Cipher, A Polarimeter Telescope Concept for Hard X-Ray Astronomy

Abstract

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 mm 2 (about 160 cm 2 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.

POLAR: A Space-borne X-Ray Polarimeter for Transient Sources

“…In view of the power and the lack of precise polarization studies of GRBs, s Several X and γ-ray polarimeters have been proposed and are under development to perform precise polarization studies of GRBs. Some examples are POLAR (Produit et al, 2005), GRAPE (Gamma-Ray Polarimeter Experiment (Bloser et al, 2006)) POET (Polarimeters for Energetic Transients (Hill et al, 2008)), CIPHER (Coded Imager and Polarimeter for High Energy Radiation (Curado da Silva et al, 2003)), PHENEX (Polarimetry for High ENErgy X rays (Gunji et al, 2007)), XPOL (Costa et al, 2007) and PO-LARIX (Costa et al, 2006). Furthermore, polarimeters designed for studying fixed sources, like PoGOLite (Kamae et al, 2008), are also able to measure the polarization of GRBs if they happen to appear in their field of view.…”

Expected performance of a hard X-ray polarimeter (POLAR) by Monte Carlo simulation

“…The detector plane of the BAT onboard Swift [22,23] may have worked as a good polarimeter, but the design of both detectors and signal processing electronics are not suitable for selecting the Compton scattering events [2]. In order to actualize polarimetry to various cosmic sources in the X-ray and gamma-ray energy range, many polarimeters have been proposed to this date: Photoelectric polarimeters based on a micropattern gas chamber (MPGC) for X-ray Evolving Universe Spectroscopy (XEUS) and POLARIX [24,25,26], Compton polarimeters like the Gamma-Ray Polarimeter (GRAPE) [27], the Polarized Gamma-ray Observer (PoGO) [28] and the smaller version PoGOLite [29], the soft gamma-ray detector (SGD) onboard NeXT [30], the Coded Imager and Polarimeter for High Energy Radiation (CIPHER) [31], the X-ray Polarimeter Experiment (XPE) [32], as well as POLAR [33], and Pair production polarimeters [34].…”

Polarimetric performance of a Laue lens gamma-ray CdZnTe focal plane prototype

“…4 A Monte Carlo simulation code based on the GEometry ANd Tracking ͑GEANT4͒ program has been developed and an extensive polarimetric study has been performed in order to evaluate the response and the performance of the detection plane to different features of in-flight polarimetric space measurements. 5 In order to compare and calibrate the simulations with measurements, a previous experiment, POLCA I ͑with small 4 ϫ 4 pixels CdTe detectors͒ was implemented at the ESRF in July 2002, where the experimental results obtained were in good agreement with those predicted by Monte Carlo simulations. 6 In the POLCA II experiment the prototype polarimeter employed was a 5 mm thick CdZnTe with the anode segmented into 16ϫ 16 pixels of 2.5 ϫ 2.5 mm 2 ͑although, due to back-end electronics channel limitations, we have only an 11ϫ 11 active pixel matrix͒.…”

“…B. Stephen, 3 A. Pisa, 4 N. Auricchio, 4 S. Del Sordo, 5 F. Frontera, 4 V. Honkimäki, 6 F. Schiavone, 3 A. Donati, 3 A gamma-ray telescope mission concept ͓gamma ray imager ͑GRI͔͒ based on Laue focusing techniques has been proposed in reply to the European Space Agency call for mission ideas within the framework of the next decade planning ͑Cosmic Vision 2015-2025͒. In order to optimize the design of a focal plane for this satellite mission, a CdZnTe detector prototype has been tested at the European Synchrotron Radiation Facility under an ϳ100% polarized gamma-ray beam.…”