Sec. 3.9] 



BETA PARTICLES 



59 



1000 



100 



coefficient is often of little use, and even when the absorption does appear to 

 be exponential the counter geometry should be specified. A more definite 

 quantity of universal adoption is the maximum range in aluminum. It is 

 less subject to the influence of scattering and does not depend on the shape 

 of the absorption curve. 



Under certain conditions the range can be estimated from the plotted 

 absorption curve, as shown in Fig. 12. The thickness of aluminum in 

 milligrams per square centimeter just sufficient to stop all beta particles is the 

 indicated range. From this value 

 the true range is obtained by add- 

 ing the equivalent absorber thickness 

 of the air path between the source 

 and the counter, the counter window, 

 and the self-absorption in the source. 

 When the absorber is a substance of 

 low atomic weight, these corrections 

 are given with sufficient accuracy by 

 adding the products of the density of 

 air, window, etc., and the path length 

 in each. Normally when thin sources 

 are used, the corrections do not total 

 more than about 10 mg per cm 2 

 equivalent of aluminum. 



This method for measuring range 

 is reliable only when the absorption 

 curve can be followed through a 

 reduction in intensity by a factor of 

 100 or more before reaching the level 



10 



cr l 



\ V Background 



mg/cm Aluminum 



Fig. 12. Absorption curve of beta particles. 

 R is the extrapolated range and R the 

 maximum range. The absorption curve 

 above is characteristic of the curves ob- 

 tained for beta particles emitted from radio- 

 active isotopes. For monoenergetic beta 

 particles the relative intensity scale would 

 be linear. 



of background activity due to radiations other than the beta particles. 

 Consequently the method cannot be used with accuracy for isotopes that 

 also emit gamma rays. Their contribution to the counting rate, assuming 

 a gamma-ray efficiency of ~ 1 per cent, may amount to several per cent 

 of the total, and since they are not appreciably attenuated by beta-particle 

 absorbers, the gamma rays tend to mask the last portion of the beta absorp- 

 tion curve. Similarly when the source activity is less than several hundred 

 times the background, it is often difficult to estimate with accuracy where 

 the absorption curve terminates. 



These difficulties are eliminated in a method of analysis developed by 

 Feather [5] which is now generally adopted as the standard procedure for 

 accurate range determinations. In addition to its accuracy as compared 

 with other methods, it is applicable to complex as well as to simple spectra, to 

 isotopes that emit gamma rays, and to relatively weak sources. 



