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Register a new userHard X-ray polarimetry with Caliste, a high performance CdTe based imaging spectrometer
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Since the initial exploration of soft gamma-ray sky in the 60s, high-energy celestial sources have been mainly characterized through imaging, spectroscopy and timing analysis. Despite tremendous progress in the field, the radiation mechanisms at work in sources such as neutrons stars and black holes are still unclear. The polarization state of the radiation is an observational parameter which brings key additional information about the physical process. This is why most of the projects for the next generation of space missions covering the tens of keV to the MeV region require a polarization measurement capability. A key element enabling this capability is a detector system allowing the identification and characterization of Compton interactions as they are the main process at play. The hard X-ray imaging spectrometer module, developed in CEA with the generic name of Caliste module, is such a detector. In this paper, we present experimental results for two types of Caliste-256 modules, one based on a CdTe crystal, the other one on a CdZnTe crystal, which have been exposed to linearly polarized beams at the European Synchrotron Radiation Facility. These results, obtained at 200-300 keV, demonstrate their capability to give an accurate determination of the polarization parameters (polarization angle and fraction) of the incoming beam. Applying a selection to our data set, equivalent to select 90 degrees Compton scattered interactions in the detector plane, we find a modulation factor Q of 0.78. The polarization angle and fraction are derived with accuracies of approximately 1 degree and 5%. The modulation factor remains larger than 0.4 when essentially no selection is made at all on the data. These results prove that the Caliste-256 modules have performances allowing them to be excellent candidates as detectors with polarimetric capabilities, in particular for future space missions.
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PoGOLite is a hard X-ray polarimeter operating in the 25-100 keV energy band. The instrument design is optimised for the observation of compact astrophysical sources. Observations are conducted from a stabilised stratospheric balloon platform at an altitude of approximately 40 km. The primary targets for first balloon flights of a reduced effective area instrument are the Crab and Cygnus-X1. The polarisation of incoming photons is determined using coincident Compton scattering and photo-absorption events reconstructed in an array of plastic scintillator detector cells surrounded by a bismuth germanate oxide (BGO) side anticoincidence shield and a polyethylene neutron shield. A custom attitude control system keeps the polarimeter field-of-view aligned to targets of interest, compensating for sidereal motion and perturbations such as torsional forces in the balloon rigging. An overview of the PoGOLite project is presented and the outcome of the ill-fated maiden balloon flight is discussed.
The imaging and spectral performance of CdTe double-sided strip detectors (CdTe-DSDs) was evaluated for the ASTRO-H mission. The charcterized CdTe-DSDs have a strip pitch of 0.25 mm, an imaging area of 3.2 cm$times$3.2 cm and a thickness of 0.75 mm. The detector was successfully operated at a temperature of $-20^circ$C and with an applied bias voltage of 250 V. By using two-strip events as well as one-strip events for the event reconstruction, a good energy resolution of 2.0 keV at 59.5 keV and a sub-strip spatial resolution was achieved. The hard X-ray and gamma-ray response of CdTe-DSDs is complex due to the properties of CdTe and the small pixel effect. Therefore, one of the issues to investigate is the response of the CdTe-DSD. In order to investigate the spatial dependence of the detector response, we performed fine beam scan experiments at SPring-8, a synchrotron radiation facility. From these experiments, the depth structure of the electric field was determined as well as properties of carriers in the detector and successfully reproduced the experimental data with simulated spectra.
We have evaluated the performance of a fine pitch CdTe Double-sided Strip Detector (CdTe-DSD), which was originally developed for the focal plane detector of a hard X-ray telescope to observe the Sun. The detector has a thickness of 750 um and has 128 strip electrodes with a 60 um strip pitch orthogonally placed on both sides of the detector and covers an energy range 4 keV to 80 keV. The study of the depth of photon interaction and charge sharing effects are of importance in order to provide good spectroscopic and imaging performance. We study the tail structure observed in the spectra caused by charge trapping and develop a new method to reconstruct the spectra based on induced charge information from both anode and cathode strips. By applying this method, energy resolutions (FWHM) of 0.76 keV and 1.0 keV can be obtained at photon energies of 14 keV and 60 keV, respectively, if the energy difference between the anode and cathode is within 1 keV. Furthermore, the tail component at 60 keV is reduced, and the energy resolution of the 60 keV peak is improved from 2.4 keV to 1.5 keV (FWHM) if the energy difference is greater than 1 keV. In order to study the imaging performance, we constructed a simple imaging system using a 5 mm thick tungsten plate that has a pinhole with a diameter of 100 um. We utilize a Ba-133 radioisotope of 1 mm in diameter as a target source in combination with a 100 um slit made from 0.5 mm thickness tungsten. We imaged the Ba-133 source behind the 100 um slit using a 30 keV peak, with a 100 um pinhole placed at the center of the source-detector distance. By applying a charge sharing correction between strips, we have succeeded in obtaining a position resolution better than the strip pitch of 60 um.
The PoGO mission, including the PoGOLite Pathfinder and PoGO+, aims to provide polarimetric measurements of the Crab system and Cygnus X-1 in the hard X-ray band. Measurements are conducted from a stabilized balloon-borne platform, launched on a 1 million cubic meter balloon from the Esrange Space Center in Sweden to an altitude of approximately 40 km. Several flights have been conducted, resulting in two independent measurements of the Crab polarization and one of Cygnus X-1. Here, a review of the PoGO mission is presented, including a description of the payload and the flight campaigns, and a discussion of some of the scientific results obtained to date.
We present a method for enhancing the sensitivity of X-ray telescopic observations with imaging polarimeters, with a focus on the gas pixel detectors (GPDs) to be flown on the Imaging X-ray Polarimetry Explorer (IXPE). Our analysis determines photoelectron directions, X-ray absorption points and X-ray energies for 1-9 keV event tracks, with estimates for both the statistical and model (reconstruction) uncertainties. We use a weighted maximum likelihood combination of predictions from a deep ensemble of ResNet convolutional neural networks, trained on Monte Carlo event simulations. We define a figure of merit to compare the polarization bias-variance trade-off in track reconstruction algorithms. For power-law source spectra, our method improves on the current planned IXPE analysis (and previous deep learning approaches), providing ~45% increase in effective exposure times. For individual energies, our method produces 20-30% absolute improvements in modulation factor for simulated 100% polarized events, while keeping residual systematic modulation within 1 sigma of the finite sample minimum. Absorption point location and photon energy estimates are also significantly improved. We have validated our method with sample data from real GPD detectors.
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