G. Spandre

Chromatic X-ray imaging with a fine pitch CdTe sensor coupled to a large area photon counting pixel ASIC

An innovative X-ray imaging sensor based on Chromatic Photon Counting technology with intrinsic digital characteristics is presented. The system counts individually the incident X-ray photons and selects them according to their energy to produce two color images per exposure. The energy selection occurs in real time and at radiographic imaging speed (GHz global counting rate). Photon counting, color mode and a very fine spatial resolution (more than 10 LP/mm at MTF50) allow to obtain a high ratio between image quality and absorbed dose. The individual building block of the imaging system is a two-side buttable semiconductor radiation detector made of a thin pixellated CdTe crystal coupled to a large area VLSI CMOS pixel ASIC. Modules with 1, 2, 4, and 8 block units have been built. The largest module has 25 × 2.5 cm2 sensitive area. Results and images obtained from testing different modules are presented.

Low energy polarization sensitivity of the Gas Pixel Detector

An X-ray photoelectric polarimeter based on the Gas Pixel Detector has been proposed to be included in many upcoming space missions to fill the gap of about 30 years from the first (and to date only) positive measurement of polarized X-ray emission from an astrophysical source. The estimated sensitivity of the current prototype peaks at an energy of about 3 keV, but the lack of readily available polarized sources in this energy range has prevented the measurement of detector polarimetric performances. In this paper we present the measurement of the Gas Pixel Detector polarimetric sensitivity at energies of a few keV and the new, light, compact and transportable polarized source that was devised and built to this aim. Polarized photons are produced, from unpolarized radiation generated with an X-ray tube, by means of Bragg diffraction at nearly 45 ◦ . The diffraction angle is constrained with two orthogonal capillary plates, which allow good collimation with limited size thanks to the 10 µm diameter holes. Polarized photons at energy as low as a few keV can be produced with a proper choice of diffracting crystal, while the maximum energy is limited by the X-ray tube voltage, since all the orders defined by the crystal lattice spacing are diffracted. The best trade-off between reasonable fluxes and high degree of polarization can be achieved selecting the degree of collimation provided by capillary plates. The employment of mosaic graphite and flat aluminum crystals allow the production of nearly completely polarized photons at 2.6, 3.7 and 5.2 keV from the diffraction of unpolarized continuum or line emission. The measured modulation factor of the Gas Pixel Detector at these energies is in good agreement with the estimates derived from a Monte Carlo software, which was up to now employed for driving the development of the instrument and for estimating its low energy sensitivity. In this paper we present the excellent polarimetric performance of the Gas Pixel Detector at energies where the peak sensitivity is expected. These measurements not only support our previous claims of high sensitivity but confirm the feasibility of astrophysical X-ray photoelectric polarimetry.

Spectral and polarimetric characterization of the Gas Pixel Detector filled with dimethyl ether

The Gas Pixel Detector belongs to the very limited class of gas detectors optimized for the measurement of X-ray polarization in the emission of astrophysical sources. The choice of the mixture in which X-ray photons are absorbed and photoelectrons propagate, deeply affects both the energy range of the instrument and its performance in terms of gain, track dimension and ultimately, polarimetric sensitivity. Here we present the characterization of the Gas Pixel Detector with a 1 cm thick cell filled with dimethyl ether (DME) at 0.79 atm, selected among other mixtures for the very low diffusion coefficient. Almost completely polarized and monochromatic photons were produced at the calibration facility built at INAF/IASF-Rome exploiting Bragg diffraction at nearly 45 degrees. For the first time ever, we measured the modulation factor and the spectral capabilities of the instrument at energies as low as 2.0 key, but also at 2.6, 3.7, 4.0, 5.2 and 7.8 key. These measurements cover almost completely the energy range of the instrument and allows to compare the sensitivity achieved with that of the standard mixture, composed of helium and DME

THE IMAGING PROPERTIES OF THE GAS PIXEL DETECTOR AS A FOCAL PLANE POLARIMETER

ABSTRACTX-rays are particularly suited to probe the physics of extreme objects. However, despite the enormousimprovements of X-ray Astronomy in imaging, spectroscopy and timing, polarimetry remains largelyunexplored. We propose the photoelectric polarimeter Gas Pixel Detector (GPD) as an instrumentcandidate to fill the gap of more than thirty years of lack of measurements. The GPD, in the focusof a telescope, will increase the sensitivity of orders of magnitude. Moreover, since it can measurethe energy, the position, the arrival time and the polarization angle of every single photon, allows toperform polarimetry of subsets of data singled out from the spectrum, the light curve or the imageof source. The GPD has an intrinsic very fine imaging capability and in this work we report on thecalibrationcampaign carriedout in 2012at the PANTER X-raytest facility of the Max-Planck-Institutfu¨r extraterrestrische Physik of Garching (Germany) in which, for the first time, we coupled it to aJET-X optics module with a focal length of 3.5 m and an angular resolution of 18 arcsec at 4.5 keV.This configuration was proposed in 2012 aboard the X-ray Imaging Polarimetry Explorer (XIPE) inresponse to the ESA call for a small mission. We derived the imaging and polarimetric performancefor extended sources like Pulsar Wind Nebulae and Supernova Remnants as case studies for the XIPEconfiguration, discussing also possible improvements by coupling the detector with advanced optics,having finer angular resolution and larger effective area, to study with more details extended objects.Keywords: X-ray polarimetry, X-ray telescope, angular resolution

Preliminary results of the LAT Calibration Unit beam tests

The calibration strategy of the GLAST Large Area Telescope (LAT) combines analysis of cosmic ray data with accelerator particle beams measurements. An advanced Monte Carlo simulation of the LAT, based on the Geant4 package, was set up to reproduce the LAT response to such radiation and to benchmark the event reconstruction and the background rejection strategy before launch and during operation. To validate the LAT simulation, a massive campaign of beam tests was performed between July and November 2006, in parallel with the LAT integration and test, on the LAT Calibration Unit. This is a detector built with spare flight modules and flight‐like readout electronics, which was exposed to a large variety of beams, representing the whole spectrum of the signal that will be detected by the LAT, using the CERN and the GSI accelerator facilities. Beams of photons (0 – 2.5 GeV), electrons (1 – 300 GeV), hadrons (π and p, a few GeV – 100 GeV) and ions (C; Xe, 1.5 GeV/n) were shot through the CU to measure the phys...

