arXiv subjects

A. Chester

Publications and source records attributed to A. Chester.

27 records · Page 2Linked to original sources

arXiv2018

Shape Coexistence and Mixing of Low-Lying $0^+$ States in $^{96}$Sr

The low energy excited $0_{2,3}^+$ states in $^{96}$Sr are amongst the most prominent examples of shape coexistence across the nuclear landscape. In this work, the neutron $[2s_{1/2}]^2$ content of the $0_{1,2,3}^+$ states in $^{96}$Sr was determined by means of the d($^{95}$Sr,p) transfer reaction at the TRIUMF-ISAC2 facility using the SHARC and TIGRESS arrays. Spectroscopic factors of 0.19(3) and 0.22(3) were extracted for the $^{96}$Sr ground and 1229~keV $0^+$ states, respectively, by fitting the experimental angular distributions to DWBA reaction model calculations. A detailed analysis of the $γ$-decay of the isomeric $0_3^+$ state was used to determine a spectroscopic factor of 0.33(13). The experimental results are compared to shell model calculations, which predict negligible spectroscopic strength for the excited $0^+$ states in $^{96}$Sr. The strengths of the excited $0_{2,3}^+$ states were also analyzed within a two-level mixing model and are consistent with a mixing strength of $a^2$=0.40(14) and a difference in intrinsic deformations of $|Δβ|=0.31(3)$. These results suggest coexistence of three different configurations in $^{96}$Sr and strong shape mixing of the two excited $0^+$ states.

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arXiv2018

Gamma-gamma angular correlation analysis techniques with the GRIFFIN spectrometer

Gamma-gamma angular correlation measurements are a powerful tool for identifying the angular momentum (spin) of excited nuclear states involved in a $γ$-ray cascade, and for measuring the multipole orders and mixing ratios of transitions. Though the physical angular correlations are fully calculable from first principles, experimental effects can make the extraction of coefficients and thus conclusions about spins and mixing ratios difficult. In this article we present data analysis techniques developed for the clover detectors of the GRIFFIN spectrometer at TRIUMF-ISAC combined with GEANT4 simulations in order to extract accurate experimental results.

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arXiv2018

Lifetimes of low-lying excited states in $^{86}_{36}$Kr$_{50}$

The evolution of nuclear magic numbers at extremes of isospin is a topic at the forefront of contemporary nuclear physics. $N=50$ is a prime example, with increasing experimental data coming to light on potentially doubly-magic $^{100}$Sn and $^{78}$Ni at the proton-rich and proton-deficient extremes, respectively. Experimental discrepancies exist in the data for less exotic systems. In $^{86}$Kr the $B(E2;2^+_1\rightarrow0^+_1)$ value - a key indicator of shell evolution - has been experimentally determined by two different methodologies, with the results deviating by $3σ$. Here, we report on a new high-precision measurement of this value, as well as the first measured lifetimes and hence transition strengths for the $2^+_2$ and $3^-_{(2)}$ states in the nucleus. The Doppler-shift attenuation method was implemented using the TIGRESS gamma-ray spectrometer and TIGRESS integrated plunger (TIP) device. High-statistics Monte-Carlo simulations were utilized to extract lifetimes in accordance with state-of-the-art methodologies. Lifetimes of $τ(2^+_1)=336\pm4\text{(stat.)}\pm20\text{(sys.)}$ fs, $τ(2^+_2)=263\pm9\text{(stat.)}\pm19\text{(sys.)}$ fs and $τ(3^-_{(2)})=73\pm6\text{(stat.)}\pm32\text{(sys.)}$ fs were extracted. This yields a transition strength for the first-excited state of $B(E2;2^+_1\rightarrow0^+)=259\pm3\text{(stat.)}\pm16\text{(sys.)}$ e$^2$fm$^4$. The measured lifetime disagrees with the previous Doppler-shift attenuation method measurement by more than $3σ$, while agreeing well with a previous value extracted from Coulomb excitation. The newly extracted $B(E2;2^+_1\rightarrow0^+_1)$ value indicates a more sudden reduction in collectivity in the $N=50$ isotones approaching $Z=40$.

