Direction-sensitive direct dark matter search experiments have been conducted using gaseous detectors. In spite of the long history of the study on the energy deposition of charged particles in materials, a full agreement between the measured results and theoretical predictions, especially in a low energy scale, are yet to be achieved. It is thus important to experimentally measure the ionization yields of recoil nuclei for the experiments with gaseous detectors using an ionization charge readout scheme. This study measured the ionization yield using a low-energy ion beam facility at Kanagawa University. The ionization yields for fluorine ions with an energy range of 5 $\sim$ 50 keV were measured using a dedicated proportional wire chamber filled with $\mathrm{CF}_{4}$ gas at 0.06 atm. The low-energy ion injection scheme into a gaseous detector was established and the ionization yield for fluorine ions was obtained to be 0.45 at 30 keV with a moderate dependence on the ion energy.
Direct searches of dark matter in a form of Weakly Interacting Massive Particles (WIMPs) with its directionality have been conducted in the world. The major strategy of the directional search is to use the track reconstruction of a nuclear recoil caused by WIMPs. Since the energy scale of nuclear recoils is at most tens of keV, their trajectories are very short. Experiments such as NEWAGE [1], DRIFT [2] and DM-TPC [3] have conducted direct dark matter searches using low pressure gaseous Time Projection Chambers (TPCs) by measuring the trajectory of nuclear recoils. Particularly, gases containing fluorine nucleus (e.g. CF 4 , CHF 3 , and SF 6 ) are widely used for spin-dependent dark matter searches with directionality lead by NEWAGE, DM-TPC, MIMAC [4], and CYGNO [5] experiments.
Along with the improvements of short-track detection technologies, studies of the detector response for low energy nuclear recoils with ∼ keV scale are also essential. In spite of the long history of the study on the energy deposition of charged particles in materials, a full agreement between the measured results and theoretical predictions, especially in the low energy scale, are yet to be achieved. One of the difficulties is that there are various methods to measure the amount of energy deposition, depending on detectors. Several direction-sensitive dark matter experiments adopt ionization charge readout. Low-energy nuclear recoils in the gas show high stopping power and are influenced by the nuclear stopping in addition to the ionization. Thus the energy deposition is a combination of complex processes including the production of excitons, multiple vibrational modes, ion-electron recombination, and thermalization. Since the yield of ionization depends on incident particles, materials, and their conditions, parameters are complicated for the calculation models such as the Lindhard theory [6] and the SRIM [7] calculation, which are widely used. In case of dark matter experiments, usually energy calibrations are carried out using mono-energy radioisotope sources (gamma or alpha rays) or X-ray generators although the ionization yields in these processes are not the same as those for the nuclear recoils due to the difference of particle type. Therefore, it is important to measure the ionization yield of the nuclear recoils in each case.
Injecting ion beams directly into the gas is a powerful method for evaluating the ionization yield through direct measurements. For example, the COMIMAC facility [8] established a low energy ion and electron beam injection scheme into a gas detector with an interface of a 1 𝜇m hole between the beamline and the gaseous chamber. In this work, we performed an ionization yield measurement injecting ion beams of known energies into a gaseous detector using an ion implantation system with an acceleration voltage of up to 200 kV at Kanagawa University in Japan. We updated the ion implantation interface to adopt ion beam injection to a gas detector and established a low-energy ion beam injection scheme at Kanagawa University in Japan. An ionization yield measurement for fluorine ions in low pressure CF 4 gas was performed with a dedicated proportional wire chamber.
Section 2 describes the setup of the ion beamline and the gaseous detector. The result of the measurement is denoted in Section 3 and Section 4 concludes this study.
The experimental setup for the measurement of the ionization yield for the ion beams with various energies is described in this section. Section 2.1 describes the accelerator at Kanagawa University. The mechanism separating the high-vacuum beamline from the gas volume of the detector is also described in this section. Section 2.2 introduces the gaseous detector for this experiment.
Figure 1 shows a schematic overview of the low-energy accelerator located at Kanagawa University. A Freeman-type ion source is employed as the ion source of the accelerator, in which the source gas is arc-discharged in an arc-chamber to stably form plasma using a hot filament and a magnetic field. The generated ion species depend on the type of source gas; there have been successes in generating light ions to heavy ions (e.g. H + , He + , B + , C + , N + , O + , F + , … Xe + ). Since fluorine nucleon recoil is assumed in various direction-sensitive dark matter search experiments, we selected F + ion beam in this study. The F + ions are produced by decomposition of a SiF 4 source gas in the arc chamber. The generated ions are extracted with a typical voltage of +30 kV and then an analyzing magnet selects the desired F + ions. The selected ions are injected in an acceleration tube with applied voltages ranging from -25 kV to +170 kV with a Cockcroft-Walton (CW) generator. The beam energy is determined by the acceleration voltage and can therefore be tuned from 5 keV to 200 keV for monovalent ions. The accelerated beam is shaped by a three-stage electrostatic quadrupole (Q-) lens system installed on the
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