One may now correct the observed Mo?? counting rate for the effects of self-scattering, back-scattering, self-absorption, and absorption due to air gap and counter window in the same manner as for the Ra D+E standard. The aliquot mounted on Mylar is taken as a good approximation to a weight- less sample on a weightless backing. | Given the absolute disintegration rate of a sample of Mo?? separated from the bomb debris, the following sequence of calculations is performed; 1. The disintegration rate is extrapolated back to the time of detonation, using the known haif-life of 66.04 hours. 2. The number of disintegrations per unit time at the time of detonation, is cpnverted into atoms of Mo?? through the relationship atoms of Mo 3. 99 _ disintegration rate x half-life In2 The number of fission events giving rise to the derived num- ber of molybdenum atoms is derived through the relationship numbers of fist:ions = atoms of Mo?? - fission yield of Mo?9 The short half-life of Mo?? may render impossible the direct confirmation of the above series of measurements. However, it has been our experi- ence that ‘the 282 day ce'** is also an excellent fission indicator and will give results similar to those from Mo 99 The determination of Ce 144 ust wait for at least two weeks after the detonation to allow the Cel 43 to decay sufficiently for accurate determination, hence our choice of Mo?? for this demonstration. b. Chemical Separation Procedure. ; Radiochemical analysis for Mo?? is routinely performed using carrier, i.e., a known amount of inert molybdenum added to the active material. Once isotopic exchange has been attained, chemical separations performed on milligram amounts of added carrier molybdenum will serve to separate the radio-molybdenum in exact propor- tion. When the carrier material is completely purified from extraneous activities, it is precipitated in weighable form and weighed to determine the fraction recovered. Tracer and carrier molybdenum ions exchange readily in hydrochloric acid medium if nitrate is present to insure the complete conversion of all cern es souk bs ae Cb “ J - 53 - A vy