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Desiccants for retrospective dosimetry using optically stimulated luminescence (OSL)
Radiation Measurements 78 (2015) 17e22
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Desiccants for retrospective dosimetry using optically stimulated luminescence (OSL) se Geber-Bergstrand*, Christian Bernhardsson, Maria Christiansson, So € ren Mattsson, There €a €f Christopher L. Ra €, Lund University, Skåne University Hospital Malmo €, 205 02 Malmo €, Sweden Medical Radiation Physics, Department of Clinical Sciences, Malmo
h i g h l i g h t s Desiccants can be used as fortuitous dosemeters using OSL. The minimum detectable dose for processed desiccants range from 8 to 450 mGy. The minimum detectable dose for natural clay was 1.8 Gy.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2014 Received in revised form 3 November 2014 Accepted 7 November 2014 Available online 8 November 2014
Optically stimulated luminescence (OSL) was used to test different kinds of desiccants for their potential use in retrospective dosimetry. Desiccants are used for the purpose of absorbing liquids and can be found in a number of items which may be found in the immediate environment of a person, including hand bags, drug packages, and the vehicles of rescue service teams. Any material exhibiting OSL properties suitable for retrospective dosimetry is a useful addition to the existing dosimetry system available in emergency preparedness. Eleven kinds of desiccants were investigated in order to obtain an overview of the fundamental OSL properties necessary for retrospective dosimetry. Measurements were made using a Risø TL/OSL reader and irradiations were achieved with the 90Sr/90Y source incorporated in the reader. Several of the desiccants exhibited promising properties as retrospective dosemeters. Some of the materials exhibited a strong as-received signal, i.e. without any laboratory irradiation, but the origin of this signal has not yet been established. The minimum detectable dose ranged from 8 to 450 mGy for ten of the materials and for one material (consisting of natural clay) the minimum detectable dose was 1.8 Gy. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Optically stimulated luminescence (OSL) Desiccants Emergency dosimetry Retrospective dosimetry
1. Introduction If there were to be a radiation incident of some kind, e.g. an accident, it might result in the exposure of people not wearing dosemeters (the general public) to ionising radiation. Such a scenario will induce the need to estimate the potential dose these people have received. When dosemeters are not available, other methods have to be used to perform these estimations. Several so called retrospective dosimetry techniques exists and have been used in real incidents. Some of them can be used to estimate radiation doses to an individual directly and some indirectly, e.g. given information regarding time and motion of an individual. There are both biological techniques, such as the dicentric assay and the micronuclei assay (IAEA, 2001), and physical techniques,
* Corresponding author. E-mail address: [email protected] (T. Geber-Bergstrand). http://dx.doi.org/10.1016/j.radmeas.2014.11.002 1350-4487/© 2014 Elsevier Ltd. All rights reserved.
such as electron paramagnetic resonance, thermoluminescence and optically stimulated luminescence (ICRU, 2002). However, all of these techniques have advantages and disadvantages (Ainsbury et al., 2011), and none are sufficient as a standalone tool after a large-scale incident. Thus, the possibility of using many different techniques and/or materials after an incident is desirable, partly to use the full extent of measuring capacities provided by different techniques and partly to make more accurate dose estimations by using several materials as references. Optically stimulated luminescence (OSL) is a process resulting in light emission from an insulator or semiconductor previously exposed to ionising radiation as the material is stimulated with light. As a result of the radiation exposure, charge carriers are trapped at metastable energy levels in the band gap of the material. During stimulation, the charge carriers recombine resulting in luminescence that is detectable by photosensitive devices (such as photomultiplier tubes). The intensity of the luminescence is
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proportional to the number of trapped charge carriers and, thus, the absorbed dose. Recently, attention has focused on finding new materials for retrospective dosimetry using OSL. Some of the most interesting materials are electronic components from portable devices (Bassinet et al., 2010; Woda et al., 2010; Inrig et al., 2008), chips €ttl, 2009; Mathur from bank and telephone cards (Woda and Spo € ksu, 2003), household salt (Bernhardsson et al., et al., 2007; Go 2009; Christiansson et al., 2012), and dental repair materials (Geber-Bergstrand et al., 2012; Veronese et al., 2010). The present study investigates the dosimetric properties of different desiccants for their possible use as fortuitous dosemeters. Desiccants are the generic name for materials used to remove excessive humidity or liquids from unwanted places; they may be used to protect various products, such as leather bags, from mold and drugs from degrading, or to absorb liquid spills, such as oil on roads. Most desiccants consist of silica gel (SiO2), activated charcoal, or montmorillonite clay, which is the main component in bentonite. Quartz, which consists of a SiO2 compound, is the most commonly used material in OSL, though for dating purposes; therefore, the hypothesis is that desiccants (containing SiO2) exposed to ionising radiation also have OSL properties. If this hypothesis is true, the initial investigations performed in this study could indicate a potential new method of dose estimation in retrospective dosimetry. A specific type of desiccant used to absorb spills on roads can be found in the vehicles of many rescue service teams. Rescue service teams often act as first responders in the event of an accident or other kind of incident and are, therefore, a group that might be exposed to ionising radiation. In the case of a radiation incident, even though the rescue service teams are supposed to have dosemeters available in their vehicles, they may not always be available in all vehicles, and even when they are they are rarely in operation because radiation incidents seldom occur. If an incident actually includes some sort of radiation, it might, therefore, not be revealed until the rescue service teams have already handled the incident. In such cases it would be beneficial to be able to use other materials present in the vehicles, such as desiccants, for postincident dose reconstruction. Similar kinds