An account of the work done during the year in setting up a counter circuit

Abstract

When an ionizing particle or radiation enters the sensitive volume of a counter an avalanche of electrons is formed by the process known as ionization by collisions. In general a counter is operated with the central wire positive with respect to the cylinder, and consequently the electrons are collected by the wire. A negative voltage pulse is thus developed. On the wire, the size of which depends upon the capacitance of the wire system and the magnitude of the collected charge. The duration and form of the pulse will depend upon the characteristics of the particular counter usedand upon the time constant of the receiving system. In general each ionizing particle produces a voltage pulse, and it is the function of the associated electrical circuits to record these pulses and thus count the number of particles entering the sensitive volume of counter. The size of the voltage pulse varies with the counter and the type of particle being detected, but the pulses can be increased to the necessary size to actuate scaling and recording devices by means of an amplifier. For counters operating in the Geiger region the pulse size is independent of the type ofionizing agent being detected, providing the voltage across the counter is kept constant, as the counter voltage is increased the pulse size varies over a considerable range until the voltage is reached at which the counter goes into continuous discharge. In consequence, for use in conjunction with different counters, working at various operating potentials, an amplifier with variable gain is necessary. The amplifier can then be set to give a suitable output pulse to the following circuits independently of the size of the input pulse. For slow counting rates the counter and amplifier can be used directly in conjunction witha suitable mechanical recorder. Many counters have recovery times as short as 10 seconds and correspondingly high counting rates. Amplifiers can be designed with RC time constants as short as 10 seconds or less, so that the results are not affected by the amplifying stage. On the other hand mechanical recorders have comparatively long time constants and if used directly would seriously decrease the possible counting rate. This is avoided by the use of electrical scaling circuits which pass on only a certain proportion of the pulses to the recording circuit. In this case the recovery time and counting rate of the whole system is governed entirely by the characteristics of the counter itself. The pulses from the last stage of the scaling circuit pass on to the recording circuit. It is the function of this circuit to use these pulses to operate some mechanical recorder. The pulses are not usually able to influence the counter directly, and so a hard valve or thyratron is used in conjunction with a telephone or Cenco counter. It will be seen from the above discussion that the electrical circuits needed for use in association with a Geiger counter are as follows 1. Voltage supply circuits for amplifier, scaling and recording circuits. 2. Pulse amplifier. 3. Scaling circuit. 4. Recording circuit. 5. Stabilised high voltage supply for Geiger counter. The arrangement of these circuits will be such as is shown in block form in diagram I.