Search

Gamma rays emitted by fission fragments of 252Cf and measured using the Gammasphere array continue to give insight into neutron rich nuclei. Using our high statistics data, we have reexamined high-spin states and linking transitions associated with octupole correlations in 143,145,146Ba. This has been a region of interest, when it was proposed that near 145Ba there was a region of octupole deformation. In an even-A system, it is possible that rotational band structures are explainable by simplex quantum numbers, s = + 1 and s = − 1. The previous bands including octupole bands in 143,145,146Ba have been extended. The new s = i octupole bands in 145Ba and new s = − 1 octupole bands in 146Ba have been identified. In addition, collective model analysis of energy displacement, rotational frequency and B(E1)/B(E2) ratios have been carried out. Evidence of the rotational consequences of octupole deformation in even-even and even-odd isotopes is given. These nuclei have trends as rotation stabilizes the vibrational degrees of freedom. The experimental results are in agreement with theoretical calculations using the interacting boson and boson-fermion models that are based on a universal nuclear energy density functional. [ABSTRACT FROM AUTHOR]

A well-type HPGe detector model with the existence of a particular dead area in the HPGe crystal was developed for use in Monte Carlo simulation codes to better model gamma-ray efficiency characteristics. Dissimilarities of counting efficiencies between simulation and calibration were successfully minimized for the case of point and volume sources. Simulations using the resulting model are applicable to the activity measurement of samples both in the hole and on top of the endcap of the well-type HPGe detector. [ABSTRACT FROM AUTHOR]

In this paper, we provide a formalism for the characterization of tracking arrays with emphasis on the proper corrections required to extract their photopeak efficiencies and peak-to-total ratios. The methods are first applied to Gammasphere, a well characterized 4 π array based on the principle of Compton suppression, and subsequently to GRETINA. The tracking efficiencies are then discussed and some guidelines as to what clustering angle to use in the tracking algorithm are presented. It was possible, using GEANT4 simulations, to scale the measured efficiencies up to the expected values for the full 4 π implementation of GRETA. [ABSTRACT FROM AUTHOR]

Abstract: A simultaneous lifetime and relative g-factor measurement of the levels in 138,142Ce was performed using the Time Dependent Recoil Into Vacuum (TDRIV) technique. The excitation mechanism was Coulomb excitation in inverse kinematics, and the experimental setup included the Yale plunger device and the Gammasphere array. The latter was used to extract angular distributions for the γ-ray transitions at various target-to-stopper distances. A factor of 0.26(8) for 138Ce was obtained relative to the literature value of in 142Ce. In addition, high-precision values of the strengths were obtained. The new data support a proposed subshell closure for the orbital at . [Copyright &y& Elsevier]

Abstract: New level schemes of 112,114,115,116,117,118Pd are established by means of , and measurements of prompt fission γ rays from 252Cf using the Gammasphere multi-detector array. Spins/parities were assigned to levels based on angular correlation measurements, level systematics and decay patterns. In the even-N isotopes 112,114,116Pd, two sets of odd-parity bands were identified and extended with spins measured in each band. The odd-parity bands with large level staggerings were interpreted as disturbed chirality with less pronounced triaxial deformations in the Pd isotopes than observed in the chiral symmetry breaking 110,112Ru with maximum triaxiality. Onset of wobbling motion was identified from the sign of the signature splitting in the γ band of even–even 114Pd, and probably also in 116Pd, as first seen in the isotone 112Ru. Maximal triaxiality in Ru and Pd isotopes is found to be reached for , 112Ru and 114Pd, 4 neutrons more than predicted in the theoretical calculations. The new data and TRS calculations allowed a systematic study of the band crossings in the even-N 112,114,116Pd and odd-N 115,117Pd isotopes. Now we find a new overall, more complex shape evolution than previously proposed from triaxial prolate in 110Pd via triaxial oblate in 112Pd to nearly oblate in 114,116Pd with a large change of the triaxial deformation parameter γ toward nearly oblate in the alignment in 114,115,116,117,118Pd, and triaxial-prolate–triaxial-oblate shape coexisting bands in 115Pd. [Copyright &y& Elsevier]

Abstract: The transition quadrupole moments, , of rotational bands in the neutron-rich, even-mass 102–108Mo and 108–112Ru nuclei were measured in the 8–16 ℏ spin range with the Doppler-shift attenuation method. The nuclei were populated as fission fragments from 252Cf fission. The detector setup consisted of the Gammasphere spectrometer and the HERCULES fast-plastic array. At moderate spin, the moments are found to be reduced with respect to the values near the ground states. Attempts to describe the observations in mean-field-based models, specifically cranked relativistic Hartree–Bogoliubov theory, illustrate the challenge theory faces and the difficulty to infer information on γ softness and triaxiality from the data. [Copyright &y& Elsevier]

