- Email: [email protected]
Optically stimulated luminescence of electronic components for forensic, retrospective, and accident dosimetry
Radiation Measurements 43 (2008) 726 – 730 www.elsevier.com/locate/radmeas
Optically stimulated luminescence of electronic components for forensic, retrospective, and accident dosimetry E.L. Inrig a,∗ , D.I. Godfrey-Smith b , S. Khanna a, a Defence Research and Development Canada—Ottawa, 3701 Carling Avenue, Ottawa, Canada b Defence Research and Development Canada—CORA, Major-General George R. Pearkes Building, 18ST, 101 Colonel Bay Drive, Ottawa, Canada
Abstract This study investigated the optically stimulated luminescence (OSL) response of electronic components found within portable electronic devices such as cell phones and pagers, portable computers, music and video players, global positioning system receivers, cameras, and digital watches. The analysis of components extracted from these ubiquitous devices was proposed for applications ranging from rapid accident dose reconstruction to the tracking and attribution of gamma-emitting radiological materials. Surface-mount resistors with alumina porcelain substrates consistently produced OSL following irradiation, with minimum detectable doses on the order of 10 mGy for a typical sample. Since the resistor ceramics were found to exhibit anomalous fading, dose reconstruction procedures were developed to correct for this using laboratory measurements of fading rates carried out over approximately 3 months. Two trials were conducted in which cellular phones were affixed to an anthropomorphic phantom and irradiated using gamma-ray sources; ultimately, analysis of the devices used in these trials succeeded in reconstructing doses in the range of 0.1.0.6 Gy. Crown Copyright © 2007 Published by Elsevier Ltd. All rights reserved. Keywords: Optically stimulated luminescence; OSL; Retrospective dosimetry; Accident dosimetry; Forensic dosimetry; Ceramics; Substrates; Electronics; Silicate; Alumina; Gamma and beta radiation
1. Introduction For more than 40 years, luminescence techniques have been used to estimate doses to exposed populations in contaminated areas through the analysis of bricks and other radiation-sensitive building materials and household objects. More recently, dose reconstruction using personal effects such as telephone chip-cards has been considered (Göksu, 2003). Applications to forensics, where the dose imparted to building materials and household chemicals is used to identify former storage locations of radiological materials, have also been explored (Larsson et al., 2005). In the current security climate, there is an increased awareness of the potential threat posed by terrorist groups, should they acquire radiological materials for use in an attack on a civilian target. In view of this threat, new technologies are Deceased.
∗ Corresponding author. Tel.: +1 613 998 6109; fax: +1 613 998 4560.
E-mail address: [email protected] (E.L. Inrig).
required for tracking the movement of such materials, ideally preventing their deployment, and for dealing with the consequences of such an attack should attempts at interdiction fail. Although there is an entire industry devoted to the protection and monitoring of radiation workers, it is clear that in the event of a radiological accident or attack involving exposure to the general public, the vast majority of those exposed will not be equipped with accredited dosimeters. In the aftermath of a widespread exposure to radiation, such as that might occur following the detonation of a radiological dispersal device (RDD) or even the deployment of a radiological exposure device (RED), health care professionals will face the overwhelming task of evaluating exposed individuals and distinguishing between the potentially exposed and the worriedwell. A rapid method of assessing external radiation exposure using commonly held personal objects (essentially “fortuitous dosimeters”) would thus be invaluable for triage purposes. Ideally, a “fortuitous dosimeter” should be positioned close to the body, be carried by a large percentage of the population, and be
1350-4487/$ - see front matter Crown Copyright © 2007 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.11.078
E.L. Inrig et al. / Radiation Measurements 43 (2008) 726 – 730
