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High Negative Ion Gain MMThGEM-Micromegas Detector for Directional Dark Matter Searches

A. G. McLean, S. Higashino, R. R. Marcelo Gregorio, K. Miuchi, N. J. C. Spooner

TL;DR

The paper tackles the challenge of achieving high gas gain in low-pressure negative-ion drift gases for directional dark matter searches. It demonstrates a MMThGEM-Micromegas detector in SF6 that achieves a NI gas gain of $1.22 \\pm 0.08 \\times 10^{5}$ and enables full 2D track directionality via a total linear regression algorithm, validated with $^{241}$Am alpha tracks. The study also shows NR-like events from a $^{252}$Cf source in a CYGNUS-m$^3$ scale vessel, with ER rejection up to 99% under simulated cuts. This work supports scalable, high-gain NI readout planes for future CYGNUS searches and provides methods for NR/ER discrimination and head-tail sensitivity.

Abstract

Low pressure gaseous Negative Ion Time Projection Chambers (NITPCs) have been used previously by the DRIFT experiment to search for a directional Dark Matter (DM) signature. The main challenge with using a Negative Ion Drift (NID) gas target is the significantly lower gas gains to which they are typically limited. Recently, a MMThGEM device has been successfully demonstrated as an excellent gain stage device in the NID gas SF$_6$; capable of producing gas gains comparable with the electron drift gas CF$_4$. The next major challenge is to extend this high gain capability to multi-dimensional readout for the purpose of particle track reconstruction. The MMThGEM is therefore ideal for coupling to a strip readout detector like a Micromegas to achieve a high gain multi-dimensional Negative Ion (NI) readout plane, which is potentially suitable for the scale up required by future searches proposed by the CYGNUS consortium. In this paper, the first high gain demonstration of such a MMThGEM-Micromegas detector in low pressure SF$_6$ is described. This includes detector characterisation in a small test vessel resulting in the largest NI gas gain ever reported, 1.22 $\pm$ 0.08 $\times$ 10$^5$ , and directionality with alpha particles. Finally, this gain characterisation and tracking capability is leveraged to measure the energy and range of events, and identify those consistent with Nuclear Recoils (NRs), in a large cubic metre scale volume of SF$_6$ for the first time.

High Negative Ion Gain MMThGEM-Micromegas Detector for Directional Dark Matter Searches

TL;DR

The paper tackles the challenge of achieving high gas gain in low-pressure negative-ion drift gases for directional dark matter searches. It demonstrates a MMThGEM-Micromegas detector in SF6 that achieves a NI gas gain of and enables full 2D track directionality via a total linear regression algorithm, validated with Am alpha tracks. The study also shows NR-like events from a Cf source in a CYGNUS-m scale vessel, with ER rejection up to 99% under simulated cuts. This work supports scalable, high-gain NI readout planes for future CYGNUS searches and provides methods for NR/ER discrimination and head-tail sensitivity.

Abstract

Low pressure gaseous Negative Ion Time Projection Chambers (NITPCs) have been used previously by the DRIFT experiment to search for a directional Dark Matter (DM) signature. The main challenge with using a Negative Ion Drift (NID) gas target is the significantly lower gas gains to which they are typically limited. Recently, a MMThGEM device has been successfully demonstrated as an excellent gain stage device in the NID gas SF; capable of producing gas gains comparable with the electron drift gas CF. The next major challenge is to extend this high gain capability to multi-dimensional readout for the purpose of particle track reconstruction. The MMThGEM is therefore ideal for coupling to a strip readout detector like a Micromegas to achieve a high gain multi-dimensional Negative Ion (NI) readout plane, which is potentially suitable for the scale up required by future searches proposed by the CYGNUS consortium. In this paper, the first high gain demonstration of such a MMThGEM-Micromegas detector in low pressure SF is described. This includes detector characterisation in a small test vessel resulting in the largest NI gas gain ever reported, 1.22 0.08 10 , and directionality with alpha particles. Finally, this gain characterisation and tracking capability is leveraged to measure the energy and range of events, and identify those consistent with Nuclear Recoils (NRs), in a large cubic metre scale volume of SF for the first time.
Paper Structure (6 sections, 3 equations, 11 figures)

This paper contains 6 sections, 3 equations, 11 figures.

Table of Contents

  1. Introduction
  2. Detector Configuration and Experimental Setup
  3. Effective Detector Gas Gain with $^{55}$Fe X-rays
  4. Directionality with $^{241}$Am Alpha Particle Tracks
  5. $^{252}$Cf Nuclear Recoils in a CYGNUS-m$^3$ Scale Vessel
  6. Conclusions

Figures (11)

  • Figure 1: (a) Cross sectional diagram of the coupled MMThGEM-Micromegas detector. (b) Circuit diagram of the MMThGEM resistor chain used for HV biasing.
  • Figure 2: (a) Image of the coupled MMThGEM-Micromegas TPC assembly mounted to the door of the test vessel. (b) Image of the test vessel with brown kapton window containing the coupled MMThGEM-Micromegas TPC assembly mounted inside.
  • Figure 3: Diagram of small test vessel showing the positioning of radioactive sources around the TPC volume.
  • Figure 4: Sum total charge spectrum with the signal integral method as measured on the LG channels.
  • Figure 5: An example z-axis exposure event (left) and y-axis exposure event (right). Points above a 40 mV threshold are indicated by magenta markers and the TLR fit is indicated by a red line.
  • ...and 6 more figures