Measurement of Absolute Fission Yields in the Fast Neutron-Induced Fission of Actinides: ²³⁸U, ²³⁷Np, ²³⁸Pu, ²⁴⁰Pu, ²⁴³Am, and ²⁴⁴Cm by Track-Etch-cum-Gamma Spectrometry

The absolute fission yields of 46 fission products in ²³⁸U (99.9997 at.%), 46 fission products in ²³⁷Np, 27 fission products in ²³⁸Pu (99.21 at.%), 30 fission products in ²⁴⁰Pu (99.48 at.%), 30 fission products in ²⁴³Am (99.998 at.%), and 32 fission products in ²⁴⁴Cm (99.43 at.%) induced by fast neutrons were determined using a fission track-etch-cum-gamma spectrometric technique. In the case of highly alpha-active and sparingly available actinides - e.g., ²³⁸Pu, ²⁴⁰Pu, ²⁴³Am, and ²⁴⁴Cm - a novel recoil catcher technique to collect the fission products on a Lexan polycarbonate foil followed by gamma-ray spectrometry was developed during the course of this work. This completely removed interferences from (a) gamma rays of daughter products in secular equilibrium with the target nuclide (e.g., ²⁴³Am-²³⁹Np), (b) activation products of the catcher foil [e.g., ²⁴Na from Al(n,)], and (c) activation products of the target [e.g., ²³⁸Np from ²³⁷Np(n,) and ²³⁹Np from ²³⁸U(n,)] reactions, making the gamma spectrometric analysis very simple and accurate. The high-yield asymmetric fission products were analyzed by direct gamma spectrometry, whereas the low-yield symmetric products (e.g., Ag, Cd, and Sb) as well as some of the asymmetric fission products (e.g., Br) and rare earths (in the case of ²³⁸U and ²³⁷Np) were radiochemically separated and then analyzed by gamma-ray spectrometry. The neutron spectra in the irradiation positions of the reactors were measured and delineated in the thermal to 10-MeV region using threshold activation detectors. The present data were compared with the ENDF/VI and UKFY2 evaluated data files. From the measured cumulative yields, the mass-chain yields have been deduced using charge distribution systematics. The mass yields, along with similar data for other fast neutron-induced fissioning systems, show several important features:

1. Fine structure in the interval of five mass units in even-Z fissioning systems due to odd-even effects. The fine structure decreases from lighter to heavier even-Z actinides, in accordance with their odd-even effect.

2. Higher yields in the mass regions 133 to 135, 138 to 140, and 143 to 145 and their complementary mass regions, depending on the mass of the fissioning systems due to the presence of 82n-66n, 86n-62n, and 88n-56n shells.

3. For odd-Z fissioning systems having no odd-even effect, the fine structure is very feeble and is due only to shell effects.

4. Unusually high yields observed in the mass region 133 to 139 in the fissioning system ²³⁹U* as compared to other U isotopes are explained on the basis of a higher neutron-to-proton ratio (N/Z) of ²³⁸U compared to lower-mass uranium isotopes. The [overbar], full-width at tenth-maximum, and [overbar]A_L increase with increasing mass of the fissioning systems, whereas [overbar]A_H of ~139 ± 1 remains constant throughout due to the strong preference for the formation of the deformed 88n shell, which is also favorable from the N/Z point of view.