High resolution neutron total cross section measurement ofY89

Abstract

Results of a high resolution neutron time-of-flight experiment for neutron energies $27\ensuremath{\le}E\ensuremath{\le}240$ keV on the target nucleus $^{89}\mathrm{Y} ({J}^{\ensuremath{\pi}}={\frac{1}{2}}^{\ensuremath{-}})$ are presented. The measurement employed a 5 nsec beam burst and a 250 m flight path. A multilevel shape analysis of the data was carried out. The parities of almost all observed resonances were easily determined, and for $s$ wave levels the $J$ values of most resonances were obtained. It was found that the $\ensuremath{\Sigma}{\ensuremath{\Gamma}}_{n}^{0}$ vs $E$ plot of the $l=0$, $J=1$ levels exhibited a significant change in slope over the 240 keV interval, which was interpreted as evidence for a ${1}^{\ensuremath{-}}$ doorway state. The analysis yielded an escape width ${\ensuremath{\Gamma}}^{\ensuremath{\uparrow}}=0.8$ keV and a spreading width ${\ensuremath{\Gamma}}^{\ensuremath{\downarrow}}=100$ keV, with the doorway state located at a neutron energy of 70 keV. No effect attributable to nonstatistical behavior was observed in the $p$ wave channel. Since most $s$ and $p$ wave levels were observed a relatively clean test could be made of the expected ratio of $s$ to $p$ wave level spacings. After estimating the number of missed levels (inferred through the use of the Porter-Thomas distribution), the experimental ratio of $s$ to $p$ wave spacings was found to be 2.74. This is in disagreement with the theoretical level density prediction of 2.06. The average $s$, $p$ strength functions and level spacings are ${S}_{0}=0.28\ifmmode\pm\else\textpm\fi{}0.05\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, ${S}_{1}=2.64\ifmmode\pm\else\textpm\fi{}0.03\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, ${〈D〉}_{l = 0}=4.0$ keV, ${〈D〉}_{l = 1}=1.46$ keV.NUCLEAR REACTIONS Neutron total cross section measurement of $^{89}\mathrm{Y}$, $27<E<240$ keV. Multilevel analysis, determined $E$, ${J}^{\ensuremath{\pi}}$, ${\ensuremath{\Gamma}}_{n}$ of resonances. Deduced $s$ and $p$ wave strength functions, doorway state parameters ${E}_{d}$, $\ensuremath{\Gamma}\ensuremath{\uparrow}$, $\ensuremath{\Gamma}\ensuremath{\downarrow}$.