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Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei, GRIFFIN, is a new high-efficiency γ -ray spectrometer designed for use in decay spectroscopy experiments with low-energy radioactive ion beams provided by TRIUMF’s Isotope Separator and Accelerator (ISAC-I) facility. GRIFFIN is composed of sixteen Compton-suppressed large-volume clover-type high-purity germanium (HPGe) γ -ray detectors combined with a suite of ancillary detection systems and coupled to a custom digital data acquisition system. The infrastructure and detectors of the spectrometer as well as the performance characteristics and the analysis techniques applied to the experimental data are described.

Three- and two-level mixing models are proposed to understand the doubling of states at the same spin and parity in triaxially-deformed atomic nuclei with odd numbers of protons and neutrons. The Particle-Rotor Model for such nuclei is solved using the newly proposed basis which couples angular momenta of two valence nucleons and the rotating triaxial mean-field into left-handed $|\mathcal{L}\rangle$, right-handed $|\mathcal{R}\rangle$, and planar $|\mathcal{P}\rangle$ configurations. The presence and the impact of the planar component is investigated as a function of the total spin for mass A$\approx$130 nuclei with the valence h$_{11/2}$ proton particle, valence h$_{11/2}$ neutron hole and the maximum difference between principle axes allowed by the quadrupole deformation of the mean field. It is concluded that at each spin value the higher-energy member of a doublet of states is built on the anti-symmetric combination of $|\mathcal{L}\rangle$ and $|\mathcal{R}\rangle$ and is free of the $|\mathcal{P}\rangle$ component, indicating that it is of pure chiral geometry. For the lower-energy member of the doublet, the contribution of the $|\mathcal{P}\rangle$ component to the eigenfunction first decreases and then increases as a function of the total spin. This trend as well as the energy splitting between the doublet states are both determined by the Hamiltonian matrix elements between the planar ($|\mathcal{P}\rangle$) and non-planar ($|\mathcal{L}\rangle$ and $|\mathcal{R}\rangle$) subspaces of the full Hilbert space., 12 pages, 5 figures, accepted as a Rapid Communication in Physical Review C

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 analog 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 in . × 3 in . NaI detector of 41(1)% at 1.3 MeV.

High precision measurements of the $ft$ values for superallowed Fermi $\beta$ transitions between $J^\pi~=~0^+$ isobaric analogue states allow for stringent tests of the electroweak interaction described by the Standard Model. These transitions provide an experimental probe of the Conserved-Vector-Current hypothesis, enable the most precise determination of the up-down ($V_{ud}$) element of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix, and allow one to set stringent limits on the existence of scalar currents in the weak interaction. An extensive program of superallowed branching-ratio and half-life measurements at TRIUMF's Isotope Separator and Accelerator (ISAC) facility has covered the full range of superallowed emitters, from the lightest case, $^{10}$C, to the heaviest case for which precision data are currently available, $^{74}$Rb. These experiments have been performed using a 4$\pi$ continuous-flow gas proportional $\beta$ counter, the 8$\pi$ $\gamma$-ray spectrometer, and, most recently, the new high-