensitivity of the (mass-normalized) CO 3 line and the zerodose EPR spectrum will depend on the concentration of paramagnetic centres in any given aliquot. We do not expect absolute homogeneity for trace centres, so both assumptions are unlikely to hold in all instances. Note, for example, the spurious peaks in Fig. 4, which indicate that background subtraction is not entirely successful. However, the results of this study do indicate that use of an arbitrary background spectrum and comparison with a previously determined DRC can result in accurate dose measurements with EPR of wallboard. Note that the zero-dose signal does not contain the known radiosensitive lines in gypsum. Presumably these lines have been
annealed due to the calcination process used in the manufacture of gypsum. This is very attractive from the perspective of retrospective dosimetry: the large doses acquired during the geologic burial of the gypsum do not appear to result in a confounding ‘‘preexposure’’ signal. (A dose-insensitive background is nonetheless present.) A detection limit of 1 Gy would be desirable for triage purposes (compare with the detection limit reported here: roughly 2.5 Gy). However, in an accident, the dose to wallboard is not necessarily the dose to the individual, which may be determined through the use of a scaling factor. These results indicate that EPR dosimetry of drywall could be used to determine spatially varying dose rates such as in an industrial accident. Additionally, this experiment was intended to simulate a possible forensic application: law enforcement agencies may desire to determine the presence and location of a previously removed, illicit radioactive source (Larsson et al., 2005). A dose map would, in this instance, be of use both in demonstrating the presence of a radioactive source (the measurement of dose in wallboard clearly indicates that the wallboard has been irradiated) and also possibly in extrapolating the position of the source based on the spatial variation of dose across a wallboard panel.
4. Conclusions The possibility of using wallboard (calcium sulphate dihydrate) as an EPR dosimeter has been demonstrated. The EPR intensity of the CO 3 radical increases linearly with dose, provided that the background is subtracted properly. Our protocol uses a power of 9.0 mW and modulation amplitude of 0.1 mW to enhance the CO 3 line intensity; the background is subtracted by alignment of the Mn2þ (III) line in the unknown and 0 Gy (background) spectrum. In order to demonstrate that this technique may be of use in retrospective or forensic dosimetry, wallboard panels were irradiated with a 60Co (gamma-ray) source. The resulting dose ‘‘map’’ indicates the presence and location of the irradiating source. The EPR dose measurements are accurate to within 10% at high doses (10–50 Gy) and to within 45% at doses near the detection limit (z2.5 Gy). It is likely that sensitivity to dose is variable across one
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of the wallboard panels; application of this technique in the future may require a correction for such a sensitivity variation. 5. Future directions Additional optimization is required to improve the signal-tonoise ratio and to decrease the minimum detectable dose. However, the most important unresolved issues are those of the background signal and radiation sensitivity. With diverse geological and artificial sources, the gypsum used in wallboard panels are extremely unlikely to be identical in terms of (trace) concentrations of dose-sensitive paramagnetic centres. The drywall used in this study came from one manufacturing facility; we would expect differences in the background spectrum for wallboard from other manufacturing facilities. Additionally, the dose-sensitivity of wallboard may vary with the concentration of carbonate impurities. Future work should therefore focus on sampling wallboard, as well as natural and synthetic gypsum, from a wide variety of manufacturing facilities, in order to be able to apply the technique more broadly. Finally, we plan to apply this technique to the dosimetry of neutron fields. Acknowledgements The authors would like to gratefully acknowledge the help of R. Pasuta (McMaster Nuclear Reactor) for assistance with irradiations, Dr. W.