Normal-deformed States Research Articles

CLUSTERING CORRELATIONS AND TRIAXIALITY IN THE sd-SHELL REGION

Effects of cluster correlations on triaxiality have been studied using the antisymmetrized molecular dynamics. Low-lying states in 28 Si and 40 Ca are obtained by the the generator coordinate method (GCM), and the GCM basis are calculated by energy variations with constraints on quadrupole deformation parameter β and inter-cluster distance. The GCM obtain prolate superdeformed (SD) states in 28 Si and triaxial normal-deformed (ND) states in 40 Ca , which contain prolate α-24 Mg and triaxial α-36 Ar cluster structure components, respectively. Different shapes of prolate 24 Mg and oblate 36 Ar clusters cause difference of triaxiality of total systems. Cluster correlations which have oblate cluster enhance triaxiality in excited states.

EXACTLY SOLVABLE MODEL FOR THE DECAY OF SUPERDEFORMED NUCLEI

The history and importance of superdeformation in nuclei is briefly discussed. A simple two-level model is then employed to obtain an elegant expression for the branching ratio for the decay via the E1 process in the normal-deformed band of superdeformed nuclei. From this expression, the spreading width Γ↓ for superdeformed decay is found to be determined completely by experimentally known quantities. The accuracy of the two-level approximation is verified by considering the effects of other normal-deformed states. Furthermore, by using a statistical model of the energy levels in the normal-deformed well, we can obtain a probabilistic expression for the tunneling matrix element V.

Excitation energies and spins of the yrast superdeformed band inHg191

The excitation energies and spins of the levels in the yrast superdeformed band of $^{191}$Hg have been determined from two single-step $\gamma$ transitions and the quasi-continuum spectrum connecting the superdeformed and normal-deformed states. The results are compared with those from theoretical mean-field calculations with different interactions. A discussion of pairing in superdeformed states is also included.

How to measure the spreading width for the decay of superdeformed nuclei.

A new expression for the branching ratio for the decay via the E1 process in the normal-deformed band of superdeformed nuclei is given within a simple two-level model. Using this expression, the spreading or tunneling width gamma (downward arrow) for superdeformed decay can be expressed entirely in terms of experimentally known quantities. We show how to determine the tunneling matrix element V from the measured value of gamma (downward arrow) and a statistical model of the energy levels. The accuracy of the two-level approximation is verified by considering the effects of the other normal-deformed states.

A Setup for the Search for E0 Decay of Superdeformed 236 U

Transitions between nuclear states with a large deformation difference may have large transition probabilities for E0 decay. Such an E0 decay has been found in the decay of the superdeformed 238U fission isomer back into the normal-deformed states [1]. However, a search for the E0 back-decay in 236U showed no result [2]. A setup has been developed to search for conversion electrons in this decay.

Superdeformation in [formula omitted]Pb: Observation of three additional excited bands

The superdeformed nucleus 193 Pb has been studied following a heavy-ion fusion-evaporation reaction with the Gammasphere spectrometer array. Three new superdeformed bands, two of them probably signature partners, have been observed, bringing the total number of superdeformed bands known in this nucleus to nine. An estimate of the g-factor associated with one of the previously known pairs of strongly coupled excited bands has been made, confirming their configuration. The properties of the bands are compared with the results of cranked shell model calculations; good agreement is found and configurations are assigned to each band. The two new signature partner bands are suggested to be based on the two signatures of the [512]5/2 neutron orbital. The third new band is probably based on the 7 3 intruder orbital. In addition, the high-energy region of the spectrum in coincidence with the yrast superdeformed bands has been carefully studied. The current data provide no evidence for the previously reported transitions linking the superdeformed and normal-deformed states with energies around 2.2– 2.4 MeV.

Triaxial superdeformation in 163Lu

High-spin states in 163Lu have been investigated using the Euroball spectrometer array. The previously known superdeformed band has been extended at low and high energies, and its connection to the normal-deformed states has been established. From its decay the mixing amplitude and interaction strength between superdeformed and normal states are derived. In addition, a new band with a similar dynamic moment of inertia has been found. The experimental results are compared to cranking calculations which suggest that the superdeformed bands in this mass region correspond to shapes with a pronounced triaxiality ( γ≈±20°).

