Multi-quasiparticle sub-nanosecond isomers in 178W

M Rudigier,J.-M Régis,R Shearman,J Benito,M Nakhostin,A Blazhev,S Bottoni,J Jolie,M Lebois,V Vedia,P.-A Söderström,A Kennington,P Koseoglou,S Oberstedt,V Sánchez-Tembleque,G Lotay,S Courtin,M Heine,Zs Podolyák,V Karayonchev,G Lorusso,Cristina Niță ,J N Wilson ,N Jovančević ,Liqiang Qi ,N Cieplicka-Oryńczak ,R Canavan ,L M Fraile ,Ł W Iskra ,William Edward Witt ,D T Doherty ,K Hadyńska-Klȩk ,Michel Brunet ,P M Walker ,P H Regan ,K O Zell

doi:10.1016/j.physletb.2019.135140

Abstract

We report on the first measurement of the half-lives of Kπ=11− and 12+ four-quasiparticle states in the even-even nucleus 178W. The sub-nanosecond half-lives were measured by applying the centroid shift method to data taken with LaBr3(Ce) scintillator detectors of the NuBall array at the ALTO facility in Orsay, France. The half-lives of these states only became experimentally accessible by the combination of several experimental techniques - scintillator fast timing, isomer spectroscopy with a pulsed beam, and the event-by-event calorimetry information provided by the NuBall array. The measured half-lives are 476(44)ps and 275(65)ps for the Iπ=11− and 12+ states, respectively. The decay transitions include weakly hindered E1 and E2 branches directly to the ground-state band, bypassing the two-quasiparticle states. This is the first such observation for an E1 transition. The interpretation of the small hindrance hinges on mixing between the ground-state band and the t-band.

Highlights

The A ≈ 180 region of deformed nuclei exemplifies the competition between individual-particle and collective degrees of freedom [1]
The experiment was performed at the ALTO accelerator facility at the IPN Orsay, France
The E1 reduced hindrance, from the 11− isomer directly to the 10+ member of the ground-state band (GSB), fν = 2.8, is very small – it is the smallest value yet observed for any K -forbidden E1 transition, and it is the only example of a bypassing E1 transition to the GSB in an even-even nuclide

Summary

Introduction

The A ≈ 180 region of deformed nuclei exemplifies the competition between individual-particle and collective degrees of freedom [1]. The bypassing transitions demonstrate an apparent collapse of the goodness of the K quantum number, providing extreme examples of so-called “anomalous” decays that were first identified from the 6qp, T1/2 = 130 ns isomer in 182Os [4], and the 4qp, T1/2 = 3.7 μs isomer in 174Hf [5] It has been a long-standing experimental challenge to measure the short half-lives of the K π = 11− and 12+ states in 178W, so that the extreme nature of their anomalous decays can be quantified. As we report, by exploiting a pulsed beam and a reaction that populates higher-lying isomers, together with an array of LaBr3(Ce) detectors combined with Ge detectors, and advanced analysis techniques, it has been possible to measure the half-lives of these states

Experimental conditions

Data analysis and results

Discussion

Conclusion