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The fusion-evaporation reaction 32S+28Si at 125-MeV beam energy was used to populate excited states in 55Fe. Combining the Gammasphere spectrometer with ancillary devices including the Microball CsI(Tl) array and a shell of neutron detectors, a comprehensive level scheme could be derived. The experimental results are compared with theoretical results from shell-model calculations. Taking into account isospin-symmetry breaking terms is found to considerably improve the shell-model description for 55Fe. This motivated a predictive case study of near-yrast states in the mirror nucleus 55Cu.

The fusion-evaporation reaction $^{28}\mathrm{Si}+^{40}\mathrm{Ca}$ at 122 MeV beam energy was used to populate excited states in the $N=Z+1$ nucleus $^{63}\mathrm{Ga}$. With the combination of the Gammasphere spectrometer and the Microball CsI(Tl) charged-particle detector array, the level scheme of $^{63}\mathrm{Ga}$ was extended by more than a factor of two in terms of number of $\ensuremath{\gamma}$-ray transitions and excited states, excitation energy $({E}_{x}g30\phantom{\rule{0.16em}{0ex}}\mathrm{MeV})$, and angular momentum $(Ig30\phantom{\rule{0.16em}{0ex}}\ensuremath{\hbar})$. Nine regular sequences of states were newly established in the high-spin part of the level scheme. The majority of these rotational band structures could be connected to the previously known part of the level scheme by high-energy $\ensuremath{\gamma}$-ray transitions in the energy range ${E}_{\ensuremath{\gamma}}=4--6\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$. Low-spin states were assessed by shell-model calculations. The high-spin rotational bands were interpreted and classified by means of cranked Nilsson-Strutinsky calculations.

Author(s): Rudolph, D; Ragnarsson, I; Andreoiu, C; Bentley, MA; Carpenter, MP; Charity, RJ; Clark, RM; Ekman, J; Fahlander, C; Fallon, P; Reviol, W; Sarantites, DG; Seweryniak, D | Abstract: The fusion-evaporation reaction S32+Si28 at 125 MeV beam energy was used to populate high-spin states in the semimagic N=28 nuclei Mn53 and Fe54. With a combination of the Gammasphere spectrometer and ancillary devices including the Microball CsI(Tl) array, extensive high-spin level schemes are derived. They exhibit rotational-like collective structures and competing single-particle excitations. The experimental results are compared with predictions from shell-model calculations, for which the inclusion of isopin-symmetry-breaking terms is found to improve the description. An interpretation of the high-spin states is put forward using cranked Nilsson-Strutinsky calculations, indicative of contributions from collective excitations beyond some 8-MeV excitation energy and highlighting the importance of the g9/2 intruder orbital in this energy range.

The fusion-evaporation reaction 28Si + 40Ca at 122 MeV beam energy was used to populate high-spin states in the odd-odd N = Z nucleus 62Ga. With the combination of the Gammasphere spectrometer and the Microball CsI(Tl) charged-particle detector array the decay scheme of 62Ga was extended beyond 10 MeV excitation energy. The onset of band structures was observed. These high-spin rotational states are interpreted and classified by means of cranked Nilsson-Strutinsky calculations.

Level lifetime and side-feeding time measurements were performed on the excited states of the normally deformed bands of Y83 using the Doppler-shift attenuation method (DSAM). The high spin states of Y83 were populated using the fusion-evaporation reaction Ni58(S32,α3p)Y83 at 135 MeV. Twenty-two level lifetime and side-feeding times were determined in most of the cases by comparing the line shapes gated with transitions above and below the state under study. Quadrupole moments determined from lifetime measurements are in the range 1.1-3.1 eb, and are similar to the ones found for some of the neighboring nuclei. The measured side-feeding times were compared with predictions made by simulations carried out with the Gammapace code. The results were in agreement with the experimental values by assuming reduced transition probabilities of the collective transitions in the continuum region, lying in the range 40-80 W.u. The discrete excited states were studied with paired cranked Nilsson-Strutinsky-Bogoliubov (CNSB) calculations carried out for the first time for an A≈80 nucleus. Unpaired cranked Nilsson-Strutinsky (CNS) calculations were used to specify configurations and study the band crossings. The measured |Qt| values show a general agreement with CNSB calculations. Cranked shell model analysis evinced that the smallest quadrupole moment appears at the sharpest band crossing of the bands studied and CNSB calculations show an increase of the deformation thereinafter. (Less)

The energy levels and γ-ray decay scheme of the positive-parity states in the Tz=12 nucleus As67 have been studied by using the Ca40(Ar36,2αp)As67 reaction at a beam energy of 145 MeV. Two new band structures have been identified which can be connected to the previously known levels. The results for these bands are compared with configuration-dependent cranked Nilsson-Strutinsky calculations. The good level of agreement between theory and experiment suggests that these structures can be interpreted in terms of configurations that involve three g92 particles and that both possess noncollective terminating states.