-H. Gong (BIMR, McMaster University) for performing XRD analysis, and R. Master (USG) for assistance with sourcing the wallboard. J.W. Thompson is grateful to Dr. T. Cousins and E. Inrig (DRDC – Ottawa) for helpful discussions regarding forensic dosimetry. This study was funded by the CRTI and the NSERC IRC. The authors are grateful to the two anonymous reviewers for helping to improve the manuscript. References Albuquerque, A.R.P.L., Isotani, S., 1982. The EPR spectra of x-ray irradiated gypsum. J. Phys. Soc. Jpn. 51, 1111–1118. Alexander, G.A., Swartz, H.M., Amundson, S.A., Blakely, W.F., Buddemeier, B., Gallez, B., Dainiak, N., Goans, R.E., Hayes, R.B., Lowry, P.C., Noska, M.A., Okunieff, P., Salner, A.L., Schauer, D.A., Trompier, F., Turteltaub, K.W., Voisin, P., Wiley Jr., A.L., Wilkins, R., 2007. BiodosEPR-2006 meeting: acute dosimetry consensus committee recommendations on biodosimetry applications in events involving uses of radiation by terrorists and radiation accidents. Radiat. Meas. 42, 972–996. Barthe, J., Kamenopoulou, V., Cattoire, B., Portal, G., 1989. Dose evaluation from textile fibers: a post-determination of initial ESR signal. Appl. Radiat. Isot. 40, 1029–1033. Blackwell, B., 1995. Electron spin resonance dating. In: Rutter, N.W., Catto, N.R. (Eds.), Dating Methods for Quaternary Deposits. Geological Association of Canada, pp. 209–268. Bradshaw, W.W., Cadena Jr., D.G., Crawford, G.W., Spetzler, H.A.W., 1962. The use of alanine as a solid dosimeter. Radiat. Res. 17, 11–21. Cutrubinis, M., Chirita, D., Savu, D., Secu, C.E., Mihai, R., Secu, M., Ponta, C., 2007. Preliminary study on detection of irradiated foodstuffs from the Romanian market. Radiat. Phys. Chem. 76, 1450–1454. Da Costa, Z.M., Pontuschka, W.M., Campos, L.L., 2005. A comparative study based on dosimetric properties of different sugars. Appl. Radiat. Isot. 62, 331–336. Da Costa, Z.M., Pontuschka, W.M., Ludwig, V., Giehl, J.M., Da Costa, C.R., Duarte, E.L., 2007. A study based on ESR, XRD and SE of signal induced by gamma irradiation in eggshell. Radiat. Meas. 42, 1233–1236. Desrosiers, M.F., 1993. EPR bone dosimetry: a new approach to spectral deconvolution problems. Appl. Radiat. Isot. 44, 81–83. Duliu, O.G., Georgescu, R., Ali, S.I., 2007. EPR investigation of some traditional oriental irradiated spices. Radiat. Phys. Chem. 76, 1031–1036. Engin, B., Aydas, C., Demirtas, H., 2006. ESR dosimetric properties of window glass. Nucl. Instrum. Methods Phys. Res. B 243, 149–155.
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Potential use of wallboard (drywall) for EPR retrospective dosimetry Jeroen W. Thompson a, *, Ibrahim Abu Atiya a, W. Jack Rink b, Doug Boreham a a b
Department of Medical Physics & Applied Radiation Sciences, McMaster University, 1280 Main St. West, Hamilton ON L8S 4K1, Canada School of Geography and Earth Sciences, McMaster University, 1280 Main St. West, Hamilton ON L8S 4K1, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 July 2008 Received in revised form 19 November 2008 Accepted 15 March 2009
Concern regarding the possibility of criminal or terrorist use of nuclear materials has led to an interest in developing the capability to measure radiation dose in a variety of natural and manufactured materials. Electron paramagnetic resonance (EPR) measurements of radiation dose following a radiological incident may aid in screening affected populations (triage) and in reconstruction of doses following accidents. One such EPR dosimeter is wallboard (drywall), a common construction material composed largely of gypsum (calcium sulphate dihydrate). We have identified the CO 3 and SO3 dose-sensitive lines in drywall and developed a measurement protocol using the intensity of CO3 line. Proper background subtraction is a major difficulty, and we demonstrate a procedure based on alignment of a contaminant Mn2þ line. As a proof-of-concept, a wallboard panel was irradiated with a 60Co source, and a twodimensional map of the absorbed dose was measured. While most aliquots yielded reasonably accurate doses, a spatially contiguous region of apparent dose-insensitivity in one panel was identified. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Electron paramagnetic resonance EPR Electron spin resonance ESR Dosimetry Retrospective dosimetry Forensic dosimetry Wallboard Drywall Gypsum Calcium sulphate dihydrate
1. Introduction Electron paramagnetic resonance (EPR, or ESR: electron spin resonance), discovered in 1944 by E. K. Zavoisky, is used for the fundamental study of free radicals and points defects, as well as the measurement of radiation dose (Weil et al., 1994). Radiationinduced EPR signals were first studied in the 1950s (Gordy et al., 1955). Later, the amino acid alanine was shown to be an EPR dosimeter (Bradshaw et al., 1962) and has been used as a transfer dosimeter for radiation therapy since the 1980s (Regulla and Deffner, 1982). EPR dosimetry has been applied to natural materials for the purposes of geochronology, first beginning with a study of dose measured in natural calcite (Ikeya, 1975); many other geological and biological materials have been studied for purposes of applying EPR dosimetry as well (Blackwell, 1995; Rink, 1997). There is growing interest in the use of EPR dosimetry following a radiological incident (such as an industrial accident or, less likely, a terrorist attack). It may be necessary in such cases to screen the
