Search

The decays of seniority isomers in the N=82 nuclei $^{150}\mathrm{Er}$ and $^{152}\mathrm{Yb}$ and in their respective N=81 isotopes $^{149}\mathrm{Er}$ and $^{151}\mathrm{Yb}$ were studied following mass separation by the Argonne Fragment Mass Analyzer. Conversion electrons were detected with Si p-i-n diodes operated at room temperature. The low-energy isomeric transitions in $^{151,152}\mathrm{Yb}$ have been observed for the first time in the electron spectra. Multipolarity assignments were made for many of the decay \ensuremath{\gamma} rays of the four nuclei.

The wealth of data collected in the study of superdeformation in {sup 151}Dy allowed for new information to be obtained on the normally deformed structures in this nucleus. At high spin several new yrast states have been identified for the first time. They were associated with single-particle excitations. Surprisingly, a sequence was identified with energy spacings characteristic of a rotational band of normal ({beta}2 {approximately} 0.2) deformation. The bandhead spin appears to be 15/2{sup -} and the levels extend up to a spin of 87/2{sup -}. A clear backbend is present at intermediate spins. While a similar band based on a bandhead of 6{sup +} is known in {sup 152}Dy, calculations suggest that this collective prolate band should not be seen in {sup 151}Dy. In the experiment described earlier in this report that is aimed at determining the deformations associated with the SD bands in this nucleus and {sup 152}Dy, the deformation associated with this band will be determined. This will provide further insight into the origin of this band.

Following the first report of superdeformed (SD) bands with identical transition energies in the pairs ({sup 151}Tb*,{sup 152}Dy), ({sup 150}Gd*, {sup 151}Tb) and ({sup 153}Dy*, {sup 152}Dy) (where * denotes an excited SD band), it was proposed by Nazarewicz et al. that the observations could be understood in a strong-coupling approach if pseudo SU(3) symmetry were invoked. In this model there are three limiting values of the decoupling parameter; i.e. a = 0, {plus_minus}1. In the first two cases mentioned above the pairs of bands have nearly identical transition energies and are interpreted as proton excitations involving the [200]1/2 pseudospin orbital coupled to the {sup 152}Dy core, for which the value of the decoupling parameter is calculated to be a =+1.

Very few experimental observables are ordinarily accessible for superdeformed (SD) states in the A {approximately} 150 and A {approximately} 190 regions. The gamma-decay out of the superdeformed bands usually proceeds directly to the normally deformed states, through highly fragmented pathways, making it difficult to determine the spins, parities and excitation energies of the SD states. The in-band E2 transitions are so collective (2 x 10{sup 3} single-particle units in the A {approximately} 190 region) that it is typically impossible to detect any of the competing M1 and E1 transitions between states in the SD well.

Gammasphere experiments in 1993-94 brought to light the existence of an excited superdeformed (SD) band in {sup 190}Hg with the unusual property of decaying entirely to the lowest (yrast) SD band over 3-4 transitions, rather than to the normally deformed states as is usually the case in the A {approximately} 150 and A {approximately} 190 regions of superdeformation. Although M1 transitions between signature-partner SD bands were previously observed in {sup 193}Hg, no such mechanism was available to explain the situation in the even-even nucleus {sup 190}Hg, whose yrast SD band has no signature partner. The best explanation appears to lie in long-standing theoretical predictions that the SD minimum in the potential energy surface would be quite soft with respect to octupole vibrations. This would lead to enhanced E1 transitions connecting the one-phonon and zero-phonon states. The data and this interpretation were published. A shortcoming of the Gammasphere experiments was that they did not allow the definitive measurement of the energies of the gamma-ray transitions connecting the two bands, due to the very weak population of the excited band ({approximately}0.05% of the {sup 190}Hg channel) and also partly, we believed, to the angular distributions of the transitions, which were peaked nearmore » 90 degrees, where Gammasphere had few detectors.« less

