HARPO, a Micromegas+GEM TPC for gamma polarimetry above 1 MeV P. Colas, on behalf of the HARPO Collaboration Polarimetry in Astrophysics The HARPO detector Seal mode: gas purity Beam test at New Subaru Results on polarization measurement and angular resolution Future

Astrophysics at MeV energies To understand phenomena at work in the emission of gamma-rays (AGN, blazars, GRB, pulsars…), as precise as possible measurements of -Energy -Direction -Polarization are necessary in all energy ranges. The advantage of a TPC for g-rays are that: A large volume of gas can be used as a target It is sensitive in all directions The multiple scattering of e+- is low 22/05/2017 P. Colas - HARPO

Astrophysics at MeV energies and polarization measurement In the 1-100 MeV range, the Compton cross-section becomes small. The pair creation keeps the memory of the photon direction and the decay plane gives the polarization direction. 22/05/2017 P. Colas - HARPO

HARPO : a Micromegas + GEM TPC 22/05/2017 P. Colas - HARPO

Readout electronics Based on T2K front – end (AFTER chip) and ML507 22/05/2017 P. Colas - HARPO

PCB architecture Power consumption very limited in space: choose a 2x1D structure to limit the number of channels. Use time measuremt to resolve ambiguities. 22/05/2017 P. Colas - HARPO

Decision was made to add a GEM stage on top of the Micromegas, to obtain a comfortable gain (x7) while keeping good stability Gas amplification 55 Fe 22/05/2017 P. Colas - HARPO

22/05/2017 P. Colas - HARPO

HARPO test in a polarized g-ray beam at New Subaru (1.74 to 74 MeV) 100% linearly polarized g by back-scattering on an electron beam Event reconstruction : use time and charge correlations to associate hits to a track Search the vertex to fully reconstruct the tracks and measure the angles 22/05/2017 P. Colas - HARPO

4 beam test events at 2.1 bar 22/05/2017 P. Colas - HARPO

Measurement method 22/05/2017 P. Colas - HARPO

Polarization sensitivity 22/05/2017 P. Colas - HARPO

Polarization sensitivity Simulated distributions for 11.8 MeV photons (isotropic photons) 22/05/2017 P. Colas - HARPO

Performance vs E Angular resolution improved by a significant factor (wrt Fermi LAT) Sensitivity to polarization in the few-MeV regions has been demonstrated (with A=10-15%) This study triggers lots of ideas for improvements. 22/05/2017 P. Colas - HARPO

Summary of the ground phase November 2014 Preparation Data taking at Hyogo Analysis The demonstration that a TPC can be used for polarimetry above 1 MeV has been done. 22/05/2017 P. Colas - HARPO

FUTURE Self-Triggering TPC Telescope for Gamma-rays (pronounce ‘Steg’) Max trigger rate ~600 Hz g conversion rate 400 Hz Compton scatt : 7 kHz Charged tracks 33 kHz/m² 22/05/2017 P. Colas - HARPO

FUTURE Altitude 35 km 64 ‘HARPO’s (32 pairs of back-to-back TPCs with a common cathode) (VERY PRELIMINARY : might evolve to 3x4x5 modules) 2 bar Argon New readout chip : ASTRE (already prototyped) 22/05/2017 P. Colas - HARPO

Thank you! 22/05/2017 P. Colas - HARPO