Search

Highly excited states, up to spin $39\ensuremath{\hbar}$, have been established in $^{174}\mathrm{W}$, using the Gammasphere array. Ultimate cranker calculations predict the appearance of triaxial, strongly deformed structures above spin $30\ensuremath{\hbar}$ in $^{174}\mathrm{W}$. A new approach was developed for a comprehensive search of the data for such structures, similar to those observed in the Lu and Hf isotopes. No evidence was found for strongly deformed bands in the W isotopes populated in this experiment. Existing rotational structures have been considerably extended, allowing for the observation of both neutron and proton alignments in a number of bands. There is evidence for the ${i}_{13/2}$ neutron and possibly both the ${h}_{9/2}$ and ${h}_{11/2}$ proton crossings. The observed neutron and proton crossing frequencies are in good agreement with predictions of Woods-Saxon cranking calculations using an empirical pair-gap energy, and they lead to an improved understanding of the underlying structure of the bands.

A Gammasphere experiment was carried out to search for triaxial strongly deformed (TSD) structures in $^{171,172}\mathrm{Hf}$ and the wobbling mode, a unique signature of nuclei with stable triaxiality. Three strongly deformed bands in $^{172}\mathrm{Hf}$ and one in $^{171}\mathrm{Hf}$ were identified through $^{48}\mathrm{Ca}$($^{128}\mathrm{Te}$, xn) reactions. Linking transitions were established for the band in $^{171}\mathrm{Hf}$ and, consequently, its excitation energies and spins (up to $111/2\ensuremath{\hbar}$) were firmly established. However, none of the $^{172}\mathrm{Hf}$ sequences were linked to known structures. Experimental evidence of triaxiality was not observed in these bands. The new bands are compared with other known strongly deformed bands in neighboring Hf isotopes. Theoretical investigations within various models have been performed. Cranking calculations with the Ultimate Cranker code suggest that the band in $^{171}\mathrm{Hf}$ and two previously proposed TSD candidates in $^{170}\mathrm{Hf}$ and $^{175}\mathrm{Hf}$ are built on proton $({i}_{13/2}{h}_{9/2})$ configurations, associated with near-prolate shapes and deformations enhanced with respect to the normal deformed bands. Cranked relativistic mean-field calculations suggest that band 2 in $^{175}\mathrm{Hf}$ has most likely a near-prolate superdeformed shape involving the $\ensuremath{\pi}{i}_{13/2}\ensuremath{\bigotimes}\ensuremath{\nu}{j}_{15/2}$ high-$j$ intruder orbitals. It is quite likely that the bands in $^{172}\mathrm{Hf}$ are similar in character to this band.

High-spin states of {sup 191,193}Au and {sup 192}Pt have been populated in the {sup 186}W({sup 11}B, xn) and {sup 186}W({sup 11}B, p4n) reactions, respectively, at a beam energy of 68 MeV and their {gamma} decay was studied using the YRAST Ball detector array at the Wright Nuclear Structure Laboratory at Yale University. The level scheme of {sup 193}Au has been extended up to I{sup {pi}}=55/2{sup +}. New transitions were observed also in {sup 191}Au and {sup 192}Pt. Particle-plus-Triaxial-Rotor (PTR) and Total Routhian Surface (TRS) calculations were performed to determine the equilibrium deformations of the Au isotopes. The predictions for oblate deformations in these nuclei are in agreement with the experimental data. Development of nonaxial shapes is discussed within the framework of the PTR model.

