M. Devlin

“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.

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.

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.

Rotational structures near 40h in La-123

The neutron-deficient nucleus La-123 was studied via the Mo-92(Ca-40,2ap) reaction at a beam energy of 184 MeV. Previously known bands were extended to a much higher spin, and in two cases the structures are now observed near 40 (h) over bar. In addition, three new sequences were identified and linked into previously known bands. The lowest (pi,alpha)=(+,-1/2) structure displays characteristics similar to those of analogous bands in La-127,La-129, which have been proposed as examples of smooth band termination. Cranked Nilsson-Strutinsky calculations were compared with the experimental data in La-123 to determine whether this band is approaching a terminating state as well.

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

Superdeformation in 91Tc

A high-spin rotational band with 11 gamma -ray transitions has barn observed in Tc-91. The dynamical moment of inertia as well as the transition quadrupole moment of 8.1(-1.4)(+1.9) eb measured for this band show the characteristics of a superdeformed band. However, the shape is more elongated than in the neighbouring A = 80-90 superdeformed nuclei. Theoretical interpretations of the band within the cranked Strutinsky approach based on two different Woods-Saxon potential parameterisations are presented. Even though an unambiguous configuration assignment proved difficult, both calculations indicate a larger deformation and at least three additional high-N intruder orbitals occupied compared to the lighter SD nuclei

Collective excitations and band termination in Nb-85

High-spin states in Nb-85 were studied using the GAMMASPHERE Ge detector array and the MICROBALL charged-particle detector system. Three gamma-ray cascades with collective rotational characteristics were observed. One of the bands exhibits a forking at the top, most likely reflecting the termination of one branch into a favoured non-collective, near spherical state. The data are interpreted in terms of cranked Strutinsky-type calculations

High-spin structures and band termination effects in 104Cd

High-spin states in the neutron-deficient isotope Cd-104 were populated using the Cr-50(Ni-58,4p)Cd-104 reaction at a beam energy of 250 MeV The level scheme has been extended using triple gamma-ray coincidences to a spin of 29h and an excitation energy of 18.2 MeV. Several collective structures involving the excitation of h(11/2) neutrons have been observed to spins approaching 30h. The high-spin structure has been compared to the results of cranked Nilsson-Strutinsky calculations.

Relative quadrupole deformations for structures in odd proton Pr nuclei near mass

The quadrupole moments for collective structures in the odd proton (Z = 59) , and nuclei have been measured using the Doppler-shift attenuation method. New information is presented on the shape-driving behaviour of the [404] proton and [541] (, ) neutron Nilsson configurations. While the involvement of the former orbital leads to quadrupole deformations that are comparable to those observed for the so-called superdeformed bands in this mass region, the values for structures that include the 1/[541] neutron are found to lie intermediate between those observed for normally deformed and superdeformed bands.

Collective dipole bands in 110,112Te: Stability against magnetic rotation

Three long, strongly coupled (ΔI=1) sequences have been identified in 110,112Te by using the GAMMASPHERE array in conjunction with the MICROBALL charged-particle array. These bands are interpreted in terms of deformed proton 1-particle–1-hole bands that reach termination at I∼40ℏ. This is the first observation of such collective dipole structures in this mass region. In contrast, many shorter dipole sequences have been associated with weakly deformed structures that generate angular momentum by the shears mechanism (magnetic rotation).