The use of thyratrons for high speed automatic counting physical phenomena

Abstract

It is sometimes necessary to count physical events which are repeated very rapidly. An example of this occurs in problems connected with radio-activity, where it is desired to count the number of α-particles entering a collecting chamber. One method of doing this is to amplify (by means of thermionic valves) the minute ionisation currents produced in the chamber by single α-particles, and to pass the amplified currents—which take the form of impul­sive surges of about 10 milliamperes—through telephones, counting the “clicks” by ear. For more rapid working, the currents are passed through an oscillograph, and a photographic record is obtained, which can be analysed at leisure after development. It is often necessary, however, in addition to recording the particles rapidly, to know the result immediately, so that further observations may be made on points of special interest. In a recent paper it has been shown that it is possible to use a mechanical relay and a counting meter (similar to a cyclometer) in conjunction with a valve amplifier for auto­matic numerical counting of α-particles. Each 10 milliampere current surge (corresponding to a particle) passes through the relay windings and closes the contacts momentarily, thus completing a local circuit containing the counting meter. Such a method of counting, while extremely simple and convenient, suffers from one great drawback. However sensitive the relay and counting meter mechanism may be, a small but finite time is required by their armatures to move to and fro in the process of counting. If, therefore, a second particle arrives before the recording of the first has been completed, it will be missed. Thus, this simple method of counting is unsuitable when the counting speed is such that there is a probability of a second particle arriving within the recording period. While it is true that a probability correction could be applied, it is evidently preferable to endeavour to make the system record all particles, however rapidly they may arrive. In the present paper it will be shown how this defect may be overcome by the use of thyratron valves in place of mechanical relays. Such valves are employed in the present work as direct current relays, and can be regarded as having negligible inertia, and no moving parts. Further, they are much more certain in action than mechanical relays, and can control larger currents for operating counting meters than could be safely carried by the contacts of delicately adjusted mechanical relays. The principle underlying the circuits which are described in the paper is that the process of counting is shared by a group of inter-connected thyratrons, which automatically switch one another into operation as need arises, so that however rapidly particles are arriving, there is always a thyratron available for each particle. The switching process is entirely electrical , and, as it involves no mechanical operations of any kind, it can be made to function extremely rapidly. The auxiliary mechanical apparatus which may be employed ( e.g ., counting meter) is given an oppor­tunity of operating comparatively slowly, and is not necessarily the governing factor which imposes a limit to the counting speed. A suggestion is also made at the end of the paper as to the possibility of constructing a “counting meter,” the dials of which consist entirely of thyratrons, and contain no moving parts of any description.