An extremity (wrist) dosemeter based on the Landauer InLight TM whole body dosemeter

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1 An extremity (wrist) dosemeter based on the Landauer InLight TM whole body dosemeter Christopher A Perks *a and Stephane Faugoin a a LANDAUER EUROPE, 33 Avenue du General Leclerc, F-92266, Fontenay-aux-Roses CEDEX, France. Abstract. A number of Landauer s clients use wrist badges to monitor extremity doses surrounding the hand. In the past, in France, we have supplied clients with standard issue InLight TM whole body dosemeters, enclosed within a soft plastic pouch. A strap was used to secure the dosemeter around the wrist. The standard algorithm for whole body dosimetry was used to determine the dose-equivalent (Hp(0.07)). Clients requested a smaller and more comfortable design. Therefore, a new design was proposed in which only the InLight case and slide are contained within a smaller plastic pouch. This design is substantially smaller than the previous wrist badge and gives the client greater comfort while allowing the badges themselves to be read out using the same readers and procedures as our standard whole body dosemeters. A neutron dosemeter can also be included in this badge. This paper gives an overview of the new design and reports on type testing of the proposed wrist badge on an ICRU pillar (wrist) phantom, in terms of the energy and angular response. It was found that, while the standard algorithm for the InLight dosemeters performed well for photons and was correctly identifying the quality of the radiation exposure (low or high energy photons, low or high energy beta particles), there was a significant under response for beta particle exposures. Minor modifications to the algorithm were adopted. Finally, the recalculated results using the modified algorithm are compared to the requirements of the international standard for extremity dosemeters (ISO 12794:2000). KEYWORDS: Optically Stimulated Luminescence, InLight, personal dosimetry. 1 Introduction The application of Optically Stimulated Luminescence (OSL) for dosimetry has been reviewed previously [1]. Landauer has developed the application of aluminium oxide doped with carbon (Al 2 O 3 :C) as an OSL detector material for its external personal radiation dosimetry services [2-6]. It is currently Landauer s preferred method because of a number of key operational and technical features: high sensitivity, improving low dose precision and enabling thin layer dosemeters; elimination of heating permits more possibilities for the design of dosemeters, including: powder coatings on clear film bases; simpler and more reliable analysis instrumentation; and better control over the amount of luminescence emitted; re-analysis of dosemeters is possible as very little of the signal is lost in each read. The system adopted in France (and a number of other countries worldwide) is the InLight system [2] which comprises a slide (containing four detector elements), which is enclosed in a case. The case has an open window and three filters which line up with the detector elements on the slides. The case is in turn enclosed in a holder which is capable of additionally including a neutron dosemeter and imaging component. To perform extremity personal dosimetry around the wrist, we supplied customers in France with standard issue InLight dosemeters enclosed within a soft plastic pouch and a band to secure the dosemeter. Clients requested a smaller and more comfortable design. We have now adopted using the InLight case and slide within a smaller plastic pouch. A neutron dosemeter can also be included in this badge. The standard algorithm, developed for the InLight dosemeter and whole body exposure * Presenting author, cperks@landauer-fr.com 1

2 was initially used for determining the dose (Hp(0.07)) from the original wrist badges. Type testing of the new design demonstrated that a small change to this algorithm would enable it to be used to assess the dose from the extremity wrist dosemeter being adopted. This paper describes the requirements for the new wrist dosemeter, its design, the type testing of the new wrist badge in terms of the energy and angular response, the modifications made to our dose calculation algorithm and compares the results with the international standard [7]. 2 Requirements The requirements for extremity dosemeters are given the appropriate international standard [7]. Most of the requirements in terms of type testing are similar to that for external whole body dosemeters and are common to the wrist badge and whole body badge regardless of the exact design of the cover of the badge. The principal differences regard the energy and angular response. 2.1 Energy response For photons and beta radiation the requirements are as follows: Photons: in the energy range from 15 kev to 3 MeV the response (Hp(0.07)) shall not vary by more than ± 50 %. Beta radiation: in the energy range (E max ) 0.5 MeV to 3 MeV the response (Hp(0.07)) shall not vary by more than ± 50 %. 2.2 Angular response The angular response (isotropy) is defined for 60 kev photons, and is as follows: When irradiated with photons of (60 ± 5) kev, the mean value of the response (Hp(0.07)) at angles of incidence of 0 o, 20 o, 40 o and 60 o from normal shall not differ from the corresponding response for normal incidence by more than ± 15 %. 3 The LANDAUER EUROPE extremity wrist badge The LANDAUER EUROPE extremity dosemeter (Figures 1 and 2) is based on the case and slide of our standard InLight dosemeter which is used for whole body monitoring. Thus the processing and read out of this dosemeter is very similar to pure standard dosemeter. The dosemeter itself (Figure 1). consists of a case that contains metal and plastic filters and a plastic slide that contains detector elements. The detector element is a layer of Al 2 O 3 sandwiched between two layers of polyester for a total thickness of 0.3mm. The filter thicknesses for the standard design InLight dosemeter adopted for the extremity wrist dosemeter are given in Table 1. Table 1: Filter thicknesses for the InLight badge Thickness /mg.cm -2 Open Window Plastic Aluminium Copper Front Back

