Luminescence dosimetry with ceramic materials for application to radiological emergencies and other incidents.

Abstract

The likelihood of the occurrence of radiological accidents which can induce significant health consequences to the members of the public has raised the importance of developing a personal radiation dosimetry system applicable to populations not monitored by dedicated dosemeters. Mobile phones are personal devices with high ubiquity and great potential for accident dosimetry applications. Alumina surface mount resistors (SMRs) are abundant in the printed circuit board of mobile phones and their potential as fortuitous dosemeters has been investigated using thermoluminescence (TL) and optically stimulated luminescence (OSL) techniques. The physical mechanism of the generation of luminescence of the alumina SMRs is, however, less known. The basic luminescence defects in SMRs were identified to be F-type centres and their emission process was shown to be temperature dependent and highly quenched at room temperature (RT). The trap environment of beta irradiated SMRs includes a series of closely spaced traps covering thermal depths between 0.9-1.4 eV; predicting an average lifetime for thermal fading at RT of ca 23 years. Trapped charges evicted by thermal or optical stimulation are likely to recombine at F-type centres and contribute to the luminescence response that is likely to be thermally assisted via the vibrational modes of the lattice. A phonon-assisted de-excitation of the trapped charge population could additionally be involved in the mechanism of athermal or anomalous fading. Based on the temperature dependence of the rate of fading, a model is presented for the anomalous fading observed where phonon-assisted and tunnelling effects alternate or operate simultaneously depending on the temperature of the material. A number of aspects related to the use of SMRs in dosimetry seem to benefit from the investigation of the physical processes, although for accurate dose reconstruction it is imperative to know the energy of the ionising radiation source and the position of the mobile phone relative to the direction of the source. For example, at low-energy exposures the dose may be over-estimated, not only due to the non-flat energy response of the alumina, but also due to the presence of several parts of the mobile phone which can increase the amount of energy deposited in alumina substrates due to backscatter effects. In addition, MCNP simulations indicated that for low-energy exposures, such as for 192Ir, differences of up to an order-of-magnitude between resistor and whole body dose are expected. Finally, to specify the most appropriate dose conversion coefficients that can be applied to estimate whole body dose from OSL / TL determinations, the knowledge of the exposure geometry is crucial.