* Corresponding author. E-mail addresses: [email protected] (J.W. Thompson), abuatii@ mcmaster.ca (I.A. Atiya), [email protected] (W.J. Rink), [email protected] (D. Boreham). 1350-4487/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2009.03.027
public in order to distinguish the exposed from the non-irradiated and the ‘‘worried well’’ (Alexander et al., 2007). EPR dosimetry may also be used to reconstruct doses well after the exposure has taken place (Ishii et al., 1990; Romanyukha et al., 1994; Skvortsov et al., 2000; International Atomic Energy Agency, 2002; Stepanenko et al., 2007). A suitable biophysical dosimeter would be most appropriate for triage purposes, assuming that the absorbed dose to the target dosimeter represents the dose to critical organs. However, other common natural or manufactured materials (so-called ‘‘fortuitous’’ dosimeters) may serve as useful EPR dosimeters. Sugar (sucrose), for example, is known to be a very sensitive EPR dosimeter, with a minimum detectable dose of approximately 100 mGy (Nakajima, 1995; Fattibene et al., 1996; Da Costa et al., 2005). Other proposed dosimeters include bone (Desrosiers, 1993; Ikeya, 1993), hair (Trivedi and Greenstock, 1993; Trompier et al., 2007), foodstuffs containing collagen, hydroxyapatite, or calcite (Ikeya et al., 1984; Ikeya, 1993; Blackwell, 1995; Engin et al., 2006; Cutrubinis et al., 2007; Maghraby, 2007; Duliu et al., 2007; Parlato et al., 2007; Yordanov and Aleksieva, 2007; Da Costa et al., 2007), window glass (Engin et al., 2006), chalk (Wieser et al., 1994), and resin and bark (Serzhant et al., 1996). Some textiles have been shown to be suitable as fortuitous EPR dosimeters. For example, cotton and untreated polypropylene fabrics have a minimum detectable dose of 1 Gy and
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0.5 Gy, respectively (Barthe et al., 1989), although it is unclear whether this applies textile fabrics generally or just the particular textile samples studied. In addition, quartz grains extracted from a granite pillar at Hiroshima were used to show that the EPR intensity profile of the germanium paramagnetic centre matched the expected intensity profile of atomic bomb radiation, suggesting that quartz-bearing construction materials may also be useful as EPR dosimeters (Ikeya, 1993). In this study, we examine the suitability of wallboard as a fortuitous EPR dosimeter. The near-ubiquity of wallboard in new construction in North America (and its increasing use in developing countries) suggest that EPR dosimetry of wallboard may be important in future radiological incidents. Although wallboard is not a biophysical dosimeterdand hence, the dose to the wallboard does not directly reflect the dose to an individualdit may still prove useful in triage, provided that the dose to the individual may be estimated through multiplication by an appropriate occupancy factor. An initial study of wallboard was performed by Haskell et al. (1996), in which a dose-response curve for wallboard was demonstrated. The goals of this study were to identify the radiosensitive EPR lines in wallboard, to develop the necessary protocols to measure dose in wallboard, and to measure a two-dimensional ‘‘map’’ of dose in irradiated wallboard panels. 1.1. Wallboard Wallboard is composed primarily of gypsum (CaSO4$2H2O), a natural monoclinic, evaporitic mineral, which may also be a component of plaster and stucco, and is an additive to cement and may also be used in soil mitigation (Panagapko, 2007). Gypsum is pulverized and calcined. The resulting hemihydrate is combined with water and hardens into the desired shape: wallboard is gypsum held between two sheets of paper. The United States are the world’s largest producer of natural (mined) gypsum (17.5 Mt in 2005), followed by Iran (11 Mt) and Canada, Thailand, and China (about 8 Mt each) (Panagapko, 2007). Synthetic gypsum also forms a significant percentage of the gypsum used to manufacture wallboard. Synthetic gypsum is largely produced at coal-fired power plants through a process called flue-gas desulphurization (FGD), but may also a by-product of titanium production (Panagapko, 2007).