of desiccants can be found inside nuclear power plants (e.g. for absorbing liquid radioactive spills), which could enable dose estimations at places that are otherwise not monitored. Bentonite clay will be used to clad the copper canisters containing high-level radioactive waste in the planned underground storage in e.g. Finland and Sweden and thus, by measuring the bentonite clay, any potential leakage could be found. In households, desiccants are found in the form of cat litter and in bags and drug packaging. Because OSL is a technique that uses light for signal read-out, all materials must be protected from light until read-out in order to preserve the signal. Fortunately, most desiccants are kept in the dark, inside a handbag or package. Larger volumes of desiccants are usually kept in thick, several layered paper bags to avoid moisture absorption before they are used, which also prevents light from reaching the material. This study was an initial investigation of the fundamental OSL properties of various kinds of desiccants in order to determine their potential as retrospective dosemeters. To the best of our knowledge, no previous studies have investigated desiccants using OSL. 2. Materials and methods All measurements were performed using a TL/OSL reader (TL/ OSL-DA-15, DTU Nutech, Technical University of Denmark, Risø campus, Roskilde, Denmark), which is described in detail elsewhere (Bøtter-Jensen et al., 2000). Irradiation was achieved using a 90 Sr/90Y source (20 MBq 2009-04-09) incorporated into the reader. The current dose rate to a thin layer of quartz grains was
approximately 0.9 mGy s1 at the sample position. Dose calibration for each desiccant material was not performed; therefore all doses are given as the dose to quartz. The irradiation set-up was calibrated against the dose in air of a 137Cs source, which was calibrated at the Danish secondary standard laboratory (State Institute for Radiation Protection, SIS, Copenhagen, Denmark). The samples were placed on stainless steel cups (∅ ¼ 12 mm) during irradiation and during all readouts. Table 1 lists information on the 11 different types of desiccants used in this study. All of the samples were collected from households and/or nearby workplaces, including the emergency service, the packaging industry, and laboratories. Therefore, these desiccants can be considered as being commonly available in Sweden. The desiccants can be divided into two main types: granulates (G) and beads (B) (Fig. 1). For the bead-like materials, as many beads as possible were placed on the cups during irradiation and read-out. For the granulate materials, a thin layer was put on the cups and used for both irradiation and read-out. The exact compositions of the different materials are difficult to know as the content information on the packaging was limited. For the bead-like materials collected from hand bags and drug packages, only “silica gel” was printed on the packaging. However, the beads have very different appearances in terms of colour, shape, and structure; thus, they are likely to include different substances in addition to the silica gel. The B5 beads were most likely an activated form of montmorillonite clay, which is a form of hydrated sodium calcium aluminium magnesium silicate hydroxide. However, also this composition is difficult to determine with certainty because the package only read “activemineral”. Most of the granulate materials were said to be composed of bentonite clay, which mainly consists of montmorillonite. Similar to the bead-like materials, differences in the appearance of the granulate materials indicate that other substances are present. These alterations may have a distinct impact on the OSL signal, which is sensitive for impurities in the crystal lattice of the material. Two of the materials, B6 and G1, had a better known composition because the content was printed on the delivery note. The chemical composition of G1 was 75% SiO2, 10% Al2O3, 7% Fe2O3, 2% MgO, 2% K2O þ Na2O, 1% TiO2, and 1% CaO. G1 is commonly used by Swedish rescue service teams for absorbing spills of water, oil, and other chemicals on firm surfaces, such as roads. B6 is a molecular sieve composed of sodium aluminosilicate. The chain of manufacturing for the different desiccants is difficult to come by due to insufficient knowledge of the exact desiccant and trade secrets for the companies involved. In particular, materials composed of clay, which is a natural product, may not have undergone any special processing during the manufacturing process, meaning that they may have a geological age (>1000 years). If the radiation induced signal in the materials do not exhibit any fading, this could lead to background signals corresponding to several Gy. All OSL decay curves were recorded for 100 s, and the OSL signal was defined as the number of counts from the photo multiplier tube (PMT) during the first 10 s subtracted by the background, which consisted of the PMT counts during the following 10 s. Unless otherwise indicated, all measurements were performed immediately after irradiation. The samples were not pre-treated before irradiation or measurement. The first measurements were made to obtain tentative read-out procedures that could be appropriate for these materials. Signal read-out was performed in various ways following a dose of 1.8 Gy (90Sr/90Y) using: i) thermoluminescence (TL) by heating the samples to 600 C, ii) continuous IR light (870 nm) of 145 mW/cm2 at room temperature without preheating, iii) continuous blue light (470 nm) of 80 mW/cm2 at room temperature without pre-heating, and iv) continuous blue
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Table 1 Summary of the applications and properties of the 11 different kinds of desiccants investigated. TL, thermoluminescence; IRSL, optically stimulated luminescence using 870 nm IR LEDs; bOSL, optically stimulated luminescence using 470 nm blue LEDs; ph, pre-heat; AsR, as-received signal (un-treated or un-irradiated sample); MDD, minimum detectable dose. þ: signal, : no or weak signal, i.e. <400 cts peak value. Sample type
Sample name
Major chemical component
Origin
TL 600
IRSL
bOSL no ph
bOSL 100 ph 120
AsR
Sensitivity [cts/mGy]
MDD [mGy]
Beads Beads Beads Beads Beads Beads Granulate Granulate Granulate Granulate Granulate
B1 B2 B3 B4 B5 B6 G1 G2 G3 G4 G5
Silica gel Silica gel Silica gel Silica gel Montmorillonite clay AlNa12SiO5 SiO2, Al2O3 Bentonite clay Bentonite clay Bentonite clay Bentonite clay
Computer bag Hand bag Hand bag Drug package Hand bag Laboratory Rescue service team vehicle Cat litter Cat litter Industry Natural clay
þ þ þ
þ þ
þ þ þ þ þ þ þ þ þ þ þ
þ þ þ þ þ þ þ þ þ þ
þ þ þ þ
0.5 0.4 0.5 0.3 13.2 1.5 3.4 2.1 5.9 8.3 1.2
450 370 110 310 10 120 40 270a 390a 20a 1800a
a
Calculated from bleached samples.
