New Type of Position-Sensitive Detectors of Ionizing Radiation Using Risetime Measurement

A new method was developed for determining the position of ionizing events by measuring the risetime of output pulses from detectors having high resistance collectors. Several prototype detectors, such as proportional counters, pulse ion chambers, and surface barrier detectors, were constructed and tested for this purpose. These detectors were treated as infinite RC lines with distributed parameters, and the position-dependent risetime of the output pulses was measured by crossover timing after double RC differentiation. With these detectors the impact locations of x rays, thermal neutrons, and alpha particles were determined. The spatial uncertainty obtained with 400 mm long proportional counters was 0.5 mm FWHM, for 22 keV x rays, 6 mm FWHM for thermal neutrons, and 0.66 mm FWHM for 242Cm alpha particles. A spatial uncertainty of 1.25 mm FWHM was obtained with a pulse ion chamber for 242Cm alpha particles. This method of determining the position of an ionizing event gives good linearity and spatial resolution with relatively simple electronic circuitry.