dose measured was greater than was geologically reasonable, which may indicate problems with determination of the natural dose rate. 2. Methods 2.1. EPR measurements and dose sensitivity All first-harmonic EPR spectra were collected with a JEOL JESFA100 X-band (9.4 GHz) spectrometer, with a TE011 cylindrical cavity and with modulation frequency of 100 kHz. Aliquots were placed inside a 5 mm (OD) Suprasil quartz EPR sample tube; aliquot masses were approximately 100 mg. The EPR spectrometer was tuned as needed using the JEOL control software. In the text, EPR scan parameters are given as follows: power, centre field halfwidth, modulation amplitude, (# of sweeps) (sweep time), time constant, gain. All irradiations were performed at the hot cell in the McMaster Nuclear Reactor. A winch was used to raise and lower a 60Co source (1.17 and 1.33 MeV gamma-rays), which was placed vertically in a holder on a tabletop. Before samples were placed in the hot cell, the dose rate at various locations was measured with a Farmer dosimeter. Exposure is converted to dose through a calibration factor (0.87). The desired dose was delivered to the samples by irradiating for the desired length of time; the dose delivered while raising and lowering the source is measured directly with the dosimeter. Uncertainties in the doses are assumed to be 5%. 2.2. Samples Half-inch wallboard panels were purchased from a local (Ancaster, Ontario, Canada) retailer. The wallboard was manufactured at the Canadian Gypsum Company (a subsidiary of United States Gypsum) manufacturing plant in Hagersville, Ontario on 27 March, 2008. According to publicly available information (see http://www.usg.com/), wallboard from this plant contains approximately 24% FGD synthetic gypsum (averaged over one year). The paper backing was peeled away, and a sample of wallboard was extracted, lightly ground with an agate mortar and pestle, and sieved to a particle size of less than 250 mm. Individual aliquots were irradiated in borosilicate culture tubes. The chemical composition of the wallboard was determined through powder X-ray diffraction (XRD).
1.2. Gypsum 2.3. Dose-response curve Dose-sensitive EPR signals in gypsum were originally studied by Wigen and Cowen (1960) and Albuquerque and Isotani (1982). Three primary paramagnetic centres have been identified in powder samples of natural and synthetic gypsum (Kasuya et al., 1991; Ikeda and Ikeya, 1992; Ikeya, 1993): G1, G2, and G3. G1 is a narrow, isotropic line at g ¼ 2.004, corresponding to the sulfite (SO 3 ) radical (electron centre). G2 is a nearly axial EPR line with gx ¼ 2.0084, gy ¼ 2.0088, and gz ¼ 2.0192, likely the carbonate (CO 3) radical (hole centre). G3 is an orthorhombic EPR line with gx ¼ 2.0029, gy ¼ 2.0027 and gz ¼ 1.9973, possibly corresponding to the CO 2 radical. EPR dosimetry has been applied to natural gypsum precipitates in several studies (Nambi, 1982; Yijian et al., 1989; Ikeda and Ikeya, 1992; Mathew et al., 2004; Ulusoy, 2004); the accumulated dose, or equivalent dose (the laboratory dose assumed to be equivalent to the actual dose), is used to determine the age of the deposit (Rink, 1997). For example, the G2 centre was used to date gypsum deposits at Mammoth Cave, Kentucky (Ikeya, 1993). The same centre was used to date gypsum precipitates from a fault surface in central California (Ikeda and Ikeya, 1992); however, the equivalent
100 mg aliquots of crushed and sieved wallboard (untreated) were irradiated with doses of 1, 5, 10, and 19 Gy. The EPR scan parameters used for the dose-response curve (DRC) are: 9.0 mW, 334.0 4.0 mT, 0.1 mT, 1 120 s, 0.1 s 1000. The 0 Gy spectrum was subtracted from the irradiated spectra in order to enhance the radiosensitive signal. The Mn2þ line was used to align the background and signal spectra; the mean-square deviation was minimized over the region 330.0–331.0 mT. 2.4. Dose measurement with irradiated panels For irradiation of whole wallboard panels, two 20 40 wallboard panels were placed on a table, perpendicular to the 60Co source. Aliquots were extracted from the panel by peeling off the paper backing and scraping the wallboard with a metal spatula. Grinding was not necessary; the aliquots were individually sieved to particle size of less than 250 mm. Unknown doses were determined by comparison with the DRC. For the panel closest to the source, the EPR scan parameters were
