[Français] , [日本語] , [中文] .

Ph D. topic 2017-2020 (our page : pdf , on adum at CNES ) CNES thesis page , PHENIICS doctoral school )

The HARPO project

The HARPO project (Hermetic ARgon POlarimeter) [ref] is a collaboration of groups from LLR ( Ecole Polytechnique & CNRS / IN2P3 ) and IRFU ( CEA Saclay ) that studies the "thin" detector concept in order to perform high performance γ-ray astronomy and polarimetry of cosmic sources. HARPO will enable us to perform gamma-ray astronomy in the MeV - GeV energy range thus bridging the sensitivity gap between Compton and pair telescopes. It will offer an improvement in angular resolution of up to a factor of ten with respect to that of the Fermi LAT. This developement has been possible thanks to successive fundings by CNRS/IN2P3, LabEx PI2O, and the French ANR.

Science case

The measurement of the degree of linear polarization of the radiation from cosmic sources in the range from microwave to X-ray energies has proven to be a powerful diagnostic for understanding their nature. Unfortunately, no polarimeter sensitive above 1 MeV has ever been sent to space thus leaving the polarimetry of the gamma-ray emission of cosmic sources an unexplored domain. Gamma-ray polarimetry would enable us to probe emission mechanisms and to distinguish between different theoretical emission models for many prominent source classes, for example blazars [ref], pulsars and gamma-ray bursts.

In addition to polarimetry studies, HARPO will allow us to explore the sensitivity gap [ref] that exists between the hard-X/soft-gamma energy range (0.1 - a few MeV), for which Compton telescopes are highly sensitive, and the high-energy gamma-ray energy range (1 - 300 GeV), in which pair-creation telescopes are highly efficient. This intermediate energy range remains a largely unexplored regime and thus the emission from astrophysical sources is not well understood at these wavelengths.

The strong degradation of the angular resolution at low photon energies for current and past gamma-ray telescopes makes the assignment of a photon to a source in crowded regions of the sky, such as the galactic centre, very difficult [ref]. The angular resolution afforded by HARPO will enable us to map such regions with unprecedented accuracy.

Realisations : "ground phase" :

The final validation of the demonstrator prior to its characterization in the beam is described at [ref].
Left : A schema of the prototype that we used for validation in beam. Right : A 4.7 MeV photon obtained from inverse Compton scattering of a Erbium laser pulse on a 0.6 GeV electron beam converts into an e+e- pair in the 2.1 bar Argon:Isobutane gas of the HARPO TPC. The 2 "maps" of the signal collected by the orthogonal x and y series of strips are shown as a function of the drift duration t of the ionization electrons.

Characterization in beam, Nov 2014

The HARPO demonstrator has been exposed to a pseudo-monochromatic beam of highly (linearly) polarized gamma rays provided by the BL01 beam line of the NewSUBARU electron storage ring, which is operated by the Laboratory of Advanced Science and Technology for Industry (LASTI) of the university of the province of Hyôgo (Japan), with the collaboration of SPring8. The gamma-ray beam is produced by inverse-Compton scattering of a pulsed laser on the electron beam. The gamma energy is maximal when the photon is scattered in the "forward" direction, so monochromaticity is achieved by collimation of the gamma beam on axis. When the laser is polarized, its polarization is then transferred almost entirely to the gamma beam. With the use of a series of laser sources (Nd:YVO4 1ω and 2ω, Erbium, CO2), and by varying the electron-beam energy from 0.6 to 1.5 GeV, we were able to take data from a gamma-ray energy of 1.7 MeV up to 74 MeV. A small fraction of the data were also taken with random polarization, for systematics studies.

The detector performed well in the high-incident gamma flux over the 3 weeks of this experimental campaign, which is described at [ref]. The analysis of these data is in progress.

Publications :

Schema of a 12 m³ space module that consists of 6, 2-by-2 back-to-back, TPCs, each of them segmented into 33 cm blocs. Conversion of a 100 MeV (left) and of a 10 MeV (right) photon. [ref].

Other expts of the gamma astronomy group at LLR :