The fusion-evaporation reaction involving a 175 MeV S-36 beam and a Sn-124 target was performed, and the emitted gamma rays were observed with the Gammasphere spectrometer. Significant additions to the level scheme of Gd-152 were made in spite of the relative weakness of the alpha 4n exit channel, being only similar to 2% of the total fusion cross-section. The high-spin behavior of Gd-152 was compared with that of other N=88 nuclei. A striking similarity was observed with Dy-154 and it is therefore suggested that the angular-momentum-induced shape changes that take place in Dy-154 also occur in Gd-152 in the 30-40h spin range. This is supported by Cranked Nilsson-Strutinsky calculations which were used to interpret the high-spin bands. It is found that a better agreement between calculation and experiment is obtained if the Z=64 shell gap increases with a decreasing number of valence particles outside the doubly-closed Gd-146(64)82 nucleus. (Less)

The systematics of the πh11/2⊗νh11/2 and πh11/2⊗νs1/2 isomeric configurations were studied for the odd‐Z N = 77 isotones near the proton dripline. The spin and parity of Iπ = 8+ and 5− were deduced for the isomers in 140Eu and 142Tb. No evidence for the expected 1+ ground‐state was found in the 144Ho decay data. The proton emission from 146Tm was restudied and the spin and parity values of Iπ = 10+ and 5− were established for 146mTm and 146gsTm, respectively. The interpretation of the observed decay properties and structure of the proton‐emitting states was made by accounting for deformation and proton and neutron coupling to the core excitations.

A classic region of band termination at high spin occurs in rare-earth nuclei with around ten valence nucleons above the Gd-146 closed core. Results are presented here for such non-collective oblate (gamma = 60 degrees) terminating states in odd-Z Ho-155, odd-odd Ho-156, and even-even Er-156, where they are compared with neighbouring nuclei. In addition to these particularly favoured states, the occurrence of collective triaxial strongly deformed (TSD) bands, bypassing the terminating states and extending to over 65 (h) over bar, is reviewed.

The quadrupole moment for the πg 9/2⊗νh 11/2 band in the 130>Pr nucleus has been measured using the Doppler-shift attenuation method. A centroid shift analysis was carried out and a value of Q0=6.1±0.4 eb, corresponding to an axial prolate deformation of β2=0.35(3), has been determined. This is the first direct experimental confirmation of the deformation-driving character of the πg 9/2 orbital in an odd-odd nucleus in the A≃135 superdeformed region.

High-spin states in $^{171}\mathrm{Ta}$ were populated in the $^{124}\mathrm{Sn}$($^{51}\mathrm{V}$,$4n$) reaction at 228 MeV to search for evidence of stable triaxial deformation. Identification of a wobbling sequence based on the previously known $\ensuremath{\pi}{i}_{13/2}$ structure would provide a unique signature for this rarely observed shape. No such sequence was identified in these data, which suggests that the island of triaxial strongly deformed bands may be smaller than once thought. However, over 200 new transitions and two new bands were placed in the level scheme and the sequence based on the $\ensuremath{\pi}{i}_{13/2}$ orbital could be observed up to spin and parity ${I}^{\ensuremath{\pi}}=(101/2{}^{+}$). The relative excitations of all the sequences were determined and the ground state of $^{171}\mathrm{Ta}$ was found to have $5/{2}^{+}$ quantum numbers, contrary to previous reports. All of the previously known structures were extended to much higher spin and their high-frequency band crossings are interpreted within the framework of the cranked shell model.

Significant extensions to the high-spin excitation spectrum of the N = 91, 92, 93 isotopes Dy-157,Dy-158,Dy-159 have been achieved using the high-efficiency gamma-ray spectrometers Euroball and Gammasphere. These nuclei were populated via weak 3n or alpha xn exit channels in fusion evaporation reactions. In Dy-157, the yrast band has been extended to I-pi = (2)/(101+) (tentatively to (2)/(105+)) with four sideband structures (two of which are new) observed in the 35-50h spin range. In Dy-158, three bands have been extended to 42(+)(44(+)),40(-), and 41(-)(43(-)), whereas in Dy-159 the yrast band is observed to (2)/(81+) ((2)/(85+)). The high-spin behavior and band crossing systematics of the Dy isotopes and of the neighboring N = 91, 92, and 93 isotones are discussed in terms of rotational alignments and shape transitions. Cranked Nilsson-Strutinsky calculations without pairing have been performed for detailed comparisons with the very high-spin states observed in Dy-157.

