68 



APPLIED RADIOACTIVITY 



The beta-ray electron spectrum, therefore, may be divided into two 

 parts: (1) the disintegration electrons forming the continuous velocity 

 spectrum, and (2) the photoelectrons, the characteristic line spectral 

 images composing the homogeneous velocity groups. 



Figure 1 1-7 shows the relative intensities of the continuous beta-ray 

 spectral velocity distributions emitted by RaB, RaC, and RaE without 

 the superimposed photoelectric emissions. They all possess different 

 but definite upper limits of velocity. These upper limits can also be 



J2 



(D 



2- 



8 10 12 

 Energy^) Volts 



14 



16 18x10 



Fig. II-7. These are the beta-ray spectra of RaB, RaE, and RaC with homo- 

 geneous velocity groups omitted. RaB and RaC are from data by Gurney [1925], 

 and RaE is reduced to the same scale from data by Madgwick [1927]. 



shown to exist by absorption methods. If aluminum is used as the 

 absorber of the beta rays from RaE, it is found that a layer of aluminum 

 1.7 mm thick must be used to absorb the electrons having the highest 

 velocity. The highest velocity electron emitted by RaC are stopped 

 by 5 mm of aluminum. 



Radium B (Table 1 1-4) shows prominent photoelectric emission 

 groups in the neighborhood of 263 kv; the very simple electron spectrum 

 of RaD possesses a moderately prominent emission of electrons at 31 kv. 



For therapeutic use these homogeneous velocity groups are not sepa- 

 rated from the continuous velocity distribution groups, but they must 

 be kept in mind when beta-ray filters are used. 



Absorption of Beta Rays 



A thin-walled glass tube of radium salt or radon gas will emit beta 

 rays and gamma radiations due to the decay products. The beta-ray 