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arXiv2017

Characteristics of GRIFFIN high-purity germanium clover detectors

The Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei, GRIFFIN, is a new experimental facility for radioactive decay studies at the TRIUMF-ISAC laboratory. The performance of the 16 high-purity germanium (HPGe) clover detectors that will make up the GRIFFIN spectrometer is reported. The energy resolution, efficiency, timing resolution, crosstalk and preamplifier properties of each crystal were measured using a combination of analogue and digital data acquisition techniques. The absolute efficiency and add-back factors are determined for the energy range of 80 - 3450 keV. The detectors show excellent performance with an average over all 64 crystals of a FWHM energy resolution of 1.89(6) keV and relative efficiency with respect to a 3"x3" NaI detector of 41(1)% at 1.3 MeV.

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arXiv2013

Two-neutron transfer reaction mechanisms in $^{12}$C($^6$He,$^{4}$He)$^{14}$C using a realistic three-body $^{6}$He model

The reaction mechanisms of the two-neutron transfer reaction $^{12}$C($^6$He,$^4$He) have been studied at 30 MeV at the TRIUMF ISAC-II facility using the SHARC charged-particle detector array. Optical potential parameters have been extracted from the analysis of the elastic scattering angular distribution. The new potential has been applied to the study of the transfer angular distribution to the 2$^+_2$ 8.32 MeV state in $^{14}$C, using a realistic 3-body $^6$He model and advanced shell model calculations for the carbon structure, allowing to calculate the relative contributions of the simultaneous and sequential two-neutron transfer. The reaction model provides a good description of the 30 MeV data set and shows that the simultaneous process is the dominant transfer mechanism. Sensitivity tests of optical potential parameters show that the final results can be considerably affected by the choice of optical potentials. A reanalysis of data measured previously at 18 MeV however, is not as well described by the same reaction model, suggesting that one needs to include higher order effects in the reaction mechanism.

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arXiv2007

Linear polarization sensitivity of SeGA detectors

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 $Q \approx 0.14$ has been measured in the 350-keV energy range for SeGA detectors using $α$-$γ$ correlations from a \nuc{249}{Cf} source.

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arXiv2007

Shape and structure of N=Z 64Ge; Electromagnetic transition rates from the application of the Recoil Distance Method to knock-out reaction

Transition rate measurements are reported for the first and the second 2+ states in N=Z 64Ge. The experimental results are in excellent agreement with large-scale Shell Model calculations applying the recently developed GXPF1A interactions. Theoretical analysis suggests that 64Ge is a collective gamma-soft anharmonic vibrator. The measurement was done using the Recoil Distance Method (RDM) and a unique combination of state-of-the-art instruments at the National Superconducting Cyclotron Laboratory (NSCL). States of interest were populated via an intermediate-energy single-neutron knock-out reaction. RDM studies of knock-out and fragmentation reaction products hold the promise of reaching far from stability and providing lifetime information for excited states in a wide range of nuclei.

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arXiv2006

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

Rare isotope beams of neutron-deficient $^{106,108,110}$Sn nuclei from the fragmentation of $^{124}$Xe were employed in an intermediate-energy Coulomb excitation experiment yielding $B(E2, 0^+_1 \to 2^+_1)$ transition strengths. The results indicate that these $B(E2,0^+_1 \to 2^+_1)$ values are much larger than predicted by current state-of-the-art shell model calculations. This discrepancy can be explained if protons from within the Z = 50 shell are contributing to the structure of low-energy excited states in this region. Such contributions imply a breaking of the doubly-magic $^{100}$Sn core in the light Sn isotopes.

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arXiv2005

Application of the Time of Flight Technique for Lifetime Measurements with Relativistic Beams of Heavy Nuclei

A novel method for picosecond lifetime measurements of excited gamma-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 first 2+ 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 gamma-ray decays from the 2+ excited state that occurred before or after traversing the degrader were measured at a different Doppler shift. The gamma-ray spectra were analyzed from the forward ring of the Segmented Germanium Array; this ring positioned at 37 deg. simultaneously provides the largest sensitivity to changes in velocity and the best energy resolution. The ratio of intensities in the peaks at different Doppler shifts gives information about the lifetime if the velocity is measured. The results and range of the application of the method are discussed.

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