Excited states in $^{137}\mathrm{Cs}$ have been studied by using the near-barrier, single-proton transfer reaction $^{10}\mathrm{B}(^{136}\mathrm{Xe},\phantom{\rule{4pt}{0ex}}^{9}\mathrm{Be}\ensuremath{\gamma})^{137}\mathrm{Cs}$ in inverse kinematics and performing particle-$\ensuremath{\gamma}$ and particle-$\ensuremath{\gamma}\ensuremath{\gamma}$ coincidence measurements. The low-spin level scheme of $^{137}\mathrm{Cs}$ has been augmented: (i) The expected set of single-proton, seniority $\ensuremath{\upsilon}=1$ states has been delineated in its entirety; (ii) Another member of the $\ensuremath{\upsilon}=3$ proton $0{g}_{7/2}$ multiplet, a low-lying $9/{2}^{+}$ level, has been identified and this set of states now approaches completion; (iii) A second $9/{2}^{+}$ and a candidate $13/{2}^{+}$ level have been observed. The results are discussed in the framework of shell-model calculations in the proton $2s1d0g0h$ shell.

Author(s): Lubna, RS; Tripathi, V; Tabor, SL; Tai, PL; Kravvaris, K; Bender, PC; Volya, A; Bouhelal, M; Chiara, CJ; Carpenter, MP; Janssens, RVF; Lauritsen, T; McCutchan, EA; Zhu, S; Clark, RM; Fallon, P; MacChiavelli, AO; Paschalis, S; Petri, M; Reviol, W; Sarantites, DG | Abstract: Excited states in the neutron-rich isotopes P33 and P34 were populated by the O18+O18 fusion-evaporation reaction at Elab=24 MeV. The Gammasphere array was used along with the Microball particle detector array to detect γ transitions in coincidence with the charged particles emitted from the compound nucleus S36. The use of Microball enabled the selection of the proton emission channel. It also helped in determining the exact position and energy of the emitted proton; this was later employed in kinematic Doppler corrections. 16 new transitions and 13 new states were observed in P33 and 21 γ rays and 20 energy levels were observed in P34 for the first time. The nearly 4π geometry of Gammasphere allowed the measurement of γ-ray angular distributions leading to spin assignments for many states. The experimental observations for both isotopes were interpreted with the help of shell-model calculations using the (0+1)ω PSDPF interaction. The calculations accounted for both the 0p-0h and 1p-1h states reasonably well and indicated that 2p-2h excitations might dominate the higher-spin configurations in both P33 and P34.

Author(s): Jones, MD; MacChiavelli, AO; Wiedeking, M; Bernstein, LA; Crawford, HL; Campbell, CM; Clark, RM; Cromaz, M; Fallon, P; Lee, IY; Salathe, M; Wiens, A; Ayangeakaa, AD; Bleuel, DL; Bottoni, S; Carpenter, MP; Davids, HM; Elson, J; Gorgen, A; Guttormsen, M; Janssens, RVF; Kinnison, JE; Kirsch, L; Larsen, AC; Lauritsen, T; Reviol, W; Sarantites, DG; Siem, S; Voinov, AV; Zhu, S | Abstract: A model-independent technique was used to determine the γ-ray strength function (γSF) of Fe56 down to γ-ray energies less than 1 MeV for the first time with GRETINA using the (p,p′) reaction at 16 MeV. No difference was observed in the energy dependence of the γSF built on 2+ and 4+ final states, supporting the Brink hypothesis. In addition, angular distribution and polarization measurements were performed. The angular distributions are consistent with dipole radiation. The polarization results show a small bias towards magnetic character in the region of the enhancement.

The Si31 nucleus was produced through the O18(O18, αn) fusion-evaporation reaction at Elab=24MeV. Evaporated α particles from the reaction were detected and identified in the Microball detector array for channel selection. Multiple γ-ray coincidence events were detected in Gammasphere. The energy and angle information for the α particles was used to determine the Si31 recoil kinematics on an event-by-event basis for a more accurate Doppler correction. A total of 22 new states and 52 new γ transitions were observed, including 14 from states above the neutron separation energy. The positive-parity states predicted by the shell-model calculations in the sd model space agree well with experiment. The negative-parity states were compared with shell-model calculations in the psdpf model space with some variations in the N=20 shell gap. The best agreement was found with a shell gap intermediate between that originally used for A≈20 nuclei and that previously adapted for P32,34. This variation suggests the need for a more universal cross-shell interaction.

