Search

The 113Ru and 113Rh nuclei, produced in the spontaneous fission of 248Cm and in proton-induced fission of 238U , were studied by means of prompt gamma spectroscopy using the EUROGAM 2 array of Anti-Compton spectrometers and the IGISOL mass separator, respectively. For the 0.5s isomer, found in 113Ru in our recent study we propose the 7/2-[523] configuration. The present data indicate spin and parity 1/2+ for the ground state in 113Ru . This and some new data on the excited levels in 113Rh allows reinterpretation and a better understanding of the β- {} decay scheme of 113Ru . The properties of 113Ru were studied in the frame of the Core-Quasiparticle Coupling model, which has well reproduced the experimental data.

Medium-spin, yrast excitations in the 138Cs nucleus, populated in the spontaneous fission of 248Cm, were observed for the first time. 138Cs was studied by means of prompt γ-ray spectroscopy using the EUROGAM2 array. The newly observed yrast cascade, built on the known 6- isomer at 80 keV, was successfully described by shell model calculations. Analogously to the 136I isotone, the 6- isomer in 138Cs has the $\ensuremath{(\pi g_{7/2} ^4 d_{5/2} \nu f_{7/2})_{6^-}}$ dominating configuration and the 7- excitation, located 175 keV above, corresponds to the $\ensuremath{(\pi g_{7/2} ^3 d_{5/2}^2 \nu f_{7/2})_{7^-}}$ as dominating configuration. Similarly as in 136I, changing the position of the d 5/2 proton orbital improves the reproduction of the data. However, in 138Cs the energy of this orbital should be increased compared to its energy in 133Sb, to get the best description, in contrast to 136I and 135Sb, where it had to be decreased. The best reproduction of excitation energies in 138Cs is obtained assuming that the πd 5/2 orbital in 138Cs is located about 100 keV higher than in 133Sb. These observations suggest that the lowering of the d 5/2 s.p. energy in 135Sb is not a physical effect due to the appearance of a neutron skin, as proposed by other authors, but rather an artifact due to some deficiency of the input data used in the shell model calculations in the region of the doubly magic 132Sn core.

A new high-spin level scheme of 149Nd has been identified, which is at variance with the one previously published. The states have been populated in the 12C + 238U fusion-fission reaction at 90MeV bombarding energy and the emitted I³-transitions were detected using the Euroball III array. The yrast band is built on the I½i13/2 sub-shell as in the neighbouring isotones. Moreover, sign of octupole vibrations has been found in 149Nd, as well as in 147Ce, where several new states decaying to the yrast band have been newly observed.

Excited states have been identified in the very neutron-deficient 122Ce nucleus. This is the first observation of this nucleus and its excited states. The ground-state rotational band has been observed up to spin 14 ℏ. The band has been assigned to 122Ce by detecting gamma rays in coincidence with evaporated charged particles and neutrons. The E ( 2 1 + ) value suggests a rather large ground-state deformation of β 2 = 0.35 , in good agreement with Hartree–Fock–Bogoliubov (HFB) mean-field calculations. The aligned angular momentum of the band has been studied and is compared with those of the neighboring even–even cerium isotopes, and to Woods–Saxon cranking calculations. The non-observation of the π ( h 11 / 2 ) 2 alignment until at least 0.4 MeV/ℏ is consistent with the extracted value of β 2 .

The projectile fragmentation of a 92 Mo beam onto a beryllium target at an incident energy of 500 A MeV has been studied at the high-resolution spectrometer FRS at GSI. The isotopic production cross-sections of the projectile fragments produced in the reaction have been determined. The data have been compared to the empirical parameterisation, EPAX, and to a microscopic nuclear reaction model, ABRABLA.

The 111Tc nucleus, populated in the spontaneous fission of 248Cm, was studied by means of prompt γ-ray spectroscopy using the EUROGAM2 array. Excited states in 111Tc were observed for the first time. Systematics of energy levels in odd-A Tc isotopes, obtained in our study of 107Tc and 109Tc provide a reliable spin and parity assignment I = 5/2+ to the head of the new band in 111Tc, interpreted as the π5/2+[422] orbital originating from the proton g9/2 shell. This level is most likely the ground state. Therefore, the (9/2+,7/2+) spin-parity assignment to the ground state of 111Tc, reported previously, is unlikely. Properties of the yrast band in 111Tc suggest prolate deformation of this band. There are hints that the deformation of 111Tc is larger than that of 109Tc, possibly due to admixtures of oblate-deformed configurations, which lower their excitation energy with increasing neutron number.