A new isomer in {sup 116}Ag with a half-life of 20(1) s has been discovered through the use of conversion electron, {beta}, and {gamma} spectroscopy of on-line mass-separated radioactivities at the Holifield Radioactive Ion Beam Facility at ORNL. The observed electron peaks at 22.5, 44.42, and 47.33 keV were interpreted as the K,L, and M conversion electron lines resulting from a 47.9-keV E3 transition associated with the decay of a second isomeric level in {sup 116}Ag. A new level structure of {sup 116}Ag is proposed, with the levels identified as the 0{sup -} ground-state and isomers at 47.9 and 128.8 keV having spin/parities of 3{sup +} and 6{sup -} respectively.

Two Gammasphere experiments have been performed in order to establish the possible triaxial nature of strongly deformed (SD) bands in 174Hf. A lifetime measurement confirmed the large deformation of the four previously observed bands in this nucleus. In addition, a thin‐target, high‐statistics experiment was carried out to search for linking transitions between the SD bands. No such transitions, which represent an experimental signature for wobbling modes, were observed. Four new SD bands were found in 174Hf together with a single SD band in 173Hf. These results indicate that the strongly deformed sequences of N ≈ 102 Hf isotopes behave differently than the TSD bands found in Lu nuclei near N = 92. The interpretation of these bands in terms of possible stable triaxial deformation is confronted with the experimental findings and UC predictions.

High-spin states in the doubly-odd $Z=77$ nucleus $^{188}\mathrm{I}\mathrm{r}$ were populated in the $^{186}\mathrm{W}(^{7}\mathrm{L}\mathrm{i},5n)$ reaction at $52\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. Two nearly degenerate $\ensuremath{\Delta}I=1$ sequences with the same parity were established. Both bands have been assigned the $\ensuremath{\pi}{h}_{9∕2}\ensuremath{\bigotimes}\ensuremath{\nu}{i}_{13∕2}$ configuration, based on the systematic behavior of these excitations in the $\mathrm{Ir}$ nuclei and on the measured values for the $B(M1)∕B(E2)$ ratios, and they are suggested as candidates for a chiral doublet.

High-spin states in {sup 174}Pt were populated via the {sup 92}Mo({sup 84}Sr,2p) and {sup 92}Mo({sup 84}Sr,2p2n) reactions. The ground-state band has been extended from I=14 to 24 (tentatively 26) and a new side band is observed up to a spin of 21 (tentatively 23). A low-frequency crossing is observed in the latter band at a rotational frequency that is similar to that seen in the ground-state band. The first and second i{sub 13/2} neutron alignments are also observed in {sup 174}Pt. Surprisingly, these crossings occur at approximately the same frequency. Total Routhian surface and cranked shell model calculations are used in an attempt to understand this behavior.

Two high-spin regularly spaced rotational bands with large dynamical moments of inertia have been identified in Hf-175 with the Gammasphere spectrometer. These new bands are very similar to the previously identified triaxial superdeformed bands in the hafnium nuclei. However, the new bands in Hf-175 have been linked into the known level scheme and thereby provide the first firm spin assignments for these structures in this region. In order to understand the new bands, theoretical calculations have been performed based on the ULTIMATE CRANKER code. The new bands in Hf-175 are deduced to be built upon highly deformed structures. No experimental evidence for triaxiality was established and this work suggests that the structure of the so-called "triaxial" superdeformed bands in the Hf nuclei may be quite different from those identified in the lighter mass Lu nuclei. Since the two highly deformed bands in Hf-175 are associated with different deformations, this work also identifies the role of the intruder orbits in polarizing the nuclear shape.