Directed and elliptic flows of neutrons and light charged particles were measured for the reaction 197Au+197Au at 400 MeV/nucleon incident energy within the ASY-EOS experimental campaign at the GSI laboratory. The detection system consisted of the Large Area Neutron Detector LAND, combined with parts of the CHIMERA multidetector, of the ALADIN Time-of-flight Wall, and of the Washington-University Microball detector. The latter three arrays were used for the event characterization and reaction-plane reconstruction. In addition, an array of triple telescopes, KRATTA, was used for complementary measurements of the isotopic composition and flows of light charged particles. From the comparison of the elliptic flow ratio of neutrons with respect to charged particles with UrQMD predictions, a value \gamma = 0.72 \pm 0.19 is obtained for the power-law coefficient describing the density dependence of the potential part in the parametrization of the symmetry energy. It represents a new and more stringent constraint for the regime of supra-saturation density and confirms, with a considerably smaller uncertainty, the moderately soft to linear density dependence deduced from the earlier FOPI-LAND data. The densities probed are shown to reach beyond twice saturation., Comment: 25 pages, 22 figures, accepted for publication in Physical Review C

Excited states in 137Xe have been studied by using the near-barrier single-neutron transfer reactions 13C(136Xe,12C )137Xe and 9Be(136Xe,8Be )137Xe in inverse kinematics.Particle- and particle- coincidence measurements have been performed with the Phoswich Wall and Digital Gammasphere detector arrays. Evidence is found for a 13/2+2 level (E = 3137 keV) and for additional high-lying 3/2– and 5/2– states. The results are discussed in the framework of realistic shell-model calculations. These calculations are also extended to the 13/2+1 and 13/2+2 levels in the N = 83 isotonic chain. Furthermore, they indicate that there is a need for a value of the neutron 0i13/2 single-particle energy (ESPE = 2366 keV) lower than the one proposed in the literature. It is also demonstrated that the population patterns of the j = l ± 1/2 single-particle states in 137Xe are different for the two targets used in these measurements and the implications of this effect are addressed.

A detailed level scheme for $^{221}\mathrm{Th}$ has been established in an experiment using the $^{18}\mathrm{O}+{}^{207}\mathrm{Pb}$ reaction at 96 MeV. The evaporation residues from this fissile system were selected with the HERCULES detector array and residue-gated $\ensuremath{\gamma}$ rays were measured with Gammasphere. Three band structures of interlinked, alternating-parity levels are observed, two of which are non-yrast. In addition, several high-lying excitations are found. The yrast band is seen up to spin-parity $37/{2}^{\ensuremath{-}}$ and $39/{2}^{+}$, beyond which a high-spin feeding transition is observed. The non-yrast sequences are interpreted as parity-doublet structures, based on a configuration similar to that of the yrast band in $^{223}\mathrm{Th}$ $(K=5/2)$. The key properties of even-odd nuclei in this mass region $[B(E1)/B(E2)$ and $B(M1)/B(E2)$ ratios, spin alignments, parity splittings] are reviewed.

Two excited well-deformed bands have been observed in the semi-magic nucleus Ni-58. One of the bands was observed to partially decay by emission of a prompt discrete alpha particle that feeds the 2949 keV 6(+) spherical yrast state in the daughter nucleus Fe-54. This constitutes the first observation of prompt alpha emission from states lying in the deformed secondary minimum of the nuclear potential. gamma -ray linking transitions via several parallel paths establish the spin. parity, and excitation energy of this deformed band in Ni-58.

A superdeformed rotational band has been identified in ${}^{36}\mathrm{Ar}$, linked to known low-spin states, and observed to its high-spin termination at ${I}^{\ensuremath{\pi}}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}16}^{+}$. Cranked Nilsson-Strutinsky and spherical shell model calculations assign the band to a configuration in which four $\mathrm{pf}$-shell orbitals are occupied, leading to a low-spin deformation ${\ensuremath{\beta}}_{2}\ensuremath{\approx}0.45$. Two major shells are active for both protons and neutrons, yet the valence space remains small enough to be confronted with the shell model. This band thus provides an ideal case to study the microscopic structure of collective rotational motion.

