Gammasphere is an array of 110 Compton suppressed large volume Ge detectors. It is approximately 100 times more powerful than its predecessors and was constructed at LBNL with the participation of other US national laboratories and universities. In the last four years, when Gammasphere was sited at the Berkeley 88-Inch Cyclotron, over 180 experiments were performed. In many experiments auxiliary detectors, designed specifically for Gammasphere, were used to enhance the data by detecting light charged particles, neutrons, x-rays, and/or heavy ions. In the area of high-spin physics we obtained many new results and insights such as a new region of superdeformation, discrete decay of superdeformed states to normal deformed states, magnetic rotation, \DeltaI=2 staggering of rotational levels, and identical bands. Results in other areas included the exploration of properties of neutron-rich nuclei from fission and deep inelastic reactions, search for strange matter and precision measurements of superallowed beta-decay to test the unitarity of the CKM matrix. I will review physics highlights as well as discuss future prospects.
[P15.02] Onset of Deformation in ^60Ni.
W.D. Weintraub, H.Q. Jin, W. Reviol, L.L. Riedinger (Univ. of Tennessee), C. Baktash, M.J. Brinkman, D.J. Dean, C.-H. Yu (ORNL), M. Devlin, D.R. LaFosse, D.G. Sarantites (Washington Univ.), M. Leddy (Univ of Manchester), I.Y. Lee, A.O. Macchiavelli (LBNL), D. Rudolph (Ludwig-Maximilians-Universität München)
High-spin states in ^60Ni were populated using the ^28Si(^36Ar,4p) reaction with beam energy of 136 MeV. Gammasphere at LBNL was used in conjunction with Microball to measure gamma rays selected for the charged-particle exit channels of interest. A total of 2 billion events was recorded, with the 4p channel to ^60Ni representing approximately 11% of the data. In our analysis, the previously known level scheme(G. Moyat \itet ~al.), Nuclear Physics \bfA318, 236 (1979). has been extended up to energy and spin of 20 MeV and 20 \hbar. The multiplicity of levels up to I=10 are well explained by shell-model calculations including the g_9/2 single-particle orbital into the fp-shell configuration space. At higher spins, evidence for rotational-like behavior increases. Two apparently rotational structures have large M1 values and are perhaps shears bands, likely involving one g_9/2 particle. Furthermore, an E2 sequence with a larger moment of inertia is observed that could correspond to other deformed structures in the region, involving two g_9/2 particles. Comparisons to calculations will be given.
[P15.03] Normal-Deformed High-Spin States in ^81Sr
I. Birriel, D.F. Winchell, J.X. Saladin, F. Cristancho, E. Landulfo, V.Q. Wood (University of Pittsburgh), C. Baktash, M.J. Brinkman, C.J. Gross, H.-Q. Jin, D. Rudolph, D.W. Stracener, C.-H. Yu (Oak Ridge National Laboratory), M. Devlin, P.-F. Hua, M. Korolija, D.R. LaFosse, F. Lerma, D.G. Sarantites (Washington University), J. Döring, G. Sylvan, S. Tabor (Florida State University), B. Cederwall, R.M. Clark, P. Fallon, I.Y. Lee, A.O. Macchiavelli, M.R. Maier, W. Rathbun (Lawrence Berkeley National Laboratory), A. Vander Mollen (Michigan State University), T.R. Werner (Warsaw University)
Normal-deformed high-spin states in ^81Sr were populated with the reactions ^58Ni(^29Si,\alpha2p) and ^58Ni(^28Si,4p) with beam energies of 128 and 130 MeV, respectively, from the 88-inch cyclotron at LBNL. Gamma rays from the reactions were detected with Gammasphere, and evaporated \alpha particles and protons were detected with Microball. Our results are consistent with previous work (D.H. Smalley et al, Nucl. Phys. A611 (1996) 96). In addition, two new negative parity rotational bands (built on a 295-241 keV cascade(S.E. Arnell et al, J.Phys. G9, (1983) 1217)) and 11 new side bands have been found. In all, the level scheme has been extended by 65 new states, along with 78 new transitions. The results will be discussed in the framework of the cranked shell model.
