368 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 13 



I f (t) (l -KM 



/.(O) - 



-\tt p—'kjt 



where X e and \/ are the decay constants of RaE and RaF, respectively. The 

 beta disintegration rate of the RaE sample at time / is then 



hit) = I e (0)er^ 



13.7. Beta-particle Standardization by Direct Measurement of the Charge 

 of the Particles. If a beta emitter is placed on an electrically insulated sup- 

 port in the center of an evacuated metal chamber, the charge due to the beta 

 particles may be observed when these particles are absorbed by the walls. 

 This method has recently been used by Failla [19] and his associates to 

 obtain standardization of some important beta-active isotopes, such as P 32 . 

 If precautions are followed to avoid secondary effects, the results obtained 

 by this method are within 5 per cent of other independent methods. The 

 value of this method lies in its independence from other methods. For 

 secondary standardization its use would probably not be convenient for 

 routine measurement. 



13.8. Indirect Standardization by Calorimetric Measurement of the Total 

 Energy. Zumwalt and his associates [20] employed an isothermal calorim- 

 eter for the determination of the rate of heat evolution from radioactive 

 phosphorus when the beta particles were stopped in the calorimeter. The 

 measurement was carried out with about 25 millicuries of P 32 at the tempera- 

 ture of liquid nitrogen, using the rate of evaporation of nitrogen at constant 

 pressure as the measure of the heat output. With measurements of this 

 kind it is necessary to determine certain constants in order to evaluate the 

 absolute disintegration rate. The total energy output is the product of the 

 emission rate and the average energy of the beta particles; hence, it is essen- 

 tial to determine the average energy independently from measurements of 

 the energy distribution of the P 32 beta particles. A small fraction of the 

 beta-particle energy is converted into Bremsstrahlung which may not be 

 completely absorbed in the calorimeter. The Bremsstrahlung accounts for 

 about 1.3 per cent of the total energy of P 32 . A small fraction of the energy 

 lost by ionization in the calorimeter also may be transformed to chemical 

 energy which will not change the temperature of the device. The over-all 

 probable error in calorimetric determinations of the total emission rate is 

 between 2 and 3 per cent. 



13.9. Ionization Measurements of Beta and Gamma Rays. Many investi- 

 gators prefer to use ionization chambers for routine determinations of the 

 disintegration rates of various radioactive isotopes. In x-ray dosimetry a 

 number of types of ionization chambers have been in use for a long time, and 

 for special problems in connection with radioactive isotopes highly sensitive 

 chambers and electrometers have been developed (see Chap. 12). 



