Ph D. topic 2017-2020 (our page :
CNES thesis page
The HARPO project
The HARPO project
(Hermetic ARgon POlarimeter)
is a collaboration of groups from
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
This developement has been possible thanks to successive fundings by
LabEx PI2O, and the French
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
different theoretical emission models for many prominent source
classes, for example blazars
pulsars and gamma-ray bursts.
In addition to polarimetry studies, HARPO will allow us to explore the
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
The angular resolution
afforded by HARPO will enable us to map such regions with
Realisations : "ground phase" :
The final validation of the
demonstrator prior to its characterization in the beam is described at
A schema of the prototype that we used for validation in beam.
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
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
We have built and validated a full (5D, either nuclear or triplet conversion)
exact (all graphs) down to threshold, polarized event generator
We have characterized the properties of a telescope based on a
thin homogeneous detector with an optimal tracking (in the presence of
track multiple scattering) and established the power laws that
describe the dominating contributions to the angular resolution and
the differential sensitivity
We have characterized the properties of a polarimeter based of
such a detector, including the dilution of the effective polarization
asymmetry due to multiple scattering, still under the same assumption
of an optimal tracking
We have designed and built a demonstrator based on a time
projection chamber (TPC) with a micromegas gas amplification
and we have validated its tracking properties using cosmic rays
As the value of the gain so measured was at the limit of a safe
routine use, we have complemented the micromegas with a pair of GEM.
We have characterized the properties of such assemblies
and we have finalized their integration into the TPC detector.
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
beam line of the
electron storage ring, which is operated by the
Laboratory of Advanced Science and Technology for Industry
of the university of the province of Hyôgo (Japan), with the collaboration of
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
The analysis of these data is in progress.
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.
Other expts of the gamma astronomy group at LLR :