Measurements of the ionization efficiency of protons in methane

Abstract

The amount of energy released by a nuclear recoil ionizing the atoms of the active volume of detection appears "quenched" compared to an electron of the same kinetic energy. This different behavior in ionization between electrons and nuclei is described by the Ionization Quenching Factor (IQF) and it plays a crucial role in direct dark matter searches. For low kinetic energies (below 50 keV), IQF measurements deviate significantly from theoretical predictions and simulations. We report measurements of the IQF for protons with kinetic energies in between 2 keV and 13 keV in 100 mbar of methane. We used the Comimac facility in order to produce the motion of nuclei and electrons of controlled kinetic energy in the active volume, and a NEWS-G SPC to measure the deposited energy. The Comimac electrons are used as reference to calibrate the detector with 7 energy points. A detailed study of systematic effects led to the final results well fitted by IQF (E K ) = E α K / (β + E α K ) with α = 0.69 ± 0.08 and β = 1.31 ± 0.17. In agreement with some previous works in other gas mixtures, we measured less ionizaa

Directionality and head-tail recognition in the keV-range with the MIMAC detector by deconvolution of the ionic signal

“…However, we have previously shown large discrepancies between IQF measurements and simulations below 50 keV [45,46] in low-pressure gas mixtures. For this reason, we have performed a measurement of the proton IQF in our gas mixture, following the procedure described in [47]. The direct comparison of the ionization energy of protons and electrons (of the same kinetic energy) sent by Comimac gives access to the protons IQF.…”