Measurement of the scintillation response of neutron scattering in NaI(Tl) scintillator for dark matter search

We have been searching for Weakly Interacting Massive Particles (WIMPs) using ultra high purity NaI(Tl) scintillation detector system(PICOLON) in Kamioka observatory. The PICOLON experimental system is based on NaI(Tl) scintillation crystals. It is necessary to know the scintillation efficiency for nuclear recoil signals. In order to measure nuclear recoil signal on NaI(Tl) scintillator, we have carried out experiments using neutrons from ²⁵²Cf source for search of WIMPs-induced nuclear recoil. The liquid scintillator has been used to identify signal on NaI(Tl) induced by scattered neutrons. The liquid scintillator was located at 50 cm away from NaI(Tl) scintillator in the 90 degree direction. The difference between the signal start times of NaI(Tl) scintillator and liquid scintillator was measured as TOF (Time Of Flight). The signal of NaI(Tl) scintillator having TOF > 12 ns was evaluated as neutron. Nuclear recoil signals are different for electron recoil signals in NaI(Tl) scintillation detector. An average arrival time was calculated nuclear recoil and electron decay under the condition of TOF>12 ns. The energy spectrum of elastic scattering and inelastic scattering were obtained from the difference of average arrival time. Quenching factor was calculated using the obtained spectrum for elastic scattering by fitting to elastic scattering simulation spectrum that was obtained from the Geant4. The obtained values of Na and I with Quenching factors in the region of 5 - 15 keV_ee were 0.20 ± 0.07 and 0.11 ± 0.02, respectively. (author)