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Astronomical applications of a micro pixel chamber

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.
20 October 2006, SPIE Newsroom. DOI: 10.1117/2.1200609.0408

Recently, micro-pattern gaseous detectors have been developed for x-ray, gamma-ray, and charged-particle imaging devices with fine position resolution. However, a discharge occasionally 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.

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.

The μ-PIC has numerous applications. When it serves as an x-ray imager with a thin (less than 1cm) gas volume, the major 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 projection 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 following sections.

X-ray polarimetry

In the x-ray band, spectroscopy, imaging, and photometry have revealed high-energy phenomena in the universe. However, some important astrophysical subjects are accessible only with x-ray polarimetry. For example, measuring the x-ray polarization 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 affected by general-relativistic effects.2 However, due to the low sensitivity of x-ray polarization, the detection of such astronomical 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 energies 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.

We designed the pixel-readout μ-PIC shown in Figure 1 and developed a system for x-ray polarimetry.4 In the test model of the pixel-readout μ-PIC, there are 16 × 16 = 256 anodes, and the pitch of the electrodes is 600μm. We conducted a beam test at a synchrotron-radiation facility to examine the polarimetry performance.4

Figure 2 (top) shows an example of a photo-electron track due to an 8 keV polarized x-ray. The image angle is φ, which is the direction of the emission of the photo-electron. The resulting distribution of φ is shown in Figure 2 (bottom). The excess of the number of events is seen at φ =120°, which is consistent with the experimental setup where the polarization angle of the x-ray beam was φ = 120°.

Figure 2. The example at the top shows an x-ray image taken with μ-PIC. The sizes of the boxes are proportional to the pulse height of the signals where φ is the angle of the image. At the bottom, we show the distribution of φ generated by 8keV polarized x-rays.

Although we successfully developed a pixel-readout μ-PIC and measured the x-ray polarization, the 600μm pitch-size is insufficient to obtain a clear image. According to a simulation,5 the modulation factor with a pitch-size of 200μm should be nearly twice that for the 600μm size. Thus, a smaller pitch μ-PIC needs to be developed.

Electron-tracking Compton telescope

The MeV gamma-ray band is another uncultivated field in astronomy. This band should provide information on new aspects of high-energy phenomena in the universe. An electron-tracking Compton telescope based on a μ-TPC6 can enhance progress in the study of this energy band. The final version of this telescope is designed to have an effective area 10 times better than the COMPTEL telescope7 at the Compton Gamma-Ray Observatory. This new telescope consists of a μ-TPC and surrounding scintillators. The recoil electrons due to Compton scattering are tracked by the μ-TPC, while the Compton-scattered gamma rays are detected by the scintillators. The electron tracks are used to determine the event plane, and thus, reduce the background.

A prototype telescope, which consists of a μ-TPC with a detection volume of 10 × 10 × 15cm3 and pixelized (6 × 6 × 13mm3) GSO (gadolinium orthosilicate) scintillators with a detection area of 15 × 15cm2 were developed. The angular resolutions of 9° (angular resolution measure) and 140° (scatter plane deviation) were obtained for 662keV gamma-rays. One characteristic property of this telescope is the ability to image wide-spread gamma-ray sources. A gamma-ray image was taken of a volume-source in which radioactive isotopes of 133Ba and 137Cs were encapsulated. Although the angular resolutions are still far from the designed values, the shapes of the spread source in the image are shown in Figure 3. Currently, we are improving the angular resolutions by optimizing the TPC gas and adopting brighter scintillators. A balloon-borne experiment with this prototype telescope is scheduled for September 2006.

Figure 3. Shown is a gamma-ray image of a volume-source (center) and a picture of the volume-source (left bottom). Radioactive isotopes of 133Ba and 137Cs are encapsulated in the silver parts of the volume-source.

Hideaki Katagiri, Kentaro Miuchi, Hironori Matsumoto, Hidetoshi Kubo, Takeshi Tsuru, Toru Tanimori
Department of Physical Science, Graduate School of Science, Hiroshima University
Higashi-Hiroshima, Japan