Development of a fast x-ray detector

Abstract

This thesis describes the development of a gas scintillation drift counter as a high counting rate X-ray detector. The new device is a modified version of the gas scintillation counter which is capable of working in a digital mode at a quite high rate without producing space charge effects. A gas scintillation drift counter has been built to investigate its behaviour using a gas mixture of argon and nitrogen. Scintillation and transport properties of the gas mixture have been studied using an Am(^241) α-particle source and a (Fe(^55)) 5.9 keV radioactive X-ray source in order to obtain optimum relative compositions of argon and nitrogen. The effects of various operating parameters on the energy resolution of the counter and the characteristics of output pulses have been extensively studied. When deciding on the counter parameters and operating conditions, a special emphasis has been placed on conditions which produce a minimum of space charge around the anode, and small pulse-widths. Pulse response, gain variations with counting rate and stability of the RCA 8575 photomultiplier used to detect scintillation light have been investigated using a light emitting diode pulser, a constant light source and primary scintillation pulses from 6.9 keV X-rays. An increase in the amplitude of output pulses and a change in the dynode potentials with pulsing rate have been observed when operating the photo tube with a resistive potential distribution network attached to its dynodes. These variations have been successfully eliminated by stabilising the dynode potentials using emitter follower configurations. J An energy resolution of 30% has been obtained using 5.9 keVX-rays with a gas amplification factor of about 20. Tests which have been done using cobalt K(_α)X-rays reveal that the resolving time of the counter system increases with the anode voltage and for a fixed discrimination level of 25 mV it varies from 43 ns at the anode voltage of 2600 V to 67 ns at 3000 V. An energy resolution of 40% has been obtained with cobalt K (_α) x-rays at a counting rate of 6 MHz and with a gas amplification factor of 15. The characteristics of the scintillation pulses are found to be degraded with increasing intensity of the X-ray beam because of coincident events. However, a counting rate of 9.5 MHz has been achieved with this counter at the expense of its energy resolution.