A new method of analysis of groups of alpha-rays.—(1) The alpha-rays from radium C, thorium C, and actinium C

Abstract

In a recent paper, Ward, Wynn-Williams, and Cave have described a method of counting accurately the number of α-particles emitted per second from a radiactive source. This method,-the practicability of which was first shown by Greinacher,-involves the amplification by means of thermionic valves of the small ionisation current caused by an α-particle in a few millimetres of its path in air. The amplification can be made linear, so that the final current which actuates the recording instrument is proportional to the ionisation produced by the particle in the ionisation chamber. Compared with that produced by an α-particle, the ionisation produced by an β -particle is too small to be recorded under working conditions. In their work on the determination of the number of α-particles emitted per second from a gram of radium, the magnified current was passed through an Einthoven galvanometer, and clear photographic records were obtained when the α-particles were entering the chamber at a rate of several hundred a minute. Under the conditions of the experiment, the β and γ-radiations were too weak to cause any disturbance, thus enabling the determination to be carried out with accuracy. For many purposes, however, it is seecential to count particles in the presence of strong γ-radiation, and to achieve this end special attention had to be paid to the amplifying and recording system. An account will be given in a later paper by Wynn-Williams and Ward, of the arrangements employed for this purpose, but it suffices to say here that the ions produced by a particle in the detecting vessel should be driven on to the collecting electrode in a time of the order of 10 -3 sec, and that the process of amplifying and recording should be sufficiently rapid to give a trace in an interval of less than 10 -3 seconds. For this purpose, a special oscillograph was constructed which has been found to be very effective. With these modifications it has been found possible to count α-particles at the rate of several hundred per minute even in the presence of strong γ-radiation.