376 ANNUAL REPORT SMITHSONIAN INSTITUTION, 196 



the ultraviolet absorption becomes pronounced at wavelengths near 

 3,000 X or near the theoretically expected 2,200 A, respectively. All 

 types show some infrared absorption and both show luminescence, 

 though it is more marked in type 1 than in type 2. Often neither type 

 is a perfect insulator, though their respective resistivities may be as 

 high as 10^* ohm cm. Then there are type 2 diamonds which are 

 colored and some which are good conductors. These latter were classi- 

 fied by Dr. Custers as type 26, so that type 2a is reserved for diamonds 

 with ultraviolet absorption at 2,200 A which are insulators. 



The divergence between the predicted properties and those which 

 are actually realized is so marked that obviously there are other 

 factors which have not yet been taken into account. "Where the theory 

 has been at fault is in leaving the energy gap between the valence 

 band and the conduction band completely free of energy levels. As 

 soon as it is accepted that some electrons may require less than 6 eV 

 to reach the conduction band, then all the difficulties are removed. 

 How can such a situation arise ? It is now fairly clear that traces of 

 impurities or defects in the lattice will produce just this effect. Im- 

 perfections of this kind, in concentrations as low as 1 part in 10 mil- 

 lion, will explain all the anomalies. 



Instead of dealing with each of the phenomena in turn, let us follow 

 the investigations which have been made in our laboratories into the 

 conductivity induced by alpha or beta particle bombardment of the 

 crystal. As we consider the effects, I believe that a general under- 

 standing of most of the other phenomena will emerge. 



It has long been known that certain crystals when bombarded by 

 energetic particles are made temporarily conducting. The pioneer 

 work in this field was done by Van Heerden and, from what has been 

 said earlier, the effect of bombarding the diamond is to raise electrons 

 from the valence band to the conduction band. If an electric field is 

 applied to the diamond, it will sweep these electrons and holes toward 

 the electrodes so that each incoming particle results in a pulse of 

 current. 



We see that the diamond behaves very much like a Geiger counter. 

 In fact it has several advantages over a conventional Geiger tube. 

 Its density is high so that the stopping power of the diamond is 

 3,000 times that of a gas. In other words, a 1 MeY beta particle 

 which would travel through a meter of air will expend all its energy 

 in 1 mm. of diamond. One cubic mm. of diamond is then as effective 

 as several cubic centimetei-s of Geiger tube and is well suited to appli- 

 cation where a small probe is necessary. The medical field is an 

 example which immediately comes to mind. Diamond has an ex- 

 tremely fast resolving time and can distinguish particles arriving at 

 intervals of lO"* sec. It is thus capable of counting at the rate of 