Comparative measurements of the lifetimes of yrast superdeformed (SD) states in ${}^{80--83}\mathrm{Sr}$, ${}^{83}\mathrm{Y}$, and ${}^{84}\mathrm{Zr}$ have been performed using the Doppler-shift attenuation method. Thus, the transition quadrupole moments ( ${Q}_{t}$) of these structures have been determined accurately. The yrast SD bands in ${}^{80--83}\mathrm{Sr}$ possess ${Q}_{t}\ensuremath{\sim}3.5e\mathrm{b}$, while the ${}^{83}\mathrm{Y}$ and ${}^{84}\mathrm{Zr}$ cases display ${Q}_{t}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4.4e\mathrm{b}$ and $5.6e\mathrm{b}$, respectively. Intruder orbital assignments based on these results demonstrate a previously unnoticed transition in the structure of the SD bands when the proton number changes from $Z\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}38$ to $Z\ensuremath{\ge}39$.

A superdeformed rotational structure is reported in the stable nucleus Zn-68. It has a transition quadrupole moment (Q(t)) of 2.5(-0.4)(+0.7) e b (beta(2) approximate to 0.4), as measured by the residual Doppler shift method. Theoretical calculations suggest that two different configurations are possible candidates for this band. One is built without h(11/2) neutrons, as seen in superdeformed bands in the A similar to 60 region, while the other involves one h(11/2) neutron in analogy with superdeformed bands in the A similar to 80 region. The second of these candidate configurations is preferred, thus establishing a new, neutron-rich (relative to the line of beta stability) region of superdeformation with Z similar to 30 and N similar to 38. (Less)

The quadrupole moments of three decoupled bands in the odd-odd nuclei {sup 130}Pr and {sup 132}Pr were measured using the Doppler shift attenuation method in the bombardment of {sup 105}Pd with {sup 35}Cl at 173 MeV. The {pi}h{sub 11/2}{circle_times}{nu}(f{sub 7/2},h{sub 9/2}) configuration was assigned to these structures on the basis of the observed in-band properties and relative deformations. New information is presented on the shape-driving behavior of the 1/2{sup {minus}}[541] (f{sub 7/2},h{sub 9/2}) intruder neutron orbital, whose involvement in a particular configuration leads to quadrupole deformation values which are intermediate between those known for normally deformed and so-called superdeformed bands in this mass region. {copyright} {ital 1999} {ital The American Physical Society}

The doubly magic superdeformed band in the N = Z nucleus Zn-60 has been identified. Linking transitions connecting this band to the yrast line provide the first spin, parity, and excitation energy measurements for superdeformed states in the A similar to 60 region. The stretched-E2 character and relatively large B(E2) values of these transitions suggest a nonstatistical decay-out process.

High-spin states in Kr-78 were studied using the Ni-58(Na-23,3p) reaction at 70 MeV and the Ni-58(Si-23,alpha 4p) reaction at 130 MeV. Prompt gamma-gamma coincidences were measured using the Pitt-FSU detector array and the GAMMASPHERE-MICROBALL array. Results from these experiments have led to 26 new excitation levels, some of which have been grouped into 3 new bands. Spins were assigned based on directional correlations of oriented nuclei. Two of the new negative-parity bands appear to form a signature-partner pair based on a two-quasineutron structure, in contrast to the previously known two-quasiproton negative-parity bands. A forking has been observed at the 24(+) state in the yrast band, which calculations suggest may result from an unpaired crossing. The available evidence suggests oblate shapes in the yrast band coexist with prolate shapes in the negative-parity bands. [S0556-2813(99)04602-6]. (Less)

High-spin states have been populated in the neutron-deficient {sup 135,136}Sm nuclei via the {sup 105}Pd({sup 35}Cl,pxn) reaction at a beam energy of 180 MeV. Transitions within the Sm nuclei were enhanced by using the Microball charged particle array and {gamma}-ray de-excitation was studied using Gammasphere. A new highly deformed band has been observed in the data and assigned to {sup 136}Sm. Comparison of the properties of the new band with Woods-Saxon cranking calculations suggests that it is built upon a {nu}(i{sub 13/2}){sup 2} configuration. The band shows evidence for an interaction at {h_bar}{omega}{approximately}0.7 MeV. This has tentatively been interpreted as resulting from a crossing between the {alpha}=+1/2 signatures of the [532]5/2 and [541]1/2 proton orbitals. The previously known {nu}i{sub 13/2} intruder band in {sup 135}Sm was also observed and extended to higher rotational frequency. Both structures show good agreement with extended total Routhian surface calculations. {copyright} {ital 1998} {ital The American Physical Society}

Gamma rays in the N=Z + 1 nucleus Y-79 were identified using the reaction Si-28(Fe-54, p2n)Y-79 at a 200 MeV beam energy and an experimental setup consisting of an array of Ge detectors and the Recoil Mass Spectrometer at Oak Ridge National Laboratory. With the help of additional gamma-gamma coincidence data obtained with Gammasphere, these gamma rays were found to form a strongly coupled rotational band with rigid-rotor-like behavior. Results of conventional Nilsson-Strutinsky cranked shell model calculations, which predict a deformation of beta(2)similar to 0.4, are in excellent agreement with the properties of this band. Similar calculations for the neighboring N=Z and N=Z + 1 nuclei are also in good agreement with experimental data. This suggests that the presence of the putative T=0 neutron-proton pairing does not significantly affect such simple observables as the moments of inertia of these bands at low spins. [S0556-2813(98)50612-7].

