1. 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

2. 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
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P. Colas - HARPO

3. Astrophysics at MeV energies and polarization measurement
In the 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.
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P. Colas - HARPO

4. HARPO : a Micromegas + GEM TPC
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P. Colas - HARPO

5. Readout electronics Based on T2K front – end (AFTER chip) and ML507
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P. Colas - HARPO

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

7. 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
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P. Colas - HARPO

8. 22/05/2017
P. Colas - HARPO

9. 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
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P. Colas - HARPO

10. 4 beam test events at 2.1 bar
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P. Colas - HARPO

11. Measurement method
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P. Colas - HARPO

12. Polarization sensitivity
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P. Colas - HARPO

13. Polarization sensitivity
Simulated distributions for 11.8 MeV photons
(isotropic photons)
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P. Colas - HARPO

14. 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.
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P. Colas - HARPO

15. 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.
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P. Colas - HARPO

16. 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
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P. Colas - HARPO

17. 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)
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P. Colas - HARPO

18. Thank you!
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P. Colas - HARPO