The island of superdeformation in the vicinity of the doubly magic {sup 152}Dy yrast superdeformed (SD) band is thought to be well understood in the framework of cranked mean field calculations. In particular, the calculations suggested that in {sup 154}Dy there should be no yrast or near yrast SD minimum in the 40-60 h spin range, where SD bands in this mass region are thought to be {sup 153}Dy nucleus, it is populated. However, with the presence of five SD bands in the neighboring necessary to ascertain if the addition of one single neutron diminishes the importance of shell effects to the extent that superdeformation can no longer be sustained. In an experiment utilizing the increased resolving power of the early implementation phase of Gammasphere, the reaction {sup 122}Sn({sup 36}S,4n) at 165 MeV was employed to populate high spin states in {sup 154}Dy. In a four-day run with 36 detectors, over one billion triple and higher fold coincidence events were recorded. One new SD band was identified and was assigned to {sup 154}Dy. From comparisons with the Im{sup (2)} moments of inertia of the SD bands in {sup 152}Dy and {sup 153}Dy, a configuration based on (514)9/2{sup 2} neutrons coupledmore » to the {sup 152}Dy SD core was proposed. One unexpected and as yet unexplained feature of this new SD band is that the transition energies are almost identical to those of an excited SD band in {sup 153}Dy. It is also worth noting that the feeding of the yrast states is similar to that achieved by the deexcitation from the ensemble of all entry states in the reaction. This observation emphasizes the statistical nature of the decay-out process. A paper reporting these results was accepted for publication.« less

The study of yrast and near-yrast structures of odd-odd nuclei to high spins is somewhat limited due to the complexity of the spectra resulting from the many proton-neutron couplings near the Fermi surface. In superdeformed nuclei, the number of available protons and neutrons near the Fermi surface is somewhat limited due to the presence of large-shell gaps which stabilize the nuclear shape. A relatively small number of available neutron and proton configurations can lead to fragmentation of the SD intensity into a number of different bands. Two good examples of this phenomenon were found in {sup 192}Tl and {sup 194}Tl where the presence of six superdeformed bands were reported in both nuclei. We reexamined {sup 192}Tl at Gammasphere using the {sup 160}Gd({sup 37}Cl,5n) reaction at 178 MeV to populate states in the superdeformed well of this nucleus. While our previous study on {sup 192}Tl at ATLAS was very successful, a number of questions remained which formed the basis of our objectives in this experiment: obtain better {gamma}-ray energies for the known transitions and identify higher spin members in each band; determine how the bands feed the known yrast states in {sup 192}Tl as well as determine the complete spectrum inmore » coincidence with the SD bands; look for M1 transitions connecting proposed signature partners; and attempt to identify other excitations in the superdeformed well. Analysis is underway and four of the six bands were confirmed. The reasons that two of the reported bands were not observed in this latest work is still under investigation. As of this time, no other superdeformed bands were identified in the data. Two of the confirmed SD bands have a constant moment of inertia and show indications of cross-talk between each other. This observation is not unexpected since the calculated M1 rates for the proposed configuration of the band, {pi}{sub 13/2} x {upsilon}j{sub 15/2}, indicate that M1 transitions linking the two SD bands should be observed.« less

There are now over 40 superdeformed (SD) bands known in the A {approximately} 150 region and in most cases the properties of these bands are understood in terms of single-particle excitations in the absence of pairing. By continuing the search for new SD bands we hope to gain insight into (1) the ordering of the proton and neutron orbitals near the Fermi surface in the SD well, (2) the effects that the alignment of those orbitals has on the moments of inertia, and (3) the collective excitations in the SD well. For {sup 150}Tb, which is one proton and one neutron away from the SD doubly-magic nucleus {sup 152}Dy, it should be possible to study SD bands based on both proton and neutron hole excitations. By adding one proton to the {sup 152}Dy nucleus (i.e. {sup 153}Ho) proton excitations above the Z = 66 shell gap can be studied. These excitations are important as calculations suggested that the proton intruder orbital N = 7 might become occupied. Interactions between this orbital and a N = 5 level may result in softness towards octupole vibrations. High spin states in {sup 150}Th and {sup 153}Ho were populated using the {sup 124}Sn({sup 31}P,5n)more » and {sup 120}Sn({sup 37}Cl,4n) reactions, respectively. In both cases the early implementation phase of Gammasphere was used to detect the decay gamma rays and over 1 x 10{sup 9} triple and higher fold coincidence events were recorded. In {sup 150}Tb, the data analysis is complete and two new SD bands were identified. The fact that Im{sup (2)} moments of inertia are sensitive to the specific high-N intruder content of the SD bands was used to suggest configurations for the two new bands. A paper reporting these results is being prepared. For {sup 153}Ho, data analysis is still in its early stages.« less

An excited superdeformed (SD) band has been observed in {sup 19O}Hg which decays to the lowest-energy (yrast) SD band rather than to the less deformed states as observed in most known SD bands in the A{approximately}150 and A{approximately}190 regions. The band exhibits properties which are in good agreement with predictions of collective octupole vibrations in the SD well of {sup 19O}Hg.