Results are presented of a systematic survey of {Delta}I=4 bifurcation in 19 superdeformed bands in A{approximately}150 Eu and Gd nuclei. The majority of bands show no sign of this regular oscillation in energy levels. The statistical significance of the few examples that exhibit the effect is examined. {copyright} {ital 1998} {ital The American Physical Society}

Eight superdeformed bands have been observed in the nuclei {sup 147,148}Eu. Some of these are identical to bands in the Z+1 nuclei {sup 148,149}Gd, and are interpreted as having the same single-particle configurations with the exception of a hole in one of the [301]1/2 proton orbitals. Some of the bands in {sup 147}Eu exhibit either upbends or downbends in their moments of inertia which are related to the crossing of {pi}6{sup 1} and {pi}6{sup 3} proton intruder configurations. These results constitute the first evidence of this crossing. {copyright} {ital 1998} {ital The American Physical Society}

An excited well-deformed rotational band has been observed in the N = Z odd-odd nucleus Cu-58. The first excited state in this band decays via gamma emission to the spherical states associated with the first minimum in the potential, thus providing for its unambiguous assignment to Cu-58. I, contrast, its bandhead decays via emission of a prompt 2.4(1) MeV proton to an excited state in the daughter nucleus Ni-57. This is the first observation of proton decay from states associated with a deformed secondary minimum in the potential. Self-consistent Hartree-Fock calculations reproduce well both the large collectivity of this band and the general trend of its moment of inertia.

Two sets of strongly coupled rotational bands have been identified in Zn-62. These bands have been observed up to the terminating states of their respective configurations. Lifetime measurements indicate that the transition quadrupole moments in these bands decrease as termination is approached. These results establish the first terminating states of rotational bands in the A similar to 60 mass region and confirm the predicted loss of collectivity associated with smooth band termination.

Four new superdeformed bands have been found in the nucleus {sup 86}Zr. The good agreement between experiment and configuration-dependent shell correction calculations suggests that three of the bands have triaxial superdeformed shapes. Such unique features in mass A{approximately}80 superdeformed bands have been predicted, but not observed experimentally until now. A fourth band in {sup 86}Zr is interesting due to a fairly constant and unusually high dynamic moment of inertia. Possible interpretations of this structure are discussed. {copyright} {ital 1998} {ital The American Physical Society}

Lifetimes of a superdeformed band in Sr-82 were measured with the centroid shift method. The measured average quadrupole moment of this band corresponds to a quadrupole deformation of beta(2) approximate to 0.49, which is slightly smaller than both the theoretical prediction, and the measured deformation of the SD band in the neighboring isotone Zr-84. Lifetimes of high spin states of three normally deformed rotational bands in Sr-82 were also measured with the Doppler shift attenuation method technique. The quadrupole moments of these normally deformed bands show a decrease at the highest spins, supporting the predicted band terminations.

An experiment was performed with the Gammasphere Ge-detector array, and the Microball charged particle detector, to search for transitions involved in the decay from the yrast superdeformed band in Eu-143. Sum spectra in coincidence with single, double, and triple gates on SD transitions were produced to search for previously reported two-step links between the superdeformed and the near-yrast ND levels. The sum spectra contained no evidence of peaks corresponding to sums of two-step linking transitions. Several discrete gamma rays were observed in coincidence with the yrast SD band. One previously reported single-step transition at 3361 keV was confirmed to decay out of the SD band.

Two strongly coupled bands with large dynamic moment of inertia values have been observed for the first time in the neutron-deficient odd-odd nucleus ${}^{130}$Pr using Gammasphere and the Microball charged-particle detector array. These bands involve the important ${g}_{9/2}$[404]9/2 proton orbital. This new information on ${}^{130}$Pr coupled with Projected Shell Model calculations provide valuable insight on rotational alignment processes taking place in this mass region, the size of the $N$=72 deformed shell gap, and the position of the ${i}_{13/2}$[660]1/2 neutron intruder orbital at the boundary of the $A$=135 superdeformed region.