3 Figure 1: The case and slide of the InLight dosemeter For the extremity wrist dosemeter the case and slide are enclosed in a heat sealed polythene pouch which is 0.2 mm thick (Figure 2), together with a paper label. The pouch is designed to allow a wrist strap to be attached. Figure 2: The LANDAUER EUROPE extremity wrist dosemeter The InLight dosemeters are readout in readers based on the widely used conventional Panasonic readers that have been modified for OSL readout, principally by employing light emitting diodes as a light source. Dosemeters can be read automatically at a speed approaching 10 seconds per dosemeter, with very little need for operator intervention. Three models of reader are available: a manual reader; a reader capable of reading out up to 200 dosemeters sequentially without operator intervention; and one capable of up to 500 dosemeters. In the two automatic readers, dosemeters are fed into the reader in 4 or 10 magazines respectively, each holding 50 dosemeters. An algorithm is used to determine the quantities of interest, including Hp(10) and Hp(0.07), from the individual responses of the four OSL detectors. In addition, the algorithm indicates the radiation quality (photon energy or beta particle) of the exposure. 3

4 4 Type testing 4.1 Preparation 80 InLight slides and cases were withdrawn from the spares stock at our Paris based offices. These were inserted and sealed inside the plastic pouches to be used for wrist badges. 4.2 Irradiation Irradiations were performed at the Health Protection Agency s irradiation facility at Chilton, UK. The irradiations were performed on an ISO standard pillar (wrist) phantom as defined in the ISO standard [7}. This comprises a water-filled hollow cylinder with PMMA walls and an outer diameter of 73 mm and a length of 300 mm. The cylinder walls have a thickness of 2.5 mm and the end faces have thickness of 10mm. The irradiations were performed in accordance with international standard procedures for photons [8] and beta particles [9]. A secondary standard ion chamber was used to determine the doses. For the majority of the irradiations, dosemeters were irradiated with a 3 msv dose with two InLight badges strapped adjacent in the middle of the phantom (Figure 3). Figure 3: The irradiation configuration For the beta irradiations (for which the beam size was smaller) and for the vertical rotations (to avoid self-shielding as the badges were rotated) only one badge was attached to the phantom. Irradiations were performed at normal incidence for the following photon and beta sources: N-20 (16 kev), N-40 (33 kev), N-80 (65 kev), N-100 (83 kev), 137 Cs (662 kev), 60 Co (1250 kev), 90 Sr/ 90 Y (E max, 2279 kev) and 85 Kr (E max, 687 kev). 85 Kr (E max, 687 kev) was used as a substitute for 204 Tl (E max, 763 kev) which is the source identified in the ISO standard [7]. In addition, the angular response was determined for 65 kev (N-80) photons up to and including ± 60 o about the vertical and horizontal axes. For the majority of the exposure conditions, four dosemeters were exposed for each source configuration. The exceptions were: two dosemeters were irradiated with 60 Co photons, three dosemeters were irradiated for the each of the beta particle sources and three each for a number of the exposures for the rotation about the vertical axis of the dosemeter. The ISO convention for defining vertical and horizontal rotations (Figure 4) was adopted. Control dosemeters accompanied the irradiated dosemeters to determine the background. 4

5 Figure 4: Convention for directional dependence 4.3 Read out The irradiated dosemeters, together with a number of control dosemeters were readout and analysed using the standard algorithm during normal operations at our facility in Fontenay-aux-Roses, Paris, France. The assessed background was subtracted from the irradiated dosemeters. 4.4 Analysis Doses were calculated using the standard algorithm (version 1.1.1) incorporated into the reader software set up for the standard assessment of whole body dosemeters in our Paris facility. Results for Hp(0.07) were compared to the exposed doses reported by the Health Protection Agency in their certificate for the irradiations performed. Further analysis was performed using a modification of the standard algorithm. 4.5 Results Energy response (photons) The energy response ((Hp0.07)) as a function of incident photon energy) of the wrist dosemeter together with the ISO limits as defined in the ISO standard [7] are presented in figure 5. Means and ± 1 standard deviation are plotted for the type test data. 5

6 Figure 5: Photon energy response 1.60 Measured/delivered dose Energy /kev Type test data ISO limit Energy response (beta radiation) The results for the beta type test data are given in table 2. Values are means ± 1 standard deviation. Table 2: Energy response (beta radiation) Beta source Measured/ reported dose (Standard algorithm) Measured/ reported dose (Modified algorithm) 90 Sr/ 90 Y 0.67 ± ± Kr 0.25 ± ± Isotropy (65 kev photons) The angular response for vertical and horizontal rotation is shown in Figure 6. Values are means ± 1 standard deviation. Figure 6: Angular response for 65 kev (N-80) photons 2.00 Measured/deliverd dose Angle Vertical rotation Horizontal rotation ISO limit ISO limit 6