Lifetimes of states in the superdeformed band in 192Hg have been measured using the Doppler-shift attenuation and the recoil-distance methods for the high-spin and the low-spin regions, respectively. Line shapes and decay curves were obtained from high-fold γ-ray coincidence spectra measured with the EUROGAM array. The transition quadrupole moments, derived from the lifetimes, are constant with an average value of Qt = 18.6 b down to the bottom of the band where it decays to the normal states. This suggests that the superdeformed potential-energy minimum is still present at that point and that the decay proceeds by tunneling.

We have studied γ-ray multiplicity distributions for the system 16 O + 152 Sm at five beam energies in the range 60–80 MeV, near and below the Coulomb barrier. The data were obtained using a large array of γ-ray detectors and an efficient evaporation-residue detector system utilizing an electrostatic deflector. The data are in good agreement with predictions of angular momentum distributions calculated using a coupled-channels model incorporating the effects of deformation.

The ground-state band of /sup 159/Tb has been Coulomb excited up to spin 23/2 by 151-MeV /sup 40/Ar ions. The lifetimes of the 9/2 to 23/2 levels have been determined by combining results obtained with the RD and DSA methods, the /b E/2//b M/1 mixing ratios for Delta /b I/=1 transitions have been measured using angular correlation techniques and the branching ratios for the levels up to spin 17/2 have been determined. The energies of the levels and the reduced /b M/1 and /b E/2 transition probabilities for their decay have been compared with the predictions of the rotational model, without and with Delta /b K/=1 admixtures. No satisfactory agreement could be achieved for all available experimental data simultaneously.

The influence of the reaction mechanism and of the nuclear structure at high angular momentum on the γ-ray deexcitation process is studied for Dy nuclei produced in a (40–80 MeV) and 12 C (80–110 MeV) induced reactions on 154, 155, 158, 160 Gd and 146, 150 Nd targets, respectively. The quasi-continuum (q.c.) γ-ray spectra for selected exit channels are decomposed into collective and statistical parts for which the γ-ray energies, the multiplicities and partly the multi-polarities are determined. Large differences in the constitution of the q.c. γ-ray cascades and in the population strengths of high-spin yrast states in 152 Dy and 156 Dy are attributed to nuclear structure effects at high angular momentum. This constitution and the centroids of the γ-ray entry regions in the energy versus angular-momentum plane are also evaluated as functions of beam energy and projectile. The γ-ray multiplicities and both coordinates of the entry region centroids increase initially with bombarding energy and then level off or even decrease at the highest energies. The observed increase is reproduced by a statistical-decay model, but at the higher bombarding energies strong deviations occur which are discussed in view of the reaction mechanisms, including incomplete fusion and non-equilibrium neutron emission, particularly absorptive breakup.

Evaporation residue cross sections and neutron multiplicity distributions have been measured for the 12 C + 144 Sm and 64 Ni + 92 Zr reactions leading to the same compound nucleous 156 Er. Statistical model calculations can account for the data in the 12 C-induced reaction. In contrast, the inhibition of neutron emission with respect to statistical model predictions seen in 64 Ni + 92 Zr cannot be explained even with the inclusion of the broad angular momentum distributions required to describe the fusion cross section data.

Regular patterns of E0 transitions were observed after 232Th(α,xn reactions. They indicate that the moments of inertia of β- and ground-state bands are nearly equal in 234U, in contrast to its even-even neighbours 232Th, 232U and 236U.

Using two Compton-suppression spectrometers in an Eγ−Eγ coincidence experiment, the yrast bands in 167,168Hf were extended up to 49 2 and 28 ħ, respectively. The i 13 2 positive parity band in 167Hf experiences backbending at a higher frequency than the first backbending in 168Hf, and no second backbending is obseerved in 168Hf. New information is threby obtained on the nature and interaction strength of the crossing bands in the vicinity of N = 96.