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the same as used to measure the DRC; for aliquots extracted from the panel farthest from the source, the same EPR parameters were used, except the number of sweeps was increased to 4. The zerodose spectrum used to construct the DRC was subtracted as the background from all spectra obtained from the wallboard panels (a comparable spectrum was obtained with 4 sweeps to use as the background for the panel farthest from the source). Due to the difficulty in obtaining useful dose measurements over the entire panel (the manipulators could not be used to move the dosimeter into the proper location), the EPR measurements were made using approximately three-quarters of each panel. Irradiation and EPR measurements were made within 10 weeks. 3. Results and discussion 3.1. EPR measurements and dose sensitivity XRD analysis determined that the wallboard was composed of 90% gypsum (CaSO4$2H2O), 4% dolomite (CaMg(CO3)2), 1% anhydrite (CaSO4), and 0.5% each of plaster of paris (CaSO4$0.5H2O), calcite (CaCO3), crystalline quartz (SiO2), and fibrous glass (SiO2), in addition to other amorphous components (2.5%); proportions are approximate and were estimated from the relative intensities of the diffraction peaks. These results compare well with the manufacturer’s Material Safety Data Sheet (MSDS), although dolomite was not listed. Initial EPR measurements revealed the presence of a narrow EPR line in the irradiated wallboard, near B ¼ 336 mT (see Fig. 1). In order to allow accurate measurements of the EPR intensity (peakto-peak height in the first-harmonic difference spectrum), it was necessary to subtract the interfering background spectrum. The previously noted line is quite clear in the difference spectrum, but an additional line appears at a slightly lower field strength, near B ¼ 335.5 mT (Fig. 1). On the basis of its uncalibrated g-value (g ¼ 2.010), the low-field line was tentatively identified as the CO 3 radical, the so-called G2 line (Ikeda and Ikeya, 1992). The CO 3 radical should give an
Fig. 1. Difference between irradiated (top: 18.8 Gy) and unirradiated wallboard EPR spectra (not mass-normalized). Uncalibrated g-values: 2.010 (CO 3 ) and 2.007 (SO3 ). EPR scan parameters: 0.5 mW, 336.2 1.0 mT, 0.01 mT, 1 30 s, 1000 .
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additional weak line at a lower field (roughly 333.8 mT in these studies); this line has not yet been observed in our wallboard samples. The high-field line (uncalibrated g ¼ 2.007) was tentatively identified as the SO 3 radical, or G1 (Ikeda and Ikeya, 1992). The intensity of each line (after background subtraction) was measured as a function of microwave power (Fig. 2) and modulation amplitude (not shown). The intensity of the CO 3 line increases with power, and saturation has not been reached by 9 mW. The SO 3 intensity saturated at a moderate power of approximately 1 mW. These results confirm identification of the CO 3 and SO3 radicals (Kasuya et al., 1991). The largest signal intensities were found with the CO 3 radical, using a microwave power of 9.0 mW and modulation amplitude of 0.1 mT. Although it may be possible in principle to obtain even greater signal intensities with larger power and modulation amplitudes, the chosen parameters were necessary to avoid clipping of the EPR spectra. 3.2. Dose-response curve The EPR spectra of irradiated wallboard are shown in Fig. 3. At a power of 9.0 mW and modulation amplitude of 0.1 mT, the SO 3 line is not clearly visible, but the CO 3 line grows with dose as a narrow peak on a large, interfering background. A large signal from Mn2þ is also visible in Fig. 3; the Mn2þ probably originates in the dolomite phase, as well as a possible contamination in the gypsum phase. One of the major challenges in obtaining reliable signal intensities is in properly performing background subtraction. If the background spectrum is not properly aligned with the signal spectrum, the signal intensities will not be accurate, and spurious peaks may arise. Shifts in the position of the resonance field are possible due to small changes in the microwave frequency. The microwave frequency, even after tuning, will not necessarily be identical between samples that have slightly different masses, or packing densities.
