Standardization of Radioactive Sources

Primary standards of radioactivity are in some ways, analogous to fixed points in temperature scale in that each can be used for comparison with unknowns not too far removed from standard. Over a thousand radio­ active species are known to exist, but only a comparatively few satisfy such basic requirements for primary standards as: known decay scheme; appro­ priate half life and disintegration energy; radiochemical and radioisotopic purity; and ease of preparation and availability. Whereas it had originally been considered that a relatively few standards covering range of {land 'Y-energies of interest would suffice, thought has been recently expressed that standards may be required for many addi­ tional radionuclides. For example, discrepancies in measurements using end­ window counters may arise in extrapolating to zero total absorber because of difference in energy distribution of {l-ray spectra of two isotopes that have approximately same maximum {l-energy. Similarly, although a given {l-'Y-standard may be suitable for determining quantitatively activity of a second radionuclide by comparison of respective beta activi­ ties, it may be totally inapplicable, for example, for use in corresponding measurements involving comparative -y-ray counting. Where chemical methods are involved in assay, interference has been found to occur from analogous compounds of same element (e.g., Na2HP3204 and Na2HP3203), as well as from different radio-species of same element in a given compound (e.g., Na2HP3204 and Na2HP3304). Radium was formerly u nique in being only radio nuclide for which establishment of radiochemical purity was equivalent to primary stand­ ardization. The curie was officially redefined, however, as reported by Evans (lb) and Paneth (2) at July, 1950, meeting of Joint Commission on Standards, Units, and Constants of Radioactivity as the quantity of any radioactive nuclide in which number of disintegrations per second is 3.700 X 1010. This new definition divorces curie unit from a special association with radon and radium. The use of term rutherford to denote 106 disintegrations per second was abandoned in 1950. The progress in acceptance of definitions for radioactivity units and standards has been reported by Evans (la, 1b), The present survey of problems involved in preparation and certification of primary standards