A high resolution neutron time-of-flight experiment for neutron energies $20\le E\le 240$ keV was carried out on the target nucleus ${}_{}{}^{140}{}_{}{}^{}\mathrm{Ce}$(0+). The measurement employed a 7.5 nsec beam burst and 250 m flight path. A multilevel shape analysis of the resonance data was performed. The plot of $\Sigma {\Gamma }_n^0$ vs $E$ for the $l=0\mathrm{}$ $J^{\pi }=\frac{1}{2^{+}}$ levels exhibited a significant change in slope at ∼ 160 keV and gives support to the doorway state picture of compound nucleus formation. The observed number of $l=0\mathrm{}$ levels vs $E$ showed an abrupt increase at 160 keV, the energy at which $S_0$ also changed. Various statistical tests were applied to see if this was an effect of missing weak levels but a definite conclusion could not be reached. The behavior of the $p$ wave strength function vs energy was found to be suggestive of doorway state structure. After correcting for missing levels the ratio of $s$ to $p$ wave level spacings was found to be 3.0; the theoretical prediction of 2.9 (for $\sigma =4.9\mathrm{}$) is in good agreement with experiment. The average $s$, $p$ strength functions and levels spacings are $S_0=1.1\mathrm{}\times {10}^{-4\mathrm{}}$, $S_1=0.34\mathrm{}\pm 0.05\times {10}^{-4\mathrm{}}$, ${〈D〉}_{l=0\mathrm{}}=4.9\mathrm{}$ keV, ${〈D〉}_{l=1\mathrm{}}=1.64\mathrm{}$ keV.

NUCLEAR REACTIONS Neutron total cross section measurement of ${}_{}{}^{140}{}_{}{}^{}\mathrm{Ce}$, $20<E<\mathrm{}240\mathrm{}$ keV. Determined $E$, $J^{\pi }$, ${\Gamma }_n$ of resonances. Deduced $s$ and $p$ wave strength functions and level spacings.

• Received 3 April 1978

DOI:https://doi.org/10.1103/PhysRevC.18.1254