Accident dosimetry with OSL on mobile phone resistors

Introduction

In case of a radiological incident the possibility exists that members of the public will receive very high doses of ionizing radiation and might need urgent medical treatment to survive. Examples of such situations are accidents with orphan sources or industrial sources, or malevolent actions with a silent source or a dirty bomb. It is then very important to perform a fast triage of potentially exposed persons in order to distinguish between the ‘worried well’, the persons needing medical follow-up and the persons needing urgent medical treatment for survival. As members of the public do not wear personal dosimeters, it is not straightforward to perform a dose assessment. Fortunately, there are different biological and physical retrospective dosimetry techniques available to perform a personal dose assessment in absence of a personal dosimeter. 

One of the most well-established physical retrospective dosimetry techniques for triage after a radiological incident involving external exposure to gamma radiation is optically stimulated luminescence (OSL) on the alumina (Al2O3) substrates of surface mounted resistors from mobile phone circuit boards. In comparison with other techniques, OSL on mobile phone resistors exhibits beneficial dosimetric characteristics such as a high sensitivity, low zero dose signal, good signal reproducibility, a linear dose response and limited energy dependence. Furthermore, although signal fading is an important issue, it is possible to correct for fading with an acceptable uncertainty by using a universal fading curve. Also, although intuitively one might expect significant variation in dosimetric properties between resistors from different manufacturers and batches, it seems that in general the dosimetric properties observed in different studies are very reproducible. The great potential of this technique was also demonstrated extensively during multiple laboratory intercomparisons and field tests organized within Eurados Working Group 10 (WG10) on Retrospective Dosimetry.

For any personal dosimetry technique, and thus also for retrospective dosimetry after a radiological incident, it is of fundamental importance to assess the complete uncertainty budget associated with a dose assessment. Based on the uncertainty budget one should then also determine the decision threshold and the detection limit of the technique. Currently SCK CEN is leading a task within Eurados WG10 with as goal to optimize and harmonize the uncertainty assessment for physical retrospective dosimetry techniques amongst all labs collaborating in WG10. Within this task it was decided to first compare the different potential uncertainty assessment techniques (GUM propagation of uncertainties, GUM Monte Carlo, Bayesian) by testing these methods on the results from previous WG10 intercomparisons. It was also decided that eventually both the dose assessment and the uncertainty assessment will be implemented in the Biotools software developed for biological retrospective dosimetry. In this way WG10 will have a single tool for all dose and uncertainty assessments. The current focus of this task is on optically stimulated luminescence (OSL) on mobile phone resistors, as this is the best established protocol at the moment. Several subtasks have been started amongst the relevant laboratories related to this:

1) Re-evaluation of the photomultiplier tube dark count rates in the different labs
2) Common calibration of the internal calibration irradiation source of the readers in the different labs
3) Signal fading study investigating effects of temperature, type of resistor and type of reader 

Objectives

In this work the student will focus on subtask 3 about signal fading. Several resistor types are available at SCK CEN to be tested. At SCK CEN there are also possibilities to store the resistors at different temperatures. Signal fading will be studied using the Risoe reader dedicated to retrospective dosimetry with variation of resistor type, storage temperature and delay time. Together with the data from the other labs, this will allow to better correct for the signal fading and to better assess the uncertainty related to this effect. The work will involve preparing and executing the experiments, analyzing the data and reporting the results. In the end this work will form an important contribution to the ongoing work within WG10 and will help in implementing the dose and uncertainty assessment for OSL on mobile phone resistors in the Biotools software and it is also planned that this work will be part of a peer reviewed scientific article. This will also be of great value for radiological emergency prepardness in Belgium.