The large multi-detector arrays such as EUROBALL and GAMMASPHERE have made possible the study of the spectroscopy of neutron-rich nuclei through the observation of discrete, prompt $\gamma$ -rays emitted following fission. Most of the information obtained has concerned yrast states and collective excitations. In the present work, a search has been made for excited bands based upon two-quasiparticle intrinsic structures. Such bands have been found in several even-even nuclei from 96Sr to 112Pd. Careful analysis of triple-coincidence spectra has been performed in order to determine branching ratios within the bands. These branching ratios are then used to establish the magnetic properties of the intrinsic structure, permitting, in most cases, the determination of which Nilsson orbits (and whether they are neutron or proton states) are contributing to the excitation. Some example results from this search are presented.

Lifetime measurements have been performed for yrast rotational states of 100Zr and 104Mo using a 252Cf source in a differential plunger arrangement, together with the EUROBALL and SAPHIR multi-detector arrays. The system has been proved capable of covering a range of lifetimes from a nanosecond to a picosecond. The transition quadrupole moments are found to decrease by around 20% between I = 4 and I = 8 in the case of 104Mo while remaining constant in 100Zr. The origin of this effect lies either in a rotation-induced shape change, or in the strong mixing between the ground and s-bands in 104Mo.

High-spin states in the neutron-deficient isotope Cd-104 were populated using the Cr-50(Ni-58,4p)Cd-104 reaction at a beam energy of 250 MeV The level scheme has been extended using triple gamma-ray coincidences to a spin of 29h and an excitation energy of 18.2 MeV. Several collective structures involving the excitation of h(11/2) neutrons have been observed to spins approaching 30h. The high-spin structure has been compared to the results of cranked Nilsson-Strutinsky calculations.

Excited states have been identified in the very neutron-deficient 112 Xe nucleus. The ground-state band has been observed up to spin 12 ℏ, and a negative-parity side band has been observed up to 13 ℏ. The bands have been assigned to 112 Xe by detecting gamma rays in coincidence with evaporated charged particles and neutrons. The energy of the 2 + 1 state in 112 Xe is lower than would be expected from both calculations and from systematics of the neighboring xenon isotopes, which indicates a larger than expected ground-state deformation. The observations of a low-lying 3 − state and large deduced B (E1) values are suggestive of increased octupole correlations, which may explain the apparently large ground-state deformation. Constrained Hartree–Fock+BCS calculations have been performed which show qualitative agreement with the trend in deformation of the very neutron-deficient xenon isotopes.

A brief introductory survey of gamma-ray detector arrays and in-beam electron spectrometers developed during the last three lustra is followed by a broad discussion of the general requirements for single and multiple in-beam e-gamma spectrometers. A detailed analysis is made of a few important tools for reducing the low-energy electron background (i.e. simultaneous E-Bp selection, true and random coincidence rates, EM static fields around the target region). This survey and discussion are illustrated by the results of several in-beam e-gamma measurements dealing with nuclear spectroscogic investigations of a few medium and heavy nuclei. (C) 1999 Elsevier Science B.V. All rights reserved.

Measurements of g-factors of excited states in barium and cerium fission fragments have been performed using time-integral perturbed angular correlations between secondary-fragment gamma rays. Larmor precession angles have been obtained for the first I-pi = 2(+) states in Ba-144,Ba-146 and Ce-146,Ce-148 as well as for the 9/2(-) state at 117 keV in Ba-143, the 7/2(-) state at 114 keV in Ba-145 and the first 4(+) state in Ce-150. The deduced g-factor fur the 2(+) state in Ba-146 provides substantiating evidence for a downward trend in 2(+) g-factors with increasing neutron number, in line with Interacting Boson Model predictions for the neutron-rich barium isotopes, assuming the quenching of the Z = 64 shell gap for N>88. The static hyperfine fields for implanted barium and cerium isotopes are found to be opposite in sign. These results constitute the first use of secondary-fragment gamma-ray emission in the determination of g-factors of excited states in fission fragments. (C) 1999 Elsevier Science B.V. All rights reserved.