Using the on-line mass separator OSTIS for studies of short-lived fission products, we found two modes for the β-decay of 98Rb with respective half-lives of 96 and 114 ms. The latter decay populates a 0+ state in 98Sr at the unusually low energy of 215.5 keV, which is by far the lowest 0+ excited state observed in even-even nuclei. The half-lives of the 0+ and the 21+ states in 98Sr were found to be respectively 25 ns and 4 ns. The transition probabilities, the reduced E0 matrix element and the observed level structure suggest shape coexistence for 98Sr: the spheroidal 01+ ground state shows a rotational band with the 2+, 4+, (6+) levels, whereas the presumably spherical 02+ excited state may be connected with the 2+ level of a vibrational band. A simple model is proposed.

Experimental data on excitation energies and B(E2) values in the even-even Ru and Pd, and odd-A Rh and Ag isotopes suggest the existence of a Spin(3) spinor symmetry and a U ( 6 2 ) supersymmetry in these nuclei, wherein the even-even isotopes are described by the SU(5) limit of the interacting boson model and the odd proton of the odd-A isotopes is in a j = 1 2 orbit.

The decays of a 16 + four-quasiparticle state and an 8 − two-quasiparticle state in 178 Hf have been studied with emphasis on K -forbidden decay modes. An M4 branch in the decay of the 16 + state was identified, locating the isomer at an excitation energy of 2446.0 keV. Upper limits were found for the intensity of the 8 − to 6 + transition and for other possible decay modes of the 16 + state. Hindrance factors for the K -forbidden transitions are compared with those for similar transitions in neighbouring nuclides.

The γ−γ energy correlation matrix obtained from 159Tb(14N,xn)reactions at 95 MeV exhibits a low-intensity central valley with smoothly decreasing width when the rotational frequency increases, indicating an increasing collective moment of inertia. Enhanced intensities in valley at h ω = 0.42 and 0.52 MeV are observed for the first time in the HF isotopes and are interpreted as due to band crossings which may involve i 13 2 ,f 7 2 neutron and h 11 2 proton or higher orbital.

High-spin states in 167,168Hf and 170, 171W have been excited by the reactions 159 Tb ( 14 N , x n ) 173 − x Hf and 155 Gd ( 20 Ne , x n ) 175 − x W . The yrast bands have been observed up to J π = 49 2 + in 167Hf, Jπ = 28+ in 168Hf, J π = 45 2 + in 171W and Jπ = 22+ in 170W. Both even-even nuclei display a strong backbend around Jπ = 14+, whereas 167Hf shows an upbend at ω ∼- 0.33 MeV and 171W exhibits a progressive gain in aligned angular momentum above ω ∼- 0.26 MeV. At even higher rotational frequencies, bridges have been found in the central valleys of the γ-γ correlation matrices at ω ∼- 0.42 and 0.52 MeV for the Hf isotopes and ω ∼- 0.45 and 0.47 MeV for the W isotopes. Deduced moments of inertia for the Hf isotopes using the correlation data show a smooth increase up to about the rigid-sphere value at ω ∼- 0.5 MeV. The data are satisfactorily accounted for by cranked shell-model calculations. In particular, a qualitative interpretation is given for the experimentally observed systematic difference in the strength of the interaction potential for the N = 95 and N = 97 isotopes of Hf and W.

Lifetimes for high-spin levels (between about 10 and 30 ħ ) in 154 Er have been measured by the recoil-distance method. Non-collective, moderately collective and collective E2 transitions from states with lifetimes between at most 10 and 400 ps have been found. All available data on this nucleus are discussed within the framework of the oblate coupling scheme with the addition of collective degrees of freedom.

The multipolarity of continuum transitions deexciting high-spin states has been deduced from measured conversion coefficients. The investigated 146Nd(20Ne, 4n or 5n) 162,161Yb reactions were selected by gating on discrete lines. The average multipolarity gradually changes from E2 at 0.5 MeV to E1 above 1.5 MeV.

A discrete γ-ray study of 155 Er has been performed. A level scheme up to spin 85/2 has been established and interpreted using the deformed Woods-Saxon cranking approximation, taking into account pairing forces. Interpretation in terms of shape coexistence, band termination and breaking of the ( Z =64, N =82) core is proposed.

Accurate intensity calibration for γ-ray detection between 200 and 600 keV can be obtained with high-multiplicity cascades following the decay of 178mHf (31 y).