R. Wadsworth

The GREAT spectrometer

Abstract The GREAT spectrometer is designed to measure the decay properties of reaction products transported to the focal plane of a recoil separator. GREAT comprises a system of silicon, germanium and gas detectors optimised for detecting the arrival of the reaction products and correlating with any subsequent radioactive decay involving the emission of protons, α particles, β particles, γ rays, X-rays or conversion electrons. GREAT can either be employed as a sensitive stand-alone device for decay measurements at the focal plane, or used to provide a selective tag for prompt conversion electrons or γ rays measured with arrays of detectors deployed at the target position. A new concept of triggerless data acquisition (total data readout) has also been developed as part of the GREAT project, which circumvents the problems and limitations of common dead time in conventional data acquisition systems.

The GREAT triggerless total data readout method

Recoil decay tagging (RDT) is a very powerful method for the spectroscopy of exotic nuclei. RDT is a delayed coincidence technique between detectors usually at the target position and at the focal plane of a spectrometer. Such measurements are often limited by dead time. This paper describes a novel triggerless data acquisition method, which is being developed for the Gamma Recoil Electron Alpha Tagging (GREAT) spectrometer, that overcomes this limitation by virtually eliminating dead time. Our solution is a total data readout (TDR) method where all channels run independently and are associated in software to reconstruct events. The TDR method allows all the data from both target position and focal plane to be collected with practically no dead-time losses. Each data word is associated with a timestamp generated from a global 100-MHz clock. Events are then reconstructed in real time in the event builder using temporal and spatial associations defined by the physics of the experiment.

The EUROBALL neutron wall - design and performance tests of neutron detectors

The mechanical design of the EUROBALL neutron wall and neutron detectors, and their performance measured with a Cm-246,Cm-248 fission source are described. The array consists of 15 pseudohexaconical detector units subdivided into three, 149 mm high, hermetically separated segments and a smaller central pentagonal unit subdivided into five segments. The detectors are filled with Bicron BC501A liquid scintillator. Each section of the hexaconical detectors is viewed by a 130 mm diameter Philips XP4512PA photomultiplier while the sections of pentagonal detectors are viewed by Philips XP4312B PMTs. The tests of n-gamma discrimination performed by zero-crossing and time-of-flight methods show a full separation of gamma- and neutron events down to 50 keV recoil electron energy. These tests demonstrate the excellent timing properties of the detectors and an average time resolution of 1.56 ns. The factors determining the efficiency of neutron detectors are discussed. The total efficiency for the full array for a symmetric fusion-evaporation reaction is predicted to be 0.30

Multiple Superdeformed Bands in

Abstract Three superdeformed bands have been observed in 194Hg. The dynamical moment of inertia J (2) of all three bands is observed to increase by 30–40% over the frequency range ħω = 0.1–0.4 MeV. This phenomena can be understood in terms of the gradual alignment of pairs of high-j intruder orbitals within the framework of the cranked Woods-Saxon and Nilsson models including pairing. The calculations together with the observed J (2) behaviour of the three bands indicate that pairing correlations in the superdeformed minimum are rather weak.

^194

Abstract The nucleus 198 Pb was populated via the 186 W( 17 O,5n) 198 Pb reaction at beam energies of 92 and 98 MeV. A highly regular rotational ΔI =1 band has been found. Angular correlation measurements confirm the transitions to be dipoles. Assuming M1 multipolarity for these transitions the experimental data are interpreted in terms of a π([505] 9 2 − ⊗[606] 13 2 + ) K π =11 − ⊗ν( i 13 2 ) 2 configuration of oblate shape. This is the first such collective oblate structure identified in the A ∼200 mass region.

Hg and Their Dynamical Moments of Inertia

T. Bloxham, A. Boston, J. Dawson, D. Dobos, S.P. Fox, M. Freer, B.R. Fulton, C. Gößling, P.F. Harrison, M. Junker, H. Kiel, J. McGrath, B. Morgan, D. Münstermann, P. Nolan, S. Oehl, Y. Ramachers, C. Reeve, D. Stewart, R. Wadsworth, J.R. Wilson, and K. Zuber School of Physics and Astronomy, University of Birmingham, B15 2TT, UK Lehrstuhl für Experimentelle Physik IV, Universität Dortmund, Otto–Hahn Str. 4,44227 Dortmund, Germany Laboratori Nazionali del Gran Sasso, S.S. 17 BIS km. 18.910, 67010, Assergi, L’Aquila, Italy Dept. of Physics, University of Liverpool, Liverpool L69 7ZE, UK Dept. of Physics and Astronomy, University of Sussex, Brighton, BN1 9QH, UK Dept. of Physics, University of Warwick, Coventry CV4 7AL, UK Dept. of Physics, University of York, Heslington, York, YO10 5DD, UK (Dated: February 5, 2008)

First observation of a collective dipole rotational band in the A∼200 mass region

Abstract The nucleus 198 Pb was populated via the 186 W( 17 O, 5n) 198 Pb reaction at beam energies of 92 and 98 MeV. Five collective rotational cascades of ΔI = 1 transitions have been found. Four are highly regular, one much more irregular. The structures are incorporated into a level scheme which extends up to approximately spin 32ħ and an excitation energy of about 10 MeV. Angular correlation measurements confirm the dipole character of the interband transitions. Their M1 multipolarity is inferred, and from this supposition the experimental data are interpreted in terms of oblate high- K two quasiproton configurations coupled to aligned neutron excitations. This interpretation is extended to include other ΔI = 1 oblate structures observed in 194–201 Pb. It is shown that the pattern of observed moments of inertia can be understood in the simple unpaired picture involving neutron i 13 2 excitations. The identical bands observed are interpreted in terms of the normal-parity weakly-coupled singlet orbital.

First results on double β-decay modes of Cd, Te, and Zn Isotopes

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.

Collective oblate dipole rotational bands in 198Pb

Abstract Differential cross section data are presented for the elastic scattering of 33 MeV tritons from a range of nuclei from 12 C to 232 Th. These data have been analysed using a phenomenological optical model. Parameters are presented for three families of the real potential. A comparison of the triton optical model potential with those from a re-analysis of 3 He scattering from fp shell nuclei has allowed the isospin dependence of the optical model potential for mass-3 projectiles to be obtained in this mass region.

Intruder bands in 108Sn

Abstract The α-decay of 228Th and the reaction 226Ra(58Ni, 60Ni)224Ra have been employed to populate states in 224Ra up to Iπ=12+. Values of the intrinsic electric dipole moment |Q1| have been deduced from the observed of B(E1)/B(E2) braching ratios. These values are ≲0.1e fm for spins up to Iπ=9−, and are thus smaller than those in 218,220Ra or 226Ra. The behavior of Q1 suggests that shell effects are important in this mass region, although theoretical calculations which include these effects do not reproduce the observed pattern.