Fig. 2. Mass-normalized EPR intensity versus power for irradiated wallboard (18.8 0.9 Gy). EPR scan parameters: (varies), 336.2 1.0 mT, 0.01 mT, 32 30 s, 0.1 s, 1000 . Corresponding background (unirradiated) spectra were obtained and subtracted: (varies), 336.2 1.0 mT, 0.01 mT, 36 30 s, 0.1 s, 1000 .
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Fig. 3. Stacked EPR spectra of crushed, irradiated wallboard. Doses given are (bottom to top): 0.0, 1.1, 5.1, 9.9, 18.8 Gy. EPR scan parameters: 9.0 mW, 334.0 4.0 mT, 0.1 mT, 1 120 s, 0.1 s, 1000.
The simplest approach to proper alignment is to calibrate the spectrum by use of a standard with known g-values. Unfortunately, the MnO standard in use at the McMaster Archaeometry and Geochronology (AGE) Laboratory was completely overwhelmed by the Mn2þ present in the wallboard samples (Fig. 3). Initial studies (optimizing power and modulation amplitude) utilized a Mathematica routine (Wolfram Research, Inc., 2007) that forced the isotropic SO 3 line to be symmetric about the x-axis in the difference spectrum. However, with the larger power and modulation amplitude used from this point forward, the SO 3 line was no longer resolved, and alignment by this method was not possible. The EPR scans were therefore extended to lower fields to include an EPR line from the contaminating Mn2þ. Alignment was performed by minimizing the mean-square deviation of the difference spectrum between 330.0 and 331.0 mT. The background spectrum used for this and all subsequent analysis was the 0 Gy spectrum shown in Fig. 3. The background-corrected spectra are shown in Fig. 4. Alignment by use of the Mn2þ (III) line is not perfect; a residual peak is visible in the spectra near 330.4 mT. Additionally, the SO 3 line is visible, though poorly resolved, to the high-field side of the CO 3 line. The intensity versus dose of the CO 3 line is plotted in Fig. 5. The intensity grows linearly with dose, and the fitted line passes very close to the origin (y-intercept of 2.248 2.069). This doseresponse curve (DRC) may be used to determine the dose in an unknown sample, assuming the unknown sample has both the same radiosensitivity as the aliquots used to determine the DRC and that the unknown has the same zero-dose background.
Fig. 4. Background-corrected EPR spectra for irradiated wallboard. Doses given are (bottom to top): 1.1, 5.1, 9.9, 18.8 Gy. The EPR scan parameters are given in Fig. 3. Arrow indicates the measured peak-to-peak intensities of the CO 3 line (see Fig. 5).
In general, the EPR doses compare well with the known doses measured with a Farmer dosimeter. A large difference is seen at the origin, for which the EPR dose (73.0 Gy) dramatically underestimates the correct dose (112.5 Gy). However, this dose is much larger than the largest dose used to generate the DRC, so the lack of accuracy is not surprising. Additionally, along the bottom and right side of the left wallboard panel, one can see that the doses are underestimated (squares, Fig. 6b): doses of 6.5, 4.4, 1.7, and 3.9 Gy were measured with EPR, which compare to the known doses of 20.6, 12.1, 6.4, and
3.3. Dose measurement with irradiated panels Wallboard panels were irradiated with a 60Co source in such a way as to deliver a wide range of doses across the panels (2– 112 Gy). EPR intensities were converted to dose through comparison with the DRC. The expected and obtained (EPR) doses are shown in Fig. 6.
Fig. 5. EPR intensity versus dose for the CO 3 line in irradiated wallboard, with background subtraction. The EPR scan parameters are given in Fig. 3. The data have been fit with a line (y ¼ mx þ b): m ¼ 1.825 0.189 and b ¼ 2.248 2.069 (R2 ¼ 0.979).
