SPIE Newsroom 10.1117/2.1200609.0408 Astronomical applications of a micro pixel chamber Hideaki Katagiri, Kentaro Miuchi, Hironori Matsumoto, Hidetoshi Kubo, Takeshi Tsuru, and Toru Tanimori A novel micro-pattern gaseous detector could expand the scope of high- energy astrophysics by providing a tool for x-ray polarimetry and gamma-ray band observations. Recently, micro-pattern gaseous detectors have been devel- oped for x-ray, gamma-ray, and charged-particle imaging de- vices with fine position resolution. However, a discharge oc- casionally occurs, especially when the gas gain is high: this is a serious problem because it causes critical damage to the electrodes. A μ-PIC (micro pixel chamber) is a novel micro-pattern gaseous detector that’s stable at a sufficient gain operation. 1 Figure 1 shows a schematic view of the μ-PIC, which consists of anodes and cathodes. The volume between the drift plane and the device is filled with a gaseous mixture. The electron cloud, which is produced by an incoming x-ray or gamma ray, drifts towards the μ-PIC under the influence of an electric field formed between it and the drift plane. The electrons are then multiplied near the anode and the signals are read from both the anode and the cathode: this allows the position, shape, and charge of the electron cloud to be measured. The μ-PIC has numerous applications. When it serves as an x-ray imager with a thin (less than 1cm) gas volume, the ma- jor applications are x-ray crystallography, solution scattering, and x-ray polarimetry. Setting a long (greater than 5cm) drift length allows the μ-PIC to be operated as a micro time projec- tion chamber ( μ-TPC) that detects three-dimensional tracks of charged particles with a fine position resolution. The μ-TPC is used as an electron-tracking Compton camera, as well as for thermal neutron imaging and dark matter search experiments. Two applications related to astronomy are described in the fol- lowing sections. X-ray polarimetry In the x-ray band, spectroscopy, imaging, and photometry have revealed high-energy phenomena in the universe. However, Figure 1. Shown is a schematic view of the μ-PIC. The pitch of the electrodes and the thickness of the gas volume vary, depending on the application. Anodes are read by strips on the rear, orthogonal to the cathode strips on large area μ-PICs. some important astrophysical subjects are accessible only with x-ray polarimetry. For example, measuring the x-ray polariza- tion as a function of energy for an accretion disc surrounding a black hole provides the space-time structure near a black hole. This is because, unlike the spectrum, polarization is strongly af- fected by general-relativistic effects. 2 However, due to the low sensitivity of x-ray polarization, the detection of such astronom- ical objects to date has been limited to the Crab Nebula. 3 For imaging polarimetry, the photo-electric effect is the most useful physical process because the cross section is largest at en- ergies of less than 20keV. To obtain the polarization angle, the photo-electron—which is ejected in the polarization direction with the maximum probability—is tracked. Therefore, a micro- pattern gas detector, such as a μ-PIC, can operate as an x-ray polarimeter. Continued on next page