Cipher, A Polarimeter Telescope Concept for Hard X-Ray Astronomy |
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| Authors: | R M Curado Da Silva E Caroli JB Stephen P Siffert |
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| Institution: | (1) Laboratoire PHASE, CNRS, 23 rue do Loess, BP 20CR, 67037 Strasbourg CEDEX 2, France;(2) Departamento de Física da Universidade de Coimbra, 3000 Coimbra, Portugal;(3) IASF-Sezione di Bologna, Via Gobetti 101, 40129 Bologna, Italy |
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| Abstract: | The polarisation of astrophysical source emission in the energy range from a few tens of keV up to the MeV region is an almost
unexplored field of high-energy astrophysics. Till date, polarimetry in astrophysics–in the energy domain from hard X-rays
up to soft γ-rays–has not been pursued due to the difficulties involved in obtaining sufficient sensitivity. Indeed for those
few instruments that are capable of performing this type of measurement (e.g. the COMPTEL instrument on the Compton Gamma-ray
Observatory and the IBIS instrument on INTEGRAL), polarimetry itself plays a secondary role in the mission objectives, as
the efficiencies (0.5% and 10% maximum, respectively) and polarimetric Q factors (0.1 and 0.3, respectively) are relatively limited. In order to perform efficient polarimetric measurements for hard
X-ray and soft gamma-ray sources, with an instrument of relatively robust and simple design, a CdTe based telescope (CIPHER:
Coded Imager and Polarimeter for High Energy Radiation) is under study. This instrument is based on a thick (10 mm) CdTe position-sensitive
spectrometer comprising four modules of 32 × 32 individual pixels, each with a surface area of 2 × 2 mm2 (about 160 cm2 total detection area). The polarimetric performance and design optimisation of the CIPHER detection surface have been studied
by use of a Monte Carlo code. This detector, due to its intrinsic geometry, can allow efficient polarimetric measurements
to be made between 100 keV and 1 MeV. In order to predict the polarimetric performance and to optimise the design and concept
of the CIPHER detection plane, a Monte Carlo code based on GEANT4 library modules was developed to simulate the detector behaviour
under a polarised photon flux. The Compton double event efficiency, as well bi-dimensional double event distribution maps
and the corresponding polarimetric modulation factor will be presented and discussed. Modulation Q factors better than 0.50 and double event total efficiencies greater than 10% were calculated in the energy range between
100 keV and 1 MeV. Herein we will present and discuss the general problems that affect polarimetric measurements in space,
such as the inclination of the source with respect to the telescope optical axis and background radiation. Q factor calculations for several beam inclinations as well as for background together with simulated astronomical sources
will be presented and discussed. |
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| Keywords: | astrophysics flight instruments Monte Carlo polarimetry semiconductor detectors |
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