Study On Scintillating Bolometer For Neutrinoless Double Beta Decay Experiments With¹⁰⁰Mo&¹¹⁶Cd Nuclides

Searching for neutrinoless double beta decay(0v??)is one of the most impor-tant research field beyond Standard Model.It is the frontier in both particle physics and nuclear physics.Almost two decades ago,the discovery of neutrino oscillation,a major achievement of particle physics,indicated that neutrinos have a non-vanishing rest mass.However,although neutrino oscillation experiments have probed the differ-ences between neutrino mass states,the absolute mass scale and mass hierarchy are still unknown.The important challenge is to determine whether neutrinos are Dirac or Ma-jorana particles.A golden channel for answering both the questions of neutrino nature and neutrino mass is neutrinoless double beta decay.Cryogenic phonon-scintillation detector with a light-heat double readout is con-sidered as one of the most promising techniques for searching for this extremely rare decay process,owing to their high detection efficiency("detector=source" approach),good energy resolution,excellent particle discrimination capability by simultaneous measurement of the heat and light channels,and low energy threshold.They also of-fer the important possibility of using compounds with a choice of different nuclei that are theoretically favorable.In addition,bolometer array can help the background-free search of the 0v?? of target nuclide to the excited states of its daughter nuclide.Low temperature detectors have been developed for about 30 years,and they are at the very beginning in China.Therefore,we proposed the use of CdMo04 crystal as the core absorber for the bolometer.CdMo04 crystal integrated with two interesting target nuclides,100Mo and 116Cd,would be a promising material to search for neutri-noless double beta decay.Both 100Mo and 116Cd have high transition energy while the y background mainly ends at 2615 keV,and reasonable natural isotopic abundance.In particular,Ningbo University can provide these molybdate crystals with good quality.Specifically,this Ph.D thesis focuses on the R&D of CdMo04 scintillating bolome-ter with both light-heat double readout for searching for neutrinoless double beta decay based on these two interesting target nuclides,100Mo and 116Cd.It includes the follow-ing two main parts:?Simulation study on the feasibility and capability of searching for 0v?? pro-cess with both 100Mo and 116Cd nuclides using a CdMo04 bolometer1.Using Monte Carlo-codes to simulate detector response of CdMo04 bolometer for electrons,gamma rays and neutrinos.The results provide the basis for the design of CdMo04 scintillating bolometer.2.We combined the DECAYO event generator and Geant4 simulation.Using the?+? coincidence and the 4? gamma veto method,most of the background from natural radionuclides is suppressed in the Range Of Interest(ROI).New limits of T1/20v??of 100Mo and 116Cd are set with one order of magnitude improvement,which indicates that the feasibility of studying both target nuclides using CdMo04 crystal.3.Using Geant4 to simulate bolometer array doing background-free search for target nuclides,100Mo and 116Cd,Ov?? to the excited states of 100Ru and 116Sn.?Experimental study on the fabrication and running of a CdMoO4 bolometer with simultaneous readout of the heat and the scintillation light1.We designed and built the platform for characterizing CdMoO4 crystal exper-imentally with wide temperature range(10-300 K),especially the fluorescence yields and decay times.The results indicate that CdMo04 crystal would be a promising ma-terial as an absorber due to the high light yield in very low temperature.2.A large cylindrical cadmium molybdate crystal with natural isotopic abundance has been used to fabricate a scintillating bolometer.The CdMo04 crystal produced at Ningbo University has a mass and size of,respectively,134.1 g and ?25 x 45 mm.3.We tested the CdMo04 crystal for about 12 h as a scintillating bolometer,ex-ploiting heat-light dual readout at 25 mK in an above-ground cryostat at CSNSM(Orsay,France).We finished doing the I-V curve and Signal-Noise-Ratio test for each channel relying on the low temperature electronics,and recorded the streamed data for physics data analyses.4.The CdMo04 detector exhibits a high energy resolution with FWHM from 5 keV to 13 keV in the energy range 0.2-2.6 MeV.The measurement demonstrated ex-cellent detector performance in terms of energy resolution and a versus ply separation power.Thanks to the simultaneous readout of heat and light,the bolometer shows a discrimination power as 21 in the 2.5-5.6 MeV region.The observed radioactive con-tamination in U/Th chains from CdMo04 crystal is in a level of a few mBq/kg.5.The preliminary results show that CdMo04 crystal integrated with two interest-ing target nuclides,100Mo and 116Cd,would be a promising material to search for Ov??process using a heat-scintillation bolometer.