Semiconductor Particle Spectrometers

Following almost ten years of incubation, solid state radiation detectors have, in recent years, achieved increasing emphasis and use in low energy nuclear physics.1-4 Such detectors depend on the production of free-charge carriers in crystals by the interaction of incident particles with bound electrons. Solid-state detectors are analogues of the well-known gaseous ionization chamber. In an ionization chamber, the incident particle produces electron-ion pairs; in a solid, the incident particle produces electron-hole pairs. Detection of the incident particle is then a problem of collection of the liberated charge by application of an electric field. An important difference between the solid and the gas is that many more electronhole pairs are produced in the solid than electron-ion pairs in the gas per unit energy loss of the particle. This leads to a smaller fractional statistical fluctuation in the number of carriers produced and hence to potentially higher resolution in the measurement of the incident particle energy. Other features also make the solid-state counter attractive for nuclear physics applications; among these are high density of the solid compared to a gas, small size and short pulse rise time. Collection of the liberated charge in a solid is more complicated than in a gas. Holes and electrons may be lost by recombination in the dense plasma formed along the particle track, or by trapping of the moving carriers subsequent to their separation by the electric field. These effects; which are discussed in § 2, depend on the detailed properties of the solid, the potential distribution in the device, and the ionization density along the initial track. The requirement of an electric field for charge collection makes it necessary that the crystal be an insulator. Otherwise high current would result, leading to excessive noise. The simplest type of crystal detector consists of conducting electrodes on opposite faces of an insulating crystal. Early attemptas*s*r** to produce such detectors