sufficiently sensitive to detect doses below the threshold for deterministic health effects. Since analysis is expected to be performed close to the time of exposure (within days to months), the measured dose response from the material of interest need not be completely stable so long as procedures can be developed to correct for signal fading. Our research has determined that cellular phones and other electronic devices should fulfil all of these requirements and can be used successfully for dose reconstruction. In previous studies, we demonstrated that certain novel materials employed as substrates for integrated circuits (ICs) exhibit dose response luminescence (Godfrey-Smith, 2006; Godfrey-Smith and Khanna, 2007a, b). In this study, we investigate the optically stimulated luminescence (OSL) dose response of widely used materials that are a part of surfacemount components, such as resistors, capacitors, diodes, and inductors, in modern electronic devices. The sensitivity and fading characteristics of resistor substrates were investigated in more detail. 2. Experimental details and samples Samples selected for analysis consisted both of unused surface-mount components obtained from Cal-Chip Electronics, Inc., and Bourns, Inc., and a representative selection of components removed from the circuit boards of several cellular phones from different manufacturers. The smallest rectangular resistors, such as those visible in Fig. 1, measure 1 × 0.5 mm and have white alumina porcelain substrates (typical composition approximately 97% Al2 O3 , 3% SiO2 ) that are uncoated on the underside permitting OSL measurements. Many of the capacitors studied contained dark-coloured ceramic substrates, exposed on all sides of the component, and were composed of materials such as barium titanate and calcium titanate. All measurements were performed using a RisZ TL-OSLDA-15 reader, and except where specified otherwise, samples were irradiated using the integrated 1.48 GBq (40 mCi) 90 Sr/90 Y irradiator, equipped with a removable 2.2 mm aluminium attenuator that reduced the dose rate to 4.3 mGy s−1 , as determined by cross-calibration with a reference 60 Co gamma source. For OSL measurements, stimulation light was provided by blue LEDs (470 nm), and both thermoluminescence (TL) and OSL curves were recorded with Hoya U-340 filters in the optical detection system. Following initial experiments to identify components with sufficient dose response to be of interest, 4.3 mGy to 98 Gy dose response curves for a number of resistor substrate types were constructed. The number of resistors per aliquot was dependent on the size of resistors being studied; some measure less than 1 mm in length and width, while large resistor arrays may be several mm long and wide. Signal fading was quantified by short- and long-term experiments conducted on multi-resistor aliquots of chip resistors manufactured by Cal-Chip Electronics (RM10F68R1CT) and Bourns, Inc., (CR0805-FX-1200E). Short-term fading was measured for delays of 30 s to 8.4 h following a 2.2 Gy automated beta irradiation, while long-term fading involved delays of 1 h to 90 d after a 5 Gy dose delivered by a 60 Co source.
727
Fig. 1. Section of a circuit board from RIM Blackberry. Inset: resistor sample prepared for analysis, displayed on a 1 cm grid.
In these experiments, various preheat temperatures between 120 and 200 ◦ C were used to determine the effect of the preheat temperature on the fading rate. The OSL response was normalized to the response from a 0.43 Gy test dose administered using the integrated 90 Sr/90 Y source. Two trials were conducted in which cellular phones were affixed to an anthropomorphic phantom at belt level and exposed to a radioactive source in a geometry designed to simulate the deployment of an RED under a park bench or a desk. An MGP SOR/R electronic dosimeter (ED) was affixed to each device to record an approximate dose, and an additional ED was placed mid-gut inside the phantom. In the first trial, three cellular phones were exposed to a 1.1 GBq (30 mCi) 60 Co source for approximately 3.5 d, while the second trial exposed four devices to a 74 GBq (2 Ci) 137 Cs source for 4 h; in each case, the distances between the source and the devices were in the range of 25–40 cm. In preparation for analysis, each device was disassembled under darkroom conditions and the resistors were removed from the circuit boards using a utility knife. The resistors were then soaked in methylene chloride for approximately 30 min in order to remove adhesive, used during the manufacturing process to secure the components to the circuit boards before soldering, from the bottom of each component. Resistors were placed bottom-up on a planchette covered with a thin layer of silicone oil, with as many components as would fit placed on a single planchette; in general, each device yielded enough resistors for one to two samples. For each sample, the OSL response to the trial dose was measured along with the response to four regeneration doses followed by 43 mGy test doses, with the first regeneration dose repeated. 3. Results and discussion 3.1. OSL and TL response Initial experiments demonstrated that surface-mount resistor alumina substrates consistently exhibited OSL following
728
E.L. Inrig et al. / Radiation Measurements 43 (2008) 726 – 730
Fig. 3. Depletion of TL signal by various durations of blue light stimulation for a set of six Cal-Chip 68.1 resistors (RM10F68R1CT) irradiated to 2.1 Gy.
