D. Miller

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.

Direct Reaction Experimental Studies with Beams of Radioactive Tin Ions

The tin chain of isotopes provides a unique region in which to investigate the evolution of single-particle structure, spreading from N = 50 at 100Sn, through 10 stable isotopes and the N = 82 shell closure at 132Sn out into the r-process path. Direct reactions performed on radioactive ion beams are sensitive spectroscopic tools for studying exotic nuclei. Here we present one experiment knocking out neutrons from tin isotopes that are already neutron deficient and two reactions that add a neutron to neutron-rich 130Sn. Both techniques rely on selective particle identification and the measurement of γ rays in coincidence with charged ions. We present the goals of the two experiments and the particle identification for the channels of interest. The final results will be presented in future publications.

Lifetime Measurement of the 2{sub 1}{sup +} State in {sup 20}C

Establishing how and when large N/Z values require modified or new theoretical tools is a major quest in nuclear physics. Here we report the first measurement of the lifetime of the 2(1)+ state in the near-dripline nucleus 20C. The deduced value of τ(2(1)+)=9.8±2.8(stat)(-1.1)(+0.5)(syst)  ps gives a reduced transition probability of B(E2; 2(1)+→0(g.s.)+)=7.5(-1.7)(+3.0)(stat)(-0.4)(+1.0)(syst)  e2  fm4 in good agreement with a shell model calculation using isospin-dependent effective charges.

Lifetime Measurement of the 21+ state in 20C

The mean lifetime of the 21+ state in the near drip‐line nuclide 20C was measured for the first time using the Recoil Distance Method with intermediate energy radioactive beams via a knockout reaction. The measured value of τ21+ = 9.8±2.8(stat)−1.1+0.5(syst) ps gives a reduced transition probability of B(E2;21+→0g.s.+) = 7.51.7+3.0(stat) e2fm4. This value is in good agreement with shell model calculations using a p—sd shell model space and isospin‐dependent (N—Z) effective charges.

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.

In-beam gamma-ray spectroscopy and inclusive two-proton knockout cross section measurements at N approximate to 40

In-beam {gamma}-ray spectroscopy of {sup 66,68}Fe following intermediate-energy one- and two-proton knockout from cobalt and nickel secondary beams has been performed at the National Superconducting Cyclotron Laboratory. New transitions have been observed in {sup 66}Fe and {sup 68}Fe. This is the first observation of {gamma}-ray transitions in {sup 68}Fe. In addition, {sup 64}Cr was produced using the {sup 9}Be({sup 66}Fe,{sup 64}Cr)X two-proton knockout reaction. An unexpectedly low inclusive cross section is observed for this reaction, an order of magnitude smaller than for the {sup 9}Be({sup 68}Ni,{sup 66}Fe)X reaction. This observation is discussed in terms of a significant structural difference between {sup 66}Fe and {sup 64}Cr and considerable admixtures of {nu}(pf){sup n-2}(g{sub 9/2}){sup +2} configurations in the ground and excited states of {sup 64}Cr at N=40.