DIAMONDS — LOGIE 383 



at the anode, the holes are trapped mainly in the acceptor levels and 

 the space charge is developed. Subsequent infrared radiation raises 

 electrons from the valence band to these levels occupied by the holes 

 and so reduces the space charge. This explains why the counting rate 

 improves under illumination. If the diamond is bombarded through 

 the cathode, then the electrons are trapped in the empty levels of the 

 donor states. The energies at which these lie are so far from the band 

 edges that infrared light can effect no change in the space charge. 

 This is one of the first tests that was made on the correctness of these 

 ideas and which agreed with the predictions. It also clears up a matter 

 over which disagreement has arisen between experimenters. Some 

 writers have found that infrared improves the counting ability, some 

 that it makes no difference, while yet others reported that it reduced 

 the counting rate. None mentions the electrode through which the 

 diamond was irradiated and which as we now see determines the 

 precise effect. A reduction in the counting rate can easily be imagined 

 if the diamond is bombarded through the negative electrode and had 

 not been carefully prepared beforehand to insure equilibrium condi- 

 tions with electrons in the lowest permitted energy levels. 



When we come to consider the effect of radiation of higher fre- 

 quency which has sufScient energy to produce transitions to the donor 

 levels from the valence band, our predictions are more tentative. If 

 the effect is to empty a large number of these of their electrons, then 

 a negative space charge will develop and will assume a density in the 

 crystal which is a function of the distance from the anode and thus 

 reduce the effective field considerably and over a large part of the 

 crystal. In fact, instead of about one trap per million being involved 

 in the creation of the space charge as we saw was the case under beta 

 bombardment alone, all of them could now be activated by tlie high- 

 energy illumination and could swamp the lesser effect of the bombard- 

 ment. The high-energy light has the same general etTect on the count- 

 ing rate whether the beta bombardment is through the anode or 

 through the cathode. 



On an occasion such as this, the speaker is expected to leave his 

 audience feeling that all has been explained, all is known; that the 

 speaker is master of tlie entire gamut of loiowledge. I am afraid that 

 in this respect I must be found sadly wanting, and to one important 

 question I have no answer. I cannot tell you why some diamonds are 

 counters and others are not. However, the interplay of theory and 

 experiment proceeds and I hope that my successor in his inaugl^ral 

 address will express his amazement that we could have been so igno- 

 rant of what will then be so well understood. 



