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Within the AHEAD consortium a mission concept named ASTENA (Advanced Surveyor of Transient Events and Nuclear Astrophysics) is proposed to address the top-priority themes identified by the AHEAD Science Advisory Group: Gamma-Ray Bursts and Nuclear Astrophysics. GRBs are among the most intriguing phenomena of the Universe, which thanks to their vast luminosities can be used to probe the first billion years of cosmic history, i.e. the era of first stars and black-holes. In spite of great advancements in the GRB astronomy since the BeppoSAX discovery of afterglows, several issues concerning both the prompt emission and the afterglow are still open. Concerning the prompt emission, for example, the emission mechanism of the radiation and the energy dissipation site (internal shocks? external shocks? photosphere?) are far from being understood. What is required is an accurate determination of the photon spectrum from few keV up to tens of MeV, and importantly, a measurement of the polarization of the radiation. The emission of the afterglow has been deeply investigated with Swift in the energy band from 0.5 to 10 keV, showing that an understanding of the origin of the emission mechanism requires spectral information extending to much higher energies, as already suggested by a few studies at < 60 keV (e.g., Kouveliotou et al. 2013, ApJ 779, L1). Landmark progress on this issue therefore requires polarization capabilities and a passband extending well beyond 60 keV. Concerning nuclear astrophysics, a fundamental issue concerns the origin of the 511 keV positron annihilation line discovered with INTEGRAL/SPI in the Galactic center. According to the INTEGRAL results the emission is diffuse, but the poor imaging capability of INTEGRAL (at the best with a resolution of 12 arcmin with ISGRI) does not allow one to establish whether what appears diffuse is indeed the superposition of the emission from point-like sources, such as micro-quasars. The important role played by micro-quasars as sources of positron annihilation line emission has also been established with INTEGRAL (Siegert et al. 2016, Nature 531, 341). Another open issue in nuclear astrophysics concerns the determination and understanding of the nuclear burning processes in Type-1a supernovae. This requires a study of the intensity and time behavior of the expected lines emitted by the heavy elements produced in supernova explosions. Instrument concept to address the IWG requirements. With the above considerations in mind, we propose to perform a feasibility study of a configuration of two instruments: a) a wide field monitor/spectrometer (WFM/S), with a passband from 1 keV to 20 MeV, made of a suitable number of detection modules, each consisting of an array of long bars of scintillator with very small cross section, and readout from both sides with solid state thin detectors (e.g. Silicon Drift Detectors, SDD). One of the SDD is used as soft X-ray Position Sensitive Detector. A possible crystal material is CsI(Tl), but also other faster crystals such as LSO(Ce) or CeBr3 should be examined. The detector modules are coupled to a light coded mask, for obtaining a GRB localization accuracy of order of ~1 arcmin between 1 and 30/50 keV. The number of modules, equipped with collimators, should be sufficient to achieve the required sensitivity to GRBs. The order of magnitude of the total detection area is 18000 cm2. The modules are slightly misaligned with each other tin order o achieve a wide FOV (> 1 sr). b) a narrow field telescope (NFT), made of a broad-band Laue lens (50 – 600/700 keV) of a 20 m focal length, based on the exploitation of bent crystals, like those under development in Ferrara (FOV= 3.5 arcmin, angular resolution ≈20”). The NFT is coupled to a high efficiency (>80% above 600 keV) focal plane position sensitive detector, with 3D spatial resolution of at least 300 µm in the (X,Y) plane, fine spectroscopic response (1% @511 keV) and with polarization sensitivity. With the WFM/S, we expect to accurately determine the energy spectrum of GRB prompt emission in the broadest band ever achieved with a single instrument, to measure the gamma-ray polarization of, at least, the brightest GRBs and to search for electromagnetic counterparts of Gravitational Wave events. In addition, with adequate scintillator bars and fast electronics, the Lorentz invariance for the brightest events can be tested. With the NFT, which is >~100 times more sensitive at a few hundred keV than any other past or planned mission, we can carry out for the first time a long-sought study of the afterglow spectrum of GRBs up to high energies (600/700 keV), including its polarization level. We can also establish, thanks to its high angular resolution (about 20”), whether the 511 keV positron annihilation line is due to the superposition of emission from point-like sources. In addition, we can address many Legacy Science topics mentioned in the Call, such as the origin of the high energy emission from magnetars, the first determination of the spectrum of blazars out to z~8 in between the two Synchrotron and Compton bumps, the determination of the sources that give rise to the gamma-ray diffuse background. For example, one could determine the high-energy cutoff from spectra of relatively bright AGN and study how this depends on the physics of the accretion (e.g. BH mass, Eddington ratio). We emphasize that the unprecedented sensitivity of the NFT and the combination with the WFM/S implies a large discovery space of this configuration. Moreover, such an instrument concept, thanks to the lightweight of the Laue lens and compactness of the wide field instrument, is expected to be within the limits imposed by an ESA Medium Size Mission.

