Non-Collective Properties of the Continuum Gamma Rays Feeding Two High-Spin Isomers in¹⁵²Er

Abstract

The decay and feeding of two yrast isomer (1 -26h and Z -36h respectively) in lS2Er is measured. The isomeric delays are used to separate the spectrum of y-rays from the highest angular momentum states from the spectrum due to the decay of low spin states. Strong dipole as well as quadrupole radiation dominates the yrast cascades which feed into the Z -36h isomer, and the correlation spectrum for the feeding transitions does not indicate any collective structure for spins below 50h. It is tentatively concluded that the near magic nucleus ls1Er remains spherical or oblate up to this high spin value.

Observation of high-spin states in theN=84 nucleusEr152and comparison with shell-model calculations

“…6},the half-lives of the two high-spin isomers could be determined. The lower one at 9.7-MeV excitation energy shows a half-life of 35(4} ns, comparable to the previously reported 32 ns [9]. The half-life of the high-lying isomer at 13.…”

Energy inversion of thef 7/2 andh 9/2 neutrons in yrast states of154Yb

“…The 3.23MeV level most likely is 11-since [8] The 16 + to 14 + transition energy ( Fig. 2) decreases significantly in ~S4Yb compared to its lower-Z neighbours, and moreover its B(E2) value of 3.6(2)Bw is much lower than in lS2Er (~10.SBw) [10] and lS~ (9.6(ll)Bw) [3]. These results, together with the above lying 17 + and 18 + states strongly suggest =h~tp.vfv/2 hgp.…”

Lifetimes of unresolved transitions from very high spin states in nuclei nearN=82

“…Thus, this fraction of the spectrum should not be folded with the Doppler shift response function (1) to calculate the spectrum for recoil into the backing. From experimental data on the isomeric decay of 152Er [8,14] and the width of the prompt time window used in our experiment we estimate that a fraction of < 10 % of unshifted transitions can be present in the yrastbump region in the 157Er* system. The incorporation of this fraction in the analysis procedure still leads to reasonable (but somewhat worse) fits to the data.…”

“…At the highest spins the nuclei around 152Er appear well deformed [-6, 7] and the transitions are strongly collective (see above). In the low spin region they are not rotational, as is known from their discrete level schemes [8,[13][14][15]. Consequently a change in shape and structure has to take place during the de-excitation process and the transitions involved can be expected to be hindered.…”

“…Systematic investigations of continuum spectra of nuclei in different regions of the periodic table revealed marked differences between well deformed nuclei (good rotors) and nuclei near closed shells, in particular close to the N=82 shell. For instance, it was found that the yrast-bump is less developed and that there is a stronger dipole component in the more spherical systems [4][5][6][7][8]. This indicates that the de-excitation of these nuclei does not so dominantly follow collective bands as is the case in the good rotors.…”