Gas Gain Measurements From a Negative Ion TPC X-ray Polarimeter

Abstract

Abstract-Gas-based time projection chambers (TPCs) have been shown to be highly sensitive X-ray polarimeters having excellent quantum efficiency while at the same time achieving large modulation factors. To observe polarization of the prompt X-ray emission of a Gamma-ray burst (GRB), a large area detector is needed. Diffusion of the electron cloud in a standard TPC could be prohibitive to measuring good modulation when the drift distance is large. Therefore, we propose using a negative ion TPC (NITPC) with Nitromethane (CH 3 NO 2 ) as the electron capture agent. The diffusion of negative ions is reduced over that of electrons due to the thermal coupling of the negative ions to the surrounding gas. This allows for larger area detectors as the drift distance can be increased without degrading polarimeter modulation. Negative ions also travel ~200 times slower than electrons, allowing the readout electronics to operate slower, resulting in a reduction of instrument power.To optimize the NITPC design, we have measured gas gain with SciEnergy gas electron multipliers (GEMs) in single and double GEM configurations. Each setup was tested with different gas combinations, concentrations and pressures: P10 700 Torr, Ne+CO 2 700 Torr at varying concentrations of CO 2 and Ne+CO 2 +CH 3 NO 2 700 Torr. We report gain as a function of total voltage, measured from top to bottom of the GEM stack, and as a function of drift field strength for the gas concentrations listed above. Examples of photoelectron tracks at 5.9 keV are also presented.Index Terms-X-ray polarimetry, negative ion time projection chamber, x-ray detectors.

Gaseous and dual-phase time projection chambers for imaging rare processes

“…Other TPCs that emphasize image reconstruction, in astronomy and applied science, involve x-ray polarimetry through the accurate reconstruction of the photoelectron (e.g. [413][414][415][416]), γ-ray polarimetry through pair production [77], Compton Cameras [417] and thermal neutron detectors [418]. They are particularly synergetic with many TPCs described in this review.…”

Simulation of photoelectric X-ray polarimetry and reconstruction of the photoelectron azimuthal angle

“…Therefore, we conclude that for any polarimeter that requires a drift distance comparable to or longer than 1 cm, the diffusion cannot be neglected and will greatly decrease the accuracy of the measurement and the sensitivity of the instrument. In these cases, negative ions such as nitromethane will be needed to suppress the electron diffusion [9].…”

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

“…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.…”