K.F. Flynn

The mass distribution for spontaneous fission of 252Cf

Abstract The distribution of mass in the spontaneous fission of 252Cf has been investigated by the radiochemical determination of the fission yields for 35 mass chains. Absolute fission yields were measured for four nuclides, 99Mo, 111Ag, 132Te and 140Ba. Ratios of the yields of fission product isomers to total chain yields were determined for 115gCd, 125gSn, 127mTe and 129mTe. A comparison of this work with that of previous investigators has resulted in “recommended” yields for 51 mass chains.

Half-lives of 90Sr and 137Cs☆

The half-lives of 90Sr and 137Cs have been measured both by specific activity and by direct decay over periods of 6–10 years. The results of the decay measurements are 28·0 ± 0·4 years for 90Sr and 29·9 ± 0·5 years for 137Cs; the values obtained by specific activity measurements are 28·5 ± 0·9 years for 90Sr and 30·9 ± 0·7 years for 137Cs. Recommended “best” values for these half-lives based on current literature values are 28·1 ± 0·3 years for 90Sr and 30·0 ± 0·2 years for 137Cs.

Formation of 111Pd isomers in neutron fission of several heavy nucleides

Abstract The cumulative yields of the 111Pd isomers have been measured in fission of 232Th, 233U, 235U, 238U and 239Pu by neutrons. The ratios of the 11 2 metastable state yield to the 5 2 ground state yield are 0·0043, 0·0209, 0·0096, 0·0048 and 0·0186 for the respective nucleides. The estimated error in each value is ±10%. Independent yield isomeric ratios have been calculated for various assumptions of charge distribution. The results of these calculations are discussed in terms of the parameters which describe the charge dispersion of mass-chain 111 and the angular momentum with which the fragments are formed. An upper limit of 0·43% has been placed on the β-decay branch of 111Rh111mPd.

Precision measurement of half-life and specific activity of 236U

Abstract Using a method developed earlier [1], we have made a precision measurement of the specific activity of 236 U. Samples were prepared by molecular plating and counted in an intermediate geometry alpha proportional counter with a flat pulse height plateau. The small amount of residual non-plated uranium was corrected for by counting in a 2π-counter. A silicon junction detector was used for energy analysis to detect non- 236 U α-activity. The measured specific activity was 143604±84d/m/mg 236 , corresponding to a half-life of (2·3415±0.0014) × 10 7 yr.

Independent yields of 82Br, 86Rb, 136Cs and 150Pm in spontaneous fission of 252Cf

Independent fission yields in the spontaneous fission of 252Cf were determined by radiochemical techniques for the shielded nuclides 82Br, 86Rb, 136Cs and 150Pm. They were found to be < 3·7 × 10−6 per cent, 5·4 × 10−6 per cent, 2·2 × 10−2 per cent and 2·0 × 10−2 per cent, respectively. The corresponding fractional chain yields lead to empirical Zp (most probable charge) values which are in good agreement with Zp values determined by physical methods (K X-rays).

Mass distribution in the fission of 229Th

Abstract Yields for twenty-five mass chains in thermal-neutron-induced fission of 229 Th have been measured radiochemically. The mass-yield curve is asymmetric with the light peak centered at mass 88 and the heavy peak centered at mass 140. The peak-to-valley ratio is about 500. No significant increase in yield in the region of symmetric fission was observed.

Thermal neutron fission cross-section of 229Th

Abstract The fission cross-section of 229 Th for thermal neutrons has been determined by counting samples placed in the thermal column of the Argonne heterogeneous heavy-water reactor, CP-5 Comparison of the fission counting rates of 229 Th samples with those of 235 U and 239 Pu standards gave a cross-section value of 30·5 ± 3 barns.

Independent yields of 84Rb, 86Rb and 136Cs for thermal-neutron-induced fission of 233U, 235U and 239Pu

Abstract The independent fission yields of 84Rb (a positron emitter), 86Rb and 136Cs have been measured for thermal-neutron-induced fission of 233U, 235U and 239Pu. The yields of 84Rb are (6·6 ± 0·7) × 10−8% for 233U, (2·4 ± 0·3) × 10−9% for 235U, and (3·9 ± 0·4) × 10−8% for 239Pu. The yields of 86Rb are (9·2 ± 0·9) × 10−4% for 233U, (2·1 ± 0·2) × 10−5% for 235U, and (1·9 ± 0·2) × 10−4% for 239Pu. The yields of 136Cs are 0·105 ± 0·011% for 233U, 0·0074 ± 0·0007% for 235U, and 0·100 ± 0·010% for 239Pu. The half-life of 136Cs by least squares analysis of the decay of eleven independent samples is 13·16 ± 0·03 days.

Mass distributions for the spontaneous fission of 248Cm and 250Cf

Abstract The distribution of mass in spontaneous fission of 248Cm and 250Cf has been investigated radiochemically and with γ-ray spectrometry. Both distributions are asymmetric with peak-to-valley ratios of > 1000 for 248Cm and > 300 for 250Cf. The average light and heavy masses ( A L , A H ) are respectively 104.6, 139.9 and 105.2, 141.5. Values for the average number of neutrons emitted per fission obtained by subtracting A L + A H from the mass of the fissioning nucleus are respectively 3.5±0.5 and 3.3±0.5.

Half-lives ofPu238andCm242

The /sup 238/Pu half-life was measured at 87.71 +- 0.03 yr by observing the /sup 238/Pu ..cap alpha..-particle activity formed from the ..cap alpha.. decay of /sup 242/Cm. Samples of /sup 242/Cm were counted three times in a 262 day interval, leading to a /sup 242/Cm half-life of 162.76 +- 0.04 days. Both values agree well with measurements made by other methods.