The low-lying level structure of the neutron-deficient isotope ${}^{198}\mathrm{Rn}$ has been studied for the first time, using the ${}^{166}\mathrm{Er}{(}^{36}\mathrm{Ar},4n)$ reaction at a beam energy of 175 MeV. Evaporation residues were selected using an in-flight gas-filled separator, RITU, and implanted at the focal plane into a 16-element position-sensitive, passivated ion-implanted planar silicon detector. Prompt $\ensuremath{\gamma}$ rays in ${}^{198}\mathrm{Rn}$ were observed at the target position using the JUROSPHERE array of 24 Compton-suppressed germanium detectors, and were identified by the subsequent radioactive decay of associated recoiling ions in the silicon detector. Isotopic assignments of the nuclei produced were made on the basis of the energy and half-life of their $\ensuremath{\alpha}$ decay. The estimated production cross section for ${}^{198}\mathrm{Rn}$ in this reaction was 180 nb. The excitation energy of the ${2}_{1}^{+}$ state indicates inconsistency with theoretical predictions of large stable ground-state deformation in this isotope. The systematics of other low-lying excited states suggests the presence of deformed intruder states falling in energy with increasing neutron deficiency.

The main features of nuclear targets, recoil ion catchers and reaction chambers used in nuclear spectroscopic investigations involving in-beam multi-e-gamma spectrometers are discussed. The relative importance of the F-ray background due to the accelerated ion-target and the recoil-ion-target interaction is estimated. Its impact on the prompt low-energy electron measurements is stressed. Finally a few general features of the interplay between accelerated ion beams, targets and recoil ion catchers particularly relevant for these measurements are broadly discussed and illustrated with typical examples of in-beam e-gamma studies. (C) 1998 Elsevier Science B.V. All rights reserved.

A few accelerated ion beam requirements for in-beam e-gamma spectroscopy are briefly reviewed as well as several features of the MP Tandem accelerator of IPN-Orsay and the accelerated ion-beam transport devices leading to the experimental area of in-beam e-gamma spectroscopy. In particular, the main capabilities of the ion-sources, the ion pulsing system, the ion stripping and stabilizing devices as well as the versatility of the ion beam transport system are discussed from the point of view of the different kinds of in-beam e-gamma experiments performed in that area. (C) 1998 Elsevier Science B.V. All rights reserved.

Prompt {gamma}-ray cascades in N=83 fission product nuclei near {sup 132}Sn have been studied at Eurogam II using a {sup 248}Cm source. Cross coincidences observed between {gamma} rays from complementary light and heavy fission fragments were vital for isotopic assignments. Yrast states in the N=83 isotones {sup 134}Sb, {sup 135}Te, and {sup 136}I are reported. The interpretation of the level schemes is based mainly on results of shell model calculations using empirical proton-proton interaction energies from {sup 134}Te, and proton-neutron interactions estimated from the well-known {sup 210}Bi level spectrum. {copyright} {ital 1997} {ital The American Physical Society}

Neutron-rich nuclei ${}^{147,149,151}$Ce have been studied via the prompt $\ensuremath{\gamma}$-ray spectroscopy of ${}^{248}$Cm fission fragments. A partial level scheme for ${}^{151}$Ce is presented for the first time, along with the high-spin structure in ${}^{147,149}$Ce. A rapidly changing yrast structure with neutron number is observed and it is suggested that bands in ${}^{147}$Ce and ${}^{151}$Ce are built on the $\ensuremath{\nu}{i}_{13/2}$ Nilsson state. The level scheme of ${}^{149}$Ce displays two bands with quite similar transition energies. There is no experimental evidence for octupole deformation in any of the three nuclei.

Prompt γ radiation following spontaneous fission has been measured using the EUROGAM 2 array. Using these data and newly developed analysis techniques a number of subjects were studied, which are briefly discussed. One of these subjects, shape transition in A≈100 Sr and Zr nuclei is discussed in more detail. Quadrupole moments were determined for rotational bands in the N=58,59,60,62 and 64 Sr and Zr isotopes. Deformation parameters, which increase from β2 ≈ 0.1 at N=56 to β2 ≈ 0.4 at N=64, suggest that in strontium and zirconium isotopes shape change happens gradually between N=56 and N=62 and is probably due to an occupancy of three or more deformation-driving orbitals of h11/2 parentage.

A = 140–148 even-even barium nuclei produced in the spontaneous fission of 248Cm were studied using the EUROGAM2 array. Spins and parities of excited levels have been firmly established from triple-γ angular correlation and direction-polarization correlation measurements. Known band structures have been extended and new bands were found. Band-crossing phenomena have been studied, which suggest termination of an alternating-parity band by particle alignment in 146Ba. The new data for 148Ba confirm predictions of a local decrease of electric dipole moment in 146Ba.

