Dose estimation based on OSL signal from banknotes ...

Radiation Measurements 101 (2017) 1e6

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Dose estimation based on OSL signal from banknotes in accident dosimetry A. Mrozik, D. Kulig, B. Marczewska*, P. Bilski Institute of Nuclear Physics Polish Academy of Sciences, PL-31342 Krakow, Poland

h i g h l i g h t s
Banknotes could be used as triage dosimeters in emergency situation.
Some spots on banknote area showed relatively good dosimetric properties.
Dose reconstruction is even possible after 48 h with 25% accuracy.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 November 2016 Received in revised form 9 February 2017 Accepted 9 April 2017 Available online 22 April 2017

Our work is focused on the optically stimulated luminescence (OSL) of paper currency of Polish zloty, especially 10 PLN and 20 PLN banknotes in regard to the dose reconstruction in an emergency situation involving radiation. ‘New’ Polish banknotes with enhanced anti-counterfeiting systems were investigated in comparison with ‘old’ banknotes which are still in circulation, as well as with 1 US dollar and 5 EURO banknotes. Similar dosimetric properties for 10 PLN and 20 PLN banknotes of ‘old’ and ‘new’ types were observed: relatively good repeatability of OSL signal, linear dose response in the dose range between 0.2 and 10 Gy, and fading reaching 65% of OSL signal after 5 h and remaining almost stable at the level of 45% after 48 h. Despite a high intrinsic initial signal of the ‘new’ type of Polish banknotes of 10 PLN and 20 PLN it is still possible to reconstruct the doses higher than 0.4 Gy two days after an incident with an accuracy better than 25%. In light of these results banknotes can be considered as potential triage dosimeters for retrospective dosimetry. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Retrospective dosimetry Optically stimulated luminescence Banknotes

1. Introduction In an emergency situation if uncontrolled radiation affected people or the environment the information about radiation doses allows for the prediction of the biological consequences. In such a situation when professional personel and environmental dosimeters are not available, objects of everyday use can be perceived as dosimeters. The electronic devices and plastic cards of different types, which are so common today, are considered especially interesting objects due to the fact that they contain luminescent Al2O3, SiO2 and polymerelike materials. Small electronic devices such as mobile phones have already been the object of several studies. Woda et al. (2012) studied the modules of chip cards; the glass displays from mobile phones were tested by Discher and Mrozik (Discher et al., 2013; Discher and

* Corresponding author. E-mail address: [email protected] (B. Marczewska). http://dx.doi.org/10.1016/j.radmeas.2017.04.012 1350-4487/© 2017 Elsevier Ltd. All rights reserved.

Woda, 2013; Mrozik et al., 2014a,b; Discher et al., 2015); electronic components of personal devices were examined by Pascu et al. (2013) and Mrozik (Mrozik et al., 2014a,b). This investigation revealed that mobile phones contain some electronic components (e. g. resistors, inductors) and glass displays allowing the measurement of the doses of some mGy. Obtaining samples from a mobile phone, however, leads to its destruction. Moreover, in an emergency situation taking away and dismantling mobile phones can be stressful for the owners. It means that the potential victims have to give up their private mobile phones which leads to loss of contact with both the family and the world. Therefore there is a need to find other materials for the simple qualitative check whether there has been radiation or not. It appears that such items as paper currency (banknotes); coins; plastic cards of different types (credit and debit cards, driver's licenses, membership cards, etc.), shoes and fabrics (Sholom and McKeever, 2014) are also sensitive to radiation due to the presence of some components in their structure which can emit

