DIAMONDS — LOGIE 377 



100 million particles per second. The energy to form an electron hole 

 is about 6 eV, compared to 30 eV to produce ionization in air. It can 

 count alpha, beta, and gamma particles of high energy as efficiently 

 as a large Geiger counter. 



All this is on the credit side. There are also drawbacks, perhaps 

 the most serious, and certainly at first sight the most surprising of 

 whicli, is that only a very small percentage of diamonds make good 

 counters. Even among those that do count, there is a wide diver- 

 gence in performance. Considerable attention has been paid by 

 Champion, Dyer, Ditchburn, Willardson, and others to the correla- 

 tion between the counting ability and other physical properties. 

 Those diamonds whose absorption band is less than 2,500 A are the 

 most likely to have comiting properties. Not all of them do, and of 

 course there are some diamonds of type 1 wliich are counters also. 

 The diamonds we have been using in this lecture are, in fact, type 1. 

 Champion believes that counting diamonds are those wliich are com- 

 posed of layers of highly perfect crystalline material separated by 

 much thiimer barriers of imperfect material. Again, diamonds which 

 fluoresce are not likely to be good counters, though it does not follow 

 that nonfluorescing diamonds are sure to count. There is fewer than 

 one diamond in a thousand which can be classified as a good counter. 



The second defect of the diamond counter is due to what has been 

 termed "polarization." Many diamonds, even though they are good 

 counters when they are first bombarded, lose their efficiency if the 

 bombardment is continued. By giving the diamond a rest or by 

 heating it or, sometimes, by irradiating it with ultraviolet light or 

 gamma or beta radiation, it will recover its initial ability. 



It is now time to turn to the modifications which result from 

 defects or impurities in the diamond. The effect of these can easily 

 be shown experimentally. One way of producing a defect in a dia- 

 mond is to bombard it with neutrons. These neutrons are much 

 heavier than electrons, and when they collide with a carbon atom in 

 the lattice they knock it out of position. This kind of defect is called 

 a vacancy. The electron bonds at this point in the crystal have been 

 altered by the vacancy and the ease with which electrons at that 

 point can be liberated and transferred to the conduction band has 

 been changed; i.e., an energy level intermediate between the valence 

 and conducting bands has been created. Diamonds which have been 

 treated in this way show very marked changes in their properties. 

 Wliite diamonds become green and counting diamonds lose this 

 ability. 



There are three kinds of imperfections which may occur in crystals 

 and are of interest to us. These are : 



1. Vacancies such as those which have just been described. 



