Measurement of 1.7 to 74 MeV polarised gamma rays with the HARPO TPC

TL;DR

This paper reports on the measurement of polarized gamma rays in the 1.7 to 74 MeV range using a gaseous TPC detector, demonstrating high angular resolution and polarimetry capabilities for cosmic gamma-ray observations.

Contribution

It presents the first experimental characterization of a TPC-based gamma-ray telescope and polarimeter in the MeV-GeV range, including a demonstration with a 30 cm cubic TPC and polarized gamma-ray beam.

Findings

Successful detection of gamma rays from 1.7 to 74 MeV.

Preliminary polarisation measurements at 11 MeV.

Validation of TPC as a high-resolution gamma-ray detector.

Abstract

Current {\gamma}-ray telescopes based on photon conversions to electron-positron pairs, such as Fermi, use tungsten converters. They suffer of limited angular resolution at low energies, and their sensitivity drops below 1 GeV. The low multiple scattering in a gaseous detector gives access to higher angular resolution in the MeV-GeV range, and to the linear polarisation of the photons through the azimuthal angle of the electron-positron pair. HARPO is an R&D program to characterise the operation of a TPC (Time Projection Chamber) as a high angular-resolution and sensitivity telescope and polarimeter for {\gamma} rays from cosmic sources. It represents a first step towards a future space instrument. A 30 cm cubic TPC demonstrator was built, and filled with 2 bar argon-based gas. It was put in a polarised {\gamma}-ray beam at the NewSUBARU accelerator in Japan in November 2014. Data were taken at different photon energies from 1.7 MeV to 74 MeV, and with different polarisation configurations. The electronics setup is described, with an emphasis on the trigger system. The event reconstruction algorithm is quickly described, and preliminary measurements of the polarisation of 11 MeVphotons are shown.