A high-spin rotational cascade of six gamma rays has been observed in Zn-62. The quadrupole moment of 2.7(-0.5)(+0.7) e b measured for this band corresponds to a deformation beta(2) = 0.45(-0.07)(+0.10). The properties of this band are in excellent agreement with calculations that predict high-spin superdeformed bands in Zn-62 with deformations beta(2) = 0.41-0.49. These results establish a new region of superdeformation for nuclei with neutron and proton numbers N, Z approximate to 30-32.

Excited states in the T-z-1/2 nucleus Rb-75 were observed for the first time using the Ca-40(Ca-40,alpha p) reaction at 128 MeV. Identification was achieved using events detected by the Daresbury recoil separator in coincidence with gamma rays detected in the 45 element EUROGAM I Ge-detector array. Threefold events were used to build a decay scheme which consists of two rotational bands observed to I-pi=(45+/2) and I-pi=(33-/2). The positive parity band in Rb-75 behaves similarly to a negative-parity band in Kr-74 and contains a region of alignment at (h) over bar omega approximate to 0.75 MeV. These data, and those of Sr-77, be interpreted by treating protons and neutrons separately in a cranked shell model approach despite a recent suggestion for the presence of T=1 neutron-proton pairing correlations in the neighboring self-conjugate, odd-odd Rb-74 ground state band. Our study suggests that some experimental observables such as the energy levels and moments of inertia, may not be able to differentiate between different T=1 pairing phases in these T-z=1/2 nuclei. (Less)

A systematic study of the decay properties of {sup 164}Yb compound nuclei with excitation energies of 100{endash}300 MeV was performed. The emission patterns of light charged particles detected in coincidence with evaporation residues produced in {sup 64}Ni+{sup 100}Mo and {sup 16}O+{sup 148}Sm reactions have been measured and compared to statistical model predictions. There is found to be significant disagreement between the experiment and the model calculations in the magnitude of the multiplicities and the peak position in the energy spectra. For deuterons and tritons, significant nonstatistical components were present at all excitation energies. The proton energy spectrum did not exhibit any entrance-channel dependence at an excitation energy of {approximately}170 MeV. However for {alpha} particles, there is an enhancement in the spectrum at the lowest kinetic energies for the more symmetric reaction. It is suggested that the emission and subsequent decay of {sup 5}He fragments, can account for a large fraction of the apparent, and often reported, decrease in the Coulomb barrier for {alpha} particle emission. {copyright} {ital 1997} {ital The American Physical Society}

The fusion-evaporation reaction {sup 58}Ni({sup 32}S,2{alpha}p) has been used to study the neutron-deficient isotope {sup 81}Y. Multiple particle {gamma}-ray coincidences have been detected by the GAMMASPHERE array combined with the MICROBALL charged-particle detector system. Gamma-ray spectra with an improved resolution have been achieved from an event-by-event determination of the nucleus recoil momentum, thus allowing a precise Doppler-shift correction. In this way a resolution enhancement by a factor 2 was obtained for a 1 MeV {gamma} line. During the analysis an E{sub {gamma}}-E{sub {gamma}} matrix as well as an E{sub {gamma}}-E{sub {gamma}}-E{sub {gamma}} cube have been used to extend the previously known level scheme to higher spin (I{approx}(57)/(2)) and excitation energy (E{sub x}{approx}17MeV). More than 100 new {gamma} rays and 80 new levels have been added to the level scheme and six new bands have been established. The interpretation of these bands in terms of the cranking model and their comparison with similar bands in neighboring nuclei is discussed. {copyright} {ital 1997} {ital The American Physical Society}

High-spin states in Kr-77 were identified via analysis of prompt gamma-gamma coincidences in the Ni-58(Si-29,2 alpha 2p) reaction at 128 MeV at the Gammasphere facility. Evaporation channel selecti ...

We have used the Ca-48+Yb-176 reaction to study the population of high-spin states in neutron-rich nuclei by deep-inelastic reactions. Using Gammasphere, we observed gamma transitions from nuclei s ...

