Gamma-spectrometry and residual nuclide analysis of pion irradiated PbWO4 crystals

The electromagnetic calorimeter of the CMS detector at CERN, Geneva will consist of PbWO₄ crystals and be exposed to a hadron flux of 10⁶ cm^-2·s^-1, mostly pions, during its operation. We have used FLUKA and DETRA codes for advance prediction of the activation of the detector. To assess the accuracy of these calculations, a small PbWO₄ crystal was irradiated in a 345 MeV/c pion beam of the PSI to a fluence of 1.6·10¹² cm^-2. The resulting activation was measured using an HPGe-detector after cooling times varying from a few minutes to 14 months. The spectra were analyzed using the SAMPO 90 code for peak search and area determination and the SHAMAN code for radionuclide identification and quantification. The spectra were extremely complex and the first ones measured not useful due to violent peak overlap and pile-up. The number of found peaks in the spectra we analyzed varied between 841 and 128 peaks depending on the cooling time. The corresponding number of nuclides identified per spectrum varied between 116 and 15. The comparisons between the predicted time-development of the nuclide composition by FLUKA/DETRA and the analyzed results show that the activities of nuclides agree excellently for the most important nuclides and very well even for the less abundant ones. The total dose rate in the vicinity of the activated crystals, including its time dependence, is very well reproduced by the FLUKA/DETRA calculations.