DIAMONDS — LOGIE 381 



the trapping occurs at impurity centers. The lifetime will then be 

 proportional to the concentration of these. 



The falling off in the counting rate must be due to a lowering of 

 the effective electric field inside the diamond. It cannot be due to a 

 change in the niunber of electrons produced by each beta particle, nor 

 is the lifetime likely to be altered since the number of traps changes 

 very little. What happens is that the trapped electrons or holes pro- 

 duce a space charge in the diamond which has the effect of reducing 

 the electric field. Not many trapped electrons are required to do 

 this. If only 1 trap in 10 million is filled with electrons, it is sufficient 

 to reduce the internal field to zero in the most favorable conditions, 

 but the precise effect will depend on the position of the center of 

 gravity of the space charge. To discover whether a space charge is 

 really produced, the external field was removed when polarization 

 had been established and pulses obtained in the reverse sense were 

 recorded. The diamond continues to count mider these circumstances 

 for a few minutes and until the space charge is reduced to a very 

 low value. If the diamond is not bombarded and no electric field is 

 applied, the space charge may persist for a very long time. We have 

 observed it over a period of days. It will be appreciated that the 

 diamond has to be carefully prepared to msure that no residual space 

 charge remains when a series of tests is to be conducted. The pro- 

 cedure which we have found to be the most satisfactory is to irradiate 

 the diamond for 12 hours with no field applied and in the dark before 

 starting an experiment. By this means sufficient free holes and elec- 

 trons are generated to achieve the equivalent of thermal equilibrium. 

 Only when these precautions are taken are the results reproducible. 



Electrons can enter and leave the diamond fairly readily through 

 the electrodes if the diamond has been activated so as to produce free 

 electrons and holes. No particular precautions are necessary to make 

 good contact between the diamond and the metal electrodes applied 

 to it. In fact, if it is suspended on a thread between two metal elec- 

 trodes, but touching only one of them, then the diamond is found to 

 become charged. The process is quite a slow one and depends on the 

 intensity of the irradiation which, in turn, controls the number of 

 electrons in the conduction band. A diamond suspended in this way 

 is found to swing from one electrode to the other and then back again 

 when it has acquired charge of the opposite polarity. 



Let us return to the conventional arrangement of the diamond be- 

 tween two electrodes, and with beta particles entering through a hole 

 in the anode. Most of the free electrons will reach the anode before 

 being trapped, because the electron pairs are produced fairly close to 

 this electrode. The holes, on the other hand, which have to traverse 

 the few millimeters to the cathode are likely to have most of their 