[P15.04] High-spin States in the Neutron Deficient Nucleus ^107Sb
D.R. LaFosse, C.J. Chiara, D.B. Fossan, G.J. Lane, J.M. Sears, J.F. Smith, K. Starosta (SUNY at Stony Brook), M. Devlin, F. Lerma, D.G. Sarantites (Washington University), A.J. Boston, E.S. Paul (University of Liverpool)
Odd-mass _51Sb nuclei have been shown to possess many interesting structure features. The phenomenon of smooth band termination was first discovered in ^109Sb, and the strongly coupled (\pi g_9/2)^-1 bands are now being studied in terms of the tilted axis cranking model, for example. Now, due to the sensitivity of Gammasphere coupled to the Microball, detailed study of the neutron deficient isotope ^107Sb is possible. The reaction employed, ^58Ni(^58Ni,2\alphap), has a very small cross section, and thus less than 10^7 2\alphap-gated events were accumulated. However the resulting data was largely free of contaminating nuclei and an extensive level scheme has been constructed. As of this writing, a single decoupled rotational band and a candidate strongly coupled band have been identified. The dynamic moment of inertia of the decoupled band drops precipitously at the highest frequencies, a hallmark of smooth band termination. This and other aspects of the bands will be discussed, as well as a comparison of these bands to others in neighboring nuclei (^108,109Sb, ^106Sn).
[P15.05] Structure of odd-odd ^112I
K. Starosta, C.J. Chiara, D.B. Fossan, D.R. LaFosse, G.J. Lane, J.M. Sears, J.F. Smith (SUNY at Stony Brook), M. Devlin, D.G. Sarantites (Washington Univ.), I.-Y. Lee, A.O. Macchiavelli (LBNL), A.J. Boston, E.S. Paul (Univ. of Liverpool)
High spin states in ^112I have been studied using the ^58Ni(^58Ni,3pn) reaction at 250 MeV. The experimental set-up consisted of GAMMASPHERE with 83 75-80%-efficient HPGe detectors coupled with the MICROBALL and an array of 15 NE213 scintillators for neutron detection. Coincident-triples cubes gated by 3p, 4p and 3pn were sorted and analysed. DCO matrices are being extracted for spin information. A preliminary level scheme has been built consisting of six high spin \DeltaI=2 sequences and a weak \DeltaI=1 band. The interpretation of the observed structures is aided by comparisons with systematic properties of the neighboring odd-odd and odd isotopes and odd isotones; new information for ^111Te extracted from the 4pn channel of this study is helpful in this regard. With the Fermi level for both the protons and neutrons being in the vicinity of the d_5/2, g_7/2 and h_11/2 orbitals, the \pih_11/2øtimes \nuh_11/2 configuration competes for the yrast level sequence in neutron defficient odd-odd Iodines, showing in ^116I and ^118I signature inversion consistent with odd-odd Cs, La, Pr systematics. Possible structure assignments in ^112I will be discussed.
[P15.06] Multiple Band Structures in ^131Nd and ^130Nd.
W. Reviol, H.Q. Jin, L.L. Riedinger, B.H. Smith, N.P. Yoder (University of Tennessee), A. Galindo-Uribarri (Oak Ridge National Laboratory), D.G. Sarantites, D. LaFosse, J.N. Wilson (Washington University), S.M. Mullins (Australian National University)
Rotational bands have been assigned to the neutron-deficient nuclei ^130,131Nd through the analysis of a Gammasphere plus Microball experiment at Berkeley, using the ^40Ca on ^94Mo reaction at 180 MeV. Whereas we have analyzed the exit channels with 3 protons leading to light Pr isotopes, here we report on the results for the 2p gated gamma-ray data, which are dominated by ^130,131Nd. The new level scheme for ^131Nd consists of four bands, significantly extending the earlier data(D. Watson, University of Liverpool Annual Report (1987/88).), which we assign to the 7/2[523], 5/2[402], 1/2[411], and perhaps 1/2[541] neutron configurations. The observed signature splitting in the last one is much smaller than expected for a 1/2[541] orbital. Calculations to address this issue will be presented. In addition to the yrast band of ^130Nd,(R.\ Wadsworth et al., Z.\ Phys.\ A333, 411 (1989).) we assign bands built on the high and low K couplings of the 7/2[523] and 5/2[402] orbitals, the \gamma-vibrational band and one other sequence. Besides these normal-deformed bands, we also observe one band of enhanced or super deformation in each case.
