HARPO, prototype of a gamma-ray polarimeter: Results of a polarised photon beam test between 1.7 and 74 MeV

Abstract

Access to the photon polarisation in the 1-100 MeV energy range is a challenge for the next generation of space telescopes. The current telescopes in space are almost blind in this energy range, mainly due to the degradation of the angular resolution of e+e-pair and due to elastic scattering in the matter. Pair-conversion detector technologies as gaseous detectors are a promising alternative to the technologies based on tungsten-converter/thin-sensitive-layer stacks such as COS-B/EGRET/Fermi-LAT, firstly to improve the single-photon angular resolution and secondly for the polarisation information. The use of a time projection chamber (TPC) as a target and a tracking detector will improve by up to one order of magnitude the single-photon angular resolution (0.5 • @100 MeV) with respect to the Fermi-LAT (5 • @100 MeV), and by up to a factor of three with respect to what can be expected for silicon detectors (1.0-1.5 • @100 MeV). With such a good angular resolution, a TPC can close the sensitivity gap at the level of 10 −6 MeV/cm 2 .s) between 3 and 300 MeV despite having e lower sensitive mass. Furthermore, this good single-track angular resolution allows us to measure the linear polarisation fraction. The HARPO (Hermetic ARgon POlarimeter) detector prototype that we built is a high pressure (0.5-4 bar) low pile-up and low-diffusion gas detector. We will present the results of its highstatistics characterisation in the 1.7-74 MeV fully-polarised and non-polarised gamma-ray beam provided by the BL01 line at NewSUBARU. The excellent value of the polarisation asymmetry dilution factor that we measured opens the possibility of having a polarimeter in space working in the MeV-GeV energy range. In conclusion, we will present the design of a balloon-flight prototype ST3G (Self-Triggered Time projection chamber as a Gamma-ray Telescope) which is being developed. We will discuss its expected performance.

Optimal measurement of charged particle momentum from multiple scattering: Bayesian analysis of filtering innovations

“…In these "active targets", the incident γ converts and the produced electrons are tracked in the same structure. The single track angular resolution of these detectors, if tracking is performed with an optimal method in the presence of multiple scattering such as a Kalman filter [2], is so good that polarimetry has been predicted to be possible despite the dilution of the polarization asymmetry induced by multiple scattering [3], and has actually been demonstrated by the characterization of a TPC prototype on beam [4,5,6]. Other projects plan to use all-silicon active targets, that is, without any additional tungsten converters [8,7].…”

ICRC Rapporteur: Space Based Gamma-ray Astronomy

“…AMEGO is a proposed MeV telescope making use of solid state detectors. STG3 is on the right [73] which is a large TPC instrument also optimized for the MeV range.…”

“…[73] presented on HARPO and ST3G (Figure 19), a high pressure time projection chamber (TPC) which would have excellent polarization sensitivity and almost 4π acceptance. They have demonstrated a prototype in a gamma-ray beam (1.74 -74 MeV) and proved the ability to measure polarization modulation.…”

A Bethe-Heitler 5D polarized photon-to-e+e-pair conversion event generator

“…This work originated from a theoretical and experimental development of low density, homogeneous detectors such as a gas time-projection chamber as a high-performance γ-ray telescope and polarimeter in the γ → e + e − regime, that is, above 1 MeV [1,2]. In contrast with past and current pair telescopes, the single-track angular resolution of such detectors can be so good that…”

“…• polarimetry has been predicted to be possible despite the dilution of the polarization asymmetry induced by multiple scattering [4] and has actually been demonstrated by the characterization of a TPC prototype on beam [6,7,2].…”