C. Vaman

Z = 50 shell gap near 100Sn from intermediate-energy Coulomb excitations in even-mass 106-112Sn isotopes.

Rare isotope beams of neutron-deficient 106,108,110Sn from the fragmentation of 124Xe were employed in an intermediate-energy Coulomb excitation experiment. The measured B(E2,0(1)(+)-->2(1)(+)) values for 108Sn and 110Sn and the results obtained for the 106Sn show that the transition strengths for these nuclei are larger than predicted by current state-of-the-art shell-model calculations. This discrepancy might be explained by contributions of the protons from within the Z = 50 shell to the structure of low-energy excited states in this region.

Application of the time-of-flight technique for lifetime measurements with relativistic beams of heavy nuclei

Abstract A novel method for picosecond lifetime measurements of excited γ -ray emitting nuclear states has been developed for fast beams from fragmentation reactions. A test measurement was carried out with a beam of 124Xe at an energy of ∼ 55 MeV / u . The beam ions were Coulomb excited to the 2 1 + state on a movable target. Excited nuclei emerged from the target and decayed in flight after a distance related to the lifetime. A stationary degrader positioned downstream with respect to the target was used to further reduce the velocity of the excited nuclei. As a consequence, the γ -ray decays from the 2 1 + excited state that occurred before or after traversing the degrader were measured at a different Doppler shift. The γ -ray spectra were analyzed from the forward ring of the Segmented Germanium Array; this ring positioned at 37 ∘ simultaneously provides the largest sensitivity to changes in β and the best-energy resolution. The ratio of intensities in the peaks at different Doppler shifts gives information about the lifetime if the velocity β is measured. The results and range of the application of the method are discussed.

Linear polarization sensitivity of SeGA detectors

Abstract Parity is a key observable in nuclear spectroscopy. Linear polarization measurements of γ -rays are a probe to access the parities of energy levels. Utilizing the segmentation of detectors in the Segmented Germanium Array (SeGA) at the NSCL and analyzing the positions of interaction therein allows the detectors to be used as Compton polarimeters. Unlike other segmented detectors, SeGA detectors are irradiated from the side to utilize the transversal segmentation for better Doppler corrections. Sensitivity in such an orientation has previously been untested. A linear polarization sensitivity Q ≈ 0.14 has been measured in the 350-keV energy range for SeGA detectors using α – γ correlations from a Cf 249 source.

Digital data acquisition modules for instrumenting large segmented germanium detector arrays

XIA LLC has developed a 16-channel digital gamma-ray spectrometer (DGF Pixie-16) and associated electronics for instrumenting segmented germanium detector systems with large numbers of channels. The Pixie-16 is a CompactPCI/PXI-based data acquisition module equipped with 100 MHz 12-bit digitizers, signal processing field programmable gate arrays and a digital signal processor. Housed in a custom 6U PXI chassis, its operation can be extended to multiple chassis by using modules which distribute clock and triggers between chassis in LVDS format to achieve high speed and low distortion transmission. XIA LLC has collaborated with the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University in the construction of a fully Digital Data Acquisition System (DDAS) for instrumenting the 594-channel Segmented Germanium Array (SeGA) at NSCL. Using its gamma-ray tracking capability and consequently improved precision in determining gamma-ray interaction positions in the SeGA detectors, the DDAS will significantly improve their ability to determine gamma-ray emission angles and thus achieve better Doppler corrections. In this paper, we demonstrate the capability to achieve sub-nanosecond timing resolution when capturing waveforms in modules across multiple chassis in response to common global triggers. We then discuss the NSCL DDAS firmware solution, especially considering its capability for data acquisition with near zero dead times.

Electromagnetic properties of nuclear chiral partners

Nuclear chirality is a novel manifestation of spontaneous symmetry breaking resulting from an orthogonal coupling of angular momentum vectors. In triaxial nuclei with an odd number of protons and neutrons three perpendicular angular momenta provided by the valence particles and collective rotation can form two systems of opposite handedness. The electromagnetic properties of the resulting doublet bands offer the opportunity to probe formation of chiral geometry according to the model predictions. The status of current theoretical and experimental investigation of electromagnetic properties of the doublet bands is discussed.

