106 research outputs found
Measurement of 1.7 to 74 MeV polarised gamma rays with the HARPO TPC
Current {\gamma}-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 1
GeV. 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 program to characterise the operation of a TPC (Time
Projection Chamber) as a high angular-resolution and sensitivity telescope and
polarimeter for {\gamma} rays from cosmic sources. It represents a first step
towards a future space instrument. A 30 cm cubic TPC demonstrator was built,
and filled with 2 bar argon-based gas. It was put in a polarised {\gamma}-ray
beam at the NewSUBARU accelerator in Japan in November 2014. Data were taken at
different photon energies from 1.7 MeV to 74 MeV, 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 11 MeVphotons
are shown.Comment: Proceedings VCI201
Backward-angle photoproduction of mesons on the proton at = 1.5--2.4 GeV
Differential cross sections and photon beam asymmetries for
photoproduction have been measured at = 1.5--2.4 GeV and at the
scattering angles, --1 cos --0.6. The
energy-dependent slope of differential cross sections for -channel
production has been determined. An enhancement at backward angles is found
above = 2.0 GeV. This is inferred to be due to the -channel
contribution and/or resonances. Photon beam asymmetries have been obtained for
the first time at backward angles. A strong angular dependence has been found
at 2.0 GeV, which may be due to the unknown high-mass resonances.Comment: 12 pages, 4 figures, submitted to PL
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