High-angular-resolution and high-sensitivity, gamma-ray detector for precision gamma-ray astronomy and polarimetry in the MeV - GeV energy range – HARPO

A TPC is a volume of matter (here : gas) immersed into a (here homogeneous) electric field, in which the «tracks« produced by the photon conversion, that is the trajectories of the electron and of the positron ionise the gas. The so produced electrons drift along the field and are amplified, collected and time sampled by the dedicated electronics.
A segmentation of the anode provides a 2D map along the two coordinates transverse to the field x and y, and the drift duration provides an estimate of the 3rd coordinate z. At a reasonable cost in terms of electronics one obtains a fine 3D image of the event.

We have built a prototype and we have characterized its tracking properties with single tracks (cosmic rays) in the laboratory.

We have then moved the prototype to a fully polarised gamma-ray beam produced by inverse Compton scattering of a laser beam on the high energy (GeV) electron beam of the NewSUBARU storage ring, which is operated by the LASTI (U. of Hyôgo, Japan). By a variation of the laser wavelength and of the electron beam energy we were able to cover the gamma energy range 1.7 - 74 MeV.

Besides, we have written an event generator that samples the full 5D probability density function, which is exact down from threshold, and polarized. We have also developped a simulation of the experiment, the high-energy physics part of which is performed by Geant4, and the TPC part is by our private software

We have operated the apparatus with the same gas fill in a sealed mode, without any recirculation/purification. Constant monitoring showed a small gas degradation over the three weeks of the data taking.

As the micro pattern amplifiiers (gas electron amplifier GEM and micromegas) are known to be prone to sparking, we monitored these and showed that their rate doesn't suffer any increase in beam.

We were able to log in events at an average rate of 50 Hz, inducing a small deadtime give the digitization rate of 1.5 ms, and this despite the high incoming rate of background events of several kHz, thanks to a dedicated trigger system.
We have logged mode than 60 million events, more than half of which are due to gamma conversion in the TPC gas.

Most of these data have been taken with an almost fully polarised gamma beam. We have rotated the detector around the beam axis on a regular basis so as to get insight to possible systematic biases induced by the non-cylindrical symmetry of the detector. For the same reason, a small fraction of the data were taken with a depolarized beam.

Data analysis is in progress.

9 conference presentations.
links at llr.in2p3.fr/~dbernard/polar/harpo-t-p.html
2 main ones :
- final prototype qualification before transportation to the beam facility
inspirehep.net/record/1301206
- the data taking
inspirehep.net/record/1351887
and a couple of posters at conferences.

We are requesting the funding of an R&D program aiming at characterizing the use of an existing technology in high-energy physics (HEP), a time projection chamber (TPC), for a new application, the high-precision detection of high-energy photons, and in particular of their polarization fraction.
The ultimate goal, on a longer term, is the launch of such a detector in space to study gamma photons from cosmic sources.

We are developing a novel concept for a gamma-ray detector based on a TPC, which will be:

- the first polarimeter for cosmic gamma rays in the energy range MeV-GeV, such as those emitted by active galaxy nuclei (AGN), gamma-ray bursts (GRB) and pulsars. Polarimetry is performed by the analysis of the angular properties of triplet conversion events (gamma e- -> e+e- e-), that are reconstructed in the TPC.

- a telescope with an angular resolution improvement of one order of magnitude over previous telescopes in this energy range. This instrument allows to fill the sensitivity gap between energy ranges in which the Compton telescopes (0.01 - 5 MeV) and telescopes using pair conversion in a high-Z converter (> 0.1 GeV) are most sensitive.

- A dead-time-free GRB detector, since it is based on a TPC.

The realization of the proposed ground-based characterization is a prerequisite for a future spatial mission.
In addition to the characterization of a prototype exposed to a linearly polarized gamma-ray beam from an accelerator, two important deliverables will be:
- the first experimental validation of polarization asymmetries at low energy, the knowledge of which is needed for polarized gamma-ray astronomy but also to validate experimentally a number of assumptions made in the theoretical computations based on QED.
- the implementation of an exact Monte Carlo (MC) generator of conversion events in the HEP simulation software Geant4, i.e. a generation of the full 5D probability density function (PDF).