Study of Negative-Ion TPC Using μ-PIC for Directional Dark Matter

. Negative-ion time projection chambers(TPCs) have been studied for low-rate and high-resolution applications such as dark matter search experiments. Recently, a full volume ﬁducialization in a self-triggering TPC was realized. This innovative technology demonstrated a signiﬁcant reduction in the background with MWPC-TPCs. We studied negative-ion TPC using the µ -PIC + GEM system and obtained su ﬃ cient gas gain with CS 2 gas and SF 6 gas at low pressures. We expect an improvement in detector sensitivity and angular resolution with better electronics.


Introduction
Weakly interacting massive particles(WIMPs) are one of the candidates for dark matter. Among the various direct search experiments, the detection of nuclear recoil tracks is thought to provide strong evidence of dark matter detection. Gas detectors are among the possible nuclear track detection methods, and several groups have developed detectors and performed underground directionsensitive dark matter searches [1][2][3][4][5].
Since the expected event rate is extremely low, a dark matter detector needs to be large(cubic-meter scale). Therefore, the electron diffusions during drift are large compared with the track lengths of the recoil nuclei(up to several mm) in conventional TPCs. Since spatial and angular resolutions are deteriorated by diffusion, the drift length is limited to several tens of cm with electron-drift detectors [2]. Transverse diffusion could be suppressed using a strong magnetic field parallel to the drift field but it would increase the cost and complexity of the system.
Negative-ion TPCs have been studied for low-rate and high-resolution applications [6]. The advantage of negative-ion TPCs is that the ion diffusions can be strongly suppressed at low pressures and without a magnetic field. The DRIFT (Directional Recoil Identification from Tracks) group has pioneered the study of negative-ion TPCs and demonstrated a strong potential for fine tracking performance [7]. Recently a discovery of "minority carriers" in CS 2 gas broadened its potential, and the measurement of absolute Z-position in self-triggering TPCs became possible. Minority carriers appeared after adding a few percent O 2 to the original gas. Although the mechanism is not completely understood, several types of negative ions are thought to drift each with slightly different a e-mail: tomonori@stu.kobe-u.ac.jp velocities. Using this technique, DRIFT was able to significantly improve their detector sensitivity [8].
Electron capture, ion transport and electron release in the gas amplification process need to occur in negative ion TPC. While forming negative ions has been well studied theoretically and experimentally, the detachment process is less well understood [9]. Negative ion gases that give sufficient gas gains are very limited; carbon disulfide(CS 2 ) and nitromethane(CH 3 NO 2 ) are one of the few possible choices [10]. These gases are not very safe and are not easy to use in an underground laboratory.
A safe negative-ion gas has long been sought as a TPC gas, and recently a GEM-TPC with SF 6 negative ion drift gas was reported [11]. In addition, they observed minority carriers without any additional gas. Unlike CS 2 , SF 6 is non-toxic, and can be handled easily while retaining the same advantages as CS 2 gas. Fluorine has a large cross section for WIMP-nuclear spin-dependent interaction; therefore, it has good properties as a target. SF 6 is generally used as high voltage insulating gas. Its electron affinity is very large, approximately 1.1 eV. Therefore, we need strong electric fields for electron-detachment and gas multiplication to occur. Micro-pattern gaseous detectors(MPGDs) are known to have strong electric fields and would be a suitable device to use with SF 6 as a TPC gas.
NEWAGE (NEw generation WIMP-search with Advanced Gaseous tracking device Experiment) has been using a µ-PIC based TPC for direction-sensitive dark matter search with CF 4 gas. We achieved a 90% confidential level direction-sensitive spin-dependent cross section limit of 557 pb for a WIMP mass of 200 GeV/c 2 [5]. The main background restricting the detector sensitivity is radioactive contamination in the µ-PIC. We initiated a new study to reduce the radioactive background using two approaches. The first approach is to develop a low back-  The obtained total gas gain as a function of the voltage supplied at the anode electrodes of the µ-PIC and the voltage supplied to the GEM are shown in Fig.5 and Fig.6, respectively. The highest gas gain was approximately 1800, while the require gas gain was 1000 · 76 T orr P . This means we need to improve the S/N by factor two, which could be achieved with a dedicated amplifier.
Consequently, the µ-PIC+GEM system also worked well with SF 6 . As noted in CS 2 experiments, we can expect better angular resolution because of the longer nuclear track.

Future work
We have confirmed that the µ-PIC+GEM system worked well with SF 6 and CS 2 . For CS 2 , we obtained sufficient performance; however, because of gas safety issues, we will pursue studies of negative ion µTPC with SF 6 .
As our next step, we are constructing a prototype µTPC the drift length of which is 30 cm. We will attempt to observe the minority peak and to demonstrate measurement of the absolute Z position. Furthermore, we plan to develop readout electronics for negative ion µTPC. An ASIC chip with a multi-channel slow shaping amplifier is being developed for liquid argon TPC by the KEK e-sys group. This system can be adopted to negative ion TPCs with a small modification. We will then test the detector performance. We expect better spatial and angular resolution because of the smaller diffusion. A spatial resolution of ∼0.2 cm along the Z axis is demonstrated by the DRIFT detector [8]; therefore we expect to reduce the background from the µ-PIC by two orders of magnitude by rejecting events from the µ-PIC plane. We expect an improvement of at least two orders of magnitude in the sensitivity because of full fiducialization.