Techniques and detectors for polarimetry in X-ray astronomy Soffitta, P.; Baldini, L.; Bellazzini, R. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
09/2003, Volume:
510, Issue:
1
Journal Article
Peer reviewed
Open access
Polarimeters flown so far were based on the
Thomson scattering and
Bragg diffraction with intrinsically limited sensitivity. In the present paper, we review the experiments based on those techniques ...and discuss possible optimization and implementation for X-ray astronomy.
Context. The flux of gamma rays with energies greater than 100 MeV is dominated by diffuse emission coming from cosmic-rays (CRs) illuminating the interstellar medium (ISM) of our Galaxy through the ...processes of Bremsstrahlung, pion production and decay, and inverse-Compton scattering. The study of this diffuse emission provides insight into the origin and transport of cosmic rays. Aims. We searched for gamma-ray emission from the Small Magellanic Cloud (SMC) in order to derive constraints on the cosmic-ray population and transport in an external system with properties different from the Milky Way. Methods. We analysed the first 17 months of continuous all-sky observations by the Large Area Telescope (LAT) of the Fermi mission to determine the spatial distribution, flux and spectrum of the gamma-ray emission from the SMC. We also used past radio synchrotron observations of the SMC to study the population of CR electrons specifically. Results. We obtained the first detection of the SMC in high-energy gamma rays, with an integrated >100 MeV flux of (3.7 ± 0.7) × 10-8 ph cm-2 s-1, with additional systematic uncertainty of ≤16%. The emission is steady and from an extended source ~3° in size. It is not clearly correlated with the distribution of massive stars or neutral gas, nor with known pulsars or supernova remnants, but a certain correlation with supergiant shells is observed. Conclusions. The observed flux implies an upper limit on the average CR nuclei density in the SMC of ~15% of the value measured locally in the Milky Way. The population of high-energy pulsars of the SMC may account for a substantial fraction of the gamma-ray flux, which would make the inferred CR nuclei density even lower. The average density of CR electrons derived from radio synchrotron observations is consistent with the same reduction factor but the uncertainties are large. From our current knowledge of the SMC, such a low CR density does not seem to be due to a lower rate of CR injection and rather indicates a smaller CR confinement volume characteristic size.
The micro-groove detector Bellazzini, R.; Bozzo, M.; Brez, A. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
03/1999, Volume:
424, Issue:
2
Journal Article
Peer reviewed
Open access
We introduce the Micro-Groove Detector (MGD), a new type of two-dimensional position-sensitive gas proportional counter produced using advanced Printed Circuit Board (PCB) technology. The MGD is ...based on a thin kapton foil, clad with gold-plated copper on both sides. An array of micro-strips at a typical pitch of 200
μm is defined on the top metal layer. Using as a protection mask the metal left after the patterning, charge amplifying micro-grooves are etched into the kapton layer. These end on a second micro-strip pattern defined on the bottom metal plane. The two arrays of micro-strips can have an arbitrary relative orientation and so can be used for read-out to obtain 2-D positional information. First results from our systematic assessment of this device are reported: gas gain
>15
000
, rate capability above 10
6
mm
−2
s
−1, energy resolution 22% at 5.4
keV, no significant charging or aging effects up to 5
mC/cm and full primary charge collection efficiency even at high drift fields.
The micro-gap chamber
Nuclear instruments and methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment/Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
10/1993
Journal Article
In astronomy there are basically four kinds of observations to extract the information carried by electromagnetic radiation: photometry, imaging, spectroscopy, and polarimetry. By optimal ...exploitation of the first three techniques, X-ray astronomy has been able to unveil the violent world of compact high-energy sources. Here, we report on a new instrument that brings high efficiency also to X-ray polarimetry, the last unexplored field of X-ray astronomy. It will then be possible to resolve the internal structures of compact objects, which otherwise would remain inaccessible even to X-ray interferometry. The new instrument derives the polarization information from the track of the photoelectron imaged by a finely subdivided gas pixel detector. Its great improvement of sensitivity (at least two order of magnitude) will allow direct exploration of the most dramatic objects of the X-ray sky.
