Cornelius Beausang

GRETA: the gamma-ray energy-tracking array. Status of the development and physics opportunities

A gamma-ray energy-tracking array (GRETA) consisting of a 4π array of highly segmented Ge detectors is a new concept for the detection of gamma radiation. In such an array, all of the individual interactions of each gamma-ray are identified by their energies and positions, the three-dimensional positions being determined by pulse-shape analysis of the signals from the segmented Ge detectors. Then, using tracking algorithms based on the properties of photoabsorption, Compton scattering and pair production, the gamma-ray scattering sequences can be reconstructed, even if many gamma-rays hit the array at the same time. Such a detector will have new and much improved capabilities compared to the current generation of gamma-ray spectrometers and will be vital to fully exploit the most exotic beams from future radioactive beam facilities. One implementation of the tracking array concept, called GRETA, is currently under development in the US. In this presentation the progress of the GRETA project will be reviewed and future plans and prospects will be discussed.

B(E2) values from low-energy Coulomb excitation at an ISOL facility: the N=80,82 Te isotopes

B(E2;0+1→2+1) values for the unstable, neutron-rich nuclei 132,134Te were determined through Coulomb excitation, in inverse kinematics, of accelerated beams of these nuclei. The systematics of measured B(E2) values from the ground state to the first excited state have been extended to the N=82 shell closure in the Te nuclei and have been compared with the predictions of different theories. The measurements were performed at the Holifield Radioactive Ion Beam Facility (HRIBF) using the GRAFIK detector. The success of this approach, which couples a 5.7% efficient through-well NaI(Tl) γ-ray detector with thin foil microchannel plate beam detectors, also demonstrates the feasibility for Coulomb excitation studies of neutron-rich nuclei even further from the valley of beta stability, both at present-generation ISOL facilities and at the proposed Rare Isotope Accelerator.

Spin dependence of d5/2 neutron configurations in the wave functions of 92,94Zr from g-factor measurements

Abstract g factors of the 2 + 1 and 4 + 1 states in 92,94 Zr have been measured via Coulomb excitation of isotopically pure 92,94 Zr beams in inverse kinematics in combination with the transient field technique. The high precision results, g ( 92 Zr; 2 + 1 ) = −0.180(10) and g ( 94 Zr; 2 + 1 ) = −0.329(15) as well as the new data on the 4 + 1 state, g ( 92 Zr; 4 + 1 ) = −0.50(11), provide, for the first time, clear evidence of the dominant role of d 5/2 neutrons in the wave functions of these nuclei just above the N =50 shell closure. Moreover, the data imply increased purity of the d 5/2 neutron component in the wave function with increasing spin.

Competing core and single particle excitations in the 2+1 state in 44Ca

Abstract The g factor of the 2 1 + state in 44 Ca has been measured using the transient field technique and Coulomb excitation in inverse kinematics. A positive value of g(2 1 + , 44 Ca )=+0.12±0.05 was deduced. The result suggests the ( fp ) 4 ν configuration competes with excitations of the 40 Ca core. A simple model which considers that the wave function for the 2 1 + state consists of approximately equal admixtures of spherical four valence neutron configurations and a deformed core configuration accounts for the experimentally observed g factor as well as the previously measured quadrupole moment.

Nature of excited 0+ states in 154Sm

Abstract Lifetimes of the 0 2 + and 2 γ + states in 154 Sm were measured using the Doppler-shift Attenuation Method following Coulomb excitation. The experiment employed the gamma-ray detector array YRAST Ball at Yale University in conjunction with an array of photo-electric cells. 154 Sm was identified as one of the few deformed nuclei where the first excited 0 + state is the β -vibration of the ground state. The 0 3 + level has no collective decay to the ground state band and is interpreted as a spherical shape coexisting state based on its decay properties and calculations in the framework of the interacting boson model (IBM) and geometric collective model (GCM).

SASSYER: An old instrument for new physics at Yale

Abstract The Wright Nuclear Structure Laboratory has recently acquired a gas-filled recoil separator previously used at Berkeley National Laboratory for heavy-element synthesis. The separator will be used to separate reaction recoils from primary beam particles and fission products following target bombardment. Commissioning of the separator has recently been completed, and the structure of 203 Rn investigated.

LIFETIMES OF YRAST AND EXCITED SUPERDEFORMED STATES IN 150GD : EFFECT OF PARTICLE-HOLE EXCITATIONS ON THE DEFORMATION

The quadrupole moments and deformations have been measured for six superdeformed bands in Gd-150. The results indicate evidence for deformation driving properties of both the high-hi intruder and a ...

Quadrupole moment measurements of TSD1 and TSD2 bands in 167Lu

The triaxial strongly deformed (TSD) bands in 167Lu were populated by the 123Sb(48Ca, 4n) reaction with a beam energy of 203 MeV. Gamma rays, requiring five fold or more in prompt coincidence, were detected with the Gammasphere spectrometer. Of particular interests are TSD bands 1 and 2 which have previously been interpreted as zero phonon and one phonon wobbling bands, respectively. Using the Doppler shift attenuation method (DSAM), a preliminary transition quadrupole moment of 6.9+0.3−0.3 eb was extracted for the TSD1 band. Data analysis continues for TSD2 which is considerably more weakly populated.

Isomers and seniority in the trans-Pb nuclei

Low-energy excited states of 210 Ra and 208 Ra were investigated at the Wright Nuclear Structure Laboratory of Yale University. Fusion evaporation recoils were selected using the gas-filled spectrometer, SASSYER. Delayed γ-rays, following isomeric decays, were detected at the focal plane of SASSYER with a small array of HPGe detectors. Transitions following the proposed J π = 8 + isomers were observed, and the half-lives measured. The experiments are discussed and results compared to expectations from the seniority scheme.

Surrogate Nuclear Reactions using STARS

The results from two surrogate reaction experiments using the STARS (Silicon Telescope Array for Reaction Studies) spectrometer are presented. The surrogate method involves measuring the particle and/or γ‐ray decay probabilities of excited nuclei populated via a direct reaction. These probabilities can then be used to deduce neutron‐induced reaction cross sections that lead to the same compound nuclei. In the first experiment STARS coupled to the GAMMASPHERE γ‐ray spectrometer successfully reproduce surrogate (n,γ), (n,n′γ) and (n,2nγ) cross sections on 155,156Gd using 3He‐induced reactions. In the second series of experiments an energetic deuteron beam from the ESTU tandem at the Wright Nuclear Structure Lab at Yale University was used to obtain the ratio of fission probabilities for 238U/236U and 237U/239U populated using the 236,238U(d,d′f) and 236,238U(d,pf) reactions. Results from these experiments are presented and the implications for the surrogate reaction technique are discussed.