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luminescent light under stimulation. These materials can serve as evidence of their earlier irradiation. It seems that the banknotes of low denominations, which are often found in our pockets and whose loss is not painful, could be an interesting alternative. Our work is focused on the optically stimulated luminescence (OSL) of paper currency of Polish zloty, especially 10 PLN and 20 PLN banknotes, their OSL signal and fading in regard to the dose reconstruction in emergency situations. This investigation was inspired by the change in Polish banknote protection systems which was implemented in Spring (2014) and concerned 4 of 5 types of banknotes (10, 20, 50 and 100 PLN). The last denomination worth 200 PLN was entered into circulation in February 2016. ‘New’ Polish banknotes with better protection (anti-counterfeiting systems) were tested in comparison with ‘old’ banknotes which are still in circulation, as well as with 1 USD dollar and 5 EURO banknotes. The aim of the work was to investigate the OSL properties of particular parts of the banknotes in regard to the presence of their intrinsic initial signal, their sensitivity to radiation, repeatability of OSL signal, dose response and fading; in terms of their potential application as triage dosimeters in emergency situations. 2. Materials and methods Several ‘new type’10 PLN and 20 PLN banknotes as well their ‘old’ equivalents, 1 USD and 5 EUR banknotes were the objects of the study. Since Spring 2014 better protected Polish banknotes of denominations of 10, 20, 50 and 100 PLN have appeared in circulation slowly replacing the ‘old’ ones which will be in use until their natural destruction. Polish banknotes are made from cotton similar to EUR banknotes, whereas US dollar banknotes use paper that consists of 75% cotton and 25% linen (Sholom and McKeever., 2014). 80 - 100 spots were distinguished on the area of each banknote depending on the size of the banknote (a 10 PLN banknote is smaller than a 20 PLN one). OSL signal was checked on both sides of the banknote (obverse and reverse), but the presented results were received from the obverse side. The location of the spots representative for 10 PLN and for 20 PLN banknotes are presented in Fig. 1a and b, respectively. The diameter of the spots was of 8 mm, adequate to the dimension of the cups in an OSL reader. The luminescence of the samples (particular parts of the banknotes) was investigated with OSL method in an automated luminescent TL/OSL reader (model DA-20) produced by Risoe National Laboratory, Denmark. OSL measurements were done using blue diodes (470 nm ± 30 nm) with a total power of 80 mW/cm2 at the sample position. The readouts were performed with U340 optical filter transmitting 250e400 nm light wavelength. The Risoe reader is equipped with a beta irradiator holding 90Sr/90Y source and maximum energy of 2.27 MeV for irradiation of the samples with a dose rate of about 64.6 mGy. The calibration of 90Sr/90Y source was performed using MCP dosimeters in regard to 137Cs gamma-ray reference source calibrated in terms of kerma in air. Between irradiation and readout the samples were kept in darkness assuming that the banknotes are usually stored in a wallet protected from light. The preliminary investigation relied on the irradiation and readouts of the spots of banknotes in order to plot maps of the intrinsic initial native signal and with sensitivity to the ionizing radiation of particular parts of the banknotes. From each banknote (‘new’ and ‘old’ 10 and 20 PLN, 1 USD and 5 EUR) some spots were selected for further study of repeatability, linearity and fading. The intrinsic initial native signal is an OSL signal which can be measured during the first readout on the samples untreated by any irradiation but only in the new type of polish banknotes. The old type does not have any initial intrinsic signal. Initial signal comes from the banknote's origin, chemical composition, treatment,

Fig. 1. The ‘new’ 10 PLN (a) and 20 PLN (b) banknotes with the location of investigated spots.

printing ink and different collaterals. Initial signal is an undesirable phenomenon and distorts the radiation induced signal. Cleaning didn't remove any initial signal in banknotes. Initial signal is irretrievably erased during the first OSL readout. All systematic investigations of repeatability, dose response and fading were carried out on samples after their first OSL readout (which means without initial signal). The use of a ‘fresh’ sample with initial signal is always clearly marked in the text. At the beginning of our experiments the repeatability of the signal of selected areas from different banknotes was checked. Generally, the same areas of the banknotes (28± 1 mm2) presented similar properties but some deviation was noticed, therefore each spot should be treated individually. This individual approach means that the calibration should be performed and calculated for each separate spot. These deviations can arise from the individual attributes of the banknotes which are caused by its wear rate. OSL signal was read out for 300 s while recording the signal every 0.1 s. For calculation the sum of the first 5 points (recorded in first 0.5 s) minus last 5 points (last 0.5 s between 299.6 and 300.0 s) was taken. This method was always applied except in Chapter 3.7 where another signal calculation method was implemented (see text).

3. Results and discussion 3.1. OSL signal of particular spots In the first step of the investigation OSL signal was checked for some spots randomly chosen from the banknotes. The exemplary results for the ‘old’ type of 20 PLN banknote are presented in Fig. 2A and for the ‘new’ type in Fig. 2B. Routinely, each spot taken from a “fresh” banknote was at first irradiated with 1 Gy and then read out (curve a). Next the background level (the second readout of the same sample) was measured (curve b). The second irradiation performed on the same spot gives only a signal coming from

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Fig. 3. Mapping of the native intrinsic OSL signal (no irradiation e A) and OSL signal after irradiation with 1 Gy (B) of ‘new’ type of 20PLN.