Abstract: A particle- coincidence technique has been used to measure the fusion cross-section in the system 12C+12C at low beam energies. Based on these results, it should be possible to measure this important fusion cross-section down to the 10pb level within a week-long run. [Copyright &y& Elsevier]

Abstract: Four previously known rotational bands in 76Rb have been extended to moderate spins using the Gammasphere and Microball γ ray and charged particle detector arrays and the 40Ca(40Ca, 3pn) reaction at a beam energy of 165 MeV. The properties of two of the negative-parity bands can only readily be interpreted in terms of the highly successful Cranked Nilsson–Strutinsky model calculations if they have the same configuration in terms of the number of particles, but they result from different nuclear shapes (one near-oblate and the other near-prolate). These data appear to constitute a unique example of shape coexisting structures at medium spins. [Copyright &y& Elsevier]

Abstract: The level scheme of odd-Z neutron-rich 141Cs (, ) was extended and expanded and that of 144Cs () was identified for the first time by means of coincidence measurements of prompt γ rays in the spontaneous fission of 252Cf with Gammasphere. Spin/parity was assigned to the levels based on angular correlations and level systematics in 141,143Cs. Parity doublets characteristic of both simplex quantum number and were proposed in 141Cs. The tests by using rotational frequency ratio imply octupole vibrations in 141Cs and 143Cs. values and electric dipole moments were calculated for 141Cs, and re-determined for 143Cs. It was found that values of 141Cs are simplex-dependent and the average value is one order of magnitude larger than that of 143Cs, and the deduced dipole moment of 141Cs is considerably larger than that of 143Cs, and comparable to the isotone 142Ba. For 144Cs the yrast sequence looks like a well-deformed rotational band, but no octupole band structure was identified in this nucleus. The overall variations of in 141,143,144Cs exhibit a pronounced drop of dipole moment with increasing neutron number in this odd-Z isotopic chain, which may be analogous in nature to the quenching of observed in even–even 146Ba (, ) and 224Ra (, ), and to the drop of in the odd-Z neutron-rich 147La (, ) reported by our collaboration. [Copyright &y& Elsevier]

Abstract: Excited states in 124Xe have been studied via the 12C(124Xe, 124Xe∗) Coulomb excitation reaction. Their population cross-sections relative to the state have been determined from the γ-ray yields observed with Gammasphere. More than twenty absolute E2 strengths for seven off-yrast, low-spin states of 124Xe have been deduced for the first time. The absolute values indicate pronounced symmetry, even for the off-yrast states with high quantum number τ, while the symmetry is substantially broken. [Copyright &y& Elsevier]

Abstract: An interlock system is described that allows volatile targets to be prepared in an evaporator, sealed under vacuum, transported to the -array Gammasphere, and inserted into the reaction chamber for experiments, while maintaining a vacuum better than . The performance of the system is demonstrated for lithium metal targets. A comparison is made between lithium targets covered by a gold evaporation to reduce oxidation and open metal targets transferred in the interlock. [Copyright &y& Elsevier]

Abstract: The potential of incorporating three-quanta annihilation into positron emission tomography (PET) to detect local tissue chemistry was investigated using the GAMMASPHERE facility, which features a spherical array of 110 Compton-suppressed high-purity germanium (HPGe) detectors. Fluoro-2-deoxyglucose (FDG) containing was introduced into 11 samples of haemolysed blood, serum, cell concentrate and whole blood, some of which had been either oxygenated or deoxygenated. The relative three-quanta yield was estimated from the reduced counts in the full-energy photopeak at 511keV. Compton-suppression produced larger effects in the calculations than time-gating, suggesting its importance for this method of estimation. The relative three-quanta yields were found to vary as much as 11% between the samples. This level of sensitivity to different biological samples commends three-quanta annihilation for molecular imaging. [Copyright &y& Elsevier]

Abstract: The yrast and near-yrast level scheme of light neutron-rich 143La (, ) is reinvestigated and expanded and that of 144La () is proposed for the first time by measuring prompt γ rays from the spontaneous fission of 252Cf at Gammasphere. Spins/parities are assigned to the lowest-lying levels of 143,144La based on the early studies of decay, and the assignments for high-spin levels of 143,144La are made by measuring internal conversion coefficients and following the level systematics of the neighboring heavier La isotopes and even–even Ba and Ce nuclei. The ratios, energy displacements and rotational frequency ratios of the new parity-doublets of 143,144La indicate that octupole deformations/correlations also develop in these nuclei. The band-crossings observed in a rotational frequency range of 0.31 to 0.34 MeV for the two even-parity bands in 143La but being absent in 144La are interpreted as due to alignment of a pair of neutrons in 143La. [Copyright &y& Elsevier]

