Light induced fading in the OSL response of Al 2 O 3 :C L. Benevides a , A. Romanyukha a , F. Hull a , M. Duffy a , S. Voss a , M. Moscovitch b, * a The US Naval Dosimetry Center, Bethesda, MD 20889, USA b Department of Radiation Medicine, Georgetown University Medical Center, 3970 Reservoir Road NW, WA 20057, USA article info Article history: Received 15 August 2009 Received in revised form 2 December 2009 Accepted 13 December 2009 Keywords: OSL Optically stimulated luminescence Al 2 O 3 :C Light induced fading Dosimetry abstract We are reporting the results of an investigation designed to determine the magnitude of the light induced fading associated with the OSL response of Al 2 O 3 :C. Unlike previous studies where bare, radi- ation sensitive OSL elements were exposed directly to light, most of the experiments described here were conducted using sealed commercially available OSL dosimeters. During light exposure the OSL sensitive elements were kept inside a standard commercially available plastic badge. A commercial OSL system was used for these experiments in an attempt to simulate typical field use conditions. Both light induced signal and light induced fading were considered, however no measurable light induced signal could be identified. Light induced fading effects, however, were significant, up to 55% loss of OSL signal following daylight exposure of 45 days. The possibility that dose information may be easily erased, intentionally or accidentally, could impose significant restrictions on the ability of the US Navy to defend, if needed, the reported personnel dose levels. Ó 2010 Published by Elsevier Ltd. 1. Introduction Aluminum-oxide doped with carbon (Al 2 O 3 :C) was initially introduced as a high sensitivity thermoluminescent (TL) material (Akselrod et al., 1990). Many had hoped that this promising new TL material would have a significant impact on a variety of radiation dosimetry applications including personal and environmental monitoring. Unfortunately, it was soon discovered that Al 2 O 3 :C responds to visible light in a way that makes it unsuitable for use as a TLD material, in addition it was also very dependent on the heating profile. The concern was that information might be lost as a result of light induced fading or alternatively, light could induce TL signal and lead to false-positives or ‘‘over-reporting’’ of dose information (Moscovitch et al., 1993). This disadvantage of Al 2 O 3 :C as a TL dosimeter created an opportunity to use this material as an optically stimulated luminescence (OSL) dosimeter (McKeever et al.,1996). At about the same time, work on the development of cooled optically stimulated dosimetry (COSL) in plastic materials was done at Pacific Northwest National Laboratory in the US (Miller and Yoder, 1996). The same mechanism that led to the light induced fading in the ‘‘TL mode,’’ in Al 2 O 3 :C became the basis for the readout in the ‘‘OSL mode.’’ More recently, concern was expressed that by switching to ‘‘OSL mode’’ the light sensitivity remains an issue; any opening in the badge, even microscopic, can result in the total loss of dose information (Mckeever and Moscovitch, 2003). Recently, Gronchi et al. (2008) reported significant light induced OSL fading using Al 2 O 3 :C, where the sensitive elements were exposed directly to light. No systematic study, however, was conducted to determine the degree of light induced fading for a typical fully assembled commercial OSL dosimeter. For this type of dosimeter, it is reason- able to assume that one of the design goals was to provide complete protection of the OSL elements from exposure to light during use, handling and processing. The US Navy is currently evaluating commercially available OSL dosimeters based on Al 2 O 3 :C for potential use in variety of applica- tions including medical, environmental and personnel dosimetry. Due to the significant impact that light induced fading may have on the ability of a dosimetry system to report accurate dose values, a systematic study was undertaken to determine the severity of this effect. In the following sections we report the results of this study. 2. Materials and methods An ensemble of Landauer InLightÔ personnel dosimetry badges (Bøtter-Jensen et al., 2003) was used in this study. Each dosimeter is composed of three parts: (a) an ‘‘OSL card’’, which is basically a plastic carrier of four bare Al 2 O 3 :C OSL elements; (b) a holder that contains four radiation modifying filters, and is designed to contain the ‘‘OSL card’’ in a light-tight environment, and (c) a hanger made of clear plastic, which contains the holder and is designed to be attached to the body of the wearer (Fig. 1). Two of the four filters are made of copper and of tin, respectively – these are the only metal * Corresponding author. Tel.: þ1 202 687 8993. E-mail address: moscovim@georgetown.edu (M. Moscovitch). Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas 1350-4487/$ – see front matter Ó 2010 Published by Elsevier Ltd. doi:10.1016/j.radmeas.2009.12.018 Radiation Measurements 45 (2010) 523–526