Perpustakaan Digital ITB

The observation of neutrino avor oscillations by Super-Kamiokande col- laboration and Sudbury Neutrino Observatory has established that neutrino is a non-zero rest mass particle. Those experiments provided the only scale of mass and upper limits on the half-life of the decay where the mass of the heaviest neutrino mass eigenstate can't be less than 0.05 eV. Up to now, no one knows what is the mass of the neutrino and it is unclear that neutrinos are Dirac particle or Majorana particle. Neutrinoless double-beta (0v) decay is a decay mode of an atomic nucleus where the two neutrons convert into two protons and two electrons. If this decay occurs, the neutrino is it's own an- tiparticle, meaning Majorana particle. The observation of 0v decay would determine whether the neutrino is a Majorana or Dirac particle and provide information on the absolute mass of the neutrino. The Advanced Mo-based Rare process Experiment (AMoRE) is an experiment using cryogenic tech- nique that aims to search the neutrino mass through 0v decay of 100Mo in 48deplCa100MoO4 crystals discoveries. AMoRE experiment requires extremely low radioactive background contributions from crystals and detector materials. To reach those goals, it is extremely important to use ultra-clean materials to intensify the detector performance following by sophisticated background re- jection technique and an excellent understanding of background contributions. However, crystals naturally can be contaminated with radioactive isotopes such as 238U, 232Th, 235U, and their daughter decay that might be produced from the chemical powder that is used in crystal growing or during the cleaning process. Additionally, the other concern about background contribution is also coming from surface contamination, impurities attached to the exposed surface of the crystal or other detector components. Some of the long-lived decay such as 238U or 232Th might be implanted inside the surface of the material. 0v is - like events, however, surface contamination can produce degraded 's making continuum energy distribution that extends to double- energies, obscuring a possible signal of 0v. Due to the short-range of particles and recoil nuclei, contamination can be originated from the crystal and the detector components that directly facing the crystal. To suppress the radioactive back- ground, understanding of background sources and complete simulation that accurately models the background-energy spectra measured by the detector are important. Simulation plays a primary role in background identication and reduction through background modeling. Properly study about the mea- sured background relies on Monte Carlo (MC) simulation to clarify the possible background sources and its contamination. In this study, eects of radioactive isotopes such as 238U, 232Th, and 235U including their daughter isotopes inside the crystal and 210Pb inside the re ector are simulated. Additionally, to reduce the presence of possible contaminants on the surface, understanding of surface contamination is also carried out. The eect of surface contamination is inves- tigated by simulating some events randomly within the depths to produce the background spectra at various surface depths. To clarify the contributions of background sources and surface contamination quantitatively, a Monte Carlo simulation is conducted by using the GEANT4 toolkit. All the background sources in the simulation are compared and tted to the measured data using log-Likelihood Fitting by constraining known activities (mBq/kg) as conser- vative upper limits and t parameters. Meanwhile, the unknown fractions of the background spectra are set as oating parameters to quantify the un- known fractions of the background compositions. The MC background spectra are well described the measured data. Through study that was conducted, a successfully understanding of the backgrounds is obtained.