EUROGAM Array Research Articles

High spin structures of 122Xe

High — spin states in 122Xe have been investigated by γ − γ coincidence measurements using the EUROGAM array. The reaction 96Zr(30Si,4n)122Xe at a beam energy of 135 MeV was used to populate states of 122Xe. The level scheme has been extended up to ∼ 13.5 MeV and also enriched as compared to previous studies. New structures of competing dipole to quadrupole transitions have been observed. Experimental B(M1)/B(E2) ratios have been extracted for these structures and compared with theoretical estimates. They have also been compared with similar structures in Xe, Ba and Ce nuclei.

Octupole correlations in neutron rich, odd-A lanthanum nuclei.

Neutron-rich, odd-Z nuclei $^{145}\mathrm{La}$ and $^{147}\mathrm{La}$ populated in spontaneous fission of $^{248}\mathrm{Cm}$ were studied using the EUROGAM array. The experiment indicates the presence of similar octupole correlations in the studied La isotopes as found in the barium core nuclei. Alternating-parity structures were found in $^{145}\mathrm{La}$, indicating enhanced octupole correlations, as observed in $^{144}\mathrm{Ba}$, while in $^{147}\mathrm{La}$ octupole effects are weakened by alignment phenomena, as seen in the core nucleus $^{146}\mathrm{Ba}$. \textcopyright{} 1996 The American Physical Society.

Single step links of the superdeformed band in 194Pb: a measure of the absolute excitation energy, spin and parity of the superdeformed states

The EUROGAM array has been used to investigate the decay out of the yrast superdeformed (SD) band in 194Pb. Six single step decays from the lowest observed SD states to low-lying states at normal deformation (ND) have been identified. From this observation, the excitation energy of the SD band in 194Pb is established at 4877 ± 1.5 keV for the 6 + SD state. The most probable spins and parities of all members of the SD band are also deduced assuming that the SD states have even spin and positive parity.

VXI electronics for EUROGAM clover detectors

A single-width D-sized VXI card has been designed and built to fully instrument one of the new Clover detectors designed for experiments with the EUROGAM array. Each Clover detector consists of four individual germanium (Ge) crystals and an escape suppression shield comprising 16 bismuth germanate (BGO) crystals. The Clover electronics card measures energy and timing data to 13-bit accuracy from each Ge crystal for spectroscopy and measures sum energy and timing for the BGO shield. It also records a complete Ge and BGO hit pattern for each event and performs intelligent hardware escape suppression to exploit the segmented nature of the Clover detectors.

Calibration of the new composite “clover” detector as a Compton polarimeter for the EUROGAM array

The application of a composite Ge detector to the measurement of the linear polarisation of γ-rays has been investigated. The polarisation sensitivity of this device has been determined over the energy range 197–1368 keV. Comparison to a previous design of a similar detector has been made.

Highly excited Delta I=1 structures in 193Hg

Highly excited states in the nucleus 193Hg have been investigated by in-beam gamma -ray spectroscopic techniques using the EUROGAM array. The reaction 150Nd(48Ca,5n) at a beam energy of 213 MeV was used to populate states of 193Hg. The level scheme has been considerably extended (up to 10.7 MeV) and enriched from earlier studies. Two new structures of competing dipole and quadrupole transitions were observed. Experimental B(M1)/B(E2) ratios were determined for the two structures and compared with theoretical estimates. They were also compared with similar structures in the neighbouring Hg and Pb nuclei.

Intruder bands in (Z=53) 113I: Band termination interpretation.

High spin states in {sup 113}I were studied with the {sup 58}Ni({sup 58}Ni, 3{ital p}{gamma}) reaction at 240 MeV, using the EUROGAM array in Daresbury, United Kingdom, with 45 Ge detectors. A total of five deformed intruder bands extending to spins {ital I} ranging from (73/2--89/2){h_bar} have been identified, all of which reach frequencies {h_bar}{omega}{gt}1.0 MeV and excitation energies {similar_to}25 MeV. A gradual dropoff of the dynamic moment of inertia {ital scrI}{sup (2)} with increasing rotational frequency has been observed and attributed to a mode of ``soft`` band termination involving valence particle alignment. In this process, the nucleus progresses gradually, over many transitions, towards a noncollective oblate ({gamma}=+60{degree}) band terminating state.

M1 transitions between superdeformed states in 194, 195Tl: The fingerprint of the i13/2 proton intruder orbital

Recent data from the EUROGAM array have revealed dipole transitions linking signature partner superdeformed bands in 194Tl and 195Tl nuclei. Measurements of the decay branching ratios, taken together with the average quadrupole moment of the neighboring superdeformed nuclei, enable the absolute M1 strengths to be determined. From these data and using the strong coupling model, we find that two SD bands in 195Tl are due to the 81st proton being in the [642]5/2+ orbital and four from the six SD bands in 194Tl correspond to a configuration where the intrinsic spins of the single proton and single neutron are aligned.