High spin states in 140Eu were populated following the reaction 92Mo(51V,2pn) at a beam energy of 205 MeV at Yale. A total of 5 bands and 69 transitions were assigned to 140Eu. The band assignments and structures are discussed in terms of the rotational model and interpreted following a series of cranked shell model calculations.

High-spin states of $^{187}\mathrm{Ir}$ have been populated in the $^{176}\mathrm{Yb}(^{15}\mathrm{N},4n)$ reaction and measured with the YRAST Ball spectrometer. The $\ensuremath{\pi}{h}_{9∕2}$ rotational band has been extended beyond the first alignment crossing, which was found at rotational frequency $\ensuremath{\hbar}{\ensuremath{\omega}}_{c}\ensuremath{\approx}0.39\phantom{\rule{0.3em}{0ex}}\text{MeV}$. Two different scenarios for describing this crossing are considered: the alignment of an ${h}_{9∕2}$ proton or ${i}_{13∕2}$ neutron pair and it is concluded that a proton band crossing is more likely. A systematic study of the rotational alignment crossings in the $\ensuremath{\pi}{h}_{9∕2}$ bands in the $N=104,106,108$ isotopes of $_{73}\mathrm{Ta},_{75}\mathrm{Re},_{77}\mathrm{Ir},$ and $_{79}\mathrm{Au}$ is presented.

The mass A = 140 products of the 54Fe (315 MeV) + 92Mo reaction were selected by a recoil mass spectrometer and studied in a recoil ‐ delayed γ‐γ coincidence experiment. A new 7 μs isomer was identified in the drip line nucleus 140Dy. Five cascading γ transitions were assigned to the decay of the Iπ = 8− {v9/2−[514] ⊗ v7/2+[404]} K isomer via the ground state band transitions. The established structure of 140Dy is discussed with reference to the fine structure in proton decay of 141Ho.

Entry distributions in angular momentum and excitation energy have been measured for several very proton-rich isotopes of Pt and Au. This is the first systematic study of the energy-spin phase space for nuclei near and beyond the proton drip line. Comparisons are made between the distributions associated with proton-unbound Au nuclei and more stable Pt isobars. In 173Au the first evidence is seen for the limits of excitation energy and angular momentum which a nucleus beyond the proton drip line can sustain.

The observation of fine structure in proton emission allows one to measure the composition of the parent state’s wavefunction and the deformation of the daughter state. A systematic study of proton‐unbound nuclei in the rare earth region is ongoing at the HRIBF. Newly discovered fine structure in the decays of 145Tm and 141Ho are discussed, as well as advances in our experimental equipment.

The Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge is the first ISOL-type facility to provide beams of accelerated radioactive fission products. To generate these beams, products of fission induced by proton bombardment of a uranium carbide target are ionized, charge-exchanged in Cs vapor, and injected into the HRIBF tandem. Using a single stripper foil in the terminal, they can be accelerated to about 3 MeV per nucleon, ideal for Coulomb Excitation studies. Alternatively, they can be accelerated to energies above the Coulomb barrier by means of double-stripping, at the expense of a factor of about five in lost beam intensity. This paper reports on recent experiments using neutron-rich radioactive ion beams (RIBs) from the HRIBF facility. They have performed Coulomb excitation measurements of B(E2; 0{sup +} {yields} 2{sup +}) in {sup 126,128}Sn and {sup 132,134,136}Te, neutron transfer to single-particle states in {sup 135}Te, and fusion-evaporation reactions on light targets to study evaporation residues with {gamma}-{gamma}-recoil coincidence spectroscopy.

