5.2 DETECTOR TECHNOLOGIES

A number of detector technologies are used for gamma-ray imaging and spectroscopy. Many aspects of such detectors are well established, such as plastic scintillators and photomultiplier tubes. What makes dramatic progress obtainable in this field is new and developing technologies which influence all types of gamma-ray imaging. Several general areas stand out as key new technologies.

5.2.1 STRIP AND PIXEL DETECTORS

5.2.1.1 CdZnTe

CdZnTe detectors are at the threshold of becoming a widely used tool in gamma-ray astronomy. The basic properties that make them interesting are: 1) large enough band gap energy (1.6 eV) to permit room temperature operation; 2) high density (~6 g cm^-2) for good stopping power; 3) high atomic numbers (48 for Cd, 52 for Te) for photoelectric absorption up to high energies (For example, CdZnTe has a photoelectric attenuation coefficient that is more than 10 times the Compton scattering coefficient up to 110 keV compared to 60 keV for Ge and 25 keV for Si.); 4) low bias voltages of typically 200 volts compared with thousands of volts for Ge; 5) ease of electrode segmentation for fine imaging; 6) low susceptibility to contamination problems so that the detectors can be easily fabricated and handled; 7) availability of large crystals so that multi detector arrays can be fabricated at low cost; and 8) increased resistivity with introduction of Zn to improve performance over CdTe. The high density and particularly high atomic number of CdZnTe combine to give several important characteristics. The domination of photoelectric attenuation means that CdZnTe has single-site absorptions (good for imaging) throughout the low-energy gamma-ray band. Also, the large attenuation coefficient for photoelectric absorption means that CdZnTe detectors can be very thin and still efficient at stopping gamma rays. The typical size of a CdZnTe detector is 1 cm2 in area by 2 mm thickness, with the thickness limited by hole trapping effects. Recently, however, it has been shown that detectors with segmented electrodes can achieve reasonable spectroscopy using only the electron signal due to the "near-field" effect, thus enabling thicker detectors. The small sizes of CdZnTe detectors means that large-area detection planes will require arrays with hundreds or thousands of individual detectors. In order to keep the electronics power level within reasonable levels for spaceflight applications, VLSI front-end amplifiers are typically used. Power levels of a few mWatts per detector are then achievable. The ability to finely segment the contacts of CdZnTe detectors, combined with their high photoelectric attenuation coefficient, means that they are ideal for high-resolution imagers.   Detectors with strip contacts and with pixel contacts have been fabricated with pitches of 100 Fm or less. Applications of such finely segmented detectors include wide-field coded mask instruments with better than arcminute (in some cases approaching arcsecond) angular resolutions and high-sensitivity focusing hard X-ray telescopes with arcminute resolutions. Examples of instruments that incorporate CdZnTe or CdZn detectors are: the INTEGRAL imager; the BASIS, and EXIST, mission concepts; and several recently-proposed balloon instruments.

5.2.1.2 GERMANIUM

Germanium remains the only solid state detector capable of high resolution spectroscopy in the nuclear energy band. This is the reason it has been used in satellite (HEAO C-1 and INTEGRAL) and balloon missions (Bell/Sandia, Lockheed/MSFC, GRIS, Hexagone) requiring the best resolution. However, detector geometries other than the current large volume co-axial detector are needed to develop instruments with imaging capabilities and sensitivities beyond INTEGRAL. Two geometries which are being investigated are the Ge planar strip detector and the Ge "finger" detector. Both of these concepts produce pixelated detectors with appropriate position resolution for applications in coded-aperture or Compton telescope imaging systems. Ge detectors with 2-mm spatial resolution have been demonstrated in the laboratory. A Ge Compton telescope using such pixelated detectors could achieve sensitivities 10 - 50 times better than INTEGRAL and have good sensitivity to both diffuse and broadened line emissions. This is because the good energy resolution of Ge also reduces the background by improving the angular resolution in Compton telescopes so that the sensitivity improves proportionally with resolution, not as the square root of resolution.

5.2.1.3 SILICON STRIPS

Silicon microstrip detectors have been developed at accelerators and are now readily available from several commercial manufacturers. These devices are, in effect, big integrated circuits fabricated on Si. The spatial resolution is determined by the width of the semiconductor strips fabricated on the Si. Resolution of 50 Fm is easily attainable by modern photolithography. Because the Si microstrips are fabricated on thin layers of silicon, they have very good two track resolution, typically 3 times the strip pitch. Si microstrip detectors are available, currently, in 6 cm x 6 cm size. Si strip detectors are applicable to both Compton and pair telescopes.