Astronomical X-ray and Gamma-ray Observations

High energy photons can hardly be reflected on the surface of materials as optical photons can easily be, due to high penetration power. For X-rays or gamma-rays, the real part of the complex refractive index of materials is very close to 1. Thus, X-ray photons can be reflected only if the angle made by the incoming light ray and the surface of material is very small (≲ 1 deg). Getting information of the directions of incoming photons and increasing a photon collection efficiency can be realized using large, cylinder-like mirrors with much longer focal length than those required for optical telescopes (more than a few meters, typically). In the case of gamma-rays, however, this technique requires unrealistically larger optical systems. Instead, direction measurement of incoming gamma-ray photons have been realized either by tracking the secondary particles produced in a detector system by the incident photon (e.g., Fermi Large Area Telescope: Atwood et al. 2009; High Energy Stereoscopic System: Hinton & HESS Collaboration 2004), or by geometric techniques, i.e., comparing the count rates of multiple detectors distributed with different viewing angles (e.g., Fermi Gamma-ray Burst Monitor: Meegan et al. 2009), or imaging with coded aperture masks (e.g., Swift Burst Alert Telescope: Gehrels et al. 2004).

In order to measure the energy of an incident photon by a detector, interaction pro- cess between the photon and the detector material have to be considered. The dominant physical interaction between an X-ray photon and matter is photoabsorption. Thus, the energy deposit by an incident X-ray photon exactly corresponds to the energy of the photon. In X-ray detectors, certain carriers (e.g., electrons, holes, or optical photons) which are produced by the energy deposit are collected and are changed into electric signals such as the voltage. In the gamma-ray energy range (≳ 10 MeV), the dominant interaction process between photons and matter is pair production of an electron and positron. Since such high-energy electrons and positrons produce electromagnetic cascade, the total energy of the secondary particles usually has to be measured in order to know the energy of the incident photon. For this purpose, detectors made of heavy materials with high stopping power are placed on the bottom of the detector system.