A polarimeter for IXO

Abstract

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