Fig. 1. Photograph of the different desiccants investigated in this paper when put on the cups used during irradiation and read-out. Top row from the left: B1, B2, B3, B4, B5, and B6. Bottom row from the left: G1, G2, G3, G4, and G5.
light at 100 C with pre-heating at 120 C for 10 s. If the peak value of the OSL curve (as defined in the previous section) was more than 400 cts it was considered to be a signal. Of the different read-out procedures, the one using continuous blue light at room temperature was chosen for further, more detailed measurements on e.g. doseeresponse, fading, and the effect of moisture on the OSL signal. Dose response measurements were performed by irradiating one to five aliquots of desiccant per given dose, depending on how much material was available and read-out immediately after irradiation. The different doses were 270, 450, 900, and 2700 mGy. One to three samples of each material, depending on availability, were irradiated with 1800 mGy and subsequently read-out in four different rounds, when studying the effect of moisture. During rounds one and four, the samples were dry; in rounds two and three, the samples were soaked with water using a plant mister prior to irradiation. Prior to the fourth round, the samples were allowed to dry completely at room temperature for 2 weeks. For the purpose of investigating signal fading over time, the samples were irradiated with 1170 mGy and read-out immediately, 1 day, and 3 days after irradiation. In order to normalise the signals, all aliquots were exposed to a test dose of 450 mGy after the read-out; this compensated for possible variations among equally treated samples of the same desiccant (due to different sample surfaces and mass in each aliquot). During the time delay between irradiation and read-out, the samples were kept in a dark environment. To explore the optical stability of the as-received signal (AsR), i.e. the signal present without prior laboratory irradiation, the materials were put on a windowsill for
exposure to sunlight. After 4 h, 3 weeks and 2 months, respectively, two samples of each material were read-out. These measurements are the first step in the study of desiccants which will reveal whether the materials are relevant for use in retrospective dosimetry. 3. Results and discussion 3.1. Signal characteristics All of the materials tested exhibited a signal after irradiation when using OSL with blue light stimulation at room temperature without any pre-heat (Table 1). When using pre-heat at 120 C before the OSL read-out at 100 C, only the material B3 did not exhibit a signal following irradiation. IR OSL (IRSL) was less successful than blue OSL because most materials did not exhibit any signal when stimulated with IR. When using TL up to 600 C, the signals were not as high as the OSL signals, if they were present at all. In addition, most of the materials exhibited an altered visual appearance due to the heating, i.e. scorched or ash-like. Due to these results, all further read-outs for all materials were performed using blue light OSL at room temperature without pre-heating. The materials composed of clay exhibited a considerable AsR. Examples of materials with and without AsR are shown in Fig. 2. This signal may be caused by background radiation or other, non radiative, causes such as mechanical friction during manufacturing. The fact that the AsR in G4 seems to have a more apparent slow component
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a
12000 Irradiated
10000 Net OSL signal [cts]
1000 PMT counts
B6 G1
1200
800 600 400
y = 3.4x+1300 2 R = 0.98
6000 4000
y = 1.5x+270 R2 = 1.00
2000
200 0
8000
As received Bleached
0
0
20 40 60 Stimulation time [s]
80
100
0
1000 2000 Absorbed dose [mGy]
3000
Fig. 3. Dose response curves for two different desiccants, B6 and G1 (±1 standard error of the mean). The materials were irradiated with a 90Sr/90Y source at doses between 270 and 2700 mGy. The average net OSL signals from three samples of each material was used to determine the OSL signals at each given dose.
b 3500
PMT counts
3000 2500 2000
MDD ¼ As received
1500 1000
Irradiated
500 0
Bleached
0
20 40 60 Stimulation time [s]
80
100
Fig. 2. Examples of OSL signals using blue light stimulation (470 nm). The three OSL signals shown were acquired i) after irradiation with 0.9 Gy (90Sr/90Y) on a bleached sample (irradiated), ii) the as-received signal (AsR) without prior laboratory irradiation, iii) after several days of bleaching in sunlight (bleached). a, Material without AsR (G1). b, Material with AsR (G4).
than the signal from the irradiated sample (Fig. 2b) could suggest a non radiative contribution for this material. But, because the manufacturing date and process is hard to find, the contribution to the AsR is difficult to determine. If the AsR originates purely from background radiation over many years, provided that long-term fading is negligible, it would correspond to doses of approximately 7 Gy for G2, 3 Gy for G3, 2 Gy for G4, and 12 Gy for G5, which is not unlikely for geological samples. If the AsR is large, it has to be taken into account, and this phenomenon needs to be further investigated before accurate corrections can be made for the AsR. 3.2. Dose response All of the studied materials exhibited an OSL signal proportional to the given dose (Fig. 3). The sensitivities, S, defined as the net OSL signal per absorbed dose unit, for the different materials are shown in Table 1. The materials composed of clay generally had the highest sensitivity. The minimum detectable dose (MDD) is traditionally defined as the absorbed dose corresponding to a signal equal to three times the standard deviation of the background/zero dose signal (mentioned in Currie, 1999; ICRU, 1972):
1 3sOSL zero S
(1)