Fig. 2. OSL signal of ‘old’ type of 20PLN, spot No.49 (A) and ‘new type’ 20 PLN, spot No. 1 (B). Curve (a) - the sample was irradiated with 1 Gy (initial signal plus signal from irradiation, (black), curve (b) e background (red), curve (c) - again irradiation with 1 Gy (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

irradiation (curve c). OSL signal of ‘fresh’ samples after their irradiation with 1 Gy was at the level of 300e350 impulses for the ‘old’ type of banknotes (Fig. 2A) and about 700 impulses for the ‘new’ type (Fig. 2B), OSL signal coming from irradiation was at the level of 300 and 100, respectively; wherein the background for annealed samples was for both ‘new’ and ‘old’ banknotes at about 20e30 impulses. As seen in Fig. 2 in the case of ‘new’ banknotes the native intrinsic signal is dominant. 3.2. Mapping of OSL signal Full mapping of the native initial signal (i) and “pure” 1Gy signal distribution (ii) was constructed based on OSL signal from 98 spots for 20 PLN (Fig. 3) and 84 spots for 10 PLN (Fig. 4) extracted from banknotes. In Figs. 3 and 4 several spots with higher OSL intensity are presented. The OSL intensity of these spots is not reproducible between banknotes. The reason for that is probably a different history of each tested banknote, which is almost impossible to assess. It is interesting that some OSL background signals are significantly higher and amount even to about 200 impulses. These places are near the spots No. 69 i 75 in 10 PLN ‘new’ type and No. 87, 88, 94 i 95 in 20 PLN ‘new’ type. These spots are near the places with a new anti-counterfeiting system where graphic elements from both sides of the banknotes viewed in the light complement each other

Fig. 4. Mapping of the native intrinsic OSL signal (no irradiation e A) and signal after irradiation with 1 Gy (B) of ‘new type’ 10PLN.

and form a complete picture - the crown in the oval (called rectoverso). These places are not visible in Figs. 3 and 4 because the maps don't present the ‘net’ signals but the difference between the sum of the last 5 points subtracted from the sum of the first 5 points.

3.3. Reproducibility tests and dose response Reproducibility tests were performed for several spots chosen randomly from the banknote as 10 cycles of irradiation with 1 Gy followed by OSL readout. However, OSL intensity between different banknotes is not reproducible, the spots from a particular banknote have a repeatable dosimetric signal with 2% precision during several irradiation and measurement cycles.

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In the next step of the experiment the dose response of several randomly chosen spots was tested in the dose range from 0.2 to 10 Gy. Generally speaking, independently of the investigated spot position, all spots are characterized by a relatively good linearity of the signal in this dose range. As an example, the dose response for spots No. 10, 32 from 10 PLN ‘new’ type of banknote and spot No. 26 (‘new’ 20 PLN) is presented in Fig. 5. The minimal dose of 0.2 Gy is four times above the level of the background.

3.4. Fading correction The loss of the signal in the time between irradiation and readout is an undesirable disturbing factor. Fig. 6 presents the shape of the decay curve signal at the time between irradiation and readout. The fading study was performed for 21 spots extracted from 10 PLN and 20 PLN banknotes. All samples were irradiated with the dose of 1.96 Gy and the irradiation time (6 s) was significantly shorter as compared to the storage times. The OSL signal was measured from 2 min up to 3 weeks. The first point, measured immediately after irradiation, takes into account moving the

Fig. 5. The linearity of the dose response for samples No.10 and 32 (‘new’ 10 PLN) and No. 26 (‘new’ 20 PLN).

Fig. 6. Fading of OSL signal after exposition to ionizing radiation, normalized to first point of measurement, insert- the same function in log scale.

sample from the irradiation position to the measurement position within the reader. The fading for all spots was about 35% in the first 5 h, after this time the decrease begins to slow down and reaches the value of less than 45% after 48 h and practically speaking remains at this level. 3 weeks after irradiation the OSL signal amounts to 35% of the first recorded OSL signal. The data points were fitted by two-exponential decay functions of the form of y ¼ A1*exp(-x/ t1) þ A2*exp(-x/t2) þ y0. In order to verify the accuracy of the determination of decay of signal in some time after irradiation a series of measurements of the impact of a temporary loss of OSL signal at the dose of irradiation were conducted. Spots no. 5 and 14 (‘new’ 10 PLN) were irradiated with dose of 0.5 Gy and the readouts were conducted 1 and 2 days after irradiation. The results showed that after 1 and 2 days 40e50% of OSL signal remains. Table 1 presents the results of the experiment.

