Channeling effect in the CsI(Tl) used for a WIMP search

Abstract

The KIMS collaboration published a recent measurement of the WIMP-nucleon scattering with 27287.9 kg·days data. For further increases in the detector sensitivity, we study possible channeling effects in the crystal detector which are relevant to the various ions' ranges according to initial propagation directions in the crystal, and result in the modulation of light yields in the detector. From the simulation, we find that ~ 3 % of all recoil ions in isotropic motions have ranges and light yields larger than those in amorphous CsI crystals. We measure this effect with a setup, that is composed of a neutron generator, six neutron detectors, and a well characterized CsI(Tl) detector. By comparing the light yields of recoil ions moving along a symmetry axis, [110], with that from recoils moving along a random direction, we find a clue for the blocking effect which are expected to result in less light yields. Though this measurement can not determine the pulse shapes of the channeling events due to the small statistics, with a hypothesis that these events are similar to random nuclear recoil events, we reanalyze the KIMS data and conclude that the channeling effect has the effect of increasing detection sensitivity by 50 ~ 0 % for nuclear recoils from WIMPs with masses of 20 ~ 50 GeV/c2.