at the upper end of the range. We observed remarkably little change in sensitivity over more than a hundred cycles of irradiation, heating, and optical stimulation. While the dose response sensitivities of the substrates varied widely between manufacturers and production lots, a minimum detectable dose of less than 10 mGy was typical for an aliquot composed of several resistors (Fig. 4). Fig. 2. (a) Smoothed TL glow curve (5 ◦ C/s) for set of six Cal-Chip 68.1 resistors (RM10F68R1CT) irradiated to 2.1 Gy. (b) OSL response of a 73 mg sample of Cal-Chip 249 surface-mount resistors (RM10F2490CT). Samples were heated to 120 ◦ C for 10 s, OSL read at 90 ◦ C.
irradiation. Dark-coloured barium titanate and calcium titanate components, such as capacitors and inductors, gave little or no OSL response. The OSL response to a given dose was observed to be greater than the TL response, so the TL measurements served mainly to provide insight into the thermal and anomalous fading characteristics of the OSL signal. The TL and OSL curves, depicted in Fig. 2, show similar characteristics to those of Al2 O3 :C; the OSL curve shows a relatively slow decay, and the TL glow curve peaks, at approximately 89 and 190 ◦ C, are similar to those observed in some “narrow peak” Al2 O3 :C samples (Akselrod and Akselrod, 2002). A small peak is also evident around 340 ◦ C. As shown in Fig. 3, the 190 ◦ C peak is photosensitive and is almost completely depleted by 1000 s of stimulation with blue LEDs. A 10 s preheat to 120 ◦ C was identified as sufficient to eliminate the thermally unstable portion of the signal, and maximum OSL efficiency was obtained at a readout temperature of 90 ◦ C. 3.2. Sensitivity and dose response The dose response of the resistor substrates is linear over the range 0.01–100 Gy, possibly becoming slightly supralinear
3.3. Anomalous fading Fig. 5 shows that the 190 ◦ C TL peak is depleted by approximately 25% over the first 8 h following irradiation, displaying a much higher rate of fading than that predicted by purely thermal effects. The fact that increasing the temperature of the thermal pre-treatment had an insignificant effect on the fading rate provides further evidence that the observed fading is anomalous; in addition, the increased preheat significantly depleted the OSL signal, causing a reduction in the minimum detectable dose. It is worth noting (from Fig. 3) that the 340 ◦ C TL peak also exhibits light sensitivity; thus, at high doses, where signal strength is not a limitation, a strong preheat of 260 ◦ C may reduce or eliminate the observed anomalous fading, but this hypothesis remains to be tested. The observed fading behaviour is best explained by a quantum-mechanical tunnelling model, in which the probability of a trapped electron escaping by tunnelling through a potential barrier is dependent on the optical excitation energy and the tunnelling distance to the recombination centre (Huntley and Lamothe, 2001). In the past, the wisdom of attempting to correct for fading in geological dating has been questioned, in part because it is impossible to verify whether fading rates observed in the laboratory are consistent over geological time scales (Aitken, 1985); however, the relatively short intervals between irradiation and analysis in a contemporary radiation event, plus the materials’ remarkably stable
E.L. Inrig et al. / Radiation Measurements 43 (2008) 726 – 730
729
Fig. 4. Dose response of Cal-Chip 68.1 surface-mount resistors (RM10F68R1CT) from 0.0043 to 98 Gy.
