Showing papers by "R. Wadsworth published in 2014"

Evolution of Collectivity in Kr 72 : Evidence for Rapid Shape Transition

Abstract: The transition rates from the yrast ${2}^{+}$ and ${4}^{+}$ states in the self-conjugate $^{72}\mathrm{Kr}$ nucleus were studied via lifetime measurements employing the GRETINA array with a novel application of the recoil-distance method. The large collectivity observed for the ${4}^{+}\ensuremath{\rightarrow}{2}^{+}$ transition suggests a prolate character of the excited states. The reduced collectivity previously reported for the ${2}^{+}\ensuremath{\rightarrow}{0}^{+}$ transition was confirmed. The irregular behavior of collectivity points to the occurrence of a rapid oblate-prolate shape transition in $^{72}\mathrm{Kr}$, providing stringent tests for advanced theories to describe the shape coexistence and its evolution.

Test of digital neutron–gamma discrimination with four different photomultiplier tubes for the NEutron Detector Array (NEDA)

Abstract: A comparative study of the neutron–γ discrimination performance of a liquid scintillator detector BC501A coupled to four different 5 in. photomultiplier tubes (ET9390kb, R11833-100, XP4512 and R4144) was carried out. Both the Charge Comparison method and the Integrated Rise-Time method were implemented digitally to discriminate between neutrons and γ rays emitted by a 252 Cf source. In both methods, the neutron–γ discrimination capabilities of the four photomultiplier tubes were quantitatively compared by evaluating their figure-of-merit values at different energy regions between 50 keVee and 1000 keVee. Additionally, the results were further verified qualitatively using time-of-flight to distinguish γ rays and neutrons. The results consistently show that photomultiplier tubes R11833-100 and ET9390kb generally perform best regarding neutron–γ discrimination with only slight differences in figure-of-merit values. This superiority can be explained by their relatively higher photoelectron yield, which indicates that a scintillator detector coupled to a photomultiplier tube with higher photoelectron yield tends to result in better neutron–γ discrimination performance. The results of this work will provide reference for the choice of photomultiplier tubes for future neutron detector arrays like NEDA. & 2014 Elsevier B.V. All rights reserved.

Spectroscopy on the proton drip-line : Probing the structure dependence of isospin nonconserving interactions

Abstract: The recoil-β tagging technique was used to identify transitions associated with the decay of the 2 + and, tentatively, the 4 + excited states in 74 Sr. Combining these results with published data for the A = 74 isobars, triplet energy differences (TEDs) have been extracted, the heaviest case for which these values have been evaluated. State-of-the-art shell-model calculations using the JUN45 interaction and incorporating a J = 0 isospin nonconserving (INC) interaction with an isotensor strength of 100 keV can reproduce the trend in the TED data, with particularly good agreement for the 2 + state. This agreement for the TED data taken together with the fact that agreement has also been shown between shell-model calculations with the same strength of INC interaction in the f7/2 shell and recently for A = 66 strongly suggests that such an interaction exists throughout the nuclear chart and cannot have a strong dependence on details of nuclear structure such as which nuclear orbitals are occupied. It also supports the hypothesis that only a J = 0 component of the INC interaction need be included to explain the observed TEDs.

Character of particle-hole excitations in 94 Ru deduced from γ-ray angular correlation and linear polarization measurements

Abstract: Linear polarization and angular correlations of gamma-rays depopulating excited states in the neutron-deficient nucleus Ru-94(44)50 have been measured, enabling firm spin-parity assignments for several excited states in this nucleus. The deduced multipolarities of strong transitions in the yrast structure were found to be mostly of stretched M1, E1, and E2 types and, in most cases, in agreement with previous tentative assignments. The deduced multipolarity of the 1869 keV and the connecting 257 and 1641 keV transitions indicates that the state at 6358 keV excitation energy has spin parity 12(1)(-) rather than 12(3)(+) as proposed in previous works. The presence of a 12(1)(-) state is interpreted within the framework of large-scale shell-model calculations as a pure proton-hole state dominated by the pi(p(1/2)(-1)circle times g(9/2)(-5)) and pi(p(3/2)(-1) g(9/2)(-5)) configurations. A new positive-parity state is observed at 6103 keV and is tentatively assigned as 12(2)(+). The 14(1)(-) state proposed earlier is reassigned as 13(4)(-) and is interpreted as being dominated by neutron particle-hole core excitations. The strengths of several E1 transitions have been measured and are found to provide a signature of core-excited configurations.

