R. M. Curado da Silva

CIPHER, A POLARIMETER TELESCOPE CONCEPT FOR HARD X-RAY ASTRONOMY

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.

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

A gamma-ray telescope mission concept [gamma ray imager (GRI)] 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 2015-2025). 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 similar to 100% polarized gamma-ray beam. The spectroscopic, imaging, and timing performances were studied and in particular its potential as a polarimeter was evaluated. Polarization has been recognized as being a very important observational parameter in high energy astrophysics (> 100 keV) and therefore this capability has been specifically included as part of the GRI mission proposal. The prototype detector tested was a 5 mm thick CdZnTe array with an 11 x 11 active pixel matrix (pixel area of 2.5 x 2.5 mm(2)). The detector was irradiated by a monochromatic linearly polarized beam with a spot diameter of about 0.5 mm over the energy range between 150 and 750 keV. Polarimetric Q factors of 0.35 and double event relative detection efficiency of 20% were obtained. Further measurements were performed with a copper Laue monochromator crystal placed between the beam and the detector prototype. In this configuration we have demonstrated that a polarized beam does not change its polarization level and direction after undergoing a small angle (< 1 degrees) Laue diffraction inside a crystal

Polarimetry Study With a CdZnTe Focal Plane Detector

Polarimetry has been recognized as a very important observational parameter for high energy astrophysics (>; 100 keV). For this reason the capability to perform accurate polarimetric measurement of high energy cosmic sources should be included in future space mission proposals. This idea has become very appealing in the perspective of new high energy focusing telescope implementing Laue lenses. In order to optimize the focal plane design of such mission as a Compton scattering gamma-ray polarimeter, a CdZnTe detector prototype has been tested at the ESRF (European Synchrotron Radiation Facility) under a ~ 100% polarized gamma-ray beam. The prototype detector tested is a 5 mm thick CdZnTe array with the anode segmented into 16 × 16 pixels of 2.5×2.5 mm2. One of the most important factors that can affect significantly the polarimetric measurements is the direction of the incoming photons from the polarized radiation source with respect to the focal plane of the polarimeter. We have studied and analyzed the polarimetric response of the CdZnTe prototype as a function of the inclination of a polarized beam with respect to the optical axis of the detector from 0° to 10° tilt angle.

CdZnTe detector for hard x-ray and gamma-ray focusing telescope

The science drivers for a new generation soft gamma-ray mission are naturally focused on the detailed study of the acceleration mechanisms in a variety of cosmic sources. Through the development of high energy optics in the energy energy range 0.05-1 MeV it will be possible to achieve a sensitivity about two orders of magnitude better than the currently operating gamma-ray telescopes. This will open a window for deep studies of many classes of sources: from Galactic X-ray binaries to magnetars, from supernova remnants to Galaxy clusters, from AGNs (Seyfert, blazars, QSO) to the determination of the origin of the hard X-/gamma-ray cosmic background, from the study of antimatter to that of the dark matter. In order to achieve the needed performance, a detector with mm spatial resolution and very high peak efficiency is needed. The instrumental characteristics of this device could eventually allow to detect polarization in a number of objects including pulsars, GRBs and bright AGNs. In this work we focus on the characteristics of the focal plane detector, based on CZT or CdTe semiconductor sensors arranged in multiple planes and viewed by a side detector to enhance gamma-ray absorption in the Compton regime. We report the preliminary results of an optimization study based on simulations and laboratory tests, as prosecution of the former design studies of the GRI mission which constitute the heritage of this activity.

