Standardization of Radioactive Sources

INTRODUCTION Primary standards of radioactivity are in some ways, analogous to fixed points in the temperature scale in that each can be used for comparison with "unknowns" not too far removed from the standard. Over a thousand radioactive species are known to exist, but only a comparatively few satisfy such basic requirements for primary standards as: known decay scheme; appropriate 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 the range of ~3and T-energies of interest would suffice, the thought has been recently expressed that standards may be required for many additional radionuclides. For example, discrepancies in measurements using endwindow counters may arise in extrapolating to "zero total absorber" because of the difference in the energy distribution of the /~-ray spectra of two isotopes that have approximately the same maximum fl-energy. Similarly, although a given/~-~/-standard may be suitable for determining quantitatively the activity of a second radionuclide by comparison of respective beta activities, it may be totally inapplicable, for example, for use in corresponding measurements involving comparative ~,-ray counting. Where chemical methods are involved in the assay, interference has been found to occur from analogous compounds of the same element (e.g., Na2HP3~O4 and Na2HP3~O3), as well as from different radio-species of the same element in a given compound (e.g., Na~HP~O4 and Na2HP~O4). Radium was formerly unique in being the only radlonucllde for which the establishment of radiochemical purity was equivalent to primary standardization. The curie was officially redefined, however, as reported by Evans (lb) and Paneth (2) at the July, 1950, meeting of the Joint Commission Standards, Units, and Constants of Radioactivity as "the quantity of any radioactive nuclide in which the number of disintegrations per second is 3.700X10~°.’’ This new definition divorces the curie unit from a special association with radon and radium. The use of the term "rutherford" to denote 106 disintegrations per second was abandoned in 1950. The progress in the acceptance of definitions for radioactivity units and standards has been reported by Evans (la, lb). The present survey of the problems involved in the preparation and certification of primary standards