Fig. 5. Anomalous fading of 190 ◦ C TL peak from six Cal-Chip 68.1 resistors (RM10F68R1CT) irradiated to 2.2 Gy.
cycle-to-cycle OSL sensitivity, allow for more realistic extrapolations of anomalous fading over the time scale of interest. The long-term fading of the OSL signal is illustrated in Fig. 6. We have followed Huntley and Lamothe (2001) in correcting for fading during dose reconstruction. The intensity at time t following irradiation is given by g t I = Ic 1 − , (1) log10 100 tc where Ic is the luminescence intensity at some time tc following irradiation and g is the percent decrease in intensity per decade, meaning a 10-fold increase in time since irradiation. The value of g depends on the choice of tc , and the approximations used
Fig. 6. Anomalous fading of OSL signal from Bourns, Inc., resistor samples (CR0805-FX-1200E) irradiated to 5 Gy with a 60 Co gamma irradiator. The measured fading rate is 23.7%/decade for tc = 0.346 d (8.3 h).
in deriving Eq. (1) are not valid for very small values of t. In cases where the duration of the irradiation was not negligible in comparison to the time between irradiation and analysis, Eq. (1) was integrated over the irradiation period to account for fading during irradiation. 3.4. Trial results The results for one aliquot of resistors removed from a trialirradiated cellular phone are shown in Fig. 7, while results for all devices from both trials are summarized in Table 1. We
730
E.L. Inrig et al. / Radiation Measurements 43 (2008) 726 – 730
4. Conclusions The potential for the use of cellular phones and similar devices as “fortuitous” dosimeters following a radiological accident or terrorist event has been successfully demonstrated. With further development, this technique will provide a new and extremely valuable tool for rapid post-event dose assessment. Much work remains to be done, including further study to measure the variability of anomalous fading rates, to measure the effect of device operation following irradiation on the OSL signal of the components, and to explore, and ideally improve, the upper and lower detection limits of this technique. Acknowledgements
Fig. 7. Dose reconstruction for Nokia 6160i (trial 1). Because the sensitivity change between cycles was found to be negligible, regeneration doses were not normalized to test doses.
Table 1 Estimated doses from trial irradiations of mobile communication devices Device
Time between Dose from Estimated irradiation and electronic dose analysis (d) dosimeter (Sv) (Gy)
Trial 1
Nokia 6160i Motorola 120c Nokia 3360
4.1 9.1 18.0
0.23 ± 0.05 0.30 ± 0.06 0.10 ± 0.02
0.2 0.4 0.07
Trial 2
Nokia 6160i Motorola 120c Nokia 3360 RIM Blackberry
20.9 20.9 20.9 20.9
0.27 ± 0.05 0.57 ± 0.11 0.09 ± 0.02 0.10 ± 0.02
0.42 0.62 0.07 0.16
noted that thanks to these substrates’ lack of OSL sensitivity change, normalization to the 43 mGy test dose did not change the dose estimates, but it did increase their statistical noise; for this reason, the values in Table 1 are calculated from the unnormalized luminescence response. All dose estimates were within 0.02–0.15 Gy of the doses recorded by the EDs, which are nominally accurate to within 20%.
