Kasperovych D. Double beta decay of 116Cd and 150Nd nuclei

The thesis describes R&D of the low background scintillation setup for small scale low counting experiments, scintillators and materials screening. Background energy spectra of CdWO4 scintillation detector in conditions without shield, and different passive copper, lead, and active shield (muon-veto counter, hardware rejection of Cherenkov signals in the quartz light-guide of the CdWO4 detector) configurations were measured. The level of background was reduced by one order of magnitude above the energy 3 MeV and by 3 orders of magnitude in the energy interval 0.5 – 2.5 MeV. The sensitivity of the setup to radioactive contamination of scintillators and different materials samples was estimated. Double beta decay of 150Nd to the first 0+ 740.5-keV excited level of 150Sm was investigated by using a deeply purified 2381-g neodymium oxide measured by four low-background HPGe detectors for 16375 h. Radioactive contamination of the sample by 40K, nuclides of uranium and thorium chains, 138La and 176Lu has been estimated. Gamma quanta with energies 334.0 keV and 406.5 keV, expected in the decay, are observed in coincidence mode giving the half-life T_(1/2)=〖4.7〗_(-1.9)^(+4.1) (stat)±0.5(syst)×〖10〗^19 y. Radioactive contamination of enriched in 116Cd to 82% 116CdWO4 crystal scintillators by uranium-thorium daughters, 40K, and 137Cs has been analysed that allowed to obtain the most accurate value of the half-life of 116Cd relatively to the 2ν2β decay to the ground state of 116Sn as T_(1⁄2)=(2.63±0.01(stat)_(-0.12)^(+0.11) (syst))×〖10〗^19 y. Radioactive contaminations of zinc tungstate crystal scintillators, produced by low-thermal-gradient Czochralski technique in various conditions, have been investigated. The total alpha activity varies between 0.16 and 1.4 mBq/kg, while the 228Th activity is limited on the level of < 0.17 μBq/kg in the best samples. The results open possibility for application of ZnWO4 crystal scintillators in low counting experiments.