Level structure of 184W from the 183W(n, γ) reaction

Abstract

The level structure of 184W has been studied from the prompt γ-rays emitted following the capture of both thermal and 2 keV neutrons by 183W. Energies and intensities were measured for both the primary and the secondary (low-energy) prompt γ-rays. From these data, a level scheme is proposed for 184W in which all the I π = 0 +, 1 + and 2 + states below ≈ 2.0 MeV are observed. Where possible, rotational-band assignments have been made to these and other levels. Additional evidence is presented which confirms the 1130 keV state as being the band head of a K π = 2 − octupole vibrational band. Admixed K π = 0 + and 2 + bands are established at 1322 and 1386 keV, respectively, with the I π = 2 + states (at 1431 and 1386 keV) having a mutual admixture of ≈ 12%. In the energy region above 1.5 MeV, the following bands and band-head energies are identified: K π = 1 +, 1613 keV; K π = 0 +, 1614 keV; K π = 1 +, 1713 keV; K π = 2 +, 1877 keV. The neutron binding energy in 184W has been determined to be 7411.1±0.6 keV. The band structure of the 1613 keV (1 +) and 1614 keV (0 +) bands is observed to be strongly distorted, the observed A ( h ̵ 2/2 I) values being ≈ 3.6 keV and ≈ 32 keV, respectively. This strong distortion is shown to be explainable in terms of Coriolis coupling of reasonable strength between the two bands. A similar explanation is shown to account for the somewhat less anomalous A-values (22.8 keV and 14.0 keV, respectively) of the 2 + band at 1386 keV and the 3 + band at 1425 keV. The results of a phenomenological fiveband-mixing analysis involving the K π = 0 + and 2 + bands below ≈ 1.5 MeV are presented and discussed. These calculations indicate, among other things, that the direct E2 matrix element connecting the 1322 keV, K π = 0 + band and the ground-state band is quite small, possibly zero. They also indicate that a nonzero E2 matrix element exists between this excited K π = 0 + band and the γ-vibrational band and that the magnitude of this element is comparable with that between the γ-vibrational and ground-state bands. Arguments favoring and apparently refuting the interpretation of the 1322 keV, 0 + band as a “two-phonon γ-vibration” are presented.