he semiconductor detectors technology has dramatically changed the broad field of x- and γ-rays spectroscopy and imaging. Semiconductor detectors, originally developed for particle physics applications, are now widely used for x/γ-rays spectroscopy and imaging in a large variety of fields, among which, for example, x-ray fluorescence, γ-ray monitoring and localization, noninvasive inspection and analysis, astronomy, and diagnostic medicine. The success of semiconductor detectors is due to several unique 242characteristics as the excellent energy resolution, the high detection efficiency, and the possibility of development of compact and highly segmented detection systems (i.e., spectroscopic imager). Among the semiconductor devices, silicon (Si) detectors are the key detectors in the soft x-ray band (15 keV) and will continue to be the first choice for laboratory-based high-performance spectrometers system (Eberth and Simpson 2006).

CdTe array detectors are being investigated for use in a variety of applications, ranging from medical diagnosis to high-energy astronomy. Herein, we present a study of the spatial resolution of several prototype CdTe arrays of varying thickness as a function of energy and angle of the incident radiation. These measurements were performed at the European Synchrotron Research Facility at Grenoble, France in the first week of July 2002. The energy range employed was between 100 and 500 keV, while the detectors consisted of a set of CdTe arrays containing elements of thicknesses between 3 and 8 mm. The spatial response of the detectors was measured using single site events, while the possibility of using this type of array detector also for measuring the degree of polarisation of the radiation was investigated using multi-site interactions.

A compact xenon filled gas proportional scintillation counter (GPSC) having a window area of 12.5 cm/sup 2/ coupled to low weight, low volume and low power associated electronics is described to be used in a balloon gondola to fly at high altitudes (30 to 40 km) and measure X-ray spectra in the 20 to 80 keV energy range, arising from solar flares. The detector has an energy resolution of 4.3% at 59.6 keV for the full window opening, and 2.7% for a 2-mm-diameter collimated X-ray beam.

Gamma-ray astrophysics in the MeV energy range plays a crucial role for the understanding of many exciting cosmic phenomena such as stellar explosions or cosmic accelerators (e.g., Supernovae, Novae, Supernova Remants (SNRs), Gamma-Ray Bursts (GRBs), Pulsars, etc). Cadmium Telluride (CdTe) and Cadmium Zinc Telluride (CdZnTe) are very attractive materials for gamma-ray detection, which have demonstrated a great potential onboard current space missions (such as IBIS/INTEGRAL and BAT/SWIFT). However, the energy coverage of these instruments has been limited up to a few hundred keV, and there has not been yet a dedicated instrument for polarimetry. Our technological activities aim to develop a gamma-ray spectrometer based on Cd(Zn)Te detectors, with imaging and polarimetry capability for astrophysical applications, in the energy range from a few hundred keV up to a few MeV. Within this framework, a gamma-ray polarimeter based on pixelated Cd(Zn)Te detectors in planar-transverse-field (PTF) configuration, is being considered as a payload option for a balloon-borne experiment.

