Session SB03 - Structure of Heavy Nuclei (A$>$100) Using Gammasphere.
ORAL session, Wednesday afternoon, March 24
Room 203E, GWCC

[SB03.01] Excited states beyond the termination of the yrast band in ^109Sb and smooth band termination in the near-spherical nucleus, ^105In.

The ^58Ni+^54Fe reaction at 243~MeV has been used to study high-spin states in ^109Sb (3p) and ^105In (\alpha 3p) in an experiment using Gammasphere plus Microball at ANL. Three rotational bands based upon proton 3p-2h configurations were known previously in ^109Sb up to their terminating states. The bands exhibit smoothly decreasing J^(2) and Q_t corresponding to a gradual change from prolate collectivity at low spin, to a non-collective oblate shape at the band termination. Further p-h excitations are required to generate angular momentum beyond the band terminations and specific predictions have already been published. The increased sensitivity obtained with the full implementation of Gammasphere plus Microball has resulted in the identification of a new band in ^109Sb which crosses the yrast band near its termination and extends to spins beyond the terminating state. The results agree with prior calculations for the lowest 4p-3h configuration. In addition, collective bands have been observed in the nucleus ^105In, which is near-spherical in its ground state. The effects of smooth band termination in a nucleus which has a rather limited valence spin can be studied. This work is the joint effort of collaborators from LBNL, SUNY at Stony Brook, Univ. of York, Washington Univ. (St Louis), ANL and Lund Institute of Technology.

[SB03.02] Shears structure in ^109_~48Cd

The high-spin states of ^109Cd were populated via the reaction ^96Zr(^18O,5n), with the 75 MeV beam provided by the 88'' Cyclotron at LBNL. Coincident events having four or more gamma rays were detected with the Gammasphere array, consisting of 100 Ge detectors. A Pb-backed target was used in order to apply the Doppler Shift Attenuation Method to extract lifetimes of states from observed peak lineshapes. A thin-target experiment using the same reaction was also performed in order to extend the ^109Cd level scheme and accurately determine transition energies and intensities. One of the \Delta I=1 bands is believed to be a good candidate for the shears mechanism, based on a pair of high-K g_9/2 proton holes coupled to 8^+ and three low- to mid-K g_7/2 and h_11/2 valence neutrons. The experimentally deduced B(M1) values for the \Delta I = 1 band are compared with Tilted Axis Cranking calculations for a [\pi(g_9/2)^-2]_8^+ øtimes \nu(g_7/2h_11/2^2) configuration, as well as semi-classical predictions. The data show a fall-off in the B(M1) strength with increasing spin, characteristic of the shears mechanism.

[SB03.03] Spectroscopy Study of the Proton Emitter ^109I.

Excited states in proton-unbound ^109I were populated using the ^54Fe(^58Ni, p2n) reaction at a beam energy of 220 MeV. The experiment was performed at Argonne National Laboratory and the Recoil Decay Tagging (RDT) technique^(a) was used to identify \gamma rays in ^109I. Gamma rays were detected by the full array of Gammasphere at the target position, and recoils were mass-separated by the Fragment Mass Analyzer (FMA) and those with A = 109 were focused at the focal plane of the FMA. A double-sided silicon strip detector was placed at the focal plane of the FMA to detect the charged-particle decays of the recoils. The \gamma rays detected by Gammasphere were then correlated with the ground-state proton decays of ^109I. From the analysis of proton-correlated \gamma-\gamma data, the yrast decay sequence in ^109I was established. This yrast decay sequence can be tentatively assigned as built on the \pi h_11/2 state, but is very different from that reported in the previous RDT study^(a). [0.3cm]

(a) E. S. Paul et al., Phys. Rev. C51, 78 (1995).

[SB03.04] Low and Medium Spin Structure of Pd and Ru around A=110

Neutron rich nuclei around A=110 were populated via heavy ion induced fission using the reaction ^18O on ^208Pb at 91 MeV. The 91MeV ^18O beam was delivered by the 88-Inch cyclotron of the Lawrence Berkeley National Laboratory. A total of 2x10^9 4-fold and higher \gamma coincidences were collected with Gammasphere. Level schemes and branching ratios were extracted using a Radware cube. The systematics of the low and medium spin levels of Pd and Ru isotopes are compared with calculations using the Interacting Boson Model and the Geometric Collective Model. This work is supported by the US-DOE under grant numbers DE-FG02-91ER-40609 and DE-FG02-88ER-40417 and contract number DE-AC03-76SF00098.

