(Budker Institute of Nuclear Physics SB RAS)
Direct search for dark matter particles (WIMP - Weakly Interacting Massive Particle) is
produced in the detectors of recoil nuclei through observation of WIMP elastic scattering on
atomic nuclei of detector matter. The current situation with the observation signals from dark
matter particles is quite confusing. Some authors believe that one of the reasons for these
differences among different experiments is the problem of reliable calibration of energy scales
for recoil nuclei. In this regard, particularly relevant is the problem of calibration of detectors
for dark matter, and low-energy neutrinos, especially in the field of low energy of the recoil
nuclei - less than 10 keV.
In Budker Institute of Nuclear Physics we measured ionization yields of nuclear recoils in liquid argon using a two-phase Cryogenic Avalanche Detector (CRAD) with electroluminescence (EL) gap, operated in argon doped with a minor (49 ± 7 ppm) admixture of nitrogen at 87 K and 1.00 atm. The EL gap was optically read out using cryogenic PMTs located on the perimeter of the gap. We used DD generator to produce neutrons with 2.45 MeV energy.
The ionization yield in liquid Ar at an electric field of 0.56 kV/cm and 0.62 kV/cm amounted to 5.87±0.85 and 7.38±1.04 e−/keV at 233 keV, respectively. The number of electrons escaping recombination with positive ions, which we used for calibrations, gives the dominant error. The Jaffe model for nuclear recoil-induced ionization, in contrast to that of Thomas-Imel, describe the energy dependence of the ionization yield in range from 0.5 to 2.5 kV/cm.