Silicon PN Junction Surface-Barrier Detectors and their Application to the Dosimetry of X- and Gamma-Ray Beams

The recent availability of very pure semiconductors such as silicon permits the construction of solid state ionization chambers. The basic mechanism of such a device is briefly described, particularly in relation to its gas-filled counterpart. On exposure to ionizing radiation the sizes of the charge pulses formed in the device are proportional to the energy lost in the silicon and so the pulses may be treated individually or d.c. operation is possible. Under pulse operation a sensitive volume of only 0.04 ml permits dose-rate determination down to 105 rad/min and a study was made of the response to various gamma-emitting radioisotopes. The possibility of using the device under approximately wave-length independent conditions is discussed and the effect of ambient temperature considered. When d.c. measurements are carried out under applied bias difficulty is experienced due to the leakage current and it is shown that the device is best used without bias provided long minority carrier lifetime material is used. Here the photovoltaic effect is utilized and the current which flows, when measured under short-circuit conditions, is a linear function of dose-rate. Alternatively, the voltage generated across the junction may be determined by a high input impedance circuit, in which case there is a logarithmic response over a restricted dose-rate range. The current measurements are temperature independent between 0 and 50°C, but this is not true for the voltage measurements. Consideration is given to the wave-length dependence of the device from both a theoretical and experimental standpoint, and data is presented pertaining to the effect of radiation damage in relation to the various modes of use. In conclusion, the authors comment on their practical experience with these devices. They found the silicon devices to be very useful as fast-responding dose-rate meters, covering a wide range of dose-rates, and to be suitable for use in situations where their high sensitivity, small size and ruggedness are advantageous. (author)