We present the C++ implementation of an event generator for linearly-polarized gamma-ray conversions to γ→e+e− pairs. The generator uses the full 5D Bethe–Heitler probability density function for the ...generation of the γ→e+e− pair and for the recoil of the target nucleus. The same code is used for triplet generation. We present a verification of the model by comparing sample distributions with theoretical predictions.
•The C++ Geant4 Gamma Conversion Model was available.•The exact 5D Bethe–Heitler event generator.•Detailed kinematic of gamma conversion process.•The recoil of the target nucleus generation.•The same code is used for triplet generation.
HARPO is a time projection chamber (TPC) demonstrator of a gamma-ray telescope and polarimeter in the MeV-GeV range, for a future space mission. We present the evolution of the TPC performance over a ...five month sealed-mode operation, by the analysis of cosmic-ray data, followed by the fast and complete recovery of the initial gas properties using a lightweight gas circulation and purification system.
We presented in 2014 the very first data from a polarised gamma-ray beam between 1.7 and 74MeV. We now show the results of their analysis, and in particular the polarimetry measurements. With these ...results, we are establishing a new, high-performance way to do gamma-ray astronomy and, for the first time, polarimetry, in the e+e- pair regime.
We analyse the performance of a gas time projection chamber (TPC) as a high-performance gamma-ray telescope and polarimeter in the e+e− pair-creation regime. We use data collected at a gamma-ray beam ...of known polarisation. The TPC provides two orthogonal projections (x, z) and (y, z) of the tracks induced by each conversion in the gas volume. We use a simple vertex finder in which vertices and pseudo-tracks exiting from them are identified. We study the various contributions to the single-photon angular resolution using Monte Carlo simulations, compare them with the experimental data and find that they are in excellent agreement. The distribution of the azimuthal angle of pair conversions shows a bias due to the non-cylindrical-symmetric structure of the detector. This bias would average out for a long duration exposure on a space mission, but for this pencil-beam characterisation we have ensured its accurate simulation by a double systematics-control scheme, data taking with the detector rotated at several angles with respect to the beam polarisation direction and systematics control with a non-polarised beam. We measure, for the first time, the polarisation asymmetry of a linearly polarised gamma-ray beam in the low energy pair-creation regime. This sub-GeV energy range is critical for cosmic sources as their spectra are power laws which fall quickly as a function of increasing energy. This work could pave the way to extending polarised gamma-ray astronomy beyond the MeV energy regime.
Measurement of 1.7–74MeV polarised γ rays with the HARPO TPC Geerebaert, Y.; Gros, Ph; Amano, S. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
02/2017, Letnik:
845
Journal Article
Recenzirano
Current γ-ray telescopes based on photon conversions to electron-positron pairs, such as Fermi, use tungsten converters. They suffer of limited angular resolution at low energies, and their ...sensitivity drops below 1GeV. The low multiple scattering in a gaseous detector gives access to higher angular resolution in the MeV–GeV range, and to the linear polarisation of the photons through the azimuthal angle of the electron-positron pair.
HARPO is an R&D programme to characterise the operation of a TPC (Time Projection Chamber) as a high angular-resolution and sensitivity telescope and polarimeter for γ rays from cosmic sources. It represents a first step towards a future space instrument. A 30cm cubic TPC demonstrator was built, and filled with 2bar argon-based gas. It was put in a polarised γ-ray beam at the NewSUBARU accelerator in Japan in November 2014. Data were taken at different photon energies from 1.7MeV to 74MeV, and with different polarisation configurations. The electronics setup is described, with an emphasis on the trigger system. The event reconstruction algorithm is quickly described, and preliminary measurements of the polarisation of 11MeV photons are shown.
Tests of a prototype for the electromagnetic calorimeter (ECAL) of the compact muon solenoid experiment (CMS) at the large hadron collider are described. The basic unit for the endcap ECAL in CMS is ...a “supercrystal” of 25 lead tungstate crystals. Results are presented from tests of the first full-sized supercrystal in electron beams and in a 3
T magnetic field. The supercrystal was exposed to electron beams with energies from 25 to 180
GeV. An energy resolution (
σ
E/
E) of (0.48±0.01)% was measured at 180
GeV.
The performance of prototype vacuum phototriodes is presented from the first full sized supercrystal array for the CMS ECAL endcaps. The array was exposed to high-energy electrons and tested in ...magnetic fields of up to 3T, in the CERN North area, in July and August 1999. The mean VPT electron yield, normalised to a naked crystal light yield of 8photoelectrons/MeV into an HPMT, was found to be 25electrons/MeV at 3T for devices from Research Institute Electron, 35electrons/MeV for devices from Hamamatsu and 18/23electrons/MeV from Electron Tubes.
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