Abstract: Prompt –– coincidence measurements of –114 neutron-rich nuclei populated in the spontaneous fission of 252Cf were made with Gammasphere. The high statistics data, 5.7×10 11 triples and higher folds, opened up discoveries of new band structures and significant extensions of previously known bands. The new data for ground bands in 99,101Y, 101,105Nb, 105,107,109,111Tc and 111,113Rh when compared with one quasi-particle plus triaxial rotor calculation, revealed a smooth transition from axial symmetry in 99,101Y to maximize triaxiality in 111,113Rh. In 106Mo, 110,112Ru, pairs of doublet bands are established to high spins. Their constant and equal kinetic moments of inertia, small energy differences between states of the same spin and very similar B(E2)/B(M1) ratios for each band in these nuclei suggest that these are chiral doublet bands. If they are not chiral, then a new symmetry is indicted. Extended one- and new two-phonon -vibrational bands are observed in 110,112Ru. The extended one-phonon bands in 104-108Mo, and 108-112Ru exhibit an unexpected reversal in the energy level staggering pattern with spin. The first one- and two-phonon -bands in an odd- nucleus are seen in 105Mo with level energies almost identical to these same bands known in 104Mo. [Copyright &y& Elsevier]

Abstract: Isomeric two-quasiparticle states have been identified in the neutron-rich isotopes 172Er and 174Er using multi-nucleon transfer reactions with 136Xe beams incident on various targets, and γ-ray spectroscopy with Gammasphere. A candidate for the two-quasineutron state in 172Er is found at 1500 keV. In 174Er, a nuclide whose level scheme was previously unknown, a long-lived isomer is identified at 1112 keV decaying via an inhibited E1 transition and revealing the yrast sequence of 174Er. This isomer is proposed to be a two-quasineutron state, defining a sequence in the isotones extending from the well-deformed neutron-rich isotope 174Er to the neutron-deficient isotope 188Pb, where the presence of the isomer signifies a prolate minimum in an otherwise spherical well. Configuration-constrained potential-energy surface calculations are used to predict the excitation energies of the 6+ and 8− intrinsic states and as a basis for extracting the pairing force strength, , in the and isotones. [Copyright &y& Elsevier]

More than a decade after the large γ-ray detector arrays came into operation, their impact is as large as ever. A few examples from Gammasphere are discussed here. They illustrate that the arrays make substantial contributions, not only in areas where they were expected to do so such as studies of nuclei at high angular momentum, but also in other domains such as nucleosynthesis, for example. [Copyright &y& Elsevier]

Abstract: A high-efficiency evaporation-residue counter (named HERCULES) has been designed and constructed. It consists of 64 thin fast-plastic scintillators arranged in four rings covering an angular range of 3.1– at 30.9cm from the target. The device design has been optimized for use with GAMMASPHERE. Evaporation residues are well separated from elastic scattering, fission events, and other undesirable reaction debris by time-of-flight and pulse-height analysis. Each detector element can operate at rates up to 0.5MHz with signal recovery in about 10ns. The geometry, construction, light-output characteristics, stability of the measured quantities, associated electronics, and in-beam performance of the device are described. For energy-degraded fission fragments with kinetic energies of less than 45MeV, it is found that (1) the dependence of the pulse height disappears and (2) the light output per unit energy increases with decreasing energy. For evaporation residues from two reactions, in which the evaporation channels dominate and in which the residue cross-section is about 100 times smaller than the fission cross-section, angular distributions and sample -ray spectra are shown. [Copyright &y& Elsevier]

The effects introduced by 4π charged-particle detector arrays such as the Microball on lifetime measurements by Doppler-shift attenuation techniques are discussed. The effect originates from the lower efficiency for detecting charged particles at backward angles. Procedures for correcting for such “bias” are described. [Copyright &y& Elsevier]

