POLIX – The Thomson scattering X-ray polarimeter Hemanth Manikantan*, Biswajit Paul, Rishin Puthiya Veetil, Vikram Rana, Gopala Krishna M R, Rajagopala G., Sandhya, Mamatha T S, Dhi- raj K Dedhia, Nirmal Iyer, Sujay Mate, Krishnamurthy S, Irshad Md, Pooja Verma, Harikrishna Sahu, Ketan Rikame, Shirisha Vissom, Meena G, Varun Bahal and Shreenandini Anand Raman Research Institute, Sadashivanagar, Bangalore, Karnataka 560080, India U R Rao Satellite Centre, Vimanapura, Bengaluru, Karnataka 560017, India hemanthm@rri.res.in 1. Introduction POLIX is the primary scientific payload on the upcoming Indian X-ray astronomy mission XPoSat. POLIX is a Thomson scattering X-ray polarimeter working in the 8-30 keV energy band designed to study the polarised X-ray emission from various celestial X-ray sources. Being a dedicated X-ray polarization mission in this unexplored energy band, POLIX is poised to give us glimpses of a new frontier in high-energy astrophysics and allow in-depth investigations of the astrophysical processes in neutron stars and black hole sources. In this poster, we intend to brief the principle of operation and instrument configuration. Tests and calibration of the position-sensitive X-ray proportional counters are also described, along with some key scientific prospects of POLIX. 2. The principle of operation Thomson scattering is the dominant scatter- ing mechanism in the 10-30 keV energy band. The electron scattering cross-section for incom- ing polarized photons is anisotropic in the plane normal to the incoming radiation. POLIX al- lows the source photons to scatter from a pure Beryllium scatterer block, and four surrounding X-ray proportional counters detect the scattered X-rays. The satellite spins at 0.2 rpm along the viewing axis of the POLIX to reduce systematic errors from different detectors. The azimuthal variation of scattered X-ray flux signifies the lin- ear polarization direction. 3.Mechanical configuration A collimator restricts the FOV, and a Beryllium scatterer is placed below the collimator. Multi- wire proportional counters with wires oriented vertically to the scatter plane are employed to detect the scattered photons in POLIX [1]. Each proportional counter has a wire frame, accom- modating 12 anode cells grouped into 2 different detector channels of 6 cells each. The housing is filled with 800 Torr of a gas mixture containing 90 % Xe, 9 %Ar, and 1 % Methane. The My- lar window opening towards the scatterer-side allows X-rays to pass into the detectors and sup- ports the gas pressure. Identification of the cell in which scattered X-ray is incident facilitates determining the azimuthal distribution of scat- tered X-ray photons. 5. Charge division Identifying the anode cell on which the X-ray is detected is the key to finding polarization di- rection. Six high resistance Nichrome wires in a proportional counter are connected in series, making one channel. The method of charge di- vision is utilized to identify the cell in which the X-ray is absorbed. 4. Wireframe High resistivity Nichrome wires having thickness of 25 µm surrounded by cathode layer consti- tutes an anode cell. 12 main anode cells of di- mensions 3 × 3 × 30 cm surrounded by 30 veto cells having dimensions 1.5 × 1.5 × 30 cm are present. High DC voltage is applied as bias to the wires. The primary electrons deposited by an incident photon are accelerated to the anode wire leading to avalanche and the resultant de- posited charge is detected at both ends of the channel. 6. Specifications Parameter Description Photon collection area 640 cm2 Energy range 8-30 keV Field of view 3◦× 3◦ Modulation factor 40% 7. Spectral calibration 55Fe and 109Cd radioactive sources, having an emission complex at 6 keV and a line at 22.1 keV, respectively, are used for the position and spectral calibration of the proportional counters. X-rays are shined at three different locations on top of each wire. The histograms of the ratio of pulse heights (Ratio hist