We report on the development of a 3D position sensitive prototype suitable as focal plane detector for Laue lens telescope. The basic sensitive unit is a drift strip detector based on a CZT crystal, (similar to 19x8 mm(2) area, 2.4 mm thick), irradiated transversally to the electric field direction. The anode side is segmented in 64 strips, that divide the crystal in 8 independent sensor (pixel), each composed by one collecting strip and 7 (one in common) adjacent drift strips. The drift strips are biased by a voltage divider, whereas the anode strips are held at ground. Furthermore, the cathode is divided in 4 horizontal strips for the reconstruction of the third interaction position coordinate. The 3D prototype will be made by packing 8 linear modules, each composed by one basic sensitive unit, bonded on a ceramic layer. The linear modules readout is provided by a custom front end electronics implementing a set of three RENA-3 for a total of 128 channels. The front-end electronics and the operating logics (in particular coincidence logics for polarisation measurements) are handled by a versatile and modular multi-parametric back end electronics developed using FPGA technology.

A new generation of high sensitivity telescopes based on the use of Laue lenses coupled with high efficiency solid state focal plane detectors has been identified as a possibility for hard X and soft gamma ray astronomy. For this kind of space mission, polarimetry is recognized as a very important observational parameter and therefore this capability should be included as one of the primary scientific requirements. In this framework our group has realized a small experiment to demonstrate that the combination of a Laue Lens, built using Cu mosaic crystals, with a CZT pixel detector is capable of measuring the polarization of hard X ray sources contemporaneously with spectroscopy and imaging. This experiment was performed at the beginning of March 2008 using the ID15B beam line at ESRF (Grenoble). The experiment was based on a pixel CZT detector (5 mm thick array with 11×11 pixels of 2.5×2.5 mm2) with a sensitive area of 3×3 cm2 in conjunction with a mosaic Cu crystal (15×15 mm2, 4 mm thick) used in the Laue diffraction configuration. During the tests the Cu crystal was rotated so as to simulate a Laue lens ring, and the detector was moved in order to have the diffracted beam always impinging on the same pixel. The beam at the ID15B have allow us to test the response of this system, with almost 100% linearly polarized photon at ∼90, 270, 350 keV. In this paper we describe the experimental setup and we report on the preliminary results, with particular emphasis on the evaluation of possible systematic effect introduced in the detected polarization of the impinging photon by the Laue diffraction process.

The energy range above 60 keV is important for the study of many open problems in high energy astrophysics such as the role of Inverse Compton with respect to synchrotron or thermal processes in GRBs, non thermal mechanisms in SNR, the study of the high energy cut-offs in AGN spectra, and the detection of nuclear and annihilation lines. Recently the development of high energy Laue lenses with broad energy bandpasses from 60 to 600 keV have been proposed for a Hard X ray focusing Telescope (HAXTEL) in order to study the X-ray continuum of celestial sources. The required focal plane detector should have high detection efficiency over the entire operative range, a spatial resolution of about 1 mm, an energy resolution of a few keV at 500 keV and a sensitivity to linear polarization. We describe a possible configuration of the focal plane detector based on several CdTe/CZT pixelated layers stacked together to achieve the required detection efficiency at high energy. Each layer can operate both as a separate position sensitive detector and polarimeter or work with other layers to increase the overall photopeak efficiency. Each layer has a hexagonal shape in order to minimize the detector surface required to cover the lens field of view. The pixels would have the same geometry so as to provide the best coupling with the lens point spread function and to increase the symmetry for polarimetric studies., Comment: 10 pages, 9 figures

A compact xenon filled Gas Proportional Scintillation Counter having a window area of 12.5 cm/sup 2/ coupled to low weight, low volume and low power associated electronics is described to be used in a balloon gondola to fly at high altitudes (30 to 40 km) and measure X-ray spectra in the 20 to 80 keV energy range, arising from solar flares. The detector has an energy resolution of 5.0% at 59.6 keV for the full window opening.

