A Semiconductor Avalanche Detector for High Gamma Intensities

A silicon radiation detector that uses avalanche multiplication of carriers similar to a G M counter is described. The detector is a p-n diode operated in the reverse direction beyond its breakdown. At small values ( 20 - 30 μA) the breakdown current is carried by pulses appearing statistically. These pulses are triggered by minority carriers which are accelerated within the electric field of the p-n junction so that carrier avalanches are released by impact ionization. Therefore, the pulse rate is proportional to the number of minority carriers in the crystal produced by any kind of ionizing radiation (energy > 1 eV) of sufficient penetration (visible light, X and gamma rays, beta rays). The amplitude of the current pulses is constant for a selected operating point. Contrary to the G M tube the pulses turn off statistically due to fluctuations of the momentary multiplication. Therefore only an average pulse length can be denoted (0.1 - 1 μsec at currents of 20-30 μA). This average pulse length is the main criterion for selecting a suitable operating point by means of diode voltage and load resistance, because the dead-time of the detector is equal to the pulse length. The recovery time can be neglected. To stabilize the operating point against temperature variation of the diode parameters two regulating circuits were developed. They adjust the diode voltage automatically so that either the amplitude or the average pulse length remains constant. The detectors can be used in the range of 102 to 106 R/h. The dose-rate that produces one pulse per second is of the order of 1 to 10 R/h for 60Co. The theoretical energy dependence of the response for gamma energies >100 keV is proportional to R(E)/E, where E is the average energy of the secondary electrons and R(E) their path length. An absorbed dose of 107 R had no influence on the detector characteristics. (author)