The efficient methods of decomposition of γ-ray spectra, based on the Gold algorithm, are presented. They use a response matrix of Gammasphere, which was obtained by synthesis of simulated and interpolated response functions using a new developed interpolation algorithm. The decomposition method has been applied to the measured spectra of 152Eu and 56Co. The results show a very effective removal of the background counts and their concentration into the corresponding photopeaks. The peak-to-total ratio in the spectra achieved after applying the decomposition method is in the interval 0.95–0.99. In addition, a new advanced algorithm of the “boosted” decomposition has been proposed. In the spectra obtained after applying the boosted decomposition to the measured spectra, very narrow photopeaks are observed with the counts concentrated to several channels. [Copyright &y& Elsevier]

The level scheme of 18176Os has been investigated with the 150Nd(36S,5n) reaction. The low-K rotational bands built upon the 9/2+[624], 7/2−[514] and 1/2−[521] neutron configurations have been extended and other new bands established. The configurations of these low-K bands are discussed within the framework of the cranked-shell model. The lifetimes for some of the states in the 9/2+[624] and 1/2−[521] collective rotational bands were also measured using the Doppler Shift Attenuation method. The large deformations deduced are found to be consistent with those predicted from theoretical Total Routhian Surface calculations. These results support the idea that for these low-K states the nuclear shape is axially symmetric and allows the K quantum number to be defined and the associated K-selection rule to be upheld. This behaviour apparently contrasts with that of the higher-K states in 181Os. In the higher-spin regime, two new high-K intrinsic states, with Kπ=37/2+ and Kπ=41/2+, were established, along with the fragmented decay of a Kπ=33/2− intrinsic state. The configurations and excitation energies of these experimentally determined intrinsic states are found to be in excellent agreement with theoretical calculations based on a fixed shape Nilsson model plus BCS pairing. The structures on top of these intrinsic states do, however, show very different behaviour. A relatively regular high-K rotational band was observed on top of the Kπ=41/2+ state but not for the other newly-established intrinsic states. Theoretical configuration-constrained potential energy surface calculations suggest that the irregular transition sequence above the Kπ=37/2+ intrinsic bandhead state, the limited excitations observed above the other intrinsic states and the observation of fragmented and non-hindered decays, are due to these configurations being subject to an appreciable γ softness. These calculations reveal that the Kπ=41/2+ configuration is less susceptible to distortions in the γ plane than any of the other high-K states. [Copyright &y& Elsevier]

Using triple-coincidence events of prompt fission γ rays from spontaneous fission of 252Cf, we made a new analysis of the yield matrix of coincident pairs of barium (Z=56) and molybdenum (Z=42) fission fragments. Branching from γ-bands (K=2) and octupole-bands (K=0) were also measured. From this reanalysis the previously proposed “extra-hot-fission mode” (8–10 neutrons evaporated) is much weaker than first reported. In this paper, we discuss in detail the methodology, including background subtraction for triple-coincidence data. The importance of minimal compression spectra allowing least-squares peak-fitting analysis is emphasized. [Copyright &y& Elsevier]

Excited structures in the neutron-deficient nucleus 179Hg have been established for the first time using the Gammasphere spectrometer in conjunction with the fragment mass analyzer. Competing states originating from three different minima associated with nearly spherical, oblate, and prolate deformations were found. This result can be contrasted with the situation in heavier odd-mass Hg isotopes where only two minima (oblate and prolate) have been seen. The implications of these three shapes at low spin and excitation energy are discussed in the general context of shape coexistence in this mass region. [Copyright &y& Elsevier]

Abstract: The performance of new scintillator detectors based on LaBr3(Ce) technology was evaluated in conjunction with the Gammasphere spectrometer. Specifically, the lifetimes of states between 50ps and 1ns, populated in the decay of the 177m Lu isomeric state (T 1/2=160 d), have been measured. Even though the decay scheme is rather complex, it is possible to perform precise measurements because of the superior energy resolution of the LaBr3(Ce) detectors, compared to that of BaF2 scintillators, when used in combination with the power of the Gammasphere array to isolate a specific cascade. [Copyright &y& Elsevier]

Abstract: We here report on recent experiments at GAMMASPHERE employing highly inverse, near-barrier reactions with silicon dioxide targets and use the Fragment Mass Analyzer (FMA) to collect the recoils. The 16O(58Ni,2n)72Kr reaction was studied with beams of 220MeV 58Ni in excess of 15pnA. Relatively few stable beam and target combinations are available to populate excited states in 72Kr, and the small cross sections of these reactions necessitate the use of targets that can withstand high intensity beam currents. A variety of oxygen containing targets were produced, with those made of silicon dioxide performing best. Over a period of 24h of irradiation the oxygen content in these targets monotonically declined to about 1/3 of the original value. The silicon content did not appear to change. By inserting new sets of targets each 18-h-cycle an impressive data set was collected. [Copyright &y& Elsevier]