Sec. 13.9] STANDARDIZATION OF RADIOACTIVE SAMPLES 371 



at a distance r from a suitable ionization chamber is approximately 



ATrr-IWS 



R = 



epumVE, 



Unfortunately, in precision measurements some corrections are again 

 needed. Among these an important one is that due to Compton scattering. 

 Uncertainties come in also in the measurements of various quantities in the 

 above expression, notably in I, W, n, n , and even V, while E y and 5 may be 

 measured quite accurately. In measurement of /, care must be taken that 

 all ion pairs formed in the sensitive volume are collected without allowing 

 recombination or multiplication. This is best accomplished by the use of an 

 inert gas or dry air and with conveniently shaped ionization chambers which 

 have a well-defined plateau. As long as the collecting potential is held 

 within the limits of the plateau, 100 per cent of the ions are collected. The 

 biggest uncertainty at the present time exists in the value of W, the energy 

 absorbed to form one ion pair, as indicated by the fact that independent 

 methods and investigators have arrived at rather widely differing values for 

 W. One of the best ways to measure this value is by counting the number of 

 droplets produced in cloud-chamber tracks of electrons. The probable 

 error of about 1 per cent for the value quoted above places a limit on the 

 absolute accuracy of gamma-ray disintegration evaluations and also on 

 estimates of the energy absorbed in matter for one roentgen of gamma rays 

 and x-rays. Assuming this value of W to be correct, the conversion factor is 

 84 + 1 ergs per gm in air. 



Notwithstanding the experimental difficulties several excellent determina- 

 tions were made of the ionization of weighed amounts of radioisotopes, 

 especially that of radium. One gram of radium, when shielded with % mm 

 of platinum gives, at 1 meter distance, ionization in air equivalent to 0.84 + 

 0.01 rhm [22 to 25]. Knowing the absorption coefficients, the formula above 

 may be used to calculate the expected energy absorption or the ionization from 

 any radioisotope even with more than one gamma ray present. Marinelli et al. 

 [26] have recently published such data, some of which are reprinted in Table 

 37. The case of radium is more complex than most other radioactive isotopes 

 because when in equilibrium with its disintegration products a Ra sample 

 emits some 16 gamma rays with different intensities and energies. Never- 

 theless the theoretically calculated value agrees quite well with the experi- 

 mental data [27]. 



The measuring methods for ionization and energy absorption are constantly 

 being refined. One particularly suitable arrangement for the study of the 

 wall effect and scattering in ionization chambers is that constructed by 

 Failla [28]. This ionization chamber has parallel walls that form the elec- 

 trodes. The gas pressure, the distance of the walls, their material and 



