D. R. Lafosse

“The microball” Design, instrumentation and response characteristics of a 4π-multidetector exit channel-selection device for spectroscopic and reaction mechanism studies with Gammasphere

Abstract A 4π multidetector light-charged particle and light fragment detection spectrometer is described. It consists of 95 CsI(T1) scintillators closely packed to cover the angular range 4.0°–172°, arranged in 9 rings with increasing forward segmentation. The device is optimally designed to be used in conjunction with Gammasphere. The scintillator light is collected by silicon photodiodes that provide high quantum efficiency and minimal mass. The signals are processed through a charge sensitive preamplifier followed by a slow shaper. Particle identification for 1,2,3 H, 3,4 He ions and Li, Be and B ions is accomplished by pulse shape discrimination. The geometry, construction, energy calibration, gain stability, associated integrated electronics and the data acquisition system are discussed. The capabilities of this spectrometer as a channel selecting device in conjunction with Gammasphere are discussed. A second version of the device with thicker scintillators, that can stop more energetic charged particles and is useful for reaction mechanism studies, is also described.

Intruder bands in 108Sn

Abstract The nucleus 108Sn has been populated via the 54Fe(58Ni, 4p) reaction channel at a beam energy of 243 MeV. The high-spin structure is dominated by three ΔI = 2 rotational sequences. These bands can be interpreted in terms of particle-hole excitations involving the proton g 7 2 , g 9 2 and h 11 2 orbitals and also aligned neutrons from the bottom of the h 11 2 shell. Lifet measurements have also been performed using the Doppler-shift attenuation method. These data have enabled quadrupole moments to be deduced for the two strongest bands. The results yield Q0 = 2.6±0.4 e·b for the positive-parity band and 3.4 ± 0.6 fse·b for one of the proposed negative-parity bands. These values yield quadrupole deformations of β2 = 0.20 and 0.26, respectively, for the two bands. The results obtained are discussed in terms of Woods-Saxon and total routhian surface calculations.

Simulated response characteristics of Gammasphere

Abstract The response of the Ge detector array Gammasphere to γ-rays, both individually and in cascades, has been modeled with the Monte Carlo code GEANT 3. The effects on the performance of Gammasphere of including an auxiliary detector, the Microball, are investigated. The use of the existing Ge detectors and BGO suppression detectors as a γ-ray multiplicity and total γ-ray energy spectrometer is explored, and its total energy - multiplicity ( H , k ) response is simulated in detail. The utility of the Hevimet collimators in spectroscopic studies, as opposed to the use of the available ( H , k ) information, is discussed.

High-spin study of 128Ce and systematics of quasiparticle pair alignment

Abstract High-spin states have been studied in 128 Ce, produced in the 100 Mo( 32 S,4n) reaction, using the Euroball γ -ray spectrometer. A quadruples analysis ( γ 4 ) of the data has extended several bands to high spin. Systematics of quasiparticle alignments in cerium isotopes and relevant isotonic chains are discussed and compared to Woods–Saxon cranking calculations.

Enhanced deformation in light Pr nuclei

Abstract We observe for the first time rotational bands of enhanced deformation (ED) in 127 Pr and 128 Pr, and suggest that these bands are built upon πg9/2 ( [404] 9 2 ) in 127 Pr and on the πg9/2νh11/2 orbitals in 128 Pr. These are the lightest isotopes of Pr for which ED bands have been observed. A contrasting trend in the dynamical moments of inertia ( J (2) ) versus N is observed at low frequencies for the “normal-deformed” πh11/2 and “enhanced-deformed” πg9/2 bands of 127 – 131 Pr both experimentally and in self-consistent Woods-Saxon calculations. The trends in J (2) and β2 are correlated with a decreasing probability for pair scattering into the deformation-driving νh9/2/f7/2 ( [541] 1 2 ) orbital.

Characterization of the first superdeformed band in the A ≈ 80 region

Abstract The recently discovered superdeformed band in one of the strontium isotopes has been identified as belonging to 83Sr from a study with the Gammasphere array and the Microball charged-particle detector system. From its decay pattern, a spin of (81 ± 2) 2 h is deduced for the highest level of this band. Evidence for very fast transition rates and agreement of the characteristics of this band with the theoretical predictions establish the yrast superdeformed nature of this band in 83Sr.

Multiple superdeformed bands in 81Sr

Abstract Four superdeformed bands extending over five to twelve transitions have been identified in 81 Sr from a study with the Gammasphere array and the Microball charged-particle array. One of the bands shows an upbend in the dynamic moment of inertia at a rotational frequency of 1.2 MeV and all bands exhibit a nearly constant moment of inertia below that frequency.

Forking and Unusual Decay Out of Superdeformed Bands in 83Zr

Two superdeformed (SD) bands extending over eight to eleven transitions have been identified in Zr-83, The quadrupole moment of the more intense band was determined by the Residual Doppler Shift Method and is consistent with a quadrupole deformation of beta(2) approximate to 0.5. The large quadrupole moment and population intensity of the yrast SD band (approximate to 5%) in Zr isotopes relative to their isotones in Sr and Y nuclei suggest the presence of a large SD shell gap at proton number Z = 40, At the decay-out points, the Routhians of the SD bands reapproach that of the positive parity normally-deformed states which may be the reason why both of these bands feed mainly (approximate to 85%) into the positive-parity yrast band.

Superdeformed Bands in 80Sr and the Evolution of Deformation in Sr Isotopes

Four superdeformed bands are reported in Sr-80, extending known superdeformation in the Sr-38 series down to N = 42. The characteristics of these bands are discussed. Residual Doppler shifts were measured and average transition quadrupole moments (Q(t)) inferred for these new bands, These Q(t) values are compared to Q(t) values obtained for previously identified superdeformed bands in Sr81-83. The low Q(t) of 2.7(-0.6)(+0.7) eb obtained for the yrast band in Sr-80 indicates a reduction in the deformation of yrast superdeformed bands in the series Sr80-83 with decreasing N, and possibly the onset of triaxiality in superdeformed shapes

Differential quadrupole moment measurements of the 1/2{sup +}[660] (i{sub 13/2}) neutron intruder band in {sup 133}Nd and {sup 135}Nd

Quadrupole moment measurements of the 1/2{sup +}[660] (i{sub 13/2}) bands in {sup 133}Nd (N=73) and {sup 135}Nd (N=75) were performed using the Doppler-shift attenuation method. These results, coupled with the previously measured Q{sub 0} for the same configuration in {sup 137}Nd (N=77), clearly demonstrate a trend of decreasing quadrupole deformation with increasing neutron number. The larger quadrupole moment in {sup 133}Nd compared with that in {sup 135}Nd and {sup 137}Nd offers evidence for the role played by the large shell gap at N=72 for {beta}{sub 2}{approximately}0.35{endash}0.40 in stabilizing the shape at enhanced deformation. The comparison of results from gating below and above the level of interest provides information on the time scale of the sidefeeding contributions to highly deformed structures in the A{approximately}130 region. {copyright} {ital 1999} {ital The American Physical Society}