By this definition, a high sensitivity generally implies a low MDD. However, if the standard deviation of the zero dose signal is high, then the MDD will increase. This was seen for several of the materials; for example, G2 had a lower MDD than G3 although G3 had a higher sensitivity. At this point, the MDDs for the materials exhibiting an AsR were calculated from bleached samples and will thus only be valid if the contribution of the AsR may be completely accounted for i.e. if the AsR is known. The materials not exhibiting an AsR (B5, B6, and G1) had the highest sensitivity. In terms of MDD, materials not exhibiting an AsR had the lowest values (B5, B6, G1, and B3). However, a low MDD is preferably accompanied by a high sensitivity and, thus, B3 is not considered to be quite as good as the other desiccants in terms of potential use for retrospective dose assessments. The OSL signals giving the dose response were measured immediately after irradiation and thus, for materials suffering from fading, the MDDs would be higher in a real case scenario since the time delay between irradiation and read-out would be longer. Note that when using Eq. (1) to calculate the MDD the uncertainty in S is not taken into account. To take this, and also any possible covariance between S and OSLzero, into account would be preferable. The manner in which this should be performed is not straightforward and needs to be further analysed. For this reason only the traditional definition will be used here. 3.3. Moisture effects No significant influence of the absorption state on the OSL signal was found for G1, and in principle neither for B6, G2, and G3 (Fig. 4), though the values for round three were somewhat lower than for the other rounds. Therefore, the OSL signal for these materials was more or less unaffected by moisture. Two of the tested materials, G4 and G5, exhibited a deviating behaviour after being soaked the first time. These materials were more clay-like and became somewhat dissolved by moisture, which could indicate that the crystal structure was destroyed/altered. The rest of the materials exhibited different effects during the second and third round, with lower signals in the third round compared to the second round. The reason for this was unclear. Because these materials were also
T. Geber-Bergstrand et al. / Radiation Measurements 78 (2015) 17e22
250 B1 B2 B3 B4 B5 B6
200 150 100
60 B5 B6 G1 G2 G3 G4 G5
40
0
Moist 1
Moist 2
Dry 2
180 G1 G2 G3 G4 G5
160 Relative OSL signal [%]
80
20
50 0
b
100 Relative OSL signal [%]
Relative OSL signal [%]
a
140 120 100 80 60 40
21
0
1 2 3 Days after irradiation
4
Fig. 5. Fading of the OSL signals after irradiation with 1170 mGy (90Sr/90Y). The values of the relative OSL signal are the mean of three samples of each material normalised to the measurement immediately after irradiation.
Only the materials exhibiting AsR (i.e. G2, G3, G4, and G5) were used when exploring the optical stability of the AsR. After 4 h, all materials still exhibited a visible AsR. After 3 weeks, G2 and G3 exhibited a weak AsR, whereas the AsR for G4 and G5 were completely bleached. After 2 months all materials were completely bleached, corresponding to a constant background level. 4. Conclusions
20 0 Moist 1
Moist 2
Dry 2
Fig. 4. OSL signals (±1 standard error of the mean) obtained from three samples of each material after irradiation with 1800 mGy (90Sr/90Y), except for B1eB5, for which one sample of each material was used due to a lack of material. Measurements were performed in four rounds, during round one and four the samples were dry and in round two and three the samples had been soaked with water prior to irradiation. All measurements were normalised to the first dry round. a, Bead-like materials. b, Granulate materials.
inferior in regard to sensitivity, no effort was made to explore this further at this time.
3.4. Fading and bleaching Only the materials with sensitivities above 1 cts/mGy were chosen for fading measurements (Table 1). B6, G1, G3 and G5 exhibited little fading over 3 days (more than 80% of the signal remaining) whereas G2 and G4 exhibited a fading corresponding to approximately 59e66% of the signal remaining after 3 days (Fig. 5). The materials G1, G2, G3, G5 and B6 show a relatively fast initial fading, up until 1 day, and a slower or non-existent fading after that. The observation of this slow component implies that the AsR observed in these materials might be due to background radiation that has been accumulated over time. The low fading also indicates that the signal can be read-out at a time-point long after an actual exposure event. B5, however, exhibited significant fading during the 3 days, with only 6% of the initial signal remaining after one day and 5% after 3 days. In order to fully describe the fading properties of these materials, a longer time period needs to be studied.
This study shows that some desiccant materials have the potential for use as fortuitous detector materials for retrospective dosimetry, ten of the eleven materials have a MDD between 8 and 450 mGy. One of the arguments supporting this conclusion is that these materials may be found in the vicinity of radiological/nuclear accidents and other incidents, such as inside nuclear power plants, radioactive waste storages, and in the vehicles of rescue service teams. By collecting the desiccants present in rescue service vehicles, the dose to the persons in the vicinity can be estimated by means of OSL after the radiation exposure event. Among the desiccants studied here, the most promising materials were B6 and G1, which exhibit promising properties such as a low short-term fading, no moisture effect, and high sensitivity. That natural clay (G5) also shows dosimetric properties, although not quite as good, is an interesting fact since it is readily available in many places. None of these materials, however, will provide an individual dose estimate immediately because information such as dwelling time is needed; thus, these materials are perhaps best suited for non-triage purposes in retrospective dosimetry. The use of these materials could result in a reduction of the uncertainty in the dose estimation when used together with other measurements; if several different materials provides the same dose estimation it is more likely that the dose estimation is correct. Desiccants could also be used for dose estimations in less acute situations, and/or when no other materials are available. Any material exhibiting OSL properties suitable for retrospective dosimetry is a useful addition to the existing dosimetry system available in emergency preparedness. However, before any of the materials can be used in a real scenario, further investigations of the dosimetric properties are needed. This includes more information on composition and manufacturing process of the different materials and the main focus for future studies will thus include investigation of the origin of the AsR with respect to discern an accident dose.