A microstrip avalanche chamber with two stages of gas amplification

Abstract The operation of a microstrip gas chamber with two stages of gas amplification is discussed. An average gas gain for ionizing tracks of ≈ 10 is obtained while drifting the ionization electrons in a uniform electric field of 6 kV/cm, while a further factor ≈ 5 × 10 3 is obtained from the avalanche process starting close to thin anodic microstrips. The operation of the microstrip gas chamber in this regime should improve both time and spatial resolution, especially for inclined tracks.

X-Ray Polarimetry: A polarimeter for IXO

The X-ray POLarimeter (XPOL) is an instrument that will fly on-board the International X-ray Observatory (IXO). We will describe the XPOL setup in IXO and we will compare the IXO requirements with the actual prototype performance. The environmental tests performed on the XPOL prototype (thermo-vacuum, vibration and heavy ions irradiation) show that this technology is ready for a space application. 39.1 XPOL on the IXO focal plane IXO is a collaboration of NASA, ESA and JAXA, and is foreseen to fly in 2020 [1]. The optics area will be 2 m2 at 2 keV with a 20 m focal length and with an angular resolution of 5”. The focal plane of IXO will be a rotating platform hosting several instruments that will take data alternatively: a Wide Field Imager, an X-ray Microcalorimeter Spectrometer, a Hard X-ray imager, a High Time Resolution Spectrometer, and the polarimeter XPOL. Further an X-ray grating spectrometer will be continuously in operation. XPOL is a sealed Gas Pixel Detector (GPD) [2; 3], with a 50 μm beryllium window, a photo-absorption gap of 1 cm, a Gas Electron Multiplier (GEM) for the charge preamplification and a readout ASIC with a 15 × 15 mm2 active area, covered by 105600 hexagonal pixels with a 50 μm pitch. Each pixel has a complete electronic chain (preamplifier, shaper, sample and hold) with a very limited noise (50 el ENC). The gas used is a He20-DME80 (DiMethyl Ether) mixture at 1 bar. The photons that have a photoelectric interaction with the gas atoms, cause the emission of a photoelectron with an angle relative to the X-ray polarization modulated as a cos2 φ function. The ionization electrons left along the photoelectron track, are drifted towards the GEM that multiplies them, and are collected, amplified and recorded by the pixels which store the track map. The ASIC has an auto-triggering

Performance of an Ar-DME imaging photoelectric polarimeter

The possibility to perform polarimetry in the soft X-ray energy band (2-10 keV) with the Gas Pixel Detector, filled with low Z mixtures, has been widely explored so far. The possibility to extend the technique to higher energies, in combination with multilayer optics, has been also hypothesized in the past, on the basis of simulations. Here we present a recent development to perform imaging polarimetry between 6 and 35 keV, employing a new design for the GPD, filled with a Ar-DME gas mixture at high pressure. In order to improve the efficiency by increasing the absorption gap, while preserving a good parallel electric field, we developed a new configuration characterized by a wider gas cell and a wider GEM. The uniform electric field allows to maintain high polarimetric capabilities without any decrease of spectroscopic and imaging properties. We present the first measurements of this prototype showing that it is now possible to perform imaging and spectro-polarimetry of hard X-ray sources.

X-ray polarimetry in astrophysics with the Gas Pixel Detector

The Gas Pixel Detector, recently developed and continuously improved by Pisa INFN in collaboration with IASF-Roma of INAF, can visualize the tracks produced within a low Z gas by photoelectrons of few keV. By reconstructing the impact point and the original direction of the photoelectrons, the GPD can measure the linear polarization of X-rays, while preserving the information on the absorption point, the energy and the time of individual photons. Applied to X-ray Astrophysics, in the focus of grazing incidence telescopes, it can perform angular resolved polarimetry with a huge improvement of sensitivity, when compared with the conventional techniques of Bragg diffraction at 45° and Compton scattering around 90°. This configuration is the basis of POLARIX and HXMT, two pathfinder missions, and is included in the baseline design of IXO, the very large X-ray telescope under study by NASA, ESA and JAXA.

Energy-windowed, pixellated X-ray diffraction using the Pixirad CdTe detector

X-ray diffraction (XRD) is a powerful tool for material identification. In order to interpret XRD data, knowledge is required of the scattering angles and energies of X-rays which interact with the sample. By using a pixellated, energy-resolving detector, this knowledge can be gained when using a spectrum of unfiltered X-rays, and without the need to collimate the scattered radiation. Here we present results of XRD measurements taken with the Pixirad detector and a laboratory-based X-ray source. The cadmium telluride sensor allows energy windows to be selected, and the 62 μm pixel pitch enables accurate spatial information to be preserved for XRD measurements, in addition to the ability to take high resolution radiographic images. Diffraction data are presented for a variety of samples to demonstrate the capability of the technique for materials discrimination in laboratory, security and pharmaceutical environments. Distinct diffraction patterns were obtained, from which details on the molecular structures of the items under study were determined.