[P15.07] New Results in A\simeq135 Pm Nuclei
J. Pfohl (Florida State University), M.A. Riley (FSU), D.G. Sarantites (Washington University), R.K. Sheline (FSU), D.R. LaFosse, M. Devlin (Wash U.), N.J. O'Brien (University of York), D.E. Archer (LLNL), P. Fallon (LBNL), I.M. Hibbert (U. York), D.T. Joss (University of Liverpool), P.J. Nolan, E.S. Paul (U. Liverpool), J. Simpson (Daresbury Laboratory), R. Wadsworth (U. York)
An experiment was carried out using the ^35Cl + ^105Pd reaction at 180 MeV using Gammasphere in conjunction with the Microball. In this experiment a wide range of different nuclei near Z = 60 were populated via xp, x\alpha and neutron emission. The wonderful selection capabilities of the Microball allowed the clean separation of the different charged particle channels.
The separation of all the charged particle channels yielded 15 different nuclei from Z=58 Ce to Z=62 Sm and populated over 25 superdeformed bands. An experiment of this magnitude allows individual orbitals to be tracked across N and Z. Furthermore some of the global systematics regarding signature partners, dynamic moments of inertia (\Im^(2)), etc., in this mass region between ^129Ce and ^136Sm can be revealed and better understood.
Analysis of the highly-deformed (HD) partner bands in the A\simeq135 Pm and Pr nuclei suggests that the HD bands in ^136Pm do not involve the \pig_9/2[404]9/2 orbital. A comprehensive level scheme for ^135Pm and significant additions to the ^133Pm level scheme will be presented and discussed.
[P15.08] Lifetime Measurements of Superdeformed Bands in ^142Sm: A Test of the Additivity Principle for Quadrupole Moments
G Hackman, R.V.F. Janssens, D. Nisius, I. Ahmad, M.P. Carpenter, S.M. Fischer, T.L. Khoo, T. Lauritsen, P. Reiter (Argonne National Laboratory), E.F. Moore (North Carolina State University and TUNL)
Lifetime measurements with the Doppler shift attenuation method have been performed for the two superdeformed (SD) bands of ^142Sm. The inferred transition quadrupole moments, Q_0, are 11.7 \pm 0.1~eb for the yrast SD band and 13.2^+0.8_-0.7~eb for the excited SD band. These results are consistent with expectations from mean-field calculations, and indicate that the proposed additivity of contributions of individual orbitals to Q_0 is valid for SD nuclei over an extended mass range in the vicinity of ^152Dy.( This work is supported by the U.~S.\ Department of Energy under Contract Nos.\ W-31-109-ENG-38 and DE-FG05-88ER40411.)
[P15.09] Quasicontinuum gamma-ray study in ^154Dy
W.C. Ma, P.G. Varmette (Mississippi State Univ.), T.L. Khoo, T. Lauritsen, I. Ahmad, D. Blumenthal, M.P. Carpenter, B. Crowell, S.M. Fischer, D. Gassmann, R.V.F. Janssens, D. Nisius (Argonne Nat. Lab.), V. Martin (Universidad Politecnica de Madrid, Spain), J.L. Egido (Universidad Autonoma de Madrid, Spain), P.K. Bhattacharyya, C.T. Zhang, P.J. Daly, Z.W. Grabowski (Purdue Univ.), J.H. Hamilton, A.V. Ramayya (Vanderbilt Univ.)
Quasicontinuum decay \gamma-rays following the reaction ^36S(^122Sn, 4n)^154Dy were studied using Gammasphere at LBNL. To study selected regions of the I - E plane, spectra of the E2 \gamma-rays feeding different yrast spin regions were extracted. Theoretical spectra with identical gating conditions were calculated based on temperature-dependent Hartree-Fock theory, with thermal shape fluctuations included. There is an agreement in the shapes and multiplcities of the experimental and theoretical spectra, and in the evolution with spin of two broad E2 peaks. This agreement and spectral evolution suggest that phase transitions predicted in mean-field theory can be observed despite smearing from fluctuations.