[SB03.05] p-h induced band structure in ^112Te

High spin states in ^112Te have been studied using the ^58Ni(^58Ni,4p) reaction at 250 MeV with GAMMASPHERE (83 Ge), the MICROBALL, and an array of 15 neutron detectors. Coincidence 3D cubes and 4D hypercubes gated and Doppler optimized by the 4p MICROBALL channel were sorted for analyses. Five high spin bands showing the characteristic behavior for smooth band termination were observed in ^112Te. Two of these, a dipole and a decoupled band, are linked to the yrast sequence, which allows definite spin assignments. The dipole band with E2 crossovers, which was observed up to spin I=38\hbar, agrees with predictions for the [11,3] configuration to near termination with no shears effects apparent ([p_1p_2,n] refers to p_1 \pi g_9/2^-1, p_2 \pi h_11/2, and n \nu h_11/2). The _52Te isotopes, including ^110Te, reveal the first 1p-1h proton excitations showing smooth band termination properties in this region. The linked decoupled band, observed to I=46\hbar, is consistent with the calculations for the [22,4] configuration. Using estimated spins, the other decoupled bands in ^112Te appear to be related to the [22,3] and [21,3] configurations.

[SB03.06] Smooth band termination in ^113I

^113I has been studied using the ^58Ni(^58Ni,3p) reaction at 250 MeV with the GAMMASPHERE/MICROBALL setup. Coincidence cubes and a hypercube gated by 3p and \gamma-ray multiplicity/sum energy were analyzed. Multipolarities of the \gamma-rays were studied using a DCO matrix. At low spins the yrast decoupled \pih_11/2 band and two \Delta I=1 bands built on (\pig_9/2)^-1 and \pig_7/2 orbitals were observed. At medium spins an expected \pih_11/2øtimes\nuh^3_11/2 decoupled band was followed up to its maximal spin. At high spin the spectrum is dominated by 2p-2h excitation across the Z=50 gap. Seven decoupled bands showing the features of smooth band termination were observed; three of those bands are linked, which allows comparisons with calculations of Ref.~[1]. The yrast band assigned to the [22,4] configuration (see Ref.~[1] for the notation) was followed up to 101/2^+ and \hbarømega\approx 1.3~MeV. The other linked band is possibly the signature partner of the yrast band. The band assigned to the [22,3] configuration was observed up to 91/2^-. Possible assignments for the other bands will be discussed. [0.01 cm] [1]~A.V.~Afanasjev~\it~et~al.,~Nucl.~Phys.~A591,~(1995)~698.

[SB03.07] High-Spin Structure in ^116Xe and ^118Xe

Excited states in ^116Xe and ^118Xe have been studied using the ^58Ni(^64Zn,\alpha2p) and ^58Ni(^64Zn,4p) reactions, respectively, at 265 MeV with the GAMMASPHERE(101 Ge)/MICROBALL/FMA facility at ANL. Coincidence 3D cubes and 4D hypercubes gated by 4p and \alpha2p MICROBALL channels were sorted. Doppler shift corrections were optimized with recoil velocity vectors obtained by the MICROBALL channels. A preliminary analysis confirmed the level schemes reported in an earlier experiment [1] and extended them to higher angular momentum. New band structures and additional linking information have been extracted. Smooth band termination features suggested in Ref. [1] for these nuclei are being investigated with the higher-resolution data. Recent observations in ^117,119I and ^117,119Xe [2] consistent with smooth terminating bands further motivate the study of such structures in ^116Xe and ^118Xe. [0.01 cm] [1] J.M.~Sears~et al.,~Phys.~Rev.~C57, 2991~(1998)~. [0.01 cm] [2] E.S.~Paul~et al., ~(to be published)~.

[SB03.08] Superdeformation in ^128Ce

We report on the observation of a new superdeformed band in ^128Ce (N=70). The experiment was performed at ANL with Gammasphere, the Microball and the FMA. The band is the result of an analysis of the 4p evaporation channel in the ^40Ca + ^92Mo reaction at E_b= 185 MeV. The structure of the band does not involves N=6 neutron intruders indicating the importance of the underlying shell gaps present. ^128Ce is the lightest Ce isotope showing superdeformation. Unlike all the other SD bands in the Ce isotopes this band has a defined link with the low-lying transitions. We will discuss the systematics of SD bands in this region.

^* Work supported in part by US department of energy under contract No. DE-AC05-96OR22464.

[SB03.09] Rotational Structures in ^128,129Nd.

High-Spin states in ^128,129Nd were populated using the ^92Mo(^40Ca,2pXn) reaction with a beam energy of 184 MeV. The Gammasphere spectrometer at Argonne was used in conjuction with the Microball array to measure gamma rays selected for the charged-particle exit channels of interest. In our analysis, the level schemes of both nuclei have been greatly extended from what was previously known up to spins of I\approx32. Analysis of rotational alignments and band crossings will be dicussed in terms of the Cranked Shell Model.