The new neutron‐rich radioactive ion beams available from the HRIBF allow an exciting variety of measurements around the 132Sn region, including Coulomb excitation, fusion‐evaporation, and neutron transfer. Experiments using these beams also provide an excellent training ground for developing techniques to be used at the future RIA facility. The B(E2;0+ → 2+) value for first 2+ excited states of neutron‐rich 132,134,136Te and 126,128Sn have been measured by inverse‐kinematics Coulomb excitation. The B(E2) values obtained for 132,134Te are in excellent agreement with expectations based on the systematics of heavy stable Te isotopes, while that for 136Te is unexpectedly small. These results are discussed in terms of the shell model and the quasiparticle random phase approximation. Neutron transfer onto a 134Te beam, from 9Be and 13C targets to populate single‐particle states in 135Te, has also been studied. Results, and plans for future experiments with these neutron‐rich beams, are presented.

The reactions ${}^{152}\mathrm{Sm}(\ensuremath{\alpha},3n)$ at 37 MeV and ${}^{124}\mathrm{Sn}{(}^{36}\mathrm{S},\ensuremath{\alpha}3n)$ at 165 MeV were used to produce high-spin states in the transitional rare-earth nucleus ${}^{153}\mathrm{Gd}.$ Significant extensions in angular momentum and excitation energy with respect to earlier work were achieved and several new rotational sequences were observed. The rotational behavior of ${}^{153}\mathrm{Gd}$ is described in terms of quasiparticle assignments and the observed alignment properties are analyzed within the framework of the cranked shell model. Detailed comparisons with other $N=89$ nuclei are presented. These have led to corrections to the ${}^{157}\mathrm{Er}$ level scheme.

High-spin states in ${}^{128}\mathrm{Ca}$ were populated with the ${}^{92}\mathrm{Mo}$ ${(}^{40}\mathrm{Ca},2p2n)$ reaction at a beam energy of 184 MeV. The previously known ground-state band has been extended to ${I}^{\ensuremath{\pi}}{=(34}^{+})$ and four sidebands were observed. Configuration assignments for these sidebands are discussed based on their alignment behavior. A significant delay of the $\ensuremath{\nu}{h}_{11/2}$ crossing frequency is observed in the $A\ensuremath{\sim}130$ region, with the largest delays occurring consistently at $N=70.$ Cranked shell model calculations were performed in order to investigate whether a variation of deformation and/or pairing parameters can account for this phenomenon.

The $B({E2;0}^{+}\ensuremath{\rightarrow}{2}^{+})$ values for the first ${2}^{+}$ excited states of neutron-rich ${}^{132,134,136}\mathrm{Te}$ have been measured using Coulomb excitation of radioactive ion beams. The $B(E2)$ values obtained for ${}^{132,134}\mathrm{Te}$ are in excellent agreement with expectations based on the systematics of heavy stable Te isotopes, while that for ${}^{136}\mathrm{Te}$ is unexpectedly small. These results are discussed in terms of proton-neutron configuration mixing and shell-model calculations using realistic effective interactions.

Over 240 transitions and three new rotational bands have been observed in the well-deformed, odd-odd ${}_{59}^{128}{\mathrm{Pr}}_{69}$ nucleus. The high-spin states were populated in two experiments using the ${}^{92}\mathrm{Mo}{(}^{40}\mathrm{Ca},3pn)$ reaction at beam energies of 170 and 184 MeV. Several structures were confirmed in the former experiment using the Clarion and HyBall arrays with the recoil mass spectrometer at Oak Ridge National Laboratory. Gammasphere, in conjunction with the Microball, were employed in the latter experiment at Argonne National Laboratory to extend the sequences to very high spins. Rotational structures in ${}^{126}\mathrm{Pr}$ were also identified in the $\ensuremath{\alpha}\mathrm{pn}$ channel of the same reaction. The recent discrepancies of spin assignments for the yrast bands in ${}^{126,128}\mathrm{Pr}$ and the interpretation of the lowest crossing in the $\ensuremath{\pi}{h}_{11/2}$ band in ${}^{127}\mathrm{Pr}$ are discussed. An adiabatic crossing of the intruder ${i}_{13/2}$ neutron with one of the normal-deformed bands in ${}^{128}\mathrm{Pr}$ is observed at high rotational frequency. Experimental trends in the signature inversion phenomenon of the $\ensuremath{\pi}{h}_{11/2}\ensuremath{\nu}{h}_{11/2}$ bands in the $A\ensuremath{\approx}130$ region are defined and prove to be surprisingly irregular with respect to those found in the $\ensuremath{\pi}{h}_{11/2}\ensuremath{\nu}{i}_{13/2}$ bands of the $A\ensuremath{\approx}160$ region.