[P15.10] Shape Coexistence and Band Terminations in N = 89 ^155Dy
T.B. Brown, M.A. Riley (FSU), R.V.F. Janssens, I. Ahmad, D.J. Blumenthal, M.P. Carpenter (ANL), P. Fallon (LBNL), S.M. Fischer, G. Hackman (ANL), D.J. Hartley (FSU), C. Kalfas (NCSR Demokritos), T.L. Khoo (ANL), F.G. Kondev (FSU), T. Lauritsen, D. Nisius (ANL), W.C. Ma (MSU), J. Simpson (Daresbury), J.F. Sharpey-Schafer (NAC Faure), A. Afanasiev, I. Ragnarsson (Lund)
A high spin study of the nucleus ^155_66Dy_89 has been performed using the GAMMASPHERE Spectrometer. The ^124Sn(^36S,5n) reaction was employed at beam energies of 175 and 165 MeV with thick and thin targets respectively. Eight new band sequences have been identified and the previously known sequences have been extended to higher spin. The yrast states of positive and negative parity have been identified up to spin \frac1012 with quite different behavior illustrating beautifully the competition between collective prolate and terminating oblate shapes. Detailed comparisions are made with cranked Nilsson Strutinsky calculations. A new feature of the high spin calculations has been added to distinguish between neutrons in h_11/2 orbitals and other N=5 orbitals. This has been found necessary to explain the continuation of near yrast collective structures beyond spin 40 \hbar. Additionally, the lowest energy negative parity sequences are observed to return to more collective behavior near spin 45 in a similar manner as N=88 ^154Dy.
[P15.11] Additivity of the incremental alignments of superdeformed bands in the A \sim 150 region
Bachir Kharraja, Umesh Garg (University of Notre Dame)
A systematic study of the additivity of the incremental alignments of SD bands in the A \sim 150 region has been performed and the incremental alignments have been compared for bands involving intruder configurations different by up to ten particles. The results show that this additivity is fulfilled with very good accuracy for SD bands in this region. No fluctuations have been observed as a function of the rotational frequency. The neutron band crossing involving the natural-parity N = 6, orbitals has no (or minor) effect on this additivity. However, the proton intruder orbital [770]1/2 leads to a medium-to-large discrepancies within the frequency range where the interaction is observed. Finally, the incremental alignment was found to strongly depend on the configuration of the various SD bands.
[P15.12] Polarization-direction correlation measurement --- Experimental test of the PDCO methods
K. Starosta, T. Morek, Ch. Droste, S.G. Rohozi\'nski, J. Srebrny (Warsaw University), M. Bergstrem, B. Herskind (Niels Bohr Institute)
Information about spins and parities of excited states is crucial for nuclear structure studies. In ``in-beam" gamma ray spectroscopy the directional correlation (DCO) or angular distribution measurements are widely used tools for multipolarity assignment; although, it is known that neither of these methods is sensitive to electric or magnetic character of gamma radiation. Multipolarity of gamma rays may be determined when the results of the DCO analysis are combined with the results of linear polarization measurements. The large total efficiency of modern multidetector arrays allows one to carry out coincidence measurements between the polarimeter and the remaining detectors. The aim of the present study was to test experimentally the possibility of polarization-direction correlation measurements using the EUROGAM II array. The studied nucleus was ^164Yb produced in the ^138Ba(^30Si,4n) reaction at beam energies of 150 and 155 MeV. The angular correlation, linear polarization and direction-polarization correlation were measured for the strong transitions in yrast and non yrast cascades. Application of the PDCO analysis to a transition connecting a side band with the yrast band allowed one to rule out most of the ambiguities in multipolarity assignment occuring if one used angular correlations only.
[P15.13] Polarization-polarization correlation measurement --- Experimental test of the PPCO methods
Ch. Droste, K. Starosta, A. Wierzchucka, T. Morek, S.G. Rohozi\'nski, J. Srebrny, E. Weso\lowski (Warsaw University), M. Bergstrem, B. Herskind (Niels Bohr Institute)
A significant fraction of modern multidetector arrays used for "in-beam" gamma-ray spectroscopy consist of a detectors which are sensitive to linear polarization of gamma quanta. This yields the opportunity to carry out correlation measurements between the gamma rays registered in polarimeters to get information concerning spins and parities of excited nuclear states. The aim of the present work was to study the ability of the polarization- polarization correlation method (the PPCO method). The correlation between the linear polarization of one gamma quantum and the polarization of the second quantum emitted in a cascade from an oriented nucleus (due to a heavy ion reaction) was studied in detail. The appropriate formulae and methods of analysis are presented. The experimental test of the method was performed using the EUROGAM II array. The CLOVER detectors are the parts of the array used as polarimeters. The ^164Yb nucleus was produced via the ^138Ba(^30Si, 4n) reaction. It was found that the PPCO method together with the standard DCO analysis and the polarization- direction correlation method (PDCO) can be helpful for spin, parity and multipolarity assignments. The results suggest that the PPCO method can be applied to modern spectrometers in which a large number of detectors (e.g. CLOVER) are sensitive to polarization of gamma rays.