Hindered Gamow-Teller Decay to the Odd-Odd N = Z Ga-62: Absence of Proton-Neutron T=0 Condensate in A=62

Abstract: Search for a new kind of superfluidity built on collective proton-neutron pairs with aligned spin is performed studying the Gamow-Teller decay of the T=1, Jπ=0+ ground state of Ge62 into excited states of the odd-odd N=Z nucleus Ga62. The experiment is performed at GSI Helmholtzzentrum fur Schwerionenforschung with the Ge62 ions selected by the fragment separator and implanted in a stack of Si-strip detectors, surrounded by the RISING Ge array. A half-life of T1/2=82.9(14) ms is measured for the Ge62 ground state. Six excited states of Ga62, populated below 2.5 MeV through Gamow-Teller transitions, are identified. Individual Gamow-Teller transition strengths agree well with theoretical predictions of the interacting shell model and the quasiparticle random phase approximation. The absence of any sizable low-lying Gamow-Teller strength in the reported beta-decay experiment supports the hypothesis of a negligible role of coherent T=0 proton-neutron correlations in Ga62. (Less)

A new front-end high-resolution sampling board for the new-generation electronics of EXOGAM2 and NEDA detectors

Abstract: This paper presents the final design and results of the FADC Mezzanine for the EXOGAM (EXOtic GAMma array spectrometer) and NEDA (Neutron Detector Array) detectors. The measurements performed include those of studying the effective number of bits, the energy resolution using HP-Ge detectors, as well as timing histograms and ${\rm n}/\gamma $ discrimination performance. Finally, the conclusion shows how a common digitizing device has been integrated in the experimental environment of two very different detectors which combine both low-noise acquisition and fast sampling rates. Not only the integration fulfilled the expected specifications on both systems, but it also showed how a study of synergy between detectors could lead to the reduction of resources and time by applying a common strategy.

A digital front-end electronics for the neutron detector NEDA

Abstract: This paper presents the design of the NEDA (Neutron Detector Array) electronics, a first attempt to involve the use of digital electronics in large neutron detector arrays. Starting from the front-end modules attached to the PMTs (PhotoMultiplier Tubes) and ending up with the data processing workstations, a comprehensive electronic system capable of dealing with the acquisition and pre-processing of the neutron array is detailed. Among the electronic modules required, we emphasize the front-end analog processing, the digitalization, digital pre-processing and communications firmware, as well as the integration of the GTS (Global Trigger and Synchronization) system, already used successfully in AGATA (Advanced Gamma Tracking Array). The NEDA array will be available for measurements in 2016.

Spectroscopy of the neutron-deficient N=50 nucleus Rh 95

Abstract: The neutron-deficient semimagic (neutron number N = 50) Rh-95 nucleus has been produced at high spins using the projectile-target system Ca-40 + Ni-58 at 125 MeV beam energy. The gamma-decays of le ...

Lifetime measurements of the yrast 8 + and 9 + states in As 70

Abstract: The lifetimes of the yrast ${8}^{+}$ and ${9}^{+}$ states of $^{70}\mathrm{As}$ have been measured via the $\ensuremath{\gamma}$-ray lineshape method following population by the ${}^{9}\mathrm{Be}\phantom{\rule{0.16em}{0ex}}{(}^{78}\mathrm{Rb},{}^{70}\mathrm{As})$ reaction at 101.6 MeV/nucleon. The strength of the $E1\phantom{\rule{0.28em}{0ex}}{8}^{+}\ensuremath{\rightarrow}{7}^{\ensuremath{-}}$ transition is found to be $B(E1)=1.3(5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5} {e}^{2}\phantom{\rule{0.16em}{0ex}}{\text{fm}}^{2}$ or $1.2(4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ Weisskopf units (W.u.) while the ${9}^{+}\ensuremath{\rightarrow}{8}^{+} M1$ transition is found to have a strength of $B(M1)=1.5(8) {\ensuremath{\mu}}_{N}^{2}$ or $0.85(42)$ W.u. The implications for the structure of these states is discussed and found to be consistent with an assignment to a $\ensuremath{\pi}{g}_{9/2}\ensuremath{ u}{g}_{9/2}$ configuration.

Analysis and results of the 104Sn Coulomb excitation experiment

Abstract: The analysis of the Coulomb excitation experiment conducted on 104Sn required a strict selection of the data in order to reduce the large background present in the γ-ray energy spectra and identify the γ-ray peak corresponding to the Coulomb excitation events. As a result the B(E2; 0+→ 2+) value could be extracted, which established the downward trend towards 100Sn and therefore the robustness of the N=Z=50 core against quadrupole excitations.