We dedicate this paper to our dear friend and colleague Dr. Shyam M. Khanna, who passed away unexpectedly on 21 July 2007. Research was conducted at Defence R&D Canada—Ottawa, and grants to D.I. Godfrey-Smith by DRDCCORA and the Natural Sciences and Engineering Research Council of Canada supported this study. References Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. Akselrod, A.E., Akselrod, M.S., 2002. Correlation between OSL and the distribution of TL traps in Al2 O3 : C. Radiat. Prot. Dosim. 100, 217–220. Godfrey-Smith, D.I., 2006. Applicability of moissanite, a monocrystalline form of silicon carbide, to retrospective and forensic dosimetry. Radiat. Meas. 41, 976–981. Godfrey-Smith, D.I., Khanna, S.M., 2007a. Low-dose optically stimulated luminescence of exotic materials. In: Abstract, SPIE Defence and Security Symposium, vol. 6540, Orlando, USA. Godfrey-Smith, D.I., Khanna, S.M., 2007b. Optically stimulated luminescence: a unique opportunity for low dose forensic dosimetry of advanced materials. In: Extended synopsis, IAEA International Conference on Environmental Radioactivity: From Measurements and Assessments to Regulation, CN-145, Vienna, Austria. Göksu, H.Y., 2003. Telephone chip-cards as individual dosimeters. Radiat. Meas. 37, 617–620. Huntley, D.J., Lamothe, M., 2001. Ubiquity of anomalous fading in Kfeldspars and the measurement and correction for it in optical dating. Can. J. Earth Sci. 38, 1093–1106. Larsson, C., Koslowsky, V., Gao, H., Khanna, S., Estan, D., 2005. Optically stimulated luminescence in forensics. Appl. Radiat. Isot. 63, 689–695.
Optically stimulated luminescence of electronic components for forensic, retrospective, and accident dosimetry E.L. Inrig a,∗ , D.I. Godfrey-Smith b , S. Khanna a, a Defence Research and Development Canada—Ottawa, 3701 Carling Avenue, Ottawa, Canada b Defence Research and Development Canada—CORA, Major-General George R. Pearkes Building, 18ST, 101 Colonel Bay Drive, Ottawa, Canada
Abstract This study investigated the optically stimulated luminescence (OSL) response of electronic components found within portable electronic devices such as cell phones and pagers, portable computers, music and video players, global positioning system receivers, cameras, and digital watches. The analysis of components extracted from these ubiquitous devices was proposed for applications ranging from rapid accident dose reconstruction to the tracking and attribution of gamma-emitting radiological materials. Surface-mount resistors with alumina porcelain substrates consistently produced OSL following irradiation, with minimum detectable doses on the order of 10 mGy for a typical sample. Since the resistor ceramics were found to exhibit anomalous fading, dose reconstruction procedures were developed to correct for this using laboratory measurements of fading rates carried out over approximately 3 months. Two trials were conducted in which cellular phones were affixed to an anthropomorphic phantom and irradiated using gamma-ray sources; ultimately, analysis of the devices used in these trials succeeded in reconstructing doses in the range of 0.1.0.6 Gy. Crown Copyright © 2007 Published by Elsevier Ltd. All rights reserved. Keywords: Optically stimulated luminescence; OSL; Retrospective dosimetry; Accident dosimetry; Forensic dosimetry; Ceramics; Substrates; Electronics; Silicate; Alumina; Gamma and beta radiation
1. Introduction For more than 40 years, luminescence techniques have been used to estimate doses to exposed populations in contaminated areas through the analysis of bricks and other radiation-sensitive building materials and household objects. More recently, dose reconstruction using personal effects such as telephone chip-cards has been considered (Göksu, 2003). Applications to forensics, where the dose imparted to building materials and household chemicals is used to identify former storage locations of radiological materials, have also been explored (Larsson et al., 2005). In the current security climate, there is an increased awareness of the potential threat posed by terrorist groups, should they acquire radiological materials for use in an attack on a civilian target. In view of this threat, new technologies are Deceased.
∗ Corresponding author. Tel.: +1 613 998 6109; fax: +1 613 998 4560.
E-mail address: [email protected] (E.L. Inrig).
