379 



"^donors 



-O- -0- 



7///// 

 /Z^VALENCE BAND 



Figure 7. 



impurities present. It is impurities wliicli change the color of the 

 diamond by introducing energy levels at various heights. Lumi- 

 nescence is likewise due to the type of impurity in the crystal, and 

 photoconductivity can be understood as an electron exchange between 

 the bands and the impurity levels. 



The behavior of a diamond when used as a counter is also controlled 

 by the number and kind of impurity. There are about 10" in a cubic 

 centimeter of material. Since there are 10^^ carbon atoms in this 

 volume, the impurity concentration is one per million. What is of 

 interest is to know at what levels these impurities lie and how one dia- 

 mond differs from another in this respect. To investigate the energy 

 levels, we have been using beta particles from a Sr^° source and then 

 making a systematic survey of the effect of illumination on the 

 counting rate. 



A very small hole is made in one of the electrodes and the diamond 

 is irradiated through the hole wdth beta particles and with a field 

 strength of 10* volts/centimeter. A typical counting rate under 

 these conditions is 100 pulses per second. The energy of the beta par- 

 ticle is 2.2 MeV, so that each beta is likely to produce 4 X 10^ electron 

 hole pairs, and a little calculation, taking into account the size of the 