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Fig. 6. (a) Contour plot of measured EPR doses for irradiated wallboard panels. The central column of EPR doses was determined using aliquots taken from the right edge of the left panel. EPR scan parameters are given in Fig. 3. (b) Comparison of EPR and known doses (measured with a Farmer dosimeter). The dashed line is a guide for the eye. EPR doses that were measured to be zero are not included. The data plotted as squares are discussed in the text.
7.2 Gy. It is possible that the wallboard near this edge is less sensitive to radiation, presumably due to a difference in the concentration of parent centres that can be converted to CO 3 radicals through ionizing radiation. At lower doses (farther from the source), the EPR dosesdwhere the EPR dose is not zerodare all within about 1.5 Gy of the known doses. The zero-dose measurements result when a definite peakto-peak measurement cannot be made in the backgroundcorrected spectrum. In order to estimate the accuracy of the EPR dose measurements, we exclude the four points with relatively low dose-insensitivity, which form a contiguous region in the lower portion of the left panel. For the remaining points, we find between 10 and 50 Gy, the EPR dose is accurate to within 10%, between 6 and 10 Gy, to within 30%, and less than 6 Gy, to within 45%. Very roughly, the detection limit appears to be approximately 2.5 Gy. A few important assumptions have been made in this analysis. First, we have assumed that the natural radiosensitivity of any particular aliquot of synthetic gypsum is identical to all other aliquots. If this is not the case, comparison with a ‘‘universal’’ DRC (Fig. 5) is not appropriate. The results shown in Fig. 6 strongly suggest that one particular region of the wallboard panel is less radiosensitive than the aliquots used to determine the DRC, which violates the assumption made here. In the future it may be necessary to determine the radiosensitivity of each aliquot by giving an additional dose, or series of additive doses. Second, we have assumed that the zero-dose background (see Fig. 5) is the same for all aliquots used in this study. Both the radiosensitivity of the (mass-normalized) CO 3 line and the zerodose EPR spectrum will depend on the concentration of paramagnetic centres in any given aliquot. We do not expect absolute homogeneity for trace centres, so both assumptions are unlikely to hold in all instances. Note, for example, the spurious peaks in Fig. 4, which indicate that background subtraction is not entirely successful. However, the results of this study do indicate that use of an arbitrary background spectrum and comparison with a previously determined DRC can result in accurate dose measurements with EPR of wallboard. Note that the zero-dose signal does not contain the known radiosensitive lines in gypsum. Presumably these lines have been
annealed due to the calcination process used in the manufacture of gypsum. This is very attractive from the perspective of retrospective dosimetry: the large doses acquired during the geologic burial of the gypsum do not appear to result in a confounding ‘‘preexposure’’ signal. (A dose-insensitive background is nonetheless present.) A detection limit of 1 Gy would be desirable for triage purposes (compare with the detection limit reported here: roughly 2.5 Gy). However, in an accident, the dose to wallboard is not necessarily the dose to the individual, which may be determined through the use of a scaling factor. These results indicate that EPR dosimetry of drywall could be used to determine spatially varying dose rates such as in an industrial accident. Additionally, this experiment was intended to simulate a possible forensic application: law enforcement agencies may desire to determine the presence and location of a previously removed, illicit radioactive source (Larsson et al., 2005). A dose map would, in this instance, be of use both in demonstrating the presence of a radioactive source (the measurement of dose in wallboard clearly indicates that the wallboard has been irradiated) and also possibly in extrapolating the position of the source based on the spatial variation of dose across a wallboard panel.