The latest generation gamma-ray detection system, GAMMASPHERE, coupled with the Microball charged-particle detector, has made possible a new class of nuclear lifetime measurement. For the first time differential lifetime measurements free from common systematic errors for over 15 different nuclei ( >30 rotational bands in various isotopes of Ce, Pr, Nd, Pm, and Sm) have been extracted at high spin within a single experiment. This comprehensive study establishes the effective single-particle transition quadrupole moments in the A approximately 135 light rare-earth region. Detailed comparisons are made with theoretical calculations using the self-consistent cranked mean-field theory which convincingly demonstrates the validity of the additivity of single-particle quadrupole moments in this mass region.

The nuclei ${}^{91}\mathrm{Sr}$ and ${}^{92}\mathrm{Sr}$ were produced at high spin as fission fragments following the fusion reaction ${}^{36}\mathrm{S}{+}^{159}\mathrm{Tb}$ at 165 MeV. $\ensuremath{\gamma}$ rays were detected with the Gammasphere array. The level schemes of ${}^{91}\mathrm{Sr}$ and ${}^{92}\mathrm{Sr}$ were extended up to $E\ensuremath{\approx}6 \mathrm{MeV}$ and $E\ensuremath{\approx}8 \mathrm{MeV},$ respectively. Level structures in ${}^{91}\mathrm{Sr}$ and ${}^{92}\mathrm{Sr}$ were interpreted in shell-model calculations performed in the configuration space ${(0f}_{5/2}{,1p}_{3/2}{,1p}_{1/2}{,0g}_{9/2})$ for the protons and ${(1p}_{1/2}{,0g}_{9/2}{,1d}_{5/2})$ for the neutrons. Negative-parity states in the yrast sequences are described in these calculations by coupling ${3}^{\ensuremath{-}}$ proton excitations to the unpaired ${1d}_{5/2}$ neutrons. A possible reduction of the gap between the proton ${1p}_{3/2}$ and ${1p}_{1/2}$ orbitals in ${}^{92}\mathrm{Sr}$ is discussed.

The lifetimes of excited states in the yrast band in ${}^{158}\mathrm{Er}$ up to ${I}^{\ensuremath{\pi}}{=20}^{+}$ have been measured to high precision by means of the Gammasphere spectrometer using the coincidence recoil distance technique. The reaction ${}^{122}\mathrm{Sn}{(}^{40}\mathrm{Ar},4n)$ at a beam energy of 185 MeV was used. The data were analyzed using the differential decay-curve method and transition quadrupole moments were extracted. The quadrupole moment was found to gradually increase by 15% of its ${2}^{+}$ value before the first $\ensuremath{\nu}{i}_{13/2}$ alignment, after which it drops to a constant level 90% of the ${2}^{+}$ value. Detailed comparisons are made with previous measurements and Ultimate Cranker calculations.

A new $7 \ensuremath{\mu}\mathrm{s}$ isomer in the drip line nucleus ${}^{140}\mathrm{Dy}$ was selected from the products of the ${}^{54}\mathrm{Fe}$ $(315 \mathrm{MeV}{)+}^{92}\mathrm{Mo}$ reaction by a recoil mass spectrometer and studied with recoil-delayed $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidences. Five cascading $\ensuremath{\gamma}$ transitions were interpreted as the decay of an ${I}^{\ensuremath{\pi}}{=8}^{\ensuremath{-}}$ ${\ensuremath{\nu}{9/2}^{\ensuremath{-}}[514]\ensuremath{\bigotimes}\ensuremath{\nu}{7/2}^{+}[404]}$ K isomer $({T}_{1/2}=7.0(5) \ensuremath{\mu}\mathrm{s})$ via the ground-state band. The probability of proton emission from ${}^{141}\mathrm{Ho}$ to the ${0}^{+}$ ground state and to the ${2}^{+}$ excited state in ${}^{140}\mathrm{Dy}$ is discussed.

