Preparation and Performance of a Homogeneous Silicon Minimum-Ionizing Particle Detector

Following up on the work of C. R. Sun1 , we are investigating the performance of homogeneous, high-resistivity, silicon crystals as detectors for minimum-ionizing particles. In particular we are considering such a detector for use in conjunction with the operation of a hydrogen bubble chamber. Such devices would have the advantages of good spatial resolution, compactness, and stable performance in high magnetic fields. Operation would be at liquid nitrogen temperature. The detectors are cut as 2.5 mm thick wafers from an n-type ingot ~1.8 cm in diameter and with a typical resistivity of 1000 to 3000 ohm-cm. Nickel ohmic contacts are applied to each side of the wafer by a chemical plating process. In order to keep the dark current to a minimum, we try to raise the resistivity of the prospective detector close to the intrinsic level. This is accomplished by exposing the silicon, at 25°C, to a high flux of ¿ radiation from a Co60 source. Typically, after an exposure of 1.5 × 107 roentgens the silicon will have a resistivity of 4.0 × 106 ohm-cm at liquid nitrogen temperature (78°K). Using a charge sensitive amplifier, we observe pulse rise times of the order of 150 nanoseconds and signal to noise ratios greater than 10. There exists a "critical exposure level," C.E.L., for the ¿ irradiation. For radiation dosages greater than the C.E.L., the detector performance deteriorates. It seems possible to determine the C.E.L. conveniently by checking the detector resistivity at 25°C; for when the C.E.L.