First evidence for linking transitions between the superdeformed yrast band and the normal deformed states in 149Gd

Double step resolved γ-ray transitions linking the yrast superdeformed (SD) band of 149Gd to the normal deformed (ND) level scheme have been observed using the EUROGAM phase II spectrometer. The excitation energy of the 47/2 − SD state above the 7/2 − ground state has thus been determined to be 10.625 MeV. The most likely multipolarity of the linking transitions is deduced, which permits the assignment of the most probable spins for this SD band. This measurement represents the first observation of discrete γ-ray transitions linking SD and ND states in the A∼150 mass region of the nuclear chart.

Chaos Assisted Tunneling from Superdeformed States

The tunneling process that governs the decay from superdeformed to normal-deformed nuclear states is shown to be enhanced by several orders of magnitude ( ${10}^{4}--{10}^{6}$) if the normal-deformed states are chaotic at the moment of decay out. The onset of (long-range) chaos may imply that the tunneling enhancement increases with decreasing angular momentum. Experimental signatures are discussed.

Rotational quenching of the N = 72 shell gap and the role of the intruder orbital in 132Nd

Rotational structures in 132Nd have been investigated via the 104Pd(32S,2p2n) reaction at 160 MeV. Four highly-deformed bands have been observed, out of which three have been linked to the normal-deformed states. Two of them are signature partners. A mixing at high spin is observed between levels of one highly-deformed band and the ground-state band allowing for an unambiguous assignment of the spins and parity of that band. The present data brings new evidence on the role of the N = 72 shell gap to the stabilization of the highly-deformed minimum in the A = 130 mass region and on the importance of the νi132 intruder orbital in the structure of the highly-deformed bands.

Do superdeformed bands decay via electric monopole transitions?

The strength of electric monopole transitions in the decay out of superdeformed bands is discussed on the basis of the mixing of superdeformed and normal-deformed states. The decay of superdeformed bands in the mass-190 region as well as the decay of the bands in ${}^{133}$Nd and ${}^{135}$Nd is investigated. Even though the values obtained for the electric monopole strength are very large, it is shown that $E$0 transitions cannot play an important role in the decay of superdeformed bands.

Recoil-distance Doppler-shift lifetime measurement of low-lying superdeformed states in194Hg

The lifetimes of three low-lying states in the superdeformed (SD) yrast band of {sup 194}Hg were measured applying the recoil-distance Doppler-shift method. As the deduced transition quadrupole moments Q{sub t} are equal within the experimental uncertainties to those extracted from a Doppler-shift attenuation method (DSAM) measurement for the high-lying states of the band, the assumption that the decay out of SD bands does not strongly affect the structure of the corresponding states is corroborated. By a simple mixing model, the decay can be described assuming a very small admixture of normal-deformed (ND) states to the decaying SD states. The deduced ND mixing amplitudes for the yrast SD bands in {sup 192,194}Hg and {sup 194}Pb are presented along with average transition quadrupole moments for the lower parts of the excited SD bands in {sup 194}Hg. {copyright} {ital 1997} {ital The American Physical Society}

Single-step link from yrast SD band in 143Eu

A discrete γ-ray transition from the yrast SD band to the region of near-yrast discrete normal-deformed states has been identified in 143Eu. It has an energy of 3360.6 keV and an intensity corresponding to 3.2% of the full intensity of the SD band and feeds a nearly-spherical state above spin 35 2 (+) and E x=4947 keV. This agrees well with the picture of the decay out of the SD band gained previously from two-step links. The angular distribution analysis favours a stretched dipole character for the transition. Due to the complicated structure of the level scheme in the region of interest it has not been possible to place the transition exactly.

Decay out of the highly deformed bands in the odd Nd isotopes: The 137Nd nucleus.

