Yields of fission products produced by thermal-neutron fission ofTh229

Abstract

Absolute yields have been determined for 47 gamma rays emitted in the decay of 37 fission products representing 25 mass chains created during thermal-neutron fission of /sup 229/ Th. Using a Ge(Li) detector, spectra were obtained of gamma rays emitted between 15 min and 0.4 yr after very short irradiations by thermal neutrons of a 15-..mu..g sample of /sup 229/ Th. On the basis of measured gamma-ray yields and known nuclear data, yields for cumulative production of 37 fission products were deduced. The absolute overall normalization uncertainty is <8%. The results are compared with fission-product yields previously measured, with generally good agreement. On this basis, and using other measurements for masses not observed in the present experiment, a complete mass distribution for A between 76 and 152 was deduced. The measured A-chain cumulative yields from the present program make up 84% of the total light-mass (A< or =115) yield and 77% of the total heavy-mass yield. The data were analyzed to obtain values of most-probable charge (Z/sub p/) and charge-dispersion (sigma) parameters. Based upon insight gained from study of similar data obtained for thermal-neutron fission of /sup 235/U, we postulate a simple functional dependence sigma = sigma(Z/sub p/), and usingmore » this dependence obtain values of Z/sub p/(A) for 15 mass chains created during fission of /sup 229/Th. Values of Z/sub p/(A) were estimated for other mass chains based upon results of a recent study of Z/sub p/(A). Charge distributions determined using the deduced mass distribution and the deduced sets of Z/sub p/(A) and sigma(Z/sub p/) are in very good agreement with recent measurements, exhibiting a pronounced even-odd effect in elemental yields. These results may be used to predict unmeasured yields for /sup 229/Th fission.« less