7 5 Modifications to the algorithm In view of the under-response for beta particle, particularly for the lower energy 85 Kr particles, changes to the algorithm were considered to improve it. The nature of the badge, in having four elements with different individual energy responses, enables the dose calculation algorithm to determine an approximate radiation quality for the incident radiation. It was noted that, although the dose (Hp(0.07)) was under-assessed, the algorithm reported the correct values for the radiation quality. Thus a simple change to the algorithm was tested in which the results for the assessment of low energy beta particle was multiplied by a factor of four and those for the higher energy photons multiplied by a factor of 1.5. These changes to the algorithm were tested in a spreadsheet version of the algorithm. The doses assessed in this way are those give in the column for the modified algorithm in table 1. These changes have now been incorporated into the operational version of the algorithm which has been tested by comparison with these results. 6 Discussion 6.1 Energy response (photons) Within the range measured, the normal incidence energy response is comfortably within the requirements of the international standard [7]. At energies above that of 60 Co, the effect of the phantom is less and less important. Irradiations with other similar types of OSL dosemeters, using whole body phantoms, with photons of energy up to 6 MeV demonstrate that the response up to these energies is within ± 10% of the response for 137 Cs. As noted in section 5, we can report on the radiation quality assessed from the dose calculation algorithm. For all the irradiations performed with normally incident radiation, the algorithm reported correctly whether the incident radiation was high energy (< 40 kev, PL; kev, PM, > 100 kev, PH). It is also worthy of note that, without any special treatment, the results for 137 Cs were within 2% of the exposed value. This demonstrated that we are maintaining our system to well within acceptable levels of variation. 6.2 Energy response (beta radiation) For 90 Sr/ 90 Y the results demonstrated compliance with the standard although a factor 30% low. However, for 85 Kr the relative response was lower than is required by the standard (25 % compared with a minimum of 50 %). Nevertheless, as for the normal incidence photon irradiations discussed in the previous section, the reported radiation quality was consistent with the exposures (BH for 90 Sr/ 90 Y and BL for 85 Kr). The reduced response for low energy beta radiation is presumably due to the thickness of the pouch and the open window of the case. Modifications to the algorithm have corrected for this under-response. 6.3 Isotropy (65 kev photons) The angular response of the dosemeter is within the requirements of the standard with the exception of the response at -60 o for horizontal rotation. This is consistent with other data produced for the InLight badge, and it is believed to be an inherent feature of the design. 7 Concluding remarks A development of the InLight dosemeter has been adopted as LANDAUER EUROPE s extremity wrist dosemeter. Type testing revealed that doses (Hp(0.07)) assessed using this dosemeter were 7

8 significantly low. Nevertheless, simple changes to the dose calculation algorithm have been adopted which correct this under-response. The dosemeter, with results assessed using the modified algorithm, meets the requirements of the international standard [7] with the exceptions of angular response at -60 o vertical. The tendency in this condition is for over-estimation of the dose and the dosemeter has been adopted for use for our clients requiring this service. REFERENCES [1] BOTTER-JENSEN, L., MCKEEVER, S.W.S. AND WINTLE, A.G., Optically Stimulated Luminescence Dosimetry. Published by Elsevier (2003). [2] PERKS, C.A., LEROY, G., YODER, C. AND PASSMORE, C. Development of the InLight Monitoring Service for World-wide Application. In Proc 11th IRPA Congress, Madrid, May [3] FORD, R.M, HANIFY, R.D. AND PERKS, C.A. Depletion of the signal from optically stimulated luminescence detectors. In Proc 11th IRPA Congress, Madrid, May [4] AKSELROD, M.S. AND MCKEEVER, S.W.S. A radiation dosimetry system using pulsed optically stimulated luminescence. Radiation Protection Dosimetry, 81 (3): , (1999). [5] AKSELROD, M.S., LUCAS, A.C., POLF, J.C., MCKEEVER, S.W.S., Optically Stimulated Luminescence of Al203, Radiation Measurements, 29 (3 4): , (1998). [6] MCKEEVER, S.W.S., AKSELROD, M.S., COLYOTT, L.E., AGERSNAP LARSEN, N., POLF, J.C., WHITLEY, V., Characterization of Al203 for Use in Thermally and Optically Stimulated Luminescence Dosimetry, Radiation Protection Dosimetry, 84 (1 4): , (1999). [7] INTERNATIONAL STANDARDS ORGANIZATION, Nuclear Energy Radiation protection Individual thermoluminescence dosimeters for extremities and eyes. ISO 12794, First edition [8] INTERNATIONAL STANDARDS ORGANIZATION, X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of energy part 3: Calibration of area and personal dosemeters and the measurement of their response as a function of energy and angle of incidence. ISO , First edition [9] INTERNATIONAL STANDARDS ORGANIZATION, Nuclear Energy Reference betaparticle radiation, Part 3, Calibration of area and personal dosimeters and determination of their response as a function of beta energy and angle of incidence. ISO , First edition