Spectroscopy in the second potential well of 151Tb

Using the EUROGAM array, seven excited superdeformed bands have been found in the nucleus 151Tb which offer the possibility to perform a complete spectroscopy of the second potential well in this nucleus. Three of these bands are interpreted as a single proton excitation whereas the four remaining ones suggest the promotion of the last neutron from high-N into low-N orbitals. Data for proton excited bands are compared with recent fully self consistent calculations and this shows large disagreement. Furthermore, if the signature partner of the yrast configuration has been most probably identified, a doubt remains on the observation of the signature partner of the first excited band which originated, a few years ago, the field of identical bands. From the systematic investigation of neutron excitations across the N = 86 shell gap, it turns out that the four other superdeformed bands in 151Tb exhibit the characteristic features of degenerate signature partners.

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.

Shape coexistence in 138Sm and evidence for the rotational alignment of a pair of N=6 neutrons

The level scheme of 138Sm has been extended up to Ipi =(32+) using gamma -ray coincidence data collected with the Eurogam array. The 106Cd(35Cl,3p)138Sm fusion-evaporation reaction was used at a bombarding energy of 150 MeV. Several new bands have been observed and the known bands extended to higher spin. Band crossings and shape coexistence are discussed with the aid of Woods-Saxon cranking calculations (CMS and TRS). Evidence is presented for a neutron (i132/)2 pair alignment which is predicted to drive the nucleus to a prolate shape with enhanced quadrupole deformation beta 2=0.32. A strongly coupled band was also established and is interpreted as a collectively rotating oblate structure.

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.

Lifetimes of the superdeformed band in 192Hg

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 Q t = 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.

Two proton high-spin excitations and dipole bands in192Hg

The192Hg nucleus was populated in the160Gd(36S, 4n) reaction at a beam energy of E=159 MeV. Emittedγ-rays were detected with the EUROGAM array comprising 43 Compton-suppressed large volume Ge detectors. The level scheme of192Hg has been extended up to an excitation energy of E=10.4 MeV and spin I=34 ħ. Two new structures, made of competing ΔI=1 and ΔI=2 transitions have been observed and their connexions with the low-lying levels established. Their lowest levels are located at 6.304 MeV and 6.879 MeV excitation energy. The experimental results are compared with mean-field HF+BCS calculations. It is proposed that the new structures originate from deformation-aligned quasi-proton excitations π(i13/2 * h9/2)K=11 andπ (h9/2) K=8 2 , coupled to rotation-aligned quasi-neutron ν(i13/2)n and quasi-proton π(h11/2)2 excitations.

High-spin studies of neutron-deficient A=138 nuclei: 138Eu and 138Gd

The level schemes of mass A approximately 138 nuclei have been extended using data collected with the Eurogam array. The 35Cl+106Cd fusion-evaporation reaction was used at a bombarding energy of 150 MeV. Results are presented for odd-odd 138Eu and even-even 138Gd. The high-spin behaviour is discussed in terms of quasiparticle alignments of h11/2 proton and neutron pairs.

First observation of a superdeformed nucleus produced in an α xn reaction channel

Recent data from the EUROGAM array have revealed the population of the yrast superdeformed (SD) band of192Hg in the α4n exit channel of the16O+184W reaction at 113 MeV beam energy. The nucleus assignment was made on the basis of the SD band transition energies, and the observation of characteristic X-rays and lowlying yrast γ-transition of192Hg in coincidence with the SD band γ-rays. Both the feeding and decay-out patterns of the observed SD band have been found similar to the ones previously measured in the (36S,4n) reaction.

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.

The high-spin single-particle structure of 121I

The high-spin level structure of 121I has been established above the first yrast 'band termination state', which occurs at an excitation energy of 5.43 MeV with Ipi =39/2-, using data from the Eurogam array. The high-spin structure exhibits an irregular set of levels consistent with non-collective behaviour. A band terminating state at 9.24 MeV with Ipi =55/2- has been identified. Comparison with total Routhian surface cranking calculations suggest a maximally aligned ( pi h112/g72/2)(23/2-) (X)( nu h112/4)(16+) oblate configuration for this state relative to the 114Sn core.

The first results from EUROGAM : superdeformed structures in 151Tb

An experiment has been carried out using the Eurogam array which consisted of 42 escape suppressed germanium detectors in order to study the properties of the superdeformed structures in 151Tb. A first 4-dimensional analysis of the data show that these superdeformed bands decay out at different rotational frequencies, with the yrast superdeformed band decaying out at much larger rotational frequency. New γ-rays has been added to the yrast and excited superdeformed bands and the remarkable similarity in γ-ray transitions energies between the identical bands in 151Tb and 152Dy has been confirmed over a quite large range of rotational frequencies.