The low-lying level structure of the neutron-deficient isotope $^{200}\mathrm{Rn}$ has been studied using the $^{176}\mathrm{Hf}$${(}^{28}$Si,4n) reaction at a beam energy of 142 MeV. Evaporation residues were selected using an in-flight recoil mass separator, the Argonne Fragment Mass Analyzer, and implanted in a double-sided silicon strip detector. Prompt \ensuremath{\gamma} rays in $^{200}\mathrm{Rn}$ were observed at the target position using the AYEBALL array of 19 Compton-suppressed germanium detectors, and were identified by the subsequent radioactive decay of associated recoiling ions in the strip detector. Isotopic assignments of the nuclei produced were made on the basis of the mass-to-charge ratio of the recoiling ion and the energy and half-life of its \ensuremath{\alpha} decay. Previous results concerning transitions in $^{202}\mathrm{Rn}$ were confirmed. The level scheme deduced for $^{200}\mathrm{Rn}$, compared with those of heavier radon isotopes, is not consistent with the onset of deformation predicted by theoretical calculations. The estimated production cross section for $^{200}\mathrm{Rn}$ in this reaction was 5 \ensuremath{\mu}b. \textcopyright{} 1996 The American Physical Society.

High-spin states in {sup 145}Pm have been studied with TESSA3 operating at NSF Daresbury, through the {sup 134}Xe({sup 15}N,4{ital n}) compound nucleus reaction using a solid xenon target. To determine conversion coefficients and properties of low-spin excitations electron-{gamma} and {gamma}-{gamma} coincidences have been measured following the {sup 146}Nd({ital d},3{ital n}) reaction. An extended level scheme of {sup 145}Pm is proposed and interpreted in terms of single-particle excitations with collective quadrupole and octupole vibrations coupled to them. {copyright} {ital 1996 The American Physical Society.}

For the first time an ({ital n},{gamma}) reaction has been extensively studied using a large array of Compton-suppressed Ge detectors (the TESSA array). The nucleus {sup 168}Er was studied and the data show substantial improvement, in both quantity and quality, over previous coincidence data. Even though {sup 168}Er is perhaps the best studied deformed nucleus, over 250 new coincidence relations and a number of new levels in {sup 168}Er were disclosed, demonstrating the usefulness of this approach. Nuclear physics applications relate to the extension of nearly complete spectroscopy to higher excitation energies and to the study of statistical and chaotic features of the decay of low spin compound nuclear levels. {copyright} {ital 1996 The American Physical Society.}

Prompt $\ensuremath{\gamma}$-ray cascades in neutron-rich nuclei around doubly magic ${}^{132}\mathrm{Sn}$ have been studied at Eurogam II using a ${}^{248}\mathrm{Cm}$ fission source. Yrast states to above 5.5 MeV in the two- and three-proton $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}82$ isotones ${}^{134}\mathrm{Te}$ and ${}^{135}\mathrm{I}$ are reported. They are interpreted in terms of valence proton and particle-hole core excitations with the help of shell model calculations employing empirical nucleon-nucleon interactions from both ${}^{132}\mathrm{Sn}$ and ${}^{208}\mathrm{Pb}$ regions. A serious inconsistency in the accepted masses of $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}82$ isotones near ${}^{132}\mathrm{Sn}$ is discovered but not resolved.

The Eurogam-2 array has been used to study {gamma} rays emitted following the spontaneous fission of a {sup 248}Cm source. Analysis of Doppler-broadened line shapes corresponding to decays from excited rotational states in neutron-rich Sr, Zr, and Mo fission fragments has enabled the first measurements of state lifetimes in these nuclei at around {ital I}=10. The results are consistent with a predicted rotation-induced change in the triaxiality of the yrast states of the Mo isotopes that contrasts with the more stable behavior observed in the Zr and Sr nuclei. {copyright} {ital 1996 The American Physical Society.}

The level structures of the neutron-rich {sup 104}Mo and {sup 108}Mo nuclei have been investigated by observing prompt {gamma} rays emitted in the spontaneous fission of {sup 248}Cm with the EUROGAM spectrometer. Levels with spins up to 12{h_bar} have been observed and {gamma} branching obtained. The data can be satisfactorily described when {sup 104,108}Mo are considered as axially symmetric nuclei: in {sup 104}Mo, rotational bands based on the ground state, the one-phonon and the two-phonon {gamma}-vibrational states and a quasiparticle state have been observed, whereas in {sup 108}Mo the information is limited to the yrast band and the one phonon {gamma} band. {copyright} {ital 1996 The American Physical Society.}

Rotational bands based on two-quasi-neutron excitations have been observed for the first time in neutron-rich isotopes. The bandhead excitation energies have been used within the BCS model to determine the strength {ital G} of the neutron pairing interaction. Values of {ital G}=0.21 and 0.19 MeV have been obtained for {sup 100}Zr and {sup 102}Zr, respectively. For these neutron-rich nuclei the strength {ital G} is thus close to 20/{ital A} MeV, smaller than the value 23/{ital A} MeV used to describe even-even nuclei near stability in this region.

