Abstract-- Polarimetry in astrophysics had no much development in the hard X- and soft γ-ray energy range; in fact no dedicated polarimeters had ever been launched in space. Previous Monte Carlo simulations and prototype experimental measurements were made in order to evaluate the polarimetric performances of pixelised CZT matrices. The results showed that CZT based polarimeters are able to perform polarimetric measurements of short duration polarised emissions like gamma ray bursts. On the behalf of a Gamma Ray Burst Monitor (GRBM) proposal for the LOBSTER experiment, approved by ESA for a Phase A study for a future flight (2009) aboard the International Space Station (ISS), we propose a Monte Carlo polarimetric study of the 4 detection units that compose this instrument. Each of these units is a 24×12 matrix of CZT elementary crystals. Each pixel has a cross section of 8×8 mm 2 , therefore a detection unit has an active area of about 184 cm 2 . The detector thickness is 3 mm as baseline, and 5 mm in the case this thickness is crucial to exploit the GRBM as polarimeter as well. The 4 detection units have a rectangular FOV of 55º×35° FWHM and their axes misaligned with each other by 45° in the direction perpendicular to the ISS motion and 10° along the ISS direction of motion. Energy dependent polarimetric Q factor and detection efficiencies will be determined. Expected GRBM minimum detectable polarisation will be presented and discussed. I. INTRODUCTION n astrophysics, polarimetric measurements of X- and γ-ray astronomical radiation sources offer an extra observational parameter in addition to spectral and to time variability analysis. This additional observational parameter can however, give a new kind of information about constitution, physical processes and emissions of the various types of objects as, for example, the emission mechanisms that are associated with strong magnetic fields present in some astrophysical objects. Some of the celestial sources that are potential emitters of Manuscript received November 1, 2004. This work was carried in cooperation between the Departamento de Física, Universidade de Coimbra, Portugal (Unit 217/94) and the IASF – Sezione di Bologna (Istituto di Astrofisica Spaziale e Fisica Cosmica), CNR, Italy and was supported in part by FEDER through project POCTI/FNU/49561/2002 of Fundação para a Ciência e a Tecnologia, Portugal. The work of R.M. Curado da Silva was supported by the Fundação para a Ciência e Tecnologia, Portugal, through the research grant SFRH/BPD/11670/2002. R. M. Curado da Silva and N. F. C. Mendes are with the Departmento de Física, Universidade de Coimbra, P-3000 Coimbra, Portugal (telephone: ++351-239410663, e-mail: ruisilva@saturno.fis.uc.pt). E. Caroli and J. B. Stephen are with IASF - Sezione di Bologna, Via Gobetti 101, I-40129 Bologna, Italy. F. Frontera is with the Dipartimento di Fisica, Università di Ferrara, Ferrara, Italy. polarized could be for example: Pulsars, Solar Flares, Active Galactic Nuclei, Galactic Black Holes or Gamma-Ray Bursts (GRB). However, to date polarimetry in astrophysics in the energy domain from hard X-rays up to soft γ-rays has not been much developed; in fact no dedicated polarimeters have ever been launched in space or on a balloon borne experiment [1]. Indeed for those few instruments which are capable of performing this type of measurement, polarimetry itself plays a secondary role in the mission schedule, the main purpose of these instruments being to perform imaging and timing analysis of celestial sources. This is the case of the COMPTEL (Imaging Compton Telescope) instrument on board CGRO (Compton Gamma-ray Observatory) [2] and the IBIS (Imager on Board the INTEGRAL Satellite) that is part of the INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) mission [3]. With the aim of improve the polarimetric studies of hard X and soft gamma ray sources, particularly the study of polarized GRB emissions, a collaboration project has been developed between the IASF (Istituto di Astrofisica Spaziale e Fisica Cosmica) – Sezione di Bologna, CNR, Italy and the Departamento de Física da Universidade de Coimbra, Portugal, on the behalf of the GRBM (Gamma Ray Burst Monitor) on board the LOBSTER experiment. The collaboration between these two groups has already gave rise to both extensive Monte Carlo studies [4] and experimental studies of CdTe and CZT pixelised matrices [5]. The LOBSTER experiment, which was proposed by the University of Leicester in response to the Call for Mission proposals for two flexi-missions (F2 and F3 issued in October 1999), is now approved by ESA for a Phase A study for a future flight (2009) aboard the Columbus Exposed Payload Facility (CEPF) of the International Space Station (ISS) [6]. On the behalf of this project a small ancillary Gamma Ray Burst Monitor (GRBM) is assumed to be part of the final instrument configuration in order to maximize the LOBSTER science return [7]. The proposed GRBM configuration was projected to satisfy the LOBSTER requirements and compatibly with the financial resources and to improve the LOBSTER—ISS scientific goals. In order to optimise the design and the concept of this GRBM, we have carried out a study developing a Monte Carlo code based on GEANT4 library modules to simulate the response of the detector under polarised photon beams that simulate typical GRB and the detector response under some of Polarimetric Performances of a Wide Field CZT Monitor for X-and γ-Ray Astrophysics R. M. Curado da Silva, E. Caroli, J. B. Stephen, N. F. C. Mendes and F. Frontera I 0-7803-8701-5/04/$20.00 (C) 2004 IEEE