The PICOLON (Pure Inorganic Crystal Observatory for Low energy Neutr(al)ino) experiment is searching for WIMP dark matter with ultra-pure NaI(Tl) crystals at the underground laboratory in Kamioka observatory. Here, the quenching factor (QF) is the scintillation light yield ratio of nuclear recoil and electron recoil at the same energy deposit, and the QF of the NaI(Tl) is required to determine the WIMP sensitivity.
To measure QF, we irradiated NaI(Tl) scintillator with 2.45-MeV monochromatic neutrons produced by the deuteron-deuteron fusion. Scattered neutrons from NaI(Tl) were detected with liquid scintillator modules placed at scattering angles of 30, 45, and 60 degree. By removing the backgrounds with the techniques of pulse shape discrimination (PSD) and time of flight (TOF), nuclear recoil spectrum depending on the scattering angle was determined.
In this paper, we report QFs calculated from the nuclear recoil spectrum and the Geant4 simulation, comparison with the results of other groups, and prospects.