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irradiated under wet and dry conditions. Radiat. Environ. Biophys. 51, 443e449. €ksu, H.Y., 2003. Telephone chip-cards as individual dosemeters. Radiat. Meas. 37, Go 617e620. Inrig, E.L., Godfrey-Smith, D.I., Khanna, S., 2008. Optically stimulated luminescence of electronic components for forensic, retrospective, and accident dosimetry. Radiat. Meas. 43, 726e730. International Atomic Energy Agency, 2001. Cytogenic Analysis for Radiation Dose Assessment. A Manual. IAEA Technical Report Series 405. International Commission on Radiation Units and Measurements, 1972. Measurement of Low-level Radioactivity. ICRU Report 22. International Commission on Radiation Units and Measurements, 2002. Retrospective Assessment of Exposure to Ionising Radiation. ICRU Report 68. €ksu, H.Y., 2007. Radiation sensitivity Mathur, V.K., Barkyoumb, J.H., Yukihara, E.G., Go of memory chip module of an ID card. Radiat. Meas. 42, 43e48. Veronese, I., Galli, A., Cantone, M.C., Martini, M., Vernizzi, F., Guzzi, G., 2010. Study of TSL and OSL properties of dental ceramics for accidental dosimetry applications. Radiat. Meas. 45, 35e41. €ttl, T., 2009. On the use of OSL of wire-bond chip card modules for Woda, C., Spo retrospective and accident dosimetry. Radiat. Meas. 44, 548e553. Woda, C., Greilich, S., Beerten, K., 2010. On the OSL curve shape and preheat treatment of electronic components from portable devices. Radiat. Meas. 45, 746e748.
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Desiccants for retrospective dosimetry using optically stimulated luminescence (OSL) se Geber-Bergstrand*, Christian Bernhardsson, Maria Christiansson, So € ren Mattsson, There €a €f Christopher L. Ra €, Lund University, Skåne University Hospital Malmo €, 205 02 Malmo €, Sweden Medical Radiation Physics, Department of Clinical Sciences, Malmo
h i g h l i g h t s Desiccants can be used as fortuitous dosemeters using OSL. The minimum detectable dose for processed desiccants range from 8 to 450 mGy. The minimum detectable dose for natural clay was 1.8 Gy.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2014 Received in revised form 3 November 2014 Accepted 7 November 2014 Available online 8 November 2014
Optically stimulated luminescence (OSL) was used to test different kinds of desiccants for their potential use in retrospective dosimetry. Desiccants are used for the purpose of absorbing liquids and can be found in a number of items which may be found in the immediate environment of a person, including hand bags, drug packages, and the vehicles of rescue service teams. Any material exhibiting OSL properties suitable for retrospective dosimetry is a useful addition to the existing dosimetry system available in emergency preparedness. Eleven kinds of desiccants were investigated in order to obtain an overview of the fundamental OSL properties necessary for retrospective dosimetry. Measurements were made using a Risø TL/OSL reader and irradiations were achieved with the 90Sr/90Y source incorporated in the reader. Several of the desiccants exhibited promising properties as retrospective dosemeters. Some of the materials exhibited a strong as-received signal, i.e. without any laboratory irradiation, but the origin of this signal has not yet been established. The minimum detectable dose ranged from 8 to 450 mGy for ten of the materials and for one material (consisting of natural clay) the minimum detectable dose was 1.8 Gy. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Optically stimulated luminescence (OSL) Desiccants Emergency dosimetry Retrospective dosimetry
1. Introduction If there were to be a radiation incident of some kind, e.g. an accident, it might result in the exposure of people not wearing dosemeters (the general public) to ionising radiation. Such a scenario will induce the need to estimate the potential dose these people have received. When dosemeters are not available, other methods have to be used to perform these estimations. Several so called retrospective dosimetry techniques exists and have been used in real incidents. Some of them can be used to estimate radiation doses to an individual directly and some indirectly, e.g. given information regarding time and motion of an individual. There are both biological techniques, such as the dicentric assay and the micronuclei assay (IAEA, 2001), and physical techniques,
* Corresponding author. E-mail address: [email protected] (T. Geber-Bergstrand). http://dx.doi.org/10.1016/j.radmeas.2014.11.002 1350-4487/© 2014 Elsevier Ltd. All rights reserved.
such as electron paramagnetic resonance, thermoluminescence and optically stimulated luminescence (ICRU, 2002). However, all of these techniques have advantages and disadvantages (Ainsbury et al., 2011), and none are sufficient as a standalone tool after a large-scale incident. Thus, the possibility of using many different techniques and/or materials after an incident is desirable, partly to use the full extent of measuring capacities provided by different techniques and partly to make more accurate dose estimations by using several materials as references. Optically stimulated luminescence (OSL) is a process resulting in light emission from an insulator or semiconductor previously exposed to ionising radiation as the material is stimulated with light. As a result of the radiation exposure, charge carriers are trapped at metastable energy levels in the band gap of the material. During stimulation, the charge carriers recombine resulting in luminescence that is detectable by photosensitive devices (such as photomultiplier tubes). The intensity of the luminescence is
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proportional to the number of trapped charge carriers and, thus, the absorbed dose. Recently, attention has focused on finding new materials for retrospective dosimetry using OSL. Some of the most interesting materials are electronic components from portable devices (Bassinet et al., 2010; Woda et al., 2010; Inrig et al., 2008), chips €ttl, 2009; Mathur from bank and telephone cards (Woda and Spo € ksu, 2003), household salt (Bernhardsson et al., et al., 2007; Go 2009; Christiansson et al., 2012), and dental repair materials (Geber-Bergstrand et al., 2012; Veronese et al., 2010). The present study investigates the dosimetric properties of different desiccants for their possible use as fortuitous dosemeters. Desiccants are the generic name for materials used to remove excessive humidity or liquids from unwanted places; they may be used to protect various products, such as leather bags, from mold and drugs from degrading, or to absorb liquid spills, such as oil on roads. Most desiccants consist of silica gel (SiO2), activated charcoal, or montmorillonite clay, which is the main component in bentonite. Quartz, which consists of a SiO2 compound, is the most commonly used material in OSL, though for dating purposes; therefore, the hypothesis is that desiccants (containing SiO2) exposed to ionising radiation also have OSL properties. If this hypothesis is true, the initial investigations performed in this study could indicate a potential new method of dose estimation in retrospective dosimetry. A specific type of desiccant used to absorb spills on roads can be found in the vehicles of many rescue service teams. Rescue service teams often act as first responders in the event of an accident or other kind of incident and are, therefore, a group that