Polarimetry is an area of high energy astrophysics which is still relatively unexplored, even though it is recognized that this type of measurement could drastically increase our knowledge of the physics and geometry of high energy sources. For this reason, in the context of the design of a Gamma-Ray Imager based on new hard-X and soft gamma ray focusing optics for the next ESA Cosmic Vision call for proposals (Cosmic Vision 2015-2025), it is important that this capability should be implemented in the principal on-board instrumentation. For the particular case of wide band-pass Laue optics we propose a focal plane based on a thick pixelated CdTe detector operating with high efficiency between 60-600 keV. The high segmentation of this type of detector (1-2 mm pixel size) and the good energy resolution (a few keV FWHM at 500 keV) will allow high sensitivity polarisation measurements (a few % for a 10 mCrab source in 106s) to be performed. We have evaluated the modulation Q factors and minimum detectable polarisation through the use of Monte Carlo simulations (based on the GEANT 4 toolkit) for on and off-axis sources with power law emission spectra using the point spread function of a Laue lens in a feasible configuration., 10 pages, 6 pages. Accepted for publication in Experimental Astronomy

To date polarimetry in astrophysics in the energy domain from hard X-rays up to soft gamma-rays has not been pursued due to the difficulties involved in obtaining sufficient sensitivity. Indeed for those few instruments which are capable of performing this type of measurement, polarimetry itself plays a secondary role in the mission schedule, as the efficiencies and polarimetric Q factors are relatively limited. In order to perform efficient polarimetric measurements for hard X 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 . The polarimetric performance and design optimisation of the CIPHER detection surface have been studied by use of a Monte Carlo code based on GEANT4 modules. Simulations show that we can achieve a Q factor better than 0.5 and Compton double event efficiency better than 11% 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 space background radiation. Q factor calculations for several beam inclinations as well as for space background together with simulated astronomical sources will be presented and discussed.

A compact, low power (2 W) electronic system which delivers all the high voltages required for portable gas proportional scintillation counters and has an optically coupled amplification output stage is described in detail. Its measured integral linearity is better than 0.0175% (rms). X-ray spectra demonstrating the performance of system are presented.

Even though it is recognized that the study of polarization from cosmic high-energy sources can give very important information about the nature of the emission mechanism, to date very few measurements have been attempted. For several years we have proposed the use of a thick CdTe array as a position sensitive spectrometer for hard X- and soft gamma-ray astronomy, a design which is also efficient for use as a polarimeter at energies above approximately 100 keV. Herein we describe the preliminary results of our study of a polarimeter based on 4096 CdTe microcrystals that we would like to develop for a high altitude balloon experiment. We present the telescope concept with a description of each subsystem together with some results on activities devoted to the optimization of the CdTe detector units' response. Furthermore we give an evaluation of the telescope performance in terms of achievable spectroscopic and polarimetric performance. In particular we will show the results of Monte Carlo simulations developed to evaluate the efficiency of our detector as a hard X ray polarimeter.

Gas Proportional Scintillation Counters using curved grids for solid angle and reflection compensation have been described in the recent literature. They allow large radiation windows with diameters of 25 mm keeping at the same time the good energy resolutions characteristic of those X-ray detectors. However, the grids used have a spherical curvature, which does not correspond to the optimal curvature. In the present work we have calculated by computer simulation an improved shape for the curved grid. This shape can be well fitted to an ellipsoid of revolution, with a large eccentricity. A detector was designed with such an ellipsoidal grid and a radiation window 40 mm in diameter, filled with pure xenon at 927 Torr coupled to an EMI D676QB VUV photomultiplier tube having a 2'' diameter window. For the experiments envisaged, detection of solar X-rays in the 20-80 keV energy range, a 7 cm thick drift region was used, leading to efficiencies from 80% to 20%, respectively. Such a thick drift region reduces the performance mainly for soft X-rays. For 22 keV X-rays the energy resolution obtained, for a broad X-ray beam entering the full 40 mm diameter detector window, is 6.0%. Results are presented showing the variation of the energy resolution with the window diameter and a performance, for ellipsoidal grids superior to that for spherical grids. A discussion of the results obtained is presented. http://www.sciencedirect.com/science/article/B6TJM-3VR1CVW-26/1/928927b1cf3cb22ebfa404018a2bdbae