4. Conclusions The possibility of using wallboard (calcium sulphate dihydrate) as an EPR dosimeter has been demonstrated. The EPR intensity of the CO 3 radical increases linearly with dose, provided that the background is subtracted properly. Our protocol uses a power of 9.0 mW and modulation amplitude of 0.1 mW to enhance the CO 3 line intensity; the background is subtracted by alignment of the Mn2þ (III) line in the unknown and 0 Gy (background) spectrum. In order to demonstrate that this technique may be of use in retrospective or forensic dosimetry, wallboard panels were irradiated with a 60Co (gamma-ray) source. The resulting dose ‘‘map’’ indicates the presence and location of the irradiating source. The EPR dose measurements are accurate to within 10% at high doses (10–50 Gy) and to within 45% at doses near the detection limit (z2.5 Gy). It is likely that sensitivity to dose is variable across one
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of the wallboard panels; application of this technique in the future may require a correction for such a sensitivity variation. 5. Future directions Additional optimization is required to improve the signal-tonoise ratio and to decrease the minimum detectable dose. However, the most important unresolved issues are those of the background signal and radiation sensitivity. With diverse geological and artificial sources, the gypsum used in wallboard panels are extremely unlikely to be identical in terms of (trace) concentrations of dose-sensitive paramagnetic centres. The drywall used in this study came from one manufacturing facility; we would expect differences in the background spectrum for wallboard from other manufacturing facilities. Additionally, the dose-sensitivity of wallboard may vary with the concentration of carbonate impurities. Future work should therefore focus on sampling wallboard, as well as natural and synthetic gypsum, from a wide variety of manufacturing facilities, in order to be able to apply the technique more broadly. Finally, we plan to apply this technique to the dosimetry of neutron fields. Acknowledgements The authors would like to gratefully acknowledge the help of R. Pasuta (McMaster Nuclear Reactor) for assistance with irradiations, Dr. W.-H. Gong (BIMR, McMaster University) for performing XRD analysis, and R. Master (USG) for assistance with sourcing the wallboard. J.W. Thompson is grateful to Dr. T. Cousins and E. Inrig (DRDC – Ottawa) for helpful discussions regarding forensic dosimetry. This study was funded by the CRTI and the NSERC IRC. The authors are grateful to the two anonymous reviewers for helping to improve the manuscript. References Albuquerque, A.R.P.L., Isotani, S., 1982. The EPR spectra of x-ray irradiated gypsum. J. Phys. Soc. Jpn. 51, 1111–1118. Alexander, G.A., Swartz, H.M., Amundson, S.A., Blakely, W.F., Buddemeier, B., Gallez, B., Dainiak, N., Goans, R.E., Hayes, R.B., Lowry, P.C., Noska, M.A., Okunieff, P., Salner, A.L., Schauer, D.A., Trompier, F., Turteltaub, K.W., Voisin, P., Wiley Jr., A.L., Wilkins, R., 2007. BiodosEPR-2006 meeting: acute dosimetry consensus committee recommendations on biodosimetry applications in events involving uses of radiation by terrorists and radiation accidents. Radiat. Meas. 42, 972–996. Barthe, J., Kamenopoulou, V., Cattoire, B., Portal, G., 1989. Dose evaluation from textile fibers: a post-determination of initial ESR signal. Appl. Radiat. Isot. 40, 1029–1033. Blackwell, B., 1995. Electron spin resonance dating. In: Rutter, N.W., Catto, N.R. (Eds.), Dating Methods for Quaternary Deposits. Geological Association of Canada, pp. 209–268. Bradshaw, W.W., Cadena Jr., D.G., Crawford, G.W., Spetzler, H.A.W., 1962. The use of alanine as a solid dosimeter. Radiat. Res. 17, 11–21. Cutrubinis, M., Chirita, D., Savu, D., Secu, C.E., Mihai, R., Secu, M., Ponta, C., 2007. Preliminary study on detection of irradiated foodstuffs from the Romanian market. Radiat. Phys. Chem. 76, 1450–1454. Da Costa, Z.M., Pontuschka, W.M., Campos, L.L., 2005. A comparative study based on dosimetric properties of different sugars. Appl. Radiat. Isot. 62, 331–336. Da Costa, Z.M., Pontuschka, W.M., Ludwig, V., Giehl, J.M., Da Costa, C.R., Duarte, E.L., 2007. A study based on ESR, XRD and SE of signal induced by gamma irradiation in eggshell. Radiat. Meas. 42, 1233–1236. Desrosiers, M.F., 1993. EPR bone dosimetry: a new approach to spectral deconvolution problems. Appl. Radiat. Isot. 44, 81–83. Duliu, O.G., Georgescu, R., Ali, S.I., 2007. EPR investigation of some traditional oriental irradiated spices. Radiat. Phys. Chem. 76, 1031–1036. Engin, B., Aydas, C., Demirtas, H., 2006. ESR dosimetric properties of window glass. Nucl. Instrum. Methods Phys. Res. B 243, 149–155.
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