Track etch velocity study in a radon detector (LR 115, cellulose nitrate)

For radon dosimetry, complete knowledge of the detection area of the detector used for alpha particles emitted by radon and its two daughters (Po 218 and Po 214) is compulsory. Latent tracks in the detector induced by alpha particles are enlarged in a hot hydroxide sodium bath to make them observable by microscopy. This paper deals with the determination of the track etch velocity (V T). We compared two different approaches to calculate the V T: for the first one, the V T in considered as a function of the alpha particle energy. The V T is determined by using a track etch model based on the experimental study of the track diameters for normal incidence versus alpha particle energies. For the second approach, we assumed that the V T was directly correlated with the ionisation rate (I) created all along the alpha particle range in the detector. This ionisation rate, which depends directly on the alpha particle energy was calculated using the TRIm programme. The knowledge of the V T enabled us to determine the critical incidence angle for each energy under which the alpha particle did not induce an observable track. Results for each approach were compared with the experimental data obtained with a specific irradiator system. Finally, track detection efficiencies of LR 115 for radon and their standard deviations are proposed for a simple configuration.