required for tracking the movement of such materials, ideally preventing their deployment, and for dealing with the consequences of such an attack should attempts at interdiction fail. Although there is an entire industry devoted to the protection and monitoring of radiation workers, it is clear that in the event of a radiological accident or attack involving exposure to the general public, the vast majority of those exposed will not be equipped with accredited dosimeters. In the aftermath of a widespread exposure to radiation, such as that might occur following the detonation of a radiological dispersal device (RDD) or even the deployment of a radiological exposure device (RED), health care professionals will face the overwhelming task of evaluating exposed individuals and distinguishing between the potentially exposed and the worriedwell. A rapid method of assessing external radiation exposure using commonly held personal objects (essentially “fortuitous dosimeters”) would thus be invaluable for triage purposes. Ideally, a “fortuitous dosimeter” should be positioned close to the body, be carried by a large percentage of the population, and be
1350-4487/$ - see front matter Crown Copyright © 2007 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.11.078
E.L. Inrig et al. / Radiation Measurements 43 (2008) 726 – 730
sufficiently sensitive to detect doses below the threshold for deterministic health effects. Since analysis is expected to be performed close to the time of exposure (within days to months), the measured dose response from the material of interest need not be completely stable so long as procedures can be developed to correct for signal fading. Our research has determined that cellular phones and other electronic devices should fulfil all of these requirements and can be used successfully for dose reconstruction. In previous studies, we demonstrated that certain novel materials employed as substrates for integrated circuits (ICs) exhibit dose response luminescence (Godfrey-Smith, 2006; Godfrey-Smith and Khanna, 2007a, b). In this study, we investigate the optically stimulated luminescence (OSL) dose response of widely used materials that are a part of surfacemount components, such as resistors, capacitors, diodes, and inductors, in modern electronic devices. The sensitivity and fading characteristics of resistor substrates were investigated in more detail. 2. Experimental details and samples Samples selected for analysis consisted both of unused surface-mount components obtained from Cal-Chip Electronics, Inc., and Bourns, Inc., and a representative selection of components removed from the circuit boards of several cellular phones from different manufacturers. The smallest rectangular resistors, such as those visible in Fig. 1, measure 1 × 0.5 mm and have white alumina porcelain substrates (typical composition approximately 97% Al2 O3 , 3% SiO2 ) that are uncoated on the underside permitting OSL measurements. Many of the capacitors studied contained dark-coloured ceramic substrates, exposed on all sides of the component, and were composed of materials such as barium titanate and calcium titanate. All measurements were performed using a RisZ TL-OSLDA-15 reader, and except where specified otherwise, samples were irradiated using the integrated 1.48 GBq (40 mCi) 90 Sr/90 Y irradiator, equipped with a removable 2.2 mm aluminium attenuator that reduced the dose rate to 4.3 mGy s−1 , as determined by cross-calibration with a reference 60 Co gamma source. For OSL measurements, stimulation light was provided by blue LEDs (470 nm), and both thermoluminescence (TL) and OSL curves were recorded with Hoya U-340 filters in the optical detection system. Following initial experiments to identify components with sufficient dose response to be of interest, 4.3 mGy to 98 Gy dose response curves for a number of resistor substrate types were constructed. The number of resistors per aliquot was dependent on the size of resistors being studied; some measure less than 1 mm in length and width, while large resistor arrays may be several mm long and wide. Signal fading was quantified by short- and long-term experiments conducted on multi-resistor aliquots of chip resistors manufactured by Cal-Chip Electronics (RM10F68R1CT) and Bourns, Inc., (CR0805-FX-1200E). Short-term fading was measured for delays of 30 s to 8.4 h following a 2.2 Gy automated beta irradiation, while long-term fading involved delays of 1 h to 90 d after a 5 Gy dose delivered by a 60 Co source.
727
Fig. 1. Section of a circuit board from RIM Blackberry. Inset: resistor sample prepared for analysis, displayed on a 1 cm grid.
