New experimental data have been obtained on the cumulative yields of some short-lived very asymmetric products in the reactor-neutron-induced fission of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$. Fission yields of ${}_{}{}^{73}{}_{}{}^{}\mathrm{Ga}$($t_{\frac{1}{2}}=4.8\mathrm{}$ h), ${}_{}{}^{167}{}_{}{}^{}\mathrm{Ho}$($t_{\frac{1}{2}}=3.1\mathrm{}$ h), ${}_{}{}^{171}{}_{}{}^{}\mathrm{Er}$($t_{\frac{1}{2}}=7.5\mathrm{}$ h), ${}_{}{}^{173}{}_{}{}^{}\mathrm{Tm}$($t_{\frac{1}{2}}=8.2\mathrm{}$ h), and ${}_{}{}^{179}{}_{}{}^{}\mathrm{Lu}$($t_{\frac{1}{2}}=4.6\mathrm{}$ h) are reported. These yields together with those reported earlier by us for other relatively longer lived products (half-lives of the order of a few days to a week) for the mass chains 66, 67, 72, 77, 161, 172, 175, 177, 183, and 199 have been used to obtain the mass yield curve of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ which gives rise to "shoulders" in the highly asymmetric region. This is the first experimental observation of shoulders in this region of mass distribution. The data have been discussed in terms of the recent fragmentation potential calculations available in the literature. The presence of these new shoulders in the highly asymmetric mass region has been ascribed to the occurrence of a new potential energy valley in the far asymmetric mass region due to the influence of the 28-proton shell. The effect of deformed shells on mass distribution has also been discussed.

NUCLEAR REACTIONS Fission ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, reactor neutrons, fragmentation potential, 28-proton shell, deformed shells, highly asymmetric mass distribution.

• Received 7 September 1978

DOI:https://doi.org/10.1103/PhysRevC.19.1372