[SB03.10] Deformation Trends of Rotational Structures in the A\sim135 Pm Nuclei

While the quadrupole moments of various rotational structures in the A\sim135 region have been determined by different groups in the past, conclusive comparisions have been limited because of systematic differences\ in experimental setup. A global lifetime experiment was performed at the Lawrence Berkeley National Laboratory using Gammasphere coupled with the Washington University Microball charged-particle detector. The ^105Pd(^35Cl,xnypz\alpha) reaction was used at a beam energy of 173 MeV. The target consisted of a 1mg/cm^2 enriched ^105Pd foil with a 17 mg/cm^2 Au backing. Since a variety of nuclei in the A\sim135 region (Pr, Nd, Pm, and Sm) were produced under the same experimental conditions, relative deformation measurements taken from this data set were then meaningful. The Doppler-shift attenuation method (DSAM) was used to extract average quadrupole moments for both the normal deformed and highly-deformed structures in ^133,135,136Pm. These quadrupole moment values were then compared to ascertain the deformation trends in the A\sim135 Pm isotopes.

[SB03.11] New Results in A\simeq135 Promethium Nuclei

High-spin states in the A \simeq 135 region near Z=61 were populated using heavy-ion reactions at the Florida State University (FSU) Tandem-Linac and the Lawrence Berkeley National Laboratory (LBNL) 88'' cyclotron facilities. The Gammasphere spectrometer (LBNL), combined with the Washington University Microball charged particle detector, identified 15 different nuclei in the ^105_46Pd_59(^35_17Cl_18,X) @ 180 MeV reaction. The level schemes of ^133,135Pm were extended significantly and are discussed. The ^135Pm nucleus shows competing driving effects between the h_11/2 protons which stabilize a prolate shape and h_11/2 neutrons which drive the nucleus to oblate deformation. In addition, over 25 superdeformed bands were identified for 15 nuclei in the A \simeq 135 region. The promethium nuclei accounted for eight superdeformed bands, two each in ^133,135Pm and four in ^136Pm.

[SB03.12] Exotic Rotational Band in ^168Hf

Recently, two experiments were carried out using the Gammasphere detector array to search for exotic band structures in the ^168Hf mass region predicted by theoretical calculations. The semi-symmetric reaction of ^76Ge + ^96Zr was used to populate very cold compound nuclei at the highest angular momentum. The first experiment was a short test run used to find the optimum experimental parameters. A wobbling target was used in the second experiment which allowed for a higher beam current. One exotic band associated with ^168Hf has been discovered through preliminary data analysis. It consists of 11 transitions with a regular spacing of 55 keV between adjacent peaks ranging from 700 keV - 1200 keV region. Comparisons with the Ultimate Cranker calculations indicate that this band is, most likely, a triaxially superdeformed band. A lifetime measurement has been planned to confirm the nature of the band.

[SB03.13] G-Factor Measurement of Discrete, High Spin, Normal Deformation States in ^193Hg.

Average g factors of ensembles of states at high spin and energy populated in fusion evaporation reactions have been measured by the transient field technique (R. H. Mayer et al.), Phys. Rev. C 58, R2640 (1998). However, g-factor measurements of individual high energy states require the combination of the transient field technique and a time delay which insures that the recoiling nucleus traverses the ferromagnetic target layer while in the state of interest. The g factor of the 33/2^+ state in ^193Hg was measured in such an experiment using the reaction ^150Nd(^48Ca,5n)^193Hg at the 88'' Cyclotron at LBNL. \gamma--rays were detected with the Gammasphere array. The time frame was selected by gating on the Doppler shifted transitions preceding the 33/2^+ state. The method will be discussed with respect to further measurements of g factors of high spin states in both ^193Hg and ^194Hg.

[SB03.14] Formation Mechanisms of the Transfermium Isotope ^254No

Gamma rays from the nucleus ^254No, produced with the ^208Pb(^48Ca,2n) reaction at a beam energy of 215 MeV, have been identified with the Gammasphere-FMA combination at Argonne. The ground band has been identified. This abstract focusses on the formation mechanism of this deformed superheavy nucleus. Measurements will be reported of the ^254No entry distribution (in spin and energy) and of the spin and energy distributions. These distribrutions test if No nuclei are produced by neutron evaporation from the compound nucleus, in competition against fission. Preliminary work suggests that the calculated spin distribution from this process agrees with the measured one. This study will also provide `measures' of the fission barrier, its variation with angular momentum and the spin dependence of the shell-correction energy, which creates the barrier. -------------------------------------------------------------

[SB03.15] High Spin Spectroscopy of Plutonium Isotopes

One of the interesting aspects in the study of actinide nuclei is the interplay between collective rotation and octupole degrees of freedom. In a series of experiments performed with \sc gammasphere at \sc atlas, the isotopes between ^238Pu and ^244Pu were investigated with Pb beams a few percent above the Coulomb barrier. From the emerging systematics it becomes clear that the nuclear structure of ^240Pu stands apart from its heavier and lighter neighbors. A number of observables, such as a delayed proton--alignment, the energy staggering and the E1-E2 decay ratios of the octupole band all point to the presence of stronger octupole correlations in this nucleus. ^240Pu is the first well deformed nucleus where this degree of octupole correlations has been observed.

Part S of program listing