Triple angular correlations between γ rays following spontaneous fission of 248 Cm are studied using the EUROGAM array. Results, for transitions in even-even fission-product nuclei, agree with predictions of angular correlation theory. Efficient detector systems combined with new analysis techniques offer for the first time a convenient tool to determine multipolarities of prompt γ transitions in the fission-product nuclei.

The neutron-rich ${}^{106}$Mo nucleus has been studied by prompt \ensuremath{\gamma}-ray spectroscopy following the spontaneous fission of ${}^{248}$Cm. The characteristics of the rotational band built on a state at 1435 keV identifies the band head as the best candidate for a harmonic double-phonon ${K}^{\ensuremath{\pi}}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4}^{+}$ \ensuremath{\gamma}-vibrational state observed so far.

Isotopes in the light actinide region with [ital N][lt]126, [ital Z][gt]82 have been produced in the reaction of [sup 28]Si on [sup 182]W at beam energies of 164 and 170 MeV. Evaporation residues were selected using an in-flight mass separator, the Argonne Fragment Mass Analyzer, and implanted in a double-sided silicon strip detector. The charge-to-mass ratio of implanted ions, and energies and half-lives of correlated [alpha]-decay events have been used to make isotopic identifications. The [alpha] decay from the ground state of [sup 204]Ra has been observed with an [alpha]-particle energy of 7.488(12) MeV and a half-life of 45[sub [minus]21][sup +55] ms. Previous results concerning the decay of [sup 205]Ra were also confirmed.

The EUROGAM-2 array has been used to study γ rays emitted following the spontaneous fission of a 248Cm source. Analysis of Doppler-broadened lineshapes corresponding to transitions from excited rotational states in neutron-rich Sr, Zr, Mo, Ru, and Pd fission fragments has enabled measurements of state lifetimes in these nuclei at nuclear spins of I∼10. Extracted transition quadrupole moments are compared with various recent theoretical approaches, as well as with measurements at lower spin, and an interpretation of the data is made within the context of prolate-oblate shape competition in this region. © 2012 American Physical Society., EUROGAM was funded jointly by IN2P3 (France) and the EPSRC (UK). This work was partly supported by the EPSRC under grant no. GRH71161 and by the the US Department of Energy, Office of Nuclear Physics, under contract no. DE-AC02-06CH11357. The authors are also indebted for the use of 248Cm to the Office of Basic Energy Sciences, US Department of Energy, through the transplutonium element production facilities at the OakRidge National Laboratory. P.S. acknowledges support from MINECO (Spain) under Contract No. FIS2011-23565.

Excited states of $^{153}\mathrm{Eu}$ were observed using the $^{150}\mathrm{Nd}$${(}^{7}$Li,4n) reaction at a beam energy of 40 MeV. Two rotational bands with levels of spins up to 45/2\ensuremath{\Elzxh} were seen; a positive parity band built on the 5/${2}^{+}$ ground state, and a negative parity band built on the 5/${2}^{\mathrm{\ensuremath{-}}}$ level at 97.5 keV excitation energy. Transitions were also observed from levels in a band built on a 3/${2}^{+}$ state at 103.2 keV. Several features of the 5/${2}^{\ifmmode\pm\else\textpm\fi{}}$ bands suggest octupole instability or deformation, as may be expected for nuclei in this region. Levels of the same spin but opposite parity are nearly degenerate, and strong electric dipole transtions connect appropriate band members. Other features such as gyromagnetic ratios indicate an interpretation in terms of bands associated with separate orbits in a reflection-symmetric potential.

Excited states in 151 Sm were populated through the 150 Nd(α, 3n) reaction at a beam energy of 35 MeV using a 9 mg/cm 2 thick 150 Nd target. An array of twenty Compton-suppressed germanium detectors was used to detect γγ and triple-γ coincidences. The prime aim of this work was to investigate the effects of octupole correlations in odd- A nuclei in the A ∼ 150, N ∼ 88 region. A total of fifteen bands has been observed in 151 Sm and their decay patterns followed up to spins near 20 ħ. Four ΔI = 2 bands have some characteristics associated with parity-doublet structures. However, the evidence overall suggests that octupole correlations in 151 Sm are too weak to produce stable octupole deformation.

