316 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 10 



10.9. Neutron Counters. Counters intended for direct detection of 

 neutrons depend upon either the recoil of light nuclei in the filling gas and 

 from the walls, or on charged particles emitted in nuclear interactions of 

 neutrons with the material of the counter wall or filling gas. The choice 

 between these two mechanisms depends upon the energy range of the 

 neutrons to be detected. Whichever method is used, in most applications 

 neutron counters are operated in the proportional region in order to reduce 

 the less heavily ionizing background of gamma and electron radiation. This 

 is essential for measuring low neutron flux since the efficiency for neutrons is 

 small compared with that for incident charged particles. 



10.10. Slow Neutron Counters. Slow neutrons are usually detected by 

 their interaction with B 10 nuclei according to the reaction B 10 (n, a)Li 7 . 

 The reaction is accompanied by the release of 2.5 mev of which 1.6 mev is 

 contributed to the alpha particle and 0.9 mev to the recoil lithium nucleus. 

 If both particles are stopped in the filling gas, roughly 75,000 ion pairs are 

 formed, assuming an average energy loss of 33 ev per ion pair. Boron can 

 be used in slow neutron counters either as the filling gas in the form of boron 

 trifluoride [15] or as a thin coating of metal on the cathode wall. 



Boron trifluoride-filled counters are constructed similarly to gamma 

 counters, preferably with materials that do not have large thermal-neutron 

 capture cross sections, as does glass containing boron for example, and 

 filled to pressures of 10 to 760 mm Hg. The maximum pressure that can be 

 used is limited by both the increase in operating voltage and the increased 

 size of electron pulses as the boron trifluoride pressure is raised. The lowest 

 useful pressure is determined by the neutron counting efficiency desired. 



The efficiency of the counter, defined as the probability that a neutron in 

 traversing an average path length I through the counter is captured, is 

 given [6] by, 



€ = IrpLcr 

 where p = pressure 



L — Loschmidt's number 

 r = B 10 concentration, B 10 /(B 10 + B 11 ) 

 a = B 10 capture cross section for neutrons of energy E 



The neutron capture cross section of B 11 is negligible compared with B 10 

 and does not contribute appreciably to the neutron count. Higher counting 

 efficiency therefore can be attained by increasing the enhancement of B 10 

 above its natural fractional concentration of 0.18 [16]. The efficiency also 

 varies with the neutron energy since for boron a ~ E~ 1/2 . Within the experi- 

 mental accuracy, the cross section for energies between 0.01 and 10,000 ev is 

 given by 



a = — p= — 0.20 barns 



where E = neutron energy, ev 



