Electron-Scattering Study of Nuclear Levels in Cobalt, Nickel, Lead, and Bismuth

Abstract

We have observed inelastic scattering of 183-Mev electrons through angles of 40°-90° in the laboratory, leading to excitation of discrete nuclear excited states in ${\mathrm{Ni}}^{58}$, ${\mathrm{Co}}^{59}$, ${\mathrm{Pb}}^{208}$, and ${\mathrm{Bi}}^{209}$. The excitation energies were below 8 Mev. Born-approximation analysis of the measured inelastic form factors was used to deduce the multipolarities $\lambda$ (when not previously known), and, by extrapolation, the transition rates for 15 corresponding gamma transitions. A number of groups of electric transitions for $\lambda =2, 3, \mathrm{and} 4$ were observed, each group having strikingly similar form factors. In all but one of these groups the ratios $G$ of the observed gamma transition rates to the single-particle predictions were greater than 15, and for some transitions from 30 to over 100. One of the groups, in cobalt and the nickels, contains the 1.33-Mev $E2\mathrm{}$ transition to the first excited state of ${\mathrm{Ni}}^{60}$. Another group consists of fast $E3\mathrm{}$ transitions, seen in all five nuclei, from states known as the "anomalous levels." They included the transition to the first excited state in ${\mathrm{Pb}}^{208}$ ($G=31\mathrm{}$) and a transition in ${\mathrm{Bi}}^{209}$ identical in energy and form factor. Among three slow $E4\mathrm{}$ transitions in cobalt and the nickels was the 2.50-Mev $4^{+}$→$0^{+}$ transition in ${\mathrm{Ni}}^{60}$. The $E4\mathrm{}$, $E3\mathrm{}$, and an $E2\mathrm{}$ transition in ${\mathrm{Co}}^{59}$ identify states analogous to the $4^{+}$, $3^{-}$, and $2^{+}$ seen in the neighboring even-even nuclei. The last two transitions are strongly enhanced. A pair of fast 4.30-Mev $E4\mathrm{}$ transitions was observed in ${\mathrm{Pb}}^{208}$ and ${\mathrm{Bi}}^{209}$; their speed ($G=37\mathrm{}$) indicates they may constitute the lowest-energy configuration of 16-pole mode of excitation of the nuclear surface. Values of the collective vibrational parameters $C_{\lambda }$ and $B_{\lambda }$ and the degree to which some of the transitions exhaust ordinary sum rules support the conclusion that the inelastic scattering process is strongly exciting nuclear collective excitations. Some of the observed results are expected on the basis of the theory of collective vibrational excited states; some are the consequence of unidentified configurations.