The N = 88 nucleus 151Eu was studied using the 150Nd (6Li, 5n) reaction, at a bombarding energy of 47 MeV, to populate yrast and near yrast levels at medium spins. A wide variety of bands was observed, including two that could be attributed to the coupling of an h112 proton to an octupole phonon. Although strong electric dipole transitions were observed, no good evidence was found for reflection-asymmetric band structures. This suggests that any tendency towards stable octupole deformation at low or medium spins is weak in this nucleus. Band structures were examined for evidence of triaxiality. The observations are consistent with but do not require a triaxial shape.

We have assigned [gamma]-ray transitions to [sup 136]Te by examining the prompt gamma radiation following spontaneous fission of [sup 248]Cm. The transitions were identified in spectra gated on the Ru complementary fragments, and confirmed by studying prompt [gamma] rays following spontaneous fission of [sup 252]Cf. The results are examined in terms of the systematics of other tellurium and [ital N]=84 nuclei.

The two-quasiproton structure of 164Dy has been investigated in a study of the 165Ho(t, α) 164Dy reaction. The experiment was performed using a 37.3 MeV triton beam delivered by the Daresbury tandem Van de Graaff accelerator. Magnetic analysis of ejectile ions was achieved using a QMG/2 magnetic spectrometer, and ion identification and detection employed a hybrid gas-filled, multielement, focal-plane detector. The two-quasiproton strengths accessed in this reaction are discussed with reference to the detailed spectroscopy of 164Dy. In particular, a rotational band built on a Kπ = 1+ state, previously interpreted as a collective structure, was observed and assigned as a two-quasiproton excitation which at zero deformation is based on the spherical (h112)2 shell-model state. The structure of the octupole-vibrational band and that of other states in the final nucleus have been elucidated.

The two-quasiproton structure of 174Yb has been investigated in a study of the 175Lu(t,α)174Yb reaction. The reaction was performed using a 35.5 MeV triton beam delivered by the Daresbury tandem Van de Graaff accelerator. Magnetic analysis of ejectile ions was achieved using a QMG 2 magnetic spectrometer and ion identification and detection employed a hybrid gas-filled, multielement, focal plane detector. Elements of the detailed nuclear structure of 174Yb are discussed in terms of the two-quasiproton configurations populated. Of particular interest is a low-lying Kπ = 3+ band, observed in this reaction and discussed in terms of hexadecapole collectivity. No Kπ = 1+ bands were observed, and the consequences of this for a microscopic description of the magnetic-dipole-strength distribution in 174Yb are discussed.

The medium-spin structure of the nuclei {sup 93}Rb and {sup 95}Rb is studied following the neutron-induced fission of {sup 235}U at the PF1B neutron guide, using the FIFI spectrometer, and at the Lohengrin mass spectrometer of the Institut Laue-Langevin Grenoble. These nuclei, plus {sup 91}Rb, are also studied following the spontaneous fission of {sup 248}Cm and {sup 252}Cf sources, using the EUROGAM-II and Gammasphere detector arrays, respectively. A high-spin isomeric state, with a half-life of 111(11) ns, is found in {sup 93}Rb at an excitation energy of 4422.4 keV, which most likely corresponds to the fully aligned [{pi}(g{sub 9/2}) x {nu}(g{sub 7/2}h{sub 11/2})]{sub 27/2}{sup -} configuration. An analogous configuration is proposed for the 5297.9-keV level observed in {sup 91}Rb. A new E3 decay branch of the 1133.9-keV isomer in {sup 91}Rb is found, for which the rather low transition rate of B(E3)=3.8(10) W.u. is determined. The energy of the isomeric state of {sup 95}Rb is now proposed to be at 810.6 keV, with a spin of (9/2{sup +}), and its half-life determined to be T{sub 1/2}=94(7) ns. A cascade of prompt transitions is observed on top of the 810.6-keV isomer in {sup 95}Rb. The near-yrast structures of {sup 91}Rb, {supmore » 93}Rb, and {sup 95}Rb are compared to the results of shell-model calculations, which support the proposed 27/2{sup -} interpretation of states in {sup 91}Rb and {sup 93}Rb. An analogous 27/2{sup -} state is expected to occur in {sup 95}Rb, as a long-lived isomer at 3.24 MeV. No such isomeric decay could be observed in a measurement using the Lohengrin spectrometer, which shows that, if it exists, its population, following the fission of {sup 235}U, is at least four times lower than that of the analogous 27/2{sup -} isomer in {sup 97}Y.« less