The high-spin structure of odd-odd {sup 86}Nb was studied with the early implementation of GAMMASPHERE using 36 Compton-suppressed Ge detectors and the MICROBALL, a 95-element full-sphere charged-particle detector array. High-spin states were populated using the {sup 58}Ni({sup 32}S,3pn) reaction at 135 MeV with beams from the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. A number of bands were observed up to spins as high as (31{sup {minus}}). The yrast positive-parity band shares many characteristics with the {pi}g{sub 9/2}{circle_times}{nu}g{sub 9/2} bands in other odd-odd f-p-g shell nuclei. These similarities include the behavior of the moments of inertia, the magnitude and phase of the signature splitting and its phase reversal near the 10{sup +} state, and the strong alternations in the B(M1)/B(E2) strengths. The moments of inertia in the lowest pair of negative-parity bands start out with a sharp upbend and then gradually fall back to the rigid-body value. New positive- and negative-parity bands appear in the quasiparticle alignment region and eventually become yrast. Some additional positive-parity states around spin 17{h_bar} are candidates for fully or nearly fully aligned shell-model-like configurations. {copyright} {ital 1997} {ital The American Physical Society}

High-spin states of T-z = 1 nuclei were studied with the reactions Ni-58(Si-28,3 alpha)Kr-74,Ni-58(Si-28,2 alpha)Sr-78, and Ni-58(Si-28,2p2n)Zr-82 at 130 MeV beam energy. The Gammasphere array in conjunction with the 4 pi charged-particle detector array Microball was used to detect gamma rays in coincidence with evaporated light charged particles. The known pi = +, alpha = 0 yrast bands were extended to I = 28 (h) over bar at 20 MeV excitation energy. For all three nuclei, a number of positive- and negative-parity sidebands were established; altogether 15 new rotational bands were found. The data are discussed using the pairing-and-deformation self-consistent total Routhian surface (TRS) model: High-spin structures of Kr-74 and Sr-78 are governed by the shell gaps at large prolate deformation while Zr-82 seems to exhibit shape coexistence. Nearly identical bands were established which may be explained as arising from the fp orbits acting as spectators at very elongated shapes. The experimental data in these T-z = 1 nuclei are in good agreement with predictions of the TRS model using conventional T = 1 like-nucleon pairing correlations. (Less)

{Delta}I=4 bifurcation has been observed in two superdeformed bands, the newly discovered yrast superdeformed band of {sup 148}Eu, and a previously known excited band in {sup 148}Gd. Both of these bands have moments of inertia that are identical to the yrast band of {sup 149}Gd, the first superdeformed band in which this bifurcation was observed. This first observation of {Delta}I=4 bifurcation in identical superdeformed bands provides a crucial test of recent models. {copyright} {ital 1997} {ital The American Physical Society}

Second minimum structures have been observed for the first time in the doubly odd nucleus ${}_{59}^{132}$Pr. Two new bands are interpreted as based on the $\ensuremath{\pi}{g}_{9/2}[404]9/2\ensuremath{\bigotimes}\ensuremath{\nu}{i}_{13/2}[660]1/2$ configuration and a third as the $\ensuremath{\pi}{h}_{11/2}[532]5/2\ensuremath{\bigotimes}\ensuremath{\nu}{i}_{13/2}[660]1/2$ configuration. Signature splitting effects observed in the $\ensuremath{\pi}{(g}_{9/2}{)}^{\ensuremath{-}1}$ bands provide new information regarding the positive parity proton orbitals at large deformation. Results from the extracted $B(M1)/B(E2)$ ratios are consistent with a large deformation for the $\ensuremath{\pi}{(g}_{9/2}{)}^{\ensuremath{-}1}$ bands.

Four superdeformed bands have been observed in ${}^{87}\mathrm{Nb}$. Transition quadrupole moment measurements on three of the bands confirm superdeformed shapes. Two of the bands were observed to mutually interact, and cross transitions between them have been observed. This is the first time such behavior has been observed in mass 80 superdeformed bands. Once this interaction is accounted for, a second crossing appears in one of the bands. This crossing is interpreted as evidence for the occupation of the ${i}_{13/2}$ superintruder orbital, making ${}^{87}\mathrm{Nb}$ the lightest nucleus by far in which the shape-driving properties of this orbital can be studied.

Collisions between $^{48}\mathrm{Ti}$ + $^{93}\mathrm{Nb}$ at 19.1 MeV/nucleon were studied using two 4\ensuremath{\pi} detection systems. A reconstruction procedure was developed to determine the mass, kinetic, and excitation energies of the primary projectile and targetlike fragments. The results show a broad range of mechanisms. These results were compared with predictions of the quantum-molecular dynamics model. \textcopyright{} 1996 The American Physical Society.