might be exposed to ionising radiation. In the case of a radiation incident, even though the rescue service teams are supposed to have dosemeters available in their vehicles, they may not always be available in all vehicles, and even when they are they are rarely in operation because radiation incidents seldom occur. If an incident actually includes some sort of radiation, it might, therefore, not be revealed until the rescue service teams have already handled the incident. In such cases it would be beneficial to be able to use other materials present in the vehicles, such as desiccants, for postincident dose reconstruction. Similar kinds of desiccants can be found inside nuclear power plants (e.g. for absorbing liquid radioactive spills), which could enable dose estimations at places that are otherwise not monitored. Bentonite clay will be used to clad the copper canisters containing high-level radioactive waste in the planned underground storage in e.g. Finland and Sweden and thus, by measuring the bentonite clay, any potential leakage could be found. In households, desiccants are found in the form of cat litter and in bags and drug packaging. Because OSL is a technique that uses light for signal read-out, all materials must be protected from light until read-out in order to preserve the signal. Fortunately, most desiccants are kept in the dark, inside a handbag or package. Larger volumes of desiccants are usually kept in thick, several layered paper bags to avoid moisture absorption before they are used, which also prevents light from reaching the material. This study was an initial investigation of the fundamental OSL properties of various kinds of desiccants in order to determine their potential as retrospective dosemeters. To the best of our knowledge, no previous studies have investigated desiccants using OSL. 2. Materials and methods All measurements were performed using a TL/OSL reader (TL/ OSL-DA-15, DTU Nutech, Technical University of Denmark, Risø campus, Roskilde, Denmark), which is described in detail elsewhere (Bøtter-Jensen et al., 2000). Irradiation was achieved using a 90 Sr/90Y source (20 MBq 2009-04-09) incorporated into the reader. The current dose rate to a thin layer of quartz grains was
approximately 0.9 mGy s1 at the sample position. Dose calibration for each desiccant material was not performed; therefore all doses are given as the dose to quartz. The irradiation set-up was calibrated against the dose in air of a 137Cs source, which was calibrated at the Danish secondary standard laboratory (State Institute for Radiation Protection, SIS, Copenhagen, Denmark). The samples were placed on stainless steel cups (∅ ¼ 12 mm) during irradiation and during all readouts. Table 1 lists information on the 11 different types of desiccants used in this study. All of the samples were collected from households and/or nearby workplaces, including the emergency service, the packaging industry, and laboratories. Therefore, these desiccants can be considered as being commonly available in Sweden. The desiccants can be divided into two main types: granulates (G) and beads (B) (Fig. 1). For the bead-like materials, as many beads as possible were placed on the cups during irradiation and read-out. For the granulate materials, a thin layer was put on the cups and used for both irradiation and read-out. The exact compositions of the different materials are difficult to know as the content information on the packaging was limited. For the bead-like materials collected from hand bags and drug packages, only “silica gel” was printed on the packaging. However, the beads have very different appearances in terms of colour, shape, and structure; thus, they are likely to include different substances in addition to the silica gel. The B5 beads were most likely an activated form of montmorillonite clay, which is a form of hydrated sodium calcium aluminium magnesium silicate hydroxide. However, also this composition is difficult to determine with certainty because the package only read “activemineral”. Most of the granulate materials were said to be composed of bentonite clay, which mainly consists of montmorillonite. Similar to the bead-like materials, differences in the appearance of the granulate materials indicate that other substances are present. These alterations may have a distinct impact on the OSL signal, which is sensitive for impurities in the crystal lattice of the material. Two of the materials, B6 and G1, had a better known composition because the content was printed on the delivery note. The chemical composition of G1 was 75% SiO2, 10% Al2O3, 7% Fe2O3, 2% MgO, 2% K2O þ Na2O, 1% TiO2, and 1% CaO. G1 is commonly used by Swedish rescue service teams for absorbing spills of water, oil, and other chemicals on firm surfaces, such as roads. B6 is a molecular sieve composed of sodium aluminosilicate. The chain of manufacturing for the different desiccants is difficult to come by due to insufficient knowledge of the exact desiccant and trade secrets for the companies involved. In particular, materials composed of clay, which is a natural product, may not have undergone any special processing during the manufacturing process, meaning that they may have a geological age (>1000 years). If the radiation induced signal in the materials do not exhibit any fading, this could lead to background signals corresponding to several Gy. All OSL decay curves were recorded for 100 s, and the OSL signal was defined as the number of counts from the photo multiplier tube (PMT) during the first 10 s subtracted by the background, which consisted of the PMT counts during the following 10 s. Unless otherwise indicated, all measurements were performed immediately after irradiation. The samples were not pre-treated before irradiation or measurement. The first measurements were made to obtain tentative read-out procedures that could be appropriate for these materials. Signal read-out was performed in various ways following a dose of 1.8 Gy (90Sr/90Y) using: i) thermoluminescence (TL) by heating the samples to 600 C, ii) continuous IR light (870 nm) of 145 mW/cm2 at room temperature without preheating, iii) continuous blue light (470 nm) of 80 mW/cm2 at room temperature without pre-heating, and iv) continuous blue
T. Geber-Bergstrand et al. / Radiation Measurements 78 (2015) 17e22
19
Table 1 Summary of the applications and properties of the 11 different kinds of desiccants investigated. TL, thermoluminescence; IRSL, optically stimulated luminescence using 870 nm IR LEDs; bOSL, optically stimulated luminescence using 470 nm blue LEDs; ph, pre-heat; AsR, as-received signal (un-treated or un-irradiated sample); MDD, minimum detectable dose. þ: signal, : no or weak signal, i.e. <400 cts peak value. Sample type
Sample name
Major chemical component
Origin
TL 600
IRSL
bOSL no ph
bOSL 100 ph 120
AsR
Sensitivity [cts/mGy]
MDD [mGy]
Beads Beads Beads Beads Beads Beads Granulate Granulate Granulate Granulate Granulate
B1 B2 B3 B4 B5 B6 G1 G2 G3 G4 G5
Silica gel Silica gel Silica gel Silica gel Montmorillonite clay AlNa12SiO5 SiO2, Al2O3 Bentonite clay Bentonite clay Bentonite clay Bentonite clay
Computer bag Hand bag Hand bag Drug package Hand bag Laboratory Rescue service team vehicle Cat litter Cat litter Industry Natural clay
þ þ þ
þ þ
þ þ þ þ þ þ þ þ þ þ þ
þ þ þ þ þ þ þ þ þ þ
þ þ þ þ
0.5 0.4 0.5 0.3 13.2 1.5 3.4 2.1 5.9 8.3 1.2
450 370 110 310 10 120 40 270a 390a 20a 1800a
a
Calculated from bleached samples.
