The investigation to be described in this paper was one of several undertaken to resolve a curious discrepancy that had arisen in measurements of the diffusion length of graphite. Those diffusion lengths measured with the graphite layups of 100 Area piles were three to six centimeters higher than the value obtained with the Hanford Standard Pile. The fact that the 100 Area structures of the order of 40 ft. X 40 ft. X 30 ft, not including reflector, approximated an infinite medium, whereas sigma piles are of the order of 8 ft. X 8ft. X 10 ft., suggested the possibility that the diffusion length obtained by mensuration of relative fluxes and application of the conventional equations depended in some way on the size of the medium. Therefore, in order to determine if such a dependency actually exists, it was proposed to erect a series of graphite stacks of different dimensions and to make diffusion length measurements in each.

The purpose of this manual is to present available data which can be used to determine disintegration rates of radioactive samples from counting rates measured under given circumstances with routinely used laboratory instruments. There is also brief discussion on the methods by which the various factors have been determined with reference to the accuracy with which they are known and to what extent the finally quoted disintegration rate is affected by these errors. Emphasis has been placed on those isotopes of particular interest to Radiological Sciences programs and the manual is intended as a standard through the use of which comparable values can be reported from all units in the Department.