The decay of the highly deformed (HD) band in the nucleus {sup 137}Nd has been studied with the GASP array using the reactions {sup 110}Pd+{sup 30}Si and {sup 123}Sb+{sup 19}F. Four decay paths linking the HD band to the normal-deformed states have been established, which fix {ital I}{sup {pi}}=29/2{sup +} for the lowest state of the HD band lying at {ital E}{sub {ital x}}=4885 keV. The sudden disappearance, at the low spin side, of the HD bands in the {sup 133,135,137}Nd isotopes can be explained by total Routhian surface calculations through a change of the nuclear shape which is microscopically related to the transfer of the valence neutron from a {ital N}=6 to a {ital N}=4 Nilsson orbital.

Decay out of superdeformed bands in Tb isotopes

Excited states in the 150, 151, 152 Tb isotopes have been investigated using the 130Te(27 Al, xn) reaction and the EUROGAM array. The theoretical interpretation of the level schemes has been performed in the framework of the deformed independent particle model. The feeding patterns of the normal-deformed states by the superdeformed bands in 150, 151 Tb istopes have also been established. The results indicate that the decay-out mechanism of the bands is not of purely statistical nature, but depends on the configurations of both normal- and superdeformed states.

Detailed level scheme of 151Tb and the feeding of the normal-deformed states by the superdeformed bands

Excited states in the nucleus 151Tb have been investigated up to spin 91/2 ħ and energy 16.5 MeV using the 130Te( 27Al, 6n) reaction and the EUROGAM array. The theoretical interpretation of the level scheme has been performed in the framework of the deformed independent particle model. The feeding patterns of the normal-deformed states by both the yrast and first excited superdeformed bands have also been established. The results indicate that the decay-out mechanism of the bands is not of purely statistical nature, but depends on the configurations of both normal- and superdeformed states.

Deformed rotational bands in the doubly odd nuclei 134Pr and 132Pr.

The nuclei $^{132,134}\mathrm{Pr}$ have been investigated using the $^{100}\mathrm{Mo}$${(}^{37}$Cl,xn) reactions at a beam energy of 155 MeV. Gamma rays were detected with the Eurogam array. Analysis of the data has revealed the presence of two new weakly populated decoupled bands in $^{134}\mathrm{Pr}$. One of these bands has been linked into the normal-deformed states and is thought to be built on a \ensuremath{\pi}(${\mathit{h}}_{11/2}$${)}^{2}$\ensuremath{\bigotimes}\ensuremath{\nu}(${\mathit{f}}_{7/2}$,${\mathit{h}}_{9/2}$) configuration. The second band has been interpreted as being based on a \ensuremath{\pi}(${\mathit{h}}_{11/2}$${)}^{3}$\ensuremath{\bigotimes}\ensuremath{\nu}${\mathit{i}}_{13/2}$ intruder configuration within the second ${\mathrm{\ensuremath{\beta}}}_{2}$\ensuremath{\simeq}0.3 prolate minimum. The known decoupled band in $^{132}\mathrm{Pr}$ (5n reaction channel) and the highly deformed band in $^{130}\mathrm{La}$ A(\ensuremath{\alpha}3n) have also been extended. The structure of all of these bands is discussed together with similar bands in nieghboring odd-odd nuclei.

High spin states in the nucleus150Tb

Excited states in the nucleus150Tb have been investigated up to spin 39ħ and 13 MeV excitation energy using the130Te(27Al, 7n) reaction and the EUROGAM array. The theoretical interpretation of the observed states has been performed in the framework of the deformed independent particle model. The analysis of the decay out of the yrast superdeformed band indicates that normal-deformed states with spins between 24ħ and 28ħ are fed.

On the decay of the superdeformed band in194Pb

We report on new results for the decay of the superdeformed (SD) band in194Pb. No transitions linking the SD structure with the known level scheme of194Pb could be found, but the feeding intensities into the normal-deformed states are determined and limits on the one-step decay of the SD band are given.

Linking transitions from the superdeformed band in 143Eu.

Connections between the superdeformed and normal-deformed states in $^{143}\mathrm{Eu}$ have been established by measuring energy sums of two consecutive \ensuremath{\gamma}-ray transitions in the linking cascade. Six decay paths have been observed connecting discrete states in the two potential wells. The absolute excitation energy and the most likely spin of the superdeformed band have been determined directly for the first time.