A new 7 μs isomer in the drip line nucleus 140Dy was selected from the products of the 54Fe (315 MeV) + 92Mo reaction by a recoil mass spectrometer and studied with recoil ‐ delayed γ‐γ coincidences. Five cascading γ transitions were interpreted as the decay of an Iπ = 8− {v9/2−[514] ⊗ v7/2+[404]} K isomer via the ground state band.

High-spin states in ${}^{129}\mathrm{Nd}$ were populated in the reaction ${}^{92}\mathrm{Mo}{(}^{40}\mathrm{Ca},2pn)$ with beam energies of 170 and 184 MeV in two separate experiments. Over 140 new transitions were placed in a level scheme that consists of four rotational structures, three of which have been observed for the first time. The bands were assigned configurations based on their $B(M1)/B(E2)$ ratios (for the strongly coupled bands), aligned angular momentum, observed band crossings, and signature splitting. Evidence for of the observation of the deformation driving $\ensuremath{\nu}{i}_{13/2}[660]1/2$ orbital is presented. Signature splitting trends in the $\ensuremath{\nu}{h}_{11/2}$ bands of $A\ensuremath{\sim}130$ nuclei are investigated within the framework of the cranked shell model. Comparisons are made with observations in the $\ensuremath{\pi}{h}_{11/2}$ bands near $A\ensuremath{\sim}160.$

The B(E2;0(+)--2+) values for the first 2+ excited states of neutron-rich 132,134,136Te have been measured using Coulomb excitation of radioactive ion beams. The B(E2) values obtained for 132,134Te are in excellent agreement with expectations based on the systematics of heavy stable Te isotopes, while that for 136Te is unexpectedly small. These results are discussed in terms of proton-neutron configuration mixing and shell-model calculations using realistic effective interactions.

The latest generation gamma-ray detection system, GAMMASPHERE, coupled with the Microball charged-particle detector, has made possible a new class of nuclear lifetime measurement. For the first time differential lifetime measurements free from common systematic errors for over 15 different nuclei (30 rotational bands in various isotopes of Ce, Pr, Nd, Pm, and Sm) have been extracted at high spin within a single experiment. This comprehensive study establishes the effective single-particle transition quadrupole moments in the A approximately 135 light rare-earth region. Detailed comparisons are made with theoretical calculations using the self-consistent cranked mean-field theory which convincingly demonstrates the validity of the additivity of single-particle quadrupole moments in this mass region.

The chiral-twin candidate bands recently observed in {sup 136}Pm have been extended to high spins [I=(21)] using the Gammasphere {gamma}-ray spectrometer and the Microball charged-particle detector array. A more-detailed spectroscopy of the bands was possible, where the rotational alignments and B(M1)/B(E2) ratios confirm that both sequences have the {pi}h{sub 11/2}{nu}h{sub 11/2} configuration. Particle-rotor calculations of intraband and interband transition strength ratios of the chiral-twin bands are compared with experimental values for the first time. Good agreement was found between the predicted transition strength ratios and the experimental values, thus supporting the possible chiral nature of the {pi}h{sub 11/2}{nu}h{sub 11/2} configuration in {sup 136}Pm.

g-ray transitions have been identified for the first time in the near-drip-line nucleus 126 Pr, making it the lightest odd-odd praseodymium nucleus in which excited states have been reported. Evidence is presented for two rotational bands in 126 Pr, one strongly coupled and the other doubly decoupled. In addition, the preliminary reports of a band in 128 Pr are confirmed. The signature inversion phenomenon and trends in the energy staggering of the ph11/2nh11/2 bands are discussed for the Cs, La, and Pr nuclei.

