Detection of α particles with the aid of a fluorescence counter

The operation principle of the fluorescence counter, used as α particles detector, is analyzed in the first part. Detection can be done in two ways: by counting the pulses due to each α particle, or by integrating all pulses and measuring the average current obtained. In the second part, three series of measurements are presented: 1 - two fluorescent substances (zinc sulfate and anthracene) are placed in front of the photocathode of three types of photomultipliers (RCA 931A, EMI 4588, and EMI 5311). These substances are bombarded with the α radiations of a Po source and then irradiated by the β and γ radiations of a Ra source in order to study the light emission of these thin film substances when submitted to the three types of radiations. The results show that thanks to the amplitude of the emitted light pulses, the fluorescence counter, when submitted to the three types of radiations, allows to distinguish between the α radiations of the polonium and the β and γ radiations of the radium source. The output current of a 931A, when measured with a galvanometer, allows to detect Po sources with an intensity of about 10-6 curie. This is observed when its photocathode receives the light from a ZnS-Ag coating bombarded by the α particles of Po. The quantum efficiency of the counter is close to 100% for the α particles of Po. This efficiency is evaluated by comparison with the efficiency of a thin wall Geiger-Mueller counter. Moreover, when a thin crystal of anthracene is used as detector, the energy of the incident particles can be measured with a 2% preciseness. (J.S.)