Sec. 5-2] RADIATION TRANSDUCERS 301 



magnitude A, the '"Schubweg," is the average distance moved by a 

 cluster of electrons in the crystal; it depends upon the crystal 

 material and is proportional to the applied field strength. The pulse 

 size rises first linearly with the applied voltage and approaches 

 saturation at higher field strengths. In the linear part the pulse size 

 is directly proportional to the energy of the incident particle. If 

 the incident particle penetrates deeply into the crystal and causes 

 the release of electrons uniformly throughout the crystal volume, 

 Eq. (1) changes to 





d 



A crystal acts as a good counter only if the Schubweg A has a magni- 

 tude from a few tenths of a millimeter to 1 cm. The number of 

 electrons released by an incident particle of a given energy depends 

 upon the energy required per ion pair formation. In a crystal, this 

 energy is about 5 to 10 eV as compared to about 30 eV in a gas. 

 Only a fraction (e.g., 60 per cent) of the energy of the incident 

 particle is used to release electrons; a 1-MeV beta particle causes 

 in a silver chloride crystal about 1.2 x 10 5 electrons. The lowest 

 detectable energy for an incident particle is of the order of 1 keV 

 (100 to 200 electrons). 



A polarization effect takes place in some crystals. The immobile 

 holes in some crystals form a positive space charge near the negative 

 electrodes; the electrons trapped near the positive electrode form 

 a negative space charge. These space charges eventually reduce the 

 effect of the applied field and the size of the output pulses. Under 

 certain conditions, the polarization effect can be overcome by 

 periodically reversing the applied field. Another possibility to 

 reduce the positive space charges is to irradiate the crystal with 

 infrared light. The polarization effect limits the lifetime of the 

 crystal counter. If the polarization cannot be removed, a counter 

 exposed to an incident gamma radiation of 1 MeV will have a life- 

 time of 10 6 counts/cm 3 of crystal material; exposed to an alpha radia- 

 tion of 5 MeV energy, it will have a lifetime of 10 8 counts/cm 3 . The 

 lifetime is practically unlimited when the space charges are removed. 



The output pulse and, therefore, the resolving time of the counter 

 are considerably shorter than that of the ionization chamber or of 

 the Geiger counter. The resolving time depends upon the applied 

 field; for a field strength of about 5 kY/em the resolving time of the 

 silver chloride or silver bromide counter is about 0.2 //sec. that of 

 the diamond counter only 0.04 ^sec. 