3.5. Sensitivity to light The fading tests described above were performed for the storage of the samples in darkness. In order to verify light sensitivity of OSL signal in banknotes, the samples were irradiated with 4 Gy and read out in three ways: immediately after irradiation, after 10 min of keeping the samples in darkness and after 10 min exposure to routine laboratory daylight. The results revealed that OSL signal in banknotes is light sensitive. The results show that after 10 min of light exposure only 20% of the initial signal remains. Usually banknotes are placed in the wallet which gives them natural protection from the sunlight. However, the process of extracting and handling samples should be done in darkness. Summarizing, similar dosimetric properties were observed for 10 PLN and 20 PLN banknotes of ‘old’ and ‘new’ types. A noticeable difference concerns the initial signal which is dominant in the case of the ‘new’ banknotes. Both types of 10 PLN banknotes have similar properties to 20 PLN banknotes: good dose response in the dose range between 0.2 and 10 Gy, relatively good repeatability and fading reaching 40e50% after 5 h and remaining almost stable after 48 h.

3.5.1. 1 US dollar and 5 EUR banknotes 1 US dollar and 5 EUR banknotes were comparatively investigated in parallel to the Polish bills. The spots were distinguished in a similar way to the Polish banknotes (1 US dollar - 126 spots and 5 EUR - 84 spots). US dollar was examined in several places randomly chosen from the surface of the banknote. Some of the spots showed initial signal, relatively high OSL signal introduced by radiation or, no OSL signal at all. For example, spot No. 39 was characterized by a high signal amounting to 700 impulses after 1 Gy irradiation, good linearity up to 5 Gy but a very large fading causing the decrease of the signal up to less than 30% after 24 h from irradiation. 5 EUR banknote was investigated in a similar way. The spots presented no OSL signal (spots No 4, 13, 18) or lack of initial signal and 1 Gy signal at the level of 150 impulses (spot No 2), good linearity up to 5 Gy and the fading similar as for 1USD banknote. These results are in line with the paper of Sholom and McKeever (Sholom and McKeever, 2014). They also noted that some US dollar bills demonstrated an initial signal. They reported the values of fading for 1 USD bill as 39, 33 and 26% after 1, 3 and 7 days, respectively. These values are a little higher than 30% after 1 day but such a discrepancy can occurs between spots and banknotes. It can be concluded that the signal of 1 US dollar, 5 EUR, ‘old’ 10 PLN and 20 PLN bills are similar, amounting to hundreds of impulses, at the level of the background of 20e30 impulses.

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Table 1 The values of the doses measures after different time after irradiation. Dose [Gy]

Time between irradiation and readout [day]

Measured dose [Gy]

Dose after fading correction [Gy]

0.50 0.50

1 2

0.22 0.28

0.44 ± 0.10 0.61 ± 0.15

3.6. Tests of dose reconstruction To reconstruct (recover) the unknown dose which the samples received during a prior irradiation a measurement sequence known as the single-aliquot regenerative-dose (SAR) procedure (Wintle and Murray, 2006)was applied. This procedure consists of several steps of readouts and calibration with defined doses. The dose recovery tests were performed for some spots extracted from banknotes which were irradiated with unknown doses. The results of recovery tests for spots taken from banknotes are shown in Figs. 7 and 8. After the first OSL readout of the unknown dose the sample was calibrated by irradiation with the defined doses of 1.9 Gy, 2.7 Gy and 4.4 Gy and then read out again in each following step. Between each calibration step, the samples were irradiated with test dose (0.65 Gy) and read out. The results for unknown dose and calibration doses (Lx) were divided by the values of the response related to the test doses (Tx) (presented in Fig. 8). One of the points was repeated to check if it would be identical to the first one. This repeated point is called recycling point. In our case the recycling point was measured for the dose of 1.9 Gy and confirmed the correct application of the procedure. The measurements according to fast protocol were also made (Bassinet et al., 2014), in which in the first step the natural dose is measured and after this the simple calibration with only one dose is done. The recovery of the dose according to fast protocol is less accurate but shortens the time. The lack of any initial signal makes it much easier to recover the cumulated dose because simply subtracting the background from the whole signal gives the value of the signal coming from irradiation. For each sample a fading factor must be estimated and included in dose calculation. The recovery of the signal from the samples having an initial signal in a situation when hours have elapsed between irradiation and readout requires a more complicated procedure. The same problem for business cards was solved by Sholom et al. (2011). The