Excited states in {sup 145}Cs, populated following the spontaneous fission of {sup 248}Cm, were studied by means of prompt-gamma spectroscopy, using the EUROGAM2 multidetector array. A new level scheme of {sup 145}Cs was proposed. We identified a decoupled band corresponding to 1/2[550] proton configuration and interpreted the ground-state band as a mixed configuration of 1/2[440] and 3/2[422] proton orbitals. Quasiparticle-rotor calculations performed for {sup 145}Cs support such assignments. The electric dipole moment in {sup 145}Cs, D{sub 0}=0.013(4) efm, is smaller than in lighter Cs isotopes, which suggests that octupole correlations in Cs isotopes decrease at the neutron number N=90, similarly as observed in the {sup 146}Ba and {sup 147}La isotones.

Gamma rays from the reaction Si-28(Ni-58, 3p)83Y at a beam energy of 195 MeV were recorded in recoil-gamma- and gamma-gamma-coincidence mode with the POLYTESSA array and the Daresbury recoil mass separator. The known rotational bands were extended up to probable spins 49/2+ and 45/2-; many new interconnecting dipole transitions and an additional cascade were established. The bands are interpreted in terms of the cranked shell model; they exhibit two band crossings each, associated with g9/2 neutron and proton alignment. In the negative-parity yrast bands a strong signature inversion with DELTA-e' = -100 keV and a crossing pattern alternating in the favoured and unfavoured band develop. Hartree-Fock-Bogolyubov cranking calculations with a Woods-Saxon single-particle potential have been performed to illustrate the various shape changes and explain this signature inversion.

Excited levels of {sup 141}Xe, populated in spontaneous fission of {sup 248}Cm, were studied by means of prompt {gamma}-ray spectroscopy, using the EUROGAMM2 array. Level scheme of {sup 141}Xe obtained in this work shows patterns characteristic of simplex symmetry with s = +i and s = -i bands present but low value of D{sub 0} moment indicates that octupole correlations in Xe isotopes are systematically lower than in Ba nuclei.

A new rotational band has been identified in {sup 103}Zr with a proposed 3/2{sup +} band head located at 26.8 keV. Quasiparticle rotor model calculations performed in the present work suggest a {nu}3/2{sup +}[411] dominant configuration for this band. The calculations also indicate that {sup 103}Zr has axial prolate deformation.

Excited states in $^{95}\mathrm{Y}$, populated following the spontaneous fission of $^{248}\mathrm{Cm}$ and $^{252}\mathrm{Cf}$ and following fission of $^{235}\mathrm{U}$ induced by thermal neutrons, were studied by means of $\ensuremath{\gamma}$ spectroscopy using the EUROGAM2 and GAMMASPHERE multidetector Ge arrays and the LOHENGRIN fission-fragment separator, respectively. We have found a new ($17/{2}^{\ensuremath{-}}$) isomer in $^{95}\mathrm{Y}$ at 3142.2 keV with a half-life of ${T}_{1/2}=14.9(5)$ ns. Another isomer was identified in $^{95}\mathrm{Y}$ at 5022.1 keV and it was assigned a spin-parity ($27/{2}^{\ensuremath{-}}$). For this isomer a half-life of ${T}_{1/2}=65(4)$ ns was determined and four decay branches were found, including an $E3$ decay. A new $E3$ decay branch was also found for the known, 1087.5-keV isomer in $^{95}\mathrm{Y}$, for which we measured a half-life of 51.2(9) \ensuremath{\mu}s. The $B(E3)$ and $B(E1)$ transition rates, of 2.0 and $3.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ W.u., respectively, observed in $^{95}\mathrm{Y}$ are significantly lower than in the neighboring $^{96}\mathrm{Zr}$ core, suggesting that octupole correlations in this region are mainly due to the coupling of proton $\ensuremath{\Delta}j=3$ orbitals. Shell-model calculations indicate that the ($27/{2}^{\ensuremath{-}}$) isomer in $^{95}\mathrm{Y}$ corresponds to the $\ensuremath{\pi}{g}_{9/2}\ensuremath{\nu}({g}_{7/2}{h}_{11/2})$ maximally aligned configuration and that all three isomers in $^{95}\mathrm{Y}$ decay, primarily, by $M2$ transitions between proton ${g}_{9/2}$ and ${f}_{5/2}$ orbitals.