{sup 34}P({sup 7}Li,{sup 7}Be+{gamma}) Reaction at 100A MeV in Inverse Kinematics

We report on the first successful extraction of a β+ Gamow-Teller strength distribution from a radioactive isotope in an intermediate-energy charge-exchange experiment in inverse kinematics. The (7Li,7Be+γ(429  keV)) reaction at 100A  MeV was used to measure Gamow-Teller transition strengths from 34P to states in 34Si. The results show that little mixing occurs between sd and pf shell configurations for the low-lying 0+ and 2+ states even though 34Si neighbors the island of inversion and low-lying 2ℏω intruder states exist. Shell-model calculations in the sdpf model space are consistent with these findings.

Plunger lifetime measurements after Coulomb excitation at intermediate beam energies

Absolute transition probabilities of the first 2{sup +} state in {sup 110,114}Pd were remeasured using the recoil distance Doppler shift technique following projectile Coulomb excitation at intermediate beam energies for the first time. The {sup 110}Pd experiment served to check the novel technique as well as the method used for the data analysis which is based on the examination of {gamma}-ray lineshapes. Whereas the measured B(E2) value for {sup 110}Pd agrees very well with the literature, the value obtained for {sup 114}Pd differs considerably. The data is also used to test a novel concept, called the valence proton symmetry, which allows one to extrapolate nuclear properties to very neutron rich nuclei.

Smooth terminating bands in {sup 112}Te: Particle-hole induced collectivity

The Gammasphere spectrometer, in conjunction with the Microball charged-particle array, was used to investigate high-spin states in Te-112 via Ni-58(Ni-58, 4p gamma) reactions at 240 and 250 MeV. Several smooth terminating bands were established, and lifetime measurements were performed for the strongest one using the Doppler-shift attenuation method. Results obtained in the spin range 18-32h yield a transition quadrupole moment of 4.0 +/- 0.5eb, which corresponds to a quadrupole deformation epsilon(2)=0.26 +/- 0.03; this value is significantly larger than the ground-state deformation of tellurium isotopes. It was also possible to extract a transition quadrupole moment for the yrast band in Xe-114, produced via the 58Ni (58Ni, 2p gamma) reaction. A value of 3.0 +/- 0.5eb was found in the spin range 16-24h, which corresponds to a quadrupole deformation epsilon(2)=0.19 +/- 0.03. Cranked Nilsson-Strutinsky calculations are used to interpret the results. (Less)

High-spin structure of {sup 102}Ru

High-spin states in the nucleus 102Ru have been investigated via the 96Zr(13}C,alpha3n) reaction at beam energies of 51 and 58 MeV, using the EUROBALL IV gamma-ray spectrometer and the DIAMANT charged particle array. Several new high-spin bands have been established. The ground state band has been extended up to E_x \sim 12 MeV with Ipi = (26+), while the previously published negative-parity bands have been extended up to E_x \sim 11 and sim 9 MeV with I pi = (23-) and (20-), respectively. The deduced high-spin structure has been compared with Woods-Saxon TRS calculations and, on the basis of the measured Routhians, aligned angular momenta, and B(M1)/B(E2) ratios, nu h_11/2(g_{7/2},d_{5/2} configurations are suggested for the negative-parity structures.

High-spin structure ofRu102

High-spin states in the nucleus 102Ru have been investigated via the 96Zr(13}C,alpha3n) reaction at beam energies of 51 and 58 MeV, using the EUROBALL IV gamma-ray spectrometer and the DIAMANT charged particle array. Several new high-spin bands have been established. The ground state band has been extended up to E_x \sim 12 MeV with Ipi = (26+), while the previously published negative-parity bands have been extended up to E_x \sim 11 and sim 9 MeV with I pi = (23-) and (20-), respectively. The deduced high-spin structure has been compared with Woods-Saxon TRS calculations and, on the basis of the measured Routhians, aligned angular momenta, and B(M1)/B(E2) ratios, nu h_11/2(g_{7/2},d_{5/2} configurations are suggested for the negative-parity structures.