Fig. 1. Photograph of the different desiccants investigated in this paper when put on the cups used during irradiation and read-out. Top row from the left: B1, B2, B3, B4, B5, and B6. Bottom row from the left: G1, G2, G3, G4, and G5.
light at 100 C with pre-heating at 120 C for 10 s. If the peak value of the OSL curve (as defined in the previous section) was more than 400 cts it was considered to be a signal. Of the different read-out procedures, the one using continuous blue light at room temperature was chosen for further, more detailed measurements on e.g. doseeresponse, fading, and the effect of moisture on the OSL signal. Dose response measurements were performed by irradiating one to five aliquots of desiccant per given dose, depending on how much material was available and read-out immediately after irradiation. The different doses were 270, 450, 900, and 2700 mGy. One to three samples of each material, depending on availability, were irradiated with 1800 mGy and subsequently read-out in four different rounds, when studying the effect of moisture. During rounds one and four, the samples were dry; in rounds two and three, the samples were soaked with water using a plant mister prior to irradiation. Prior to the fourth round, the samples were allowed to dry completely at room temperature for 2 weeks. For the purpose of investigating signal fading over time, the samples were irradiated with 1170 mGy and read-out immediately, 1 day, and 3 days after irradiation. In order to normalise the signals, all aliquots were exposed to a test dose of 450 mGy after the read-out; this compensated for possible variations among equally treated samples of the same desiccant (due to different sample surfaces and mass in each aliquot). During the time delay between irradiation and read-out, the samples were kept in a dark environment. To explore the optical stability of the as-received signal (AsR), i.e. the signal present without prior laboratory irradiation, the materials were put on a windowsill for
exposure to sunlight. After 4 h, 3 weeks and 2 months, respectively, two samples of each material were read-out. These measurements are the first step in the study of desiccants which will reveal whether the materials are relevant for use in retrospective dosimetry. 3. Results and discussion 3.1. Signal characteristics All of the materials tested exhibited a signal after irradiation when using OSL with blue light stimulation at room temperature without any pre-heat (Table 1). When using pre-heat at 120 C before the OSL read-out at 100 C, only the material B3 did not exhibit a signal following irradiation. IR OSL (IRSL) was less successful than blue OSL because most materials did not exhibit any signal when stimulated with IR. When using TL up to 600 C, the signals were not as high as the OSL signals, if they were present at all. In addition, most of the materials exhibited an altered visual appearance due to the heating, i.e. scorched or ash-like. Due to these results, all further read-outs for all materials were performed using blue light OSL at room temperature without pre-heating. The materials composed of clay exhibited a considerable AsR. Examples of materials with and without AsR are shown in Fig. 2. This signal may be caused by background radiation or other, non radiative, causes such as mechanical friction during manufacturing. The fact that the AsR in G4 seems to have a more apparent slow component
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a
12000 Irradiated
10000 Net OSL signal [cts]
1000 PMT counts
B6 G1
1200
800 600 400
y = 3.4x+1300 2 R = 0.98
6000 4000
y = 1.5x+270 R2 = 1.00
2000
200 0
8000
As received Bleached
0
0
20 40 60 Stimulation time [s]
80
100
0
1000 2000 Absorbed dose [mGy]
3000
Fig. 3. Dose response curves for two different desiccants, B6 and G1 (±1 standard error of the mean). The materials were irradiated with a 90Sr/90Y source at doses between 270 and 2700 mGy. The average net OSL signals from three samples of each material was used to determine the OSL signals at each given dose.
b 3500
PMT counts
3000 2500 2000
MDD ¼ As received
1500 1000
Irradiated
500 0
Bleached
0
20 40 60 Stimulation time [s]
80
100
Fig. 2. Examples of OSL signals using blue light stimulation (470 nm). The three OSL signals shown were acquired i) after irradiation with 0.9 Gy (90Sr/90Y) on a bleached sample (irradiated), ii) the as-received signal (AsR) without prior laboratory irradiation, iii) after several days of bleaching in sunlight (bleached). a, Material without AsR (G1). b, Material with AsR (G4).
