First Observation of Backbending in an Actinide Nucleus

Abstract

The yrast bands in Pu andPu have been studied up to spin 26+ by Coulomb excitation using 08Pb beams of 5.1 and 5.3 MeV/u. In the case of 4Pu a pronounced backbending has been observed for the first time in an actinide nucleus. Microscopic calculations are presented which indicate that the observed anomalies in the yrast sequences of both nuclei are due to alignment effects in the i &3g& proton shell.PACS numbers: 21.10.Be, 25.70.Cd, 27.90.+b Our understanding of the structure of heavy nuclei has been influenced decisively by the discovery of the backbending effect' and its subs'equent experimental and theoretical investigations. ' These irregularities occur in the yrast sequences of many deformed nuclei in particular in the lanthanide region. It was therefore rather surprising that in our investigation of the high-spin states of the equally strongly deformed actinide nuclei "'Th, "'"'"'U, and "'Cm by Coulomb excitation with "'Pb projectiles, no pronounced backbending effect could be observed. ' ' In these nuclei the ground-state band was investigated up to spin 30' corresponding to a rotational frequency of 270 keV. Thus the spin as well as the frequency at which the backbending occurs in the lanthanides had been exceeded considerably in these experiments without observing any strong irregularity.It is generally accepted that the backbending effect is caused by the rotational alignment of high angular momentum orbitals due to the Coriolis interaction. ' In the actinide region the i»» proton and the j»» neutron shells are both in the vicinity of the Fermi surface and can thus easily align. Indeed, the measured increase of the g factor of the yrast state in "'Th and "'U with increasing spin can be attributed to an increasing contribution of the magnetic moment of an aligned i»» proton pair to the magnetic moment produced by the rotating core. ' Prom the study of the energy spectra of the odd nuclei "'U and "'Np it can also be deduced that the i»"proton alignment plays an important role in the structure of high-spin states of the actinide nuclei. ' However, a backbending effect is only expected in case the interaction between the ground-state band (g band) and the two-quasiparticle aligned band (s band) is sufficiently weak. As the calculated strength of the interaction shows a periodicity in proton (neutron) number with two subsequent minima being about 6 (10) units apart, "" the nonoccurrence of a strong backbending in the actinide isotropes studied so far might thus result from the fortuitous circumstance that all nuclei investigated so far exhibit a strong interaction between the g band and the s band. Actually the series of the go Th g2U and "Cm isotope s spans a range of && =6, i.e. , at least one oscillation in the interaction strength; consequently, the 1983 The American Physical Society

Coulomb excitation of230Th with32S,84Kr and142Nd projectiles

“…It is known that in actinides the i13/2 proton and the Jls/2 neutron orbitals are responsible for the alignment phenomena [-1-3]. A typical backbending was found until now only in the 242'244pu isotopes [4]. Besides the yrast band which in 236U [-5, 6], 238U, [-6], 232Th [-7] and 2~8Cm [-8] has been excited up to spin 30 +, a negative parity band based on the K ~ = 0-octupole vibrational state was also strongly excited in these nuclei.…”

New nuclides: Neptunium-243 and neptunium-244

“…Besides gamma-lines from the known neptunium isotopes and small contaminants the remaining gamma-rays could be assigned to 243Np and 244Np by comparison with the known level schemes of the plutonium daughters [31][32][33][34][35]. Figure 1 shows part of a gamma-ray spectrum of the neptunium activity obtained with SISAK.…”

New perspectives on the level density of compound resonances

“…In order to support this interpretation and to get more detailed information about the strength of the n-p force, backbending should be searched for in odd nuclei where the breakdown of the pairing correlation in neutron resonances is observed in the mass region 150<A<190 and around A=105. Recently backbending has also been observed in the actinide mass region [18], where strong fluctuations in the level density parameter are also observed (Figs. 1 and 6); but the studies in this mass region are not completed.…”