616 ADOLF F. VOIGT 



does not need to be as great and the individual pulses produced by- 

 radiations of all kinds can be counted. There is a voltage region in 

 which the size of the pulse given by the counter is proportional to the 

 original amount of ionization and hence is different for the different 

 types of radiation. This so-called proportional region is useful for 

 counting a-active material in cases in which it is desirable to count a 

 particles above a high background of /S radiation. A satisfactory ar- 

 rangement {8Jf) uses methane at atmospheric pressure as counter gas, 

 a wire down the center of a cylinder of 1-2 cm. diameter as counter 

 with a potential of 2500 v. The samples are placed directly in the 

 counter. The circuit used with it contains a pulse height selector 

 along with the necessary amplifier {27) . 



An important use of this principle is in neutron counters. The ele- 

 ment boron is highly sensitive to neutrons giving instantaneously the 

 nuclear reaction : 



A considerable energy is also given off as kinetic energy of the product 

 particles. This results in a large amount of ionization, producing a 

 pulse. This instrument is capable of measuring neutrons in the pres- 

 ence of high intensities of other radiation, since no other radiation will 

 yield as much ionization per particle. The boron may be in the form 

 of a coat of the free element on the inside wall or of a filling of BF3 

 gas {85). 



Geiger-Mueller Counter. A further increase in the voltage or, 

 better, a reduction of the gas pressure to about 10 mm. Hg, at which 

 pressure the required voltage is 1000-1500, brings the tube to the 

 Geiger counter region. Although more properly termed a Geiger- 

 Mueller counter tube, current usage favors the shorter term ''Geiger 

 counter'' or the abbreviation "GM tube." The gas amplification 

 becomes greater, and the avalanche spreads completely along the 

 wire. This ionization is so extensive that the tube will continue to 

 discharge if the voltage on the counter electrodes is maintained at 

 its original value. However, when the discharge occurs, the voltage 

 across the counter drops below the discharge point, and the avalanche 

 stops; when the voltage recovers its original value, the tube is 

 ready for another particle. 



This sudden lowering of the potential constitutes the pulse. It is 

 taken oft' the positive center .wire of the counter and hence it is nega- 

 tive in sign. The size of the pulse is determined by the amplification 