In these experiments, various preheat temperatures between 120 and 200 ◦ C were used to determine the effect of the preheat temperature on the fading rate. The OSL response was normalized to the response from a 0.43 Gy test dose administered using the integrated 90 Sr/90 Y source. Two trials were conducted in which cellular phones were affixed to an anthropomorphic phantom at belt level and exposed to a radioactive source in a geometry designed to simulate the deployment of an RED under a park bench or a desk. An MGP SOR/R electronic dosimeter (ED) was affixed to each device to record an approximate dose, and an additional ED was placed mid-gut inside the phantom. In the first trial, three cellular phones were exposed to a 1.1 GBq (30 mCi) 60 Co source for approximately 3.5 d, while the second trial exposed four devices to a 74 GBq (2 Ci) 137 Cs source for 4 h; in each case, the distances between the source and the devices were in the range of 25–40 cm. In preparation for analysis, each device was disassembled under darkroom conditions and the resistors were removed from the circuit boards using a utility knife. The resistors were then soaked in methylene chloride for approximately 30 min in order to remove adhesive, used during the manufacturing process to secure the components to the circuit boards before soldering, from the bottom of each component. Resistors were placed bottom-up on a planchette covered with a thin layer of silicone oil, with as many components as would fit placed on a single planchette; in general, each device yielded enough resistors for one to two samples. For each sample, the OSL response to the trial dose was measured along with the response to four regeneration doses followed by 43 mGy test doses, with the first regeneration dose repeated. 3. Results and discussion 3.1. OSL and TL response Initial experiments demonstrated that surface-mount resistor alumina substrates consistently exhibited OSL following
728
E.L. Inrig et al. / Radiation Measurements 43 (2008) 726 – 730
Fig. 3. Depletion of TL signal by various durations of blue light stimulation for a set of six Cal-Chip 68.1 resistors (RM10F68R1CT) irradiated to 2.1 Gy.
at the upper end of the range. We observed remarkably little change in sensitivity over more than a hundred cycles of irradiation, heating, and optical stimulation. While the dose response sensitivities of the substrates varied widely between manufacturers and production lots, a minimum detectable dose of less than 10 mGy was typical for an aliquot composed of several resistors (Fig. 4). Fig. 2. (a) Smoothed TL glow curve (5 ◦ C/s) for set of six Cal-Chip 68.1 resistors (RM10F68R1CT) irradiated to 2.1 Gy. (b) OSL response of a 73 mg sample of Cal-Chip 249 surface-mount resistors (RM10F2490CT). Samples were heated to 120 ◦ C for 10 s, OSL read at 90 ◦ C.
irradiation. Dark-coloured barium titanate and calcium titanate components, such as capacitors and inductors, gave little or no OSL response. The OSL response to a given dose was observed to be greater than the TL response, so the TL measurements served mainly to provide insight into the thermal and anomalous fading characteristics of the OSL signal. The TL and OSL curves, depicted in Fig. 2, show similar characteristics to those of Al2 O3 :C; the OSL curve shows a relatively slow decay, and the TL glow curve peaks, at approximately 89 and 190 ◦ C, are similar to those observed in some “narrow peak” Al2 O3 :C samples (Akselrod and Akselrod, 2002). A small peak is also evident around 340 ◦ C. As shown in Fig. 3, the 190 ◦ C peak is photosensitive and is almost completely depleted by 1000 s of stimulation with blue LEDs. A 10 s preheat to 120 ◦ C was identified as sufficient to eliminate the thermally unstable portion of the signal, and maximum OSL efficiency was obtained at a readout temperature of 90 ◦ C. 3.2. Sensitivity and dose response The dose response of the resistor substrates is linear over the range 0.01–100 Gy, possibly becoming slightly supralinear
3.3. Anomalous fading Fig. 5 shows that the 190 ◦ C TL peak is depleted by approximately 25% over the first 8 h following irradiation, displaying a much higher rate of fading than that predicted by purely thermal effects. The fact that increasing the temperature of the thermal pre-treatment had an insignificant effect on the fading rate provides further evidence that the observed fading is anomalous; in addition, the increased preheat significantly depleted the OSL signal, causing a reduction in the minimum detectable dose. It is worth noting (from Fig. 3) that the 340 ◦ C TL peak also exhibits light sensitivity; thus, at high doses, where signal strength is not a limitation, a strong preheat of 260 ◦ C may reduce or eliminate the observed anomalous fading, but this hypothesis remains to be tested. The observed fading behaviour is best explained by a quantum-mechanical tunnelling model, in which the probability of a trapped electron escaping by tunnelling through a potential barrier is dependent on the optical excitation energy and the tunnelling distance to the recombination centre (Huntley and Lamothe, 2001). In the past, the wisdom of attempting to correct for fading in geological dating has been questioned, in part because it is impossible to verify whether fading rates observed in the laboratory are consistent over geological time scales (Aitken, 1985); however, the relatively short intervals between irradiation and analysis in a contemporary radiation event, plus the materials’ remarkably stable
E.L. Inrig et al. / Radiation Measurements 43 (2008) 726 – 730
729
Fig. 4. Dose response of Cal-Chip 68.1 surface-mount resistors (RM10F68R1CT) from 0.0043 to 98 Gy.