Fig. 7. OSL signal of a ‘new’ 10PLN, spot No.5. Curve (a) - the sample was irradiated with accidental dose (black), curve (b) e background (red), curve (c) e one of calibration dose e 1.9 Gy (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

authors proposed using a difference between shapes of two OSL curves for calculating dosimetric signal. The OSL curve, which includes a dosimetric component besides the intrinsic initial signal, possesses the higher signal only in its first part. After 5 s of stimulation the signal is at the background level (Fig. 7). In comparison to OSL curves with an initial signal (a), where decay is slower, dosimetric signal escapes as fast as in the case of OSL curve (c). In order to obtain a dosimetric signal from OSL curves (a) the exponential function should be fitted to curve (a) for the range 5e200s of stimulation time (which is free from any dosimetric signal). The fitted signal is then extrapolated to zero over the whole range of stimulation time and is subtracted from the measured OSL to obtain the dosimetric signal. The exemplary recovery test according to SAR protocol is presented for spot no 5 ‘new’ type 10 PLN in Fig. 8. The spot was irradiated with an unknown dose and the time between irradiation and readout was 20 h. The value of accidental dose was corrected with fading factor. The data were satisfactory with regard to the proposed objective. The uncertainty results are a combination of uncertainty of dose response fitting and fading correction. All recovered dose were estimated with the uncertainty of less than 25%. 3.7. Portable OSL reader Some tests were performed with a small portable reader called HELIOS-1 which was constructed in cooperation with Jan Długosz Academy in Cze˛ stochowa (Marczewska et al., 2012). Configuration of HELIOS-1 modified for the present experiment consists of the stimulation and readout modules similar to TL/OSL Risoe reader, e.g. the stimulation is conducted by a ring with blue diodes (460 nm) and the signal is recorded in the region of UV light (340e400 nm). HELIOS-1 was applied in the readout of samples No. 4 and 25 from a new 10 PLN banknote. The samples were irradiated with the doses of 5 Gy and 1 Gy using Cs-137 source. The signal was

Fig. 8. Standardised dose-response as function of dose for spot No. 5 (10 PLN ‘new’ type) using SAR protocol with test dose normalization. The given test dose was 0.65 Gy. Recycling point is indicated by open square (please see in text).

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recorded for 60 s during the readout of samples in HELIOS-1 every 1s. The signals in the first seconds were visibly higher than the tail of the curve. Even for the 1 Gy irradiation the sum of the 5 first points is significantly higher as a sum of 5 first points of the background. This means that the simple, light 2 kg portable HELIOS-1 reader if available close to the incident place can be applied for the first quick assessment in triage application.

Acknowledgements Anna Mrozik has been partly supported by the EU Human Capital Operation Program, Polish Project No. POKL.04.0101-00
w 434/08-00; Dagmara Kulig by “Marian Smoluchowski Krako Research Consortium “Matter e Energy e Future” (KNOW).

4. Conclusions References In the case of a radiation incident victims should be checked if they have a banknote in their pocket or wallet or other place with no access to light. The storage of banknotes in darkness or semidarkness increases the chances of proper dose estimation. Also the estimation of time between an incident and the readout is very important. It is necessary to cut out a sample of a banknote from the incident location, put it into the reader, perform the OSL readout and the calibration of this spot with different doses. The calculation should include the fading factor. In the case of Polish zloty it is better to deal with the ‘old’ type of banknote, if available, due to the lack of any intrinsic initial signal. The potential for the use of banknotes as accidental dosimeters has been successfully demonstrated. The Polish banknotes of 10PLN and 20PLN of a ‘new’ type, which are more and more common in circulation, have a high intrinsic initial signal, opposite to the ‘old’ type of banknotes. Independently of the banknote type almost all spots were sensitive to ionizing radiation, are characterized by relatively good repeatability, linear dose response and strong fading in first 48 h which stabilizes and remains at the level of 35% of OSL signal (immediately after irradiation) for weeks. Despite a high intrinsic initial signal of the ‘new’ type of Polish banknotes of 10 PLN and 20 PLN and for 1 USD and 5 EURO it is still possible to reconstruct the doses two days after an incident with accuracy better than 25%. This estimation should be considered as a triage application before the decision if the victim was strongly affected by the radiation and before taking further steps.

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