Four highly deformed structures in the odd-odd nucleus {sub 61}{sup 136}Pm{sub 75} were observed via the {sup 105}Pd({sup 35}Cl,2p2n) reaction at 180 and 173 MeV using the GAMMASPHERE {gamma}-ray spectrometer and the Microball charged-particle detector array. Quadrupole moment measurements were performed on all of the bands. In contrast to lighter odd-Z Pm and Pr nuclei, bands based on the g{sub 9/2}[404]9/2 proton orbital were not observed. Instead, the four observed sequences are assigned as a coupling of an i{sub 13/2} neutron with the low-{omega} h{sub 11/2} and mixed d{sub 5/2}g{sub 7/2} orbitals. Comparisons with neighboring highly deformed structures are discussed and cranked Nilsson-Strutinsky calculations for {sup 136}Pm are presented. (c) 2000 The American Physical Society.

New sideband partners of the yrast bands built on the pi(h11/2)nu(h11/2) configuration were identified in 55Cs, 57La, and 61Pm N = 75 isotones of 134Pr. These bands form with 134Pr unique doublet-band systematics suggesting a common basis. Aplanar solutions of 3D tilted axis cranking calculations for triaxial shapes define left- and right-handed chiral systems out of the three angular momenta provided by the valence particles and the core rotation, which leads to spontaneous chiral symmetry breaking and the doublet bands. Small energy differences between the doublet bands suggest collective chiral vibrations.

High-spin states in {sup 131}Nd were populated in the reaction {sup 94}Mo({sup 40}Ca,2pn) at a beam energy of 180 MeV. Over 170 new transitions were placed in a level scheme that consists of seven rotational structures. The bands were given configuration assignments based on their B(M1)/B(E2) ratios (for the strongly coupled bands), aligned angular momentum, observed band crossings, and signature splitting. Several quasiparticle alignments were observed in the bands and compared with predictions from the cranked shell model. Three-band mixing calculations were performed in order to interpret the low-spin interaction observed in the [411]1/2 band. An examination of the signature splitting for the [541]1/2 bands in {sup 129,131,133}Nd revealed information regarding the parentage of the orbital as well as a signature inversion at higher spin. (c) 2000 The American Physical Society.

Two experiments have recently been performed in the A~130 highly-deformed region using GAMMASPHERE and the Microball. A large number of highly-deformed structures have been observed (many of them for the first time). In addition, the quadrupole moments for a variety of configurations, including the 9/2+ [404] (g 9/2) proton, 1/2+ [660](i 13/2) and 1/2-[541] (f 7/2,h 9/2) neutron orbitals, were measured in a wide range of nuclei using the Doppler-shift attenuation method. While the involvement of the first two orbitals leads to quadrupole deformations that are comparable to those observed for the so-called superdeformed bands in this mass region, the s 2 values for structures that include the 1/2- [541] neutron are found to lie intermediate between those observed for normally deformed and highly deformed bands. Deformation trends for the same configuration as a function of N are also established. New results on 135Pm (N =75) are presented. Some unresolved questions and perspectives for the region are discussed.

The deformation driving {nu}i{sub 13/2} [660]1/2 structure has been identified for the first time in {sup 131}Nd. It is the lightest Nd (Z=60) isotope in which a band based on this intruder orbital has been reported. A crossing between the [411]1/2 band and the [660]1/2 sequence takes place near {h_bar}{omega}{approx}0.34 MeV with a large interaction strength ({vert_bar}V{vert_bar}{sub max}=47 keV). Thus, a smooth transition from the normal to the highly deformed minimum is established in {sup 131}Nd. thinsp {copyright} {ital 1999} {ital The American Physical Society}