Fig. 5. Anomalous fading of 190 ◦ C TL peak from six Cal-Chip 68.1 resistors (RM10F68R1CT) irradiated to 2.2 Gy.
cycle-to-cycle OSL sensitivity, allow for more realistic extrapolations of anomalous fading over the time scale of interest. The long-term fading of the OSL signal is illustrated in Fig. 6. We have followed Huntley and Lamothe (2001) in correcting for fading during dose reconstruction. The intensity at time t following irradiation is given by g t I = Ic 1 − , (1) log10 100 tc where Ic is the luminescence intensity at some time tc following irradiation and g is the percent decrease in intensity per decade, meaning a 10-fold increase in time since irradiation. The value of g depends on the choice of tc , and the approximations used
Fig. 6. Anomalous fading of OSL signal from Bourns, Inc., resistor samples (CR0805-FX-1200E) irradiated to 5 Gy with a 60 Co gamma irradiator. The measured fading rate is 23.7%/decade for tc = 0.346 d (8.3 h).
in deriving Eq. (1) are not valid for very small values of t. In cases where the duration of the irradiation was not negligible in comparison to the time between irradiation and analysis, Eq. (1) was integrated over the irradiation period to account for fading during irradiation. 3.4. Trial results The results for one aliquot of resistors removed from a trialirradiated cellular phone are shown in Fig. 7, while results for all devices from both trials are summarized in Table 1. We
730
E.L. Inrig et al. / Radiation Measurements 43 (2008) 726 – 730
4. Conclusions The potential for the use of cellular phones and similar devices as “fortuitous” dosimeters following a radiological accident or terrorist event has been successfully demonstrated. With further development, this technique will provide a new and extremely valuable tool for rapid post-event dose assessment. Much work remains to be done, including further study to measure the variability of anomalous fading rates, to measure the effect of device operation following irradiation on the OSL signal of the components, and to explore, and ideally improve, the upper and lower detection limits of this technique. Acknowledgements
Fig. 7. Dose reconstruction for Nokia 6160i (trial 1). Because the sensitivity change between cycles was found to be negligible, regeneration doses were not normalized to test doses.
Table 1 Estimated doses from trial irradiations of mobile communication devices Device
Time between Dose from Estimated irradiation and electronic dose analysis (d) dosimeter (Sv) (Gy)
Trial 1
Nokia 6160i Motorola 120c Nokia 3360
4.1 9.1 18.0
0.23 ± 0.05 0.30 ± 0.06 0.10 ± 0.02
0.2 0.4 0.07
Trial 2
Nokia 6160i Motorola 120c Nokia 3360 RIM Blackberry
20.9 20.9 20.9 20.9
0.27 ± 0.05 0.57 ± 0.11 0.09 ± 0.02 0.10 ± 0.02
0.42 0.62 0.07 0.16
noted that thanks to these substrates’ lack of OSL sensitivity change, normalization to the 43 mGy test dose did not change the dose estimates, but it did increase their statistical noise; for this reason, the values in Table 1 are calculated from the unnormalized luminescence response. All dose estimates were within 0.02–0.15 Gy of the doses recorded by the EDs, which are nominally accurate to within 20%.
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