The detection of photons above 10 keV through MeV and GeV energies is challenging due to the penetrating nature of the radiation, which can require large detector volumes, resulting in correspondingly high background. In this energy range, most detectors in space are either scintillators or solid-state detectors. The choice of detector technology depends on the energy range of interest, expected levels of signal and background, required energy and spatial resolution, particle environment on orbit, and other factors. This section covers the materials and configurations commonly used from 10 keV to > 1 GeV.
Most high energy detectors in space are based on scintillators or solid-state detectors. Scintillators are the older technology and are generally used where large detector volumes and lower cost are paramount. Solid-state diode detectors, e.g. germanium, silicon, and cadmium telluride (CdTe) or cadmium zinc telluride (Cd 1-x Zn x Te or CZT), generally have better energy resolution than scintillators and a lower energy threshold, and can be more easily pixellated for fine spatial resolution if that is required. They are more expensive than scintillators and more difficult to produce, package and read out in large volumes.
For all these materials, photons are detected when their energy is transferred to electrons via photoelectric absorption, Compton scattering, or pair production (which also produces positrons, of course). The high energy particles come to a stop in the detector volume, producing ionization that is detected by varying methods.
I will review the most commonly used solid-state and scintillator materials and detector configurations. Considerable work has also been done worldwide on gas and liquid detectors and on newer or less common semiconductors and scintillators. For a much more detailed discussion of a wider range of detectors, as well as an excellent treatment of general considerations in photon counting and electronics, see the excellent textbook by Knoll [33].
The optimum configuration and material for a detector depend most strongly on the energy range of the photons to be observed. Figure 1 shows the cross-sections for photoelectric absorption, Compton scattering, and pair production for photons by elements commonly used in detectors: silicon and germanium (in solid-state detectors) and iodine and bismuth (in common scintillators).
Simple efficiency calculations based on cross-sections can assist with instrument design, particularly when photoelectric interactions are dominant, but Monte Carlo simulation is the most powerful and flexible tool. It can be used to model the response to source and background radiation and to incident particles other than photons as well. The packages most commonly used are GEANT3 and GEANT4 (GEometry ANd Tracking), originally developed at the European Organization for Nuclear Research (CERN) for accelerator applications [17; 1].
Since the cross-section for photoelectric absorption is large at low energies, low-energy detectors can be quite thin. Photoelectric absorption is also a strong function of atomic number, so that, for example, a silicon detector 1 mm thick will absorb >50% of X-rays up to 23 keV, while a 1 mm CdTe detector will do the same up to 110 keV. At these low energies, high spatial resolution is often desired when there is an imaging system using focusing optics or a coded mask (see Chapter 12). This can be accomplished with multiple small detectors, by pixellating the electrodes of solid-state detectors, or by using multiple or position-sensitive phototubes to read out a scintillator (“Anger camera” configuration, after inventor Hal Oscar Anger).
At energies of more than about 300 keV, photoelectric cross sections are small even at high atomic number, and detectors must be made large enough that photons can Compton scatter in the detector and still be photoelectrically absorbed afterwards. Even though the Compton cross section is nearly independent of atomic number, a high atomic number is still critical for stopping the dowscattered photon before it escapes the detector carrying off some of its energy. A low atomic number can be desireable for the scattering plane of a Compton telescope or for a detector or shield designed to stop charged particles or X-rays and pass γ-rays through.
In many cases the optimum solution for maximizing sensitivity will be to have separate detectors for low energies (thin) and high energies (thick). Since most cosmic sources have falling energy spectra, high-energy detectors will generally need larger area than low-energy detectors in order to reach comparable sensitivity. Even large monolithic detectors can serve as elements for a coarse imaging system when placed in a large array. The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) provides two good examples. The Spectrometer on INTEGRAL (SPI) [68] has coaxial germanium detectors with a characteristic size of 7 cm serving as pixels below a large coded mask (see Chapter 12), and the Imager on Board the INTEGRAL Spacecraft (IBIS) includes thick fingers of CsI serving as pixels beneath a finer mask than SPI’s [66]. The large germanium detectors on the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) [61] sit below rotation modulation collimators (see Chapter 12) that do not require position sensitivity.
At MeV and GeV energies, the physics of the photon interactions in matter can be exploited to reject background and determine the direction of the incoming photon.
In the range of a few hundred keV to tens of MeV, large-volume d
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