The XEUS mission incorporates two satellites: the Mirror Spacecraft with 5 m2 of collecting area at 1 keV and 2 m2 at 7 keV, and an imaging resolution of 5" HEW and the Payload Spacecraft which ...carries the focal plane instrumentation. XEUS was submitted to ESA Cosmic Vision and was selected for an advanced study as a large mission. The baseline design includes XPOL, a polarimeter based on the photoelectric effect, that takes advantage of the large effective area which permits the study of the faint sources and of the long focal length, resulting in a very good spatial resolution, which allows the study of spatial features in extended sources. We show how, with XEUS, Polarimetry becomes an efficient tool at disposition of the Astronomical community.
We devised and built a light, compact and transportable X-ray polarized source based on the Bragg diffraction at nearly 45 degrees. The source is composed by a crystal coupled to a small power X-ray ...tube. The angles of incidence are selected by means of two orthogonal capillary plates which, due to the small diameter holes (10 um) allow good collimation with limited sizes. All the orders of diffraction defined by the crystal lattice spacing are polarized up to the maximum order limited by the X-ray tube voltage. Selecting suitably the crystal and the X-ray tube, either the line or the continuum emission can be diffracted, producing polarized photons at different energies. A very high degree of polarization and reasonable fluxes can be reached with a suitable choice of the capillary plates collimation. We present the source and test its performances with the production of nearly completely polarized radiation at 2.6, 5.2, 3.7 and 7.4 keV thanks to the employment of graphite and aluminum crystals, with copper and calcium X-ray tubes respectively. Triggered by the very compact design of the source, we also present a feasibility study for an on-board polarized source, coupled to a radioactive Fe55 nuclide and a PVC thin film, for the calibration of the next generation space-borne X-ray polarimeters at 2.6 and 5.9 keV.
Nature vol. 411, pag.662-665, 7th June 2001 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 sources which otherwise would remain inaccessible, even to X-ray
interferometry1. Polarimetry could provide a direct, visual picture of the
state of matter under extreme magnetic and gravitational fields by measuring
the radiation polarized through interaction with the highly asymmetric matter
distribution (accretion disk) and with the magnetic field.
The new instrument derives the polarization information from the track of the
photoelectrons imaged by a finely subdivided gas detector. Its great
improvement of sensitivity (at least two orders of magnitude) will allow direct
exploration of the most dramatic objects of the X-ray sky.
The development of micropixel gas detectors, capable to image tracks produced in a gas by photoelectrons, makes possible to perform polarimetry of X-ray celestial sources in the focus of grazing ...incidence X-ray telescopes. HXMT is a mission by the Chinese Space Agency aimed to survey the Hard X-ray Sky with Phoswich detectors, by exploitation of the direct demodulation technique. Since a fraction of the HXMT time will be spent on dedicated pointing of particular sources, it could host, with moderate additional resources a pair of X-ray telescopes, each with a photoelectric X-ray polarimeter in the focal plane. We present the design of the telescopes and the focal plane instrumentation and discuss the performance of this instrument to detect the degree and angle of linear polarization of some representative sources. Notwithstanding the limited resources the proposed instrument can represent a breakthrough in X-ray Polarimetry.
We present a new method for the assessment of bone mineral content of the peripheral skeleton. The method employs a new photon counting and imaging area device, the multi-wire proportional counter. ...The system can work as a single photon absorptiometer at the energies of both I-125 and Gd-153. It can work also as a dual photon absorptiometer using one of the two pairs of isotopes (I-125, Gd-153) or (I-125, Am-241). Both the calcaneus and the forearm can be imaged in a few minutes, with either single or dual photon absorptiometry. To test this new method in a clinical environment we compared it with the standard dual photon absorptiometry of the forearm. The data collected in a cross-sectional study of bone mineral content and bone mineral density in a group of 76 patients show a good correlation between the two measurement techniques (r = 0.931). The multi-wire proportional counter, due to its imaging capabilities, has a much lower repositioning error. This allows its use in regions with a large gradient of bone mineral density, such as the calcaneus or the very distal site of the forearm, which recent data have indicated to be superior to more proximal measurements in identifying osteoporosis.
SPIE proceedings, Vol. 6266 2X-1, 24-31 May 2006, Orlando
(Florida), M.J. Turner, G. Hasinger eds Development of multi-layer optics makes feasible the use of X-ray telescope
at energy up to 60-80 ...keV: in this paper we discuss the extension of
photoelectric polarimeter based on Micro Pattern Gas Chamber to high energy
X-rays. We calculated the sensitivity with Neon and Argon based mixtures at
high pressure with thick absorption gap: placing the MPGC at focus of a next
generation multi-layer optics, galatic and extragalactic X-ray polarimetry can
be done up till 30 keV.
Proc.SPIE Int.Soc.Opt.Eng. 6266 (2006) 24-31 XEUS is a large area telescope aiming to rise X-ray Astronomy to the level of
Optical Astronomy in terms of collecting areas. It will be based on two
...satellites, locked on a formation flight, one with the optics, one with the
focal plane. The present design of the focal plane foresees, as an auxiliary
instrument, the inclusion of a Polarimeter based on a Micropattern Chamber. We
show how such a device is capable to solve open problems on many classes of
High Energy Astrophysics objects and to use X-ray sources as a laboratory for a
substantial progress on Fundamental Physics.
SPIE proceedings, Vol. 6266 0R-1, 24-31 May 2006, Orlando
(Florida), M.J. Turner, G. Hasinger eds X-Ray Polarimetry can be now performed by using a Micro Pattern Gas Chamber
in the focus of a ...telescope. It requires large area optics for most important
scientific targets. But since the technique is additive a dedicated mission
with a cluster of small telescopes can perform many important measurements and
bridge the 40 year gap between OSO-8 data and future big telescopes such as
XEUS. POLARIX has been conceived as such a pathfinder. It is a Small Satellite
based on the optics of JET-X. Two telescopes are available in flight
configuration and three more can be easily produced starting from the available
superpolished mandrels. We show the capabilities of such a cluster of
telescopes each equipped with a focal plane photoelectric polarimeter and
discuss a few alternative solutions.
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 sources which otherwise would remain inaccessible, even to X-ray interferometry1. Polarimetry could provide a direct, visual picture of the state of matter under extreme magnetic and gravitational fields by measuring the radiation polarized through interaction with the highly asymmetric matter distribution (accretion disk) and with the magnetic field. The new instrument derives the polarization information from the track of the photoelectrons imaged by a finely subdivided gas detector. Its great improvement of sensitivity (at least two orders of magnitude) will allow direct exploration of the most dramatic objects of the X-ray sky.
The polarisation of x-ray photons can be determined by measuring the
direction of emission of a K-shell photoelectron. Effective exploitation of
this effect below 10 keV would allow development of a ...highly sensitive x-ray
polarimeter dedicated in particular to x-ray astronomy observations. Only with
the advent of finely segmented gas detectors was it possible to detect
polarisation sensitivity based on the photoelectric effect in this energy
range. Simulation and measurements at 5.4 and 8.04 keV with a microgap gas
counter, using both a polarised and an unpolarised x-ray source, showed that
the photoelectron track in a neon-based gas mixture retains the memory of the
polarisation of the incoming photons. Possible experiments aimed at
galactic/extragalactic sources and solar flares are considered and their
sensitivity to these sources is calculated.
