Linac activation of radioisotopes and underground gammaspectrometric analyses

Abstract

After irradiating various medical linac parts with photon beams, we performed underground gamma-ray spectroscopy (GRS) measurements of the samples only a few minutes after the irradiations, in order to observe activated short-lived radioisotopes. Three samples were used: an Elekta Synergy flattening filter never irradiated before, an old flattening filter, unmounted from a linac in 2012 and a tungsten leaf, unmounted from the multileaf collimator (MLC) of the same linac in 2012. All three samples were measured before the irradiations in the underground laboratory of la Vue-des-Alpes. The new flattening filter and the leaf were then irradiated with ~200 Gy, first under 15MV and then with 6MV photon beams. The gamma counting began 20 minutes after the irradiations. Irradiations with 15MV led to (n,{\gamma}) activation of short-lived isotopes : In the leaf, the measured activity was 1556 Bq just after the irradiation and we observed various gamma lines from 187W, 57Ni and 56Mn. In the new filter, the activity was 1097 Bq and the gamma signature of 56Mn, 56Ni, 57Ni and 59Co was clearly present. The germanium detector allowed to measure gamma lines with relative intensities (branching ratio BR) down to ~0.1%, thank to its very low background. Irradiations of the new flattening filter at 6MV led to a small activation of 56Mn. Gammaspectroscopic data was taken several times after the irradiation to monitor the time evolution and the total activity. Treating patients with 15MV photon beams activate long-lived radioisotopes in the linac head. With this work, it was possible to identify these isotopes, and especially highlight the created short-lived radioisotopes.