We report on a large area (15×15
mm
2), high channel density (470
pixel/mm
2), self-triggering CMOS analog chip that we have developed as a pixelized charge collecting electrode of a Micropattern Gas ...Detector. This device represents a big step forward both in terms of size and performance, and is in fact the last version of three generations of custom ASICs of increasing complexity. The top metal layer of the CMOS pixel array is patterned in a matrix of 105,600 hexagonal pixels with a 50
μm pitch. Each pixel is directly connected to the underlying full electronics chain which has been realized in the remaining five metal and single poly-silicon layers of a 0.18
μm VLSI technology. The chip, which has customizable self-triggering capabilities, also includes a signal pre-processing function for the automatic localization of the event coordinates. Thanks to these advances it is possible to significantly reduce the read-out time and the data volume by limiting the signal output only to those pixels belonging to the region of interest. In addition to the reduced read-out time and data volume, the very small pixel area and the use of a deep sub-micron CMOS technology has allowed bringing the noise down to 50 electrons ENC.
Results from in depth tests of this device when coupled to a fine pitch (50
μm on a triangular pattern) Gas Electron Multiplier are presented. It was found that matching the read-out and gas amplification pitch allows getting optimal results. The experimental detector response to polarized and unpolarized X-ray radiation when working with two gas mixtures and two different photon energies is shown and the application of this detector for Astronomical X-ray Polarimetry is discussed. Results from a full Monte-Carlo simulation for several galactic and extragalactic astronomical sources are also reported.
We report on the performance of silicon strip sensors for GLAST, produced by Hamamatsu Photonics, Japan. The size of the sensors is 89.5 /spl times/ 89.5 mm/sup 2/. They were processed on 6-in-high ...resistivity wafers. By now, more than 1000 of ultimately 11500 sensors have been produced, and 622 have been investigated in detail. The average leakage current density is only 3.4 nA/cm/sup 2/ at 25/spl deg/C. Such a low leakage current density enables us to screen out a sensor having few strips with high leakage current by looking at the total sensor leakage current instead of measuring individual strip currents. High breakdown voltage is also achieved. Of ten sensors investigated for high-voltage breakdown, all hold bias voltage up to 500 V without significant increase in the leakage current. The faulty strip rate is about 0.01 % of 240 000 strips tested.
Thirty-two large-area Micro-Strip Gas Chambers were tested in a high-intensity, 350 MeV pion beam at PSI to prove that we had reached a Milestone for the Compact Muon Solenoid (CMS) experiment. The ...particle rate was approximately
6
kHz/
mm
2
, distributed over the whole active area of the detectors, and this rate was maintained for a total integrated time of 493 h. All of the chambers were operated with signal-to-noise values at or above that corresponding to 98% hit detection efficiency at CMS; the average
S/
N was 31. No indications of any gain instabilities or ageing effects were observed. In the official 3-week Milestone period, three strips from a total of 16
384 were damaged, a result which is 20 times lower than the minimal requirement for CMS. The spark rate of the detectors was very low and decreased with time to an average of one spark per chamber per day. The cathode voltages of 24 of the chambers were increased over a one week period to investigate the behaviour of the detectors at higher gains; the maximum
S/
N value was 2.4 times that at the normal working point. No significant increase in spark rate or strip loss rate was detected and the chambers operated stably. The detector efficiencies and imaging capabilities were also investigated. The MSGC design features and the assembly and test methodologies that enabled us to achieve these results are reported.
We present the implementation of a computed tomography (CT) iterative reconstruction strategy developed within the SYRMA-CT project for in vivo phase-contrast CT of uncompressed breast, ongoing at ...the ELETTRA synchrotron radiation facility (Trieste, Italy). Propagation-based phase-contrast imaging exploits the high spatial coherence of the monoenergetic laminar X-ray beam, as well as the large object-to-detector distance and the use of a Pixirad-8 high-resolution photon-counting CdTe detector. The signal in projection views depends on the X-ray absorption as well as on the phase shift introduced by the breast tissue in the beam path. A phase retrieval algorithm allowed recovering the projected 2-D phase information of the irradiated tissue layer, which was input to the CT reconstruction; then, the breast slices were reconstructed via a simultaneous algebraic reconstruction technique algorithm. Our iterative reconstruction-coupled with a filtering process for reducing the noise level and ring artifacts while preserving edges sharpness-showed better image quality than conventional filtered back projection (FBP) reconstruction. This phantom study showed that our iterative reconstruction produced images with higher contrast-to-noise ratio and spatial resolution than those obtained with FBP. Finally, the developed algorithm removed ring-like artifacts, without worsening the image quality.
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