The level scheme of 87Mo was investigated with the reaction Ni-58(S-32, 2pn) at projectile energies of 110-130 MeV and the reaction Ca-40(Cr-50, 2pn) at 170 MeV. Extensive spectroscopic information was collected including gamma-gamma and neutron-gated gamma-gamma-coincidences as well as coincidences with identified 87Mo residues. Level lifetimes were studied with the RDDS and DSA methods. The angular distributions of low-lying transitions were measured. The results indicate a sudden transition from single-particle-like to collective behavior which is caused by g9/2 particle alignment. The level scheme was constructed up to 12 MeV excitation energy and a probable spin of 49/2 HBAR.

We report on the observation of excited states in the niobium isotope with masses 84, 85 and 86. These nuclei lie in the shape transition region known to occur in nuclei with 35 less-than-or-equal-to Z less-than-or-equal-to 40 and N = 45. Unambiguous identification was achieved by detecting gamma-rays in coincidence with mass-separated recoils produced in heavy-ion fusion reactions. Rotational bands were observed in 84Nb and 85Nb which show decoupling schemes similar to those in the neighbouring nuclei. A rotational model was used to interpret these bands and the quasiparticle configurations involved. Two sequences of transitions were observed in 86Nb, at least one of which appears to be a rotational band.

Rotational bands up to spin of ≈10 h have been studied in even-even and odd- A nuclei in the mass 100 region, by observing prompt γ-rays from the spontaneous fission of 248 Cm. Transitions in the neutro-rich nuclei 103 Zr, 104 Zr, 107 Mo and 108 Mo have been observed for the first time, and evidence for a band in 105 Nb is presented. Rotational bands are also reported in 98–100 Sr, 99,101 Y, 100–102 Zr, 101,103 Nb and 102–106 Mo. In most cases these bands have been extended to much higher spins than previously known. The data on the even-even nuclei provide information on deformation trends in this region. The level structures of the odd- A nuclei indicates which Nilsson orbitals lie close to the Fermi level. Bands based on the 5 2 + [422] and 5 2 − [303] configurations are identified in the odd- A nuclei with Z = 39 and 41. Evidence is presented that certain bands observed in N = 61 and 63 nuclei are built on the 5 2 − [532] configuration, and the role of the neutron h 11 2 orbital in stabilizing the deformation in this region is discussed.

The gamma-decay of excited states in the neutron deficient isotopes Tc-87 and Tc-88 has been observed for the first time. The fusion-evaporation reactions Ni-58(S-32, p2n)Tc-87 and Ni-58(S-32, pn)Tc-88 at 110 MeV, Ca-40(Cr-50, p2n)Tc-87 and Ca-40(Cr-50, pn)Tc-88 at 170 MeV and Ni-58(Ar-36, alpha-pn)Tc-88 at 145 MeV beam energy were used. Unambiguous identification of these nuclei which are populated with less than 1 mb cross section was achieved with coincidences between the gamma-radiation and the evaporation residues detected in a recoil mass separator. From gamma-gamma-coincidence data, a decay scheme of 11 transitions was constructed for Tc-88, the ground state of which is suggested to have I-pi = 7- or 8+. Two transitions identified in Tc-87 follow the pattern of a g9/2 one-quasiparticle band. (Less)

The band structure of83Zr has been investigated using the Daresbury recoil separator and TESSA array with 20 Compton suppressed Ge detectors and the reaction28Si (58Ni, 2pn) at 195 MeV. DCO ratios have been used to make many new spin assignments. The three known rotational bands have been extended up to the onset of the five quasiparticle bands. On the basis of the 21/2+, 23/2+ and 25/2+ yrast and yrare states, a (3qp) structure similar to those found in the isotones79Kr and81Sr has been identified. Additional bands have been observed for the first time, one of which appears to be non-collective. Strong mixing between the various negative parity bands is evident. A multiplet of states feeding the three quasiparticle region of the negative parity yrast band has been found. The data is interpreted in the framework of the cranked shell model.

The 28Si(58Ni, 2n)84Mo reaction has been studied at 195 MeV bombarding energy. The prompt gamma rays were detected in an array of twenty, escape-suppressed Ge detectors in coincidence with the recoiling nuclei, which were identified by A and Z with a recoil separator. A 443.8±0.3 keV gamma ray was observed from 84Mo which is interpreted as the 2+−0+ transition in this nucleus. This would indicate a quadrupole deformation e2∼0.30. This result confirms that 76Sr lie at the centre of the region of deformed nuclei with A∼80.