Effects of light on the 'primed' state of CVD diamond nuclear detectors

Diamond radiation detectors produced by Chemical Vapour Deposition (CVD) have been extensively studied as nuclear detectors both for the application as tracking detectors in high energy physics experiments and for applications in nuclear industry domain. A meaningful measurement to characterise the performance of such devices is the evaluation of the charge collection distance (CCD), i.e. the mean distance the charge carriers travel before being trapped. It is well known that CCD increases with the absorbed dose (priming or pumping process). The priming of diamond is usually explained by the saturation of active traps in the diamond bulk, which are filled by charge carriers generated by the ionisation. However, such a primed state is metastable, i.e. the exposure to fight can de-pumps diamond to its initial state. The analysis of 'pumping' and 'de-pumping' processes is then useful to explain trapping mechanisms and polarisation effects, which limit the performances of diamond detectors. This paper deals with an investigation of the 'pumping' process on detector grade CVD diamond samples exposed to some doses of irradiation (X-rays or beta particles). Photoconductivity measurements carried out during monochromatic illumination as well as optical bleaching spectra of thermoluminescence glow curves highlight the passivation effects of some trap levels due to irradiation. A possible interpretation of these effects is presented and discussed, taking into account charge collection efficiency measurements carried out using Am-241 alpha particles. (C) 2002 Elsevier Science B.V. All rights reserved.