Based on measured correlations between experimental observables in the {sup 209}Bi+{sup 136}Xe reaction at {ital E}/{ital A}=28 MeV, it is shown that multiple intermediate-mass fragment (IMF) production is a dynamical process driven by the energy of relative motion of projectilelike and targetlike fragments. This kinetic energy is converted into thermal energy of the system, until a certain {open_quote}{open_quote}saturation{close_quote}{close_quote} value of approximately 3MeV/nucleon is reached. From this point on, this {open_quote}{open_quote}conventional{close_quote}{close_quote} dissipation mechanism is replaced by dynamical IMF production, constituting a new mode of energy dissipation. {copyright} {ital 1996 The American Physical Society.}

A search was undertaken to look for evidence of hyperdeformation in Gd-146,Gd-147. Three experiments employing Gammasphere for gamma-ray detection coupled with the Microball for channel selection via charged particle detection were carried out with increasing detection sensitivity and statistics. No definitive evidence for band structures that could be assigned to hyperdeformation could be found. Candidates previously reported are shown not to have properties consistent with a band structure.

High-spin states have been studied in neutron-deficient {sup 132}Nd, produced in the {sup 105}Pd({sup 35}Cl,{alpha}{ital p}3{ital n}{gamma}) reaction, using the Gammasphere {gamma}-ray spectrometer in conjunction with the Microball charged-particle detector array. Three new, weakly populated rotational bands have been found, which are believed to be associated with a superdeformed (second minimum) prolate shape. Comparison of the properties of these bands with Woods-Saxon cranking calculations suggests that they are built upon negative-parity neutron configurations that involve only one intruder orbital ({nu}{ital i}{sub 13/2}) from the {ital N}=6 shell. One of the bands has been linked into the normally deformed states allowing an estimation of the spin, parity, and excitation energy of the band. {copyright} {ital 1996 The American Physical Society.}

High-spin states in Mo-86 were studied by means of the fusion evaporation reaction Ni-58(S-32,2p2n) Mo-86 at 135 MeV beam energy. Charged-particle-gamma gamma and -gamma gamma gamma coincidences recorded with the early implementation of the GAMMASPHERE array and the MICROBALL charged-particle detection system were used to largely extend the level scheme of the T-tau=1 nucleus Mo-86 to a possible spin of I=24h at 13 MeV excitation energy. The excitation scheme is compared to neighboring nuclei. There is evidence for enhanced shell-model influence in the 4 quasiparticle region (I=12-16h). The observed (pi=+,alpha=0) sequence at spin I greater than or equal to 16 h appears to be associated with a triaxial collective rotational band. This interpretation is supported by calculations within the configuration-dependent shell-correction approach with the cranked Nilsson potential. (Less)

An exclusive measurement of slow, massive residues from the {sup 197}Au+{sup 86}Kr reaction at E/A=35 MeV has been performed in coincidence with projectile-like fragments, neutrons, as well as light- and intermediate-mass charged products. The highly efficient (double 4{pi}) detector setup used included the University of Rochester SuperBall neutron detector and the Washington University Microball. The observed large yield of slow, massive residues shows characteristics consistent with a production scenario similar to that of binary dissipative collisions. The residues result from the statistical decay of primary targetlike fragments, produced even in the most dissipative collisions identified in the present experiment. {copyright} {ital 1996 The American Physical Society.}

High energy {gamma} rays from the decay of the giant resonance in hot {sup 120}Sn nuclei were measured in the excitation energy range of 30{endash}130 MeV. The excited nuclei were populated by inelastic scattering of {alpha} particles at 40 and 50 MeV/nucleon. The resonance width was observed to increase monotonically with increasing excitation energy, from 5 MeV at the ground state to {approximately}12 MeV at the largest excitation energy. Inelastic scattering predominantly populates low angular momentum states, and the observed width increase is thus attributed to fluctuations in the nuclear shape induced by temperature. {copyright} {ital 1996 The American Physical Society.}

The fragmentation of E/A=40 MeV $^{20}\mathrm{Ne}$ projectiles into exit channels containing only p, d, t, $^{3}\mathrm{He}$, \ensuremath{\alpha}, $^{6}\mathrm{He}$, $^{6}\mathrm{Li}$, and $^{7}\mathrm{Li}$ fragments has been investigated. Relative shifts between the average longitudinal velocity of each fragment type were measured with $^{120}\mathrm{Sn}$ and $^{197}\mathrm{Au}$ targets. A post-breakup Coulomb acceleration component of these shifts was isolated. Its magnitude indicates that the projectile broke up while in near contact with the target. On the other hand, correlations between projectile fragments indicate that a significant fraction of these fragments are produced by the sequential decay of long- and short-lived unstable intermediates. A fully sequential decay process, although consistent with the experimental relative angle distributions, fails to reproduce the correlation function. Thus, a picture emerges of a prompt breakup step producing stable and unstable particles with the subsequent decay of the latter extending over a long time period.