than the signal from the irradiated sample (Fig. 2b) could suggest a non radiative contribution for this material. But, because the manufacturing date and process is hard to find, the contribution to the AsR is difficult to determine. If the AsR originates purely from background radiation over many years, provided that long-term fading is negligible, it would correspond to doses of approximately 7 Gy for G2, 3 Gy for G3, 2 Gy for G4, and 12 Gy for G5, which is not unlikely for geological samples. If the AsR is large, it has to be taken into account, and this phenomenon needs to be further investigated before accurate corrections can be made for the AsR. 3.2. Dose response All of the studied materials exhibited an OSL signal proportional to the given dose (Fig. 3). The sensitivities, S, defined as the net OSL signal per absorbed dose unit, for the different materials are shown in Table 1. The materials composed of clay generally had the highest sensitivity. The minimum detectable dose (MDD) is traditionally defined as the absorbed dose corresponding to a signal equal to three times the standard deviation of the background/zero dose signal (mentioned in Currie, 1999; ICRU, 1972):
1 3sOSL zero S
(1)
By this definition, a high sensitivity generally implies a low MDD. However, if the standard deviation of the zero dose signal is high, then the MDD will increase. This was seen for several of the materials; for example, G2 had a lower MDD than G3 although G3 had a higher sensitivity. At this point, the MDDs for the materials exhibiting an AsR were calculated from bleached samples and will thus only be valid if the contribution of the AsR may be completely accounted for i.e. if the AsR is known. The materials not exhibiting an AsR (B5, B6, and G1) had the highest sensitivity. In terms of MDD, materials not exhibiting an AsR had the lowest values (B5, B6, G1, and B3). However, a low MDD is preferably accompanied by a high sensitivity and, thus, B3 is not considered to be quite as good as the other desiccants in terms of potential use for retrospective dose assessments. The OSL signals giving the dose response were measured immediately after irradiation and thus, for materials suffering from fading, the MDDs would be higher in a real case scenario since the time delay between irradiation and read-out would be longer. Note that when using Eq. (1) to calculate the MDD the uncertainty in S is not taken into account. To take this, and also any possible covariance between S and OSLzero, into account would be preferable. The manner in which this should be performed is not straightforward and needs to be further analysed. For this reason only the traditional definition will be used here. 3.3. Moisture effects No significant influence of the absorption state on the OSL signal was found for G1, and in principle neither for B6, G2, and G3 (Fig. 4), though the values for round three were somewhat lower than for the other rounds. Therefore, the OSL signal for these materials was more or less unaffected by moisture. Two of the tested materials, G4 and G5, exhibited a deviating behaviour after being soaked the first time. These materials were more clay-like and became somewhat dissolved by moisture, which could indicate that the crystal structure was destroyed/altered. The rest of the materials exhibited different effects during the second and third round, with lower signals in the third round compared to the second round. The reason for this was unclear. Because these materials were also
T. Geber-Bergstrand et al. / Radiation Measurements 78 (2015) 17e22
250 B1 B2 B3 B4 B5 B6
200 150 100
60 B5 B6 G1 G2 G3 G4 G5
40
0
Moist 1
Moist 2
Dry 2
180 G1 G2 G3 G4 G5
160 Relative OSL signal [%]
80
20
50 0
b
100 Relative OSL signal [%]
Relative OSL signal [%]
a
140 120 100 80 60 40
21
0
1 2 3 Days after irradiation
4
Fig. 5. Fading of the OSL signals after irradiation with 1170 mGy (90Sr/90Y). The values of the relative OSL signal are the mean of three samples of each material normalised to the measurement immediately after irradiation.
Only the materials exhibiting AsR (i.e. G2, G3, G4, and G5) were used when exploring the optical stability of the AsR. After 4 h, all materials still exhibited a visible AsR. After 3 weeks, G2 and G3 exhibited a weak AsR, whereas the AsR for G4 and G5 were completely bleached. After 2 months all materials were completely bleached, corresponding to a constant background level. 4. Conclusions
20 0 Moist 1
Moist 2
Dry 2
Fig. 4. OSL signals (±1 standard error of the mean) obtained from three samples of each material after irradiation with 1800 mGy (90Sr/90Y), except for B1eB5, for which one sample of each material was used due to a lack of material. Measurements were performed in four rounds, during round one and four the samples were dry and in round two and three the samples had been soaked with water prior to irradiation. All measurements were normalised to the first dry round. a, Bead-like materials. b, Granulate materials.
inferior in regard to sensitivity, no effort was made to explore this further at this time.
3.4. Fading and bleaching Only the materials with sensitivities above 1 cts/mGy were chosen for fading measurements (Table 1). B6, G1, G3 and G5 exhibited little fading over 3 days (more than 80% of the signal remaining) whereas G2 and G4 exhibited a fading corresponding to approximately 59e66% of the signal remaining after 3 days (Fig. 5). The materials G1, G2, G3, G5 and B6 show a relatively fast initial fading, up until 1 day, and a slower or non-existent fading after that. The observation of this slow component implies that the AsR observed in these materials might be due to background radiation that has been accumulated over time. The low fading also indicates that the signal can be read-out at a time-point long after an actual exposure event. B5, however, exhibited significant fading during the 3 days, with only 6% of the initial signal remaining after one day and 5% after 3 days. In order to fully describe the fading properties of these materials, a longer time period needs to be studied.
This study shows that some desiccant materials have the potential for use as fortuitous detector materials for retrospective dosimetry, ten of the eleven materials have a MDD between 8 and 450 mGy. One of the arguments supporting this conclusion is that these materials may be found in the vicinity of radiological/nuclear accidents and other incidents, such as inside nuclear power plants, radioactive waste storages, and in the vehicles of rescue service teams. By collecting the desiccants present in rescue service vehicles, the dose to the persons in the vicinity can be estimated by means of OSL after the radiation exposure event. Among the desiccants studied here, the most promising materials were B6 and G1, which exhibit promising properties such as a low short-term fading, no moisture effect, and high sensitivity. That natural clay (G5) also shows dosimetric properties, although not quite as good, is an interesting fact since it is readily available in many places. None of these materials, however, will provide an individual dose estimate immediately because information such as dwelling time is needed; thus, these materials are perhaps best suited for non-triage purposes in retrospective dosimetry. The use of these materials could result in a reduction of the uncertainty in the dose estimation when used together with other measurements; if several different materials provides the same dose estimation it is more likely that the dose estimation is correct. Desiccants could also be used for dose estimations in less acute situations, and/or when no other materials are available. Any material exhibiting OSL properties suitable for retrospective dosimetry is a useful addition to the existing dosimetry system available in emergency preparedness. However, before any of the materials can be used in a real scenario, further investigations of the dosimetric properties are needed. This includes more information on composition and manufacturing process of the different materials and the main focus for future studies will thus include investigation of the origin of the AsR with respect to discern an accident dose.
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