A three-dimensional CZT detector as a focal plane prototype for a Laue Lens telescope

The importance of hard X-ray astronomy (>10 keV) is now widely recognized. Recently both ESA and NASA have indicated in their guidelines for the progress of X- and γ-ray astronomy in the next decade the development of new instrumentation working in the energy range from the keV to the MeV region, where important scientific issues are still open, exploiting high sensitivity for spectroscopic imaging and polarimetry observations. The development of new concentrating (e.g. multilayer mirror) telescopes for hard X-rays (10 -100 keV) and focusing instruments based on Laue lenses operating from ~60 keV up to a few MeV is particularly challenging. We describe the design of a threedimensional (3D) depth-sensing position sensitive device suitable for use as the basic unit of a high efficiency focal plane detector for a Laue lens telescope. The sensitive unit is a drift strip detector based on a CZT crystal, (10×10 mm2 area, 2.5 mm thick), irradiated transversally to the electric field direction. The anode is segmented into 4 detection cells, each comprising one collecting strip and 8 drift strips. The drift strips are biased by a voltage divider, whereas the anode strips are held at 0 V. The cathode is divided in 4 horizontal strips for the reconstruction of the Z interaction position. The 3D prototype will be made by packing 8 linear modules, each composed of 2 basic sensitive units, bonded onto a ceramic layer together with the readout electronics.

A 3D CZT hard x-ray polarimeter for a balloon-borne payload

Today it is widely recognised that a measurement of the polarization status of cosmic sources high energy emission is a key observational parameter to understand the active production mechanism and its geometry. Therefore new instrumentation operating in the hard X/soft γ rays energy range should be optimized also for this type of measurement. In this framework, we present the concept of a small high-performance spectrometer designed for polarimetry between 100 and 1000 keV suitable as a stratospheric balloon-borne payload dedicated to perform an accurate and reliable measurement of the polarization status of the Crab pulsar, i.e. the polarization level and direction. The detector with 3D spatial resolution is based on a CZT spectrometer in a highly segmented configuration designed to operate as a high performance scattering polarimeter. We discuss different configurations based on recent development results and possible improvements currently under study. Furthermore we describe a possible baseline design of the payload, which can be also seen as a pathfinder for a high performance focal plane detector in new hard X and soft gamma ray focussing telescopes and/or advanced Compton instruments. Finally we present preliminary data from Montecarlo undergoing studies to determine the best trade-off between polarimetric performance and detector design complexity.

Development status of a CZT spectrometer prototype with 3D spatial resolution for hard x-ray astronomy

The development of new focusing optics based on wide band Laue lenses operating from ~60 keV up to several hundred keV is particularly challenging. This type of hard X-ray or gamma ray optics requires a high performance focal plane detector in order to exploit to the best their intrinsic capabilities. We describe a three dimensional (3D) position sensitive detector prototype suitable as the basic module for a high efficiency Laue lens focal plane detector. This detector configuration is currently under study for use in a balloon payload dedicated to performing a high significance measurement of the polarization status of the Crab between 100 and 500 keV. The prototype is made by packing 8 linear modules, each composed of one basic sensitive unit bonded onto a thin supporting ceramic layer. Each unit is a drift strip detector based on a CZT crystal, irradiated transversally to the electric field direction. The anode is segmented into 8 detection cells, each comprising one collecting strip and 8 surrounding drift strips. The drift strips are biased by a voltage divider. The cathode is divided into 4 horizontal strips for the reconstruction of the Z interaction position. The detector readout electronics is based on RENA-3 ASIC and the data handling system uses a custom electronics based on FPGA to provide the ASIC setting, the event handling logic, and the data acquisition. This paper mainly describes the components and the status of the undergoing activities for the construction of the proposed 3D CZT prototype and shows the results of the electronics tests.

Polarimetry study with a CdZnTe focal plane detector

Polarimetry has been recognized as a very important observational parameter for high energy astrophysics (>100 keV). Therefore the capability to perform accurate polarimetric measurement of high energy cosmic sources should be included in future space mission proposals. This idea become very appealing in the last three years in the development of a new telescope mission concept based on the Laue focusing technique - the Gamma Ray Imager mission - for the next call for mission ideas in the ESA Cosmic Vision 2015-2025 plan framework. In order to optimize the design of a focal plane for this mission as Compton scattering hard X- and soft ¿-ray polarimeter, a CdZnTe detector prototype has been tested at the ESRF (European Synchrotron Radiation Facility) under an ~100% polarized gamma-ray beam. The prototype detector tested is a 5 mm thick CdZnTe array with the anode segmented into 16×16 pixels of 2.5×2.5 mm2. One of the most important factors that can affect significantly the polarimetric measurements is the direction of the incoming photons from the polarised radiation source with respect to the focal plane of the polarimeter. We have studied and analyzed the polarimetric response of the CZT prototype as a function of the direction of a polarized beam forming different inclination angles with the optical axis of the detector: 0°, 0.5°, 1°, 1.5°, 2°, 3°, 4°, 5° and 10°.

A focal plane detector design for a wide band Laue-lens telescope

The energy range above 50 keV is important for the study of many open problems in high energy astrophysics such as, non thermal mechanisms in SNR, the study of the high energy cut-offs in AGN spectra, and the detection of nuclear and annihilation lines. In the framework of the definition of a new mission concept for hard X and soft gamma ray (GRI- Gamma Ray Imager) for the next decade, the use of Laue lenses with broad energy band-passes from 100 to 1000 keV is under study. This kind of instruments will be used for deep study the hard X-ray continuum of celestial sources. This new telescope will require focal plane detectors with high detection efficiency over the entire operative range, an energy resolution of few keV at 500 keV and a sensitivity to linear polarization. We describe a possible configuration for the focal plane detector based on CdTe/CZT pixelated layers stacked together to achieve the required detection efficiency at high energy. Each layer can either operate as a separate position sensitive detector and a polarimeter or together with other layers in order to increase the overall full energy efficiency. We report on the current state of art in high Z spectrometers development and on some activities undergoing. Furthermore we describe the proposed focal plane option with the required resources and an analytical summary of the achievable performance in terms of efficiency and polarimetry.

Prospects on Low-Z Elements K Fluorescence and Actinide-Radionuclides L Fluorescence X-Ray Detection With Cooled CZT

In the present study, we show that CZT detectors can be used in a wide X-ray energy range including low-energy X-rays down to 2.1 keV at - 17° C which may be extended down to 1.5 keV by additional cooling to - 26° C. CZT detector was operated in the low energy X-ray band to detect the K fluorescence X-rays emitted by low-Z elements, such as Ca, K, Cl, S, and eventually even further P, Si, Al. Additionally, it was demonstrated the fine spectroscopic features of the CZT detector cooled to - 26° C, which make it suitable for the detection of L fluorescence X-rays emitted by high-Z elements in the ~ 11 - 22 keV range, such as Th, Pa, U, Np and Pu, resulting from the decay of actinide radionuclides ²³⁴U/²³⁵U/²³⁸U, ²³⁷Np,²³⁸Pu/²³⁹Pu, ²⁴¹Am and ²⁴⁴Cm, respectively. With this cooled CZT detector it was possible to resolve the main L X-ray lines as well as to measure its intensity. The relative intensity obtained for the Pu L X-ray lines Ll, Lα1,2, Lβ6,η, Lβ2,4, Lβ1,3,5, and Lγ1,2,3,6 was, respectively, 7.1 ±2.3, 100 ±2.5, 5.6 ±2.3, 31.9 ±2.5, 103.2 ±2.4 and 29.8 ±2.2. The Np L X-ray most intense lines Lα1,2, Lβ2,4, Lβ1,3,5, and Lγ1,2,3,6 yielded a relative intensity of 100 ±4.2, 36.4 ±3.3, 96.2 ±5.0 and 34.3 ±3.7, respectively. The overall results are in good agreement with the values found in literature. The principal performance features of the CZT detector were, also studied experimentally or estimated from data tabulated in the literature. Notably, we found a ~ 2.5-fold diminution of the electronic noise FWHM temperature sensitivity (from ~ 41 to ~ 16 eV/°C) and thus of the equivalent noise charge temperature sensitivity (from ~ 3.8 to ~ 1.5e^- /°C) when the temperature decreases from - 1 to - 26°C. The influence of several properties, such as the charge mobility and lifetime, band gap energy, detectors leakage current and capacitance, on CZT detector performance features was also addressed.