318 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 10 



that can be detected is B = B /a. For hydrogen, B — B Q , and the scatter- 

 ing cross section for neutron energies above 0.050 mev varies as \/E^~. 

 Therefore, the sensitivity of hydrogen-filled counters varies approximately 

 as [17] 



JB-H 



0-s) 



The sensitivity rises rapidly for neutrons with greater than the bias energy 

 and then remains relatively constant in the energy interval 



1.57 B < E < 9.6B. 



The existence of a threshold is an important property of the hydrogen-filled 

 proportional counter since it provides a rapid although rough means for 

 determining neutron energies. This is done by altering the bias of a dis- 

 criminator to pass only pulses due to recoil nuclei with energies equal to or 

 greater than B . 



An alternative form of proton-recoil counter employs thin hydrogenous 

 radiators mounted within the counter tube [17,18]. Radiators are frequently 

 prepared by evaporation of glycerol tristearate on thin platinum disks in a 

 vacuum. The thickness of the hydrogenous layer is usually of the order of 

 100 mg per cm 2 . The same characteristics are observed as for hydrogen- 

 filled counters provided that the filling gas has a sufficiently high stopping 

 power to stop all hydrogen recoils within the gas volume and does not itself 

 recoil with greater than the bias energy B . Heavy, inert gases such as 

 argon, krypton, and xenon are the most satisfactory filling gases for this 

 reason. 



The elastic scattering cross section of hydrogen and most other substances 

 for fast neutrons is in the order of 1 barn; hence the efficiency of counters 

 used in this energy range is necessarily low. In addition to counts from 

 recoil nuclei, counts will be registered from competing neutron processes 

 occurring in the counter wall and in the filling gases, liberating protons, 

 deuterons, alpha particles, or gamma rays. The cross sections for these 

 interactions in nearly all substances is also in the order of 1 barn. Thus, 

 if a boron trifluoride counter is used for detecting very fast neutrons, dis- 

 charges are initiated by alpha particles from the reaction B(n, a)lA as well 

 as by recoil boron and fluorine nuclei. 



10.12. Accuracy of Counting Measurements. An estimate of the accuracy 

 of counting measurements on the activity of a sample is determined, for the 

 most part, by simple statistical procedures. The methods employed are 

 based upon the fact that each disintegration is a statistically independent 

 event since it is in no way affected by a preceding event or the means by 

 which it is detected. The distribution of disintegrations in time and the 



