Table II. Fission Yield of Mo ?? from y235 as a Function of Bombarding Neutron Energy Energy (Mev) 0 Percent fission yield 6.142) - 0.95 6.1018) 4.85 5.45(4) 14.2 5,1 7{®) (a) 5 Terrell, W. E. Scott, J. S. Gilmore, and C. O. Minkkinen, Phys. Rev. 92, 1091A (1953). (b) 5 Terrell, W. E. Scott, J. S. Gilmore, and C.O. Minkkinen, Item 55- 136 (Presented at the American Physical Society Meeting, Sept. 3, 1953, Albuquerque, New Mexico). The beta proportional counters (Figure 15) used to determine Mo 99 disintegration rates are calibrated using Pb 210 _ pi?! - Po*!° samples of known disintegration rates obtained from the National Bureau of Standards. Pertinent data for each standard are provided by the National Bureau of Standards. These Ra D+E samples are counted in the same position relative to the counter as the Mo’’ 99 is to be counted. A thin (7mg /em*) aluminum 210 4s from Po } absorber is placed close to the counter to absorb any a radiations and B radiations from Pb 210. The observed counting rate is corrected for absorption by air gap, absorber, and 3.4 mg /em® aluminum counter window, back-scattering from the Pd-Ag mount, and self-absorption of the PbO, electro deposit (see Appendix III for sample calculation). The corrected _ counting rate is divided by the known absolute disintegration rate of the standard sample to give the efficiency, i.e., the solid angle of the counter for a weightless sample mounted on a weightless mounting. Previously, a carrier-free sample of Mo’ 99 was prepared and a meas- | ured portion evaporated carefully on a thin Mylar (organic polymer) film supported on a frame. Another measured portion of the active solution was added to a known amountof inactive molybdenum in solution as Mo(VI). solution was well mixed and heated to 90°C for 15 minutes. The Then PbMoO, was precipitated, washed twice with water, once with acetone, and dried for one hour at 130°C. After cooling to room temperature, samples-of vary- ing weight were prepared on thin aluminum dishes. All samples were mounted on thick aluminumholders and counted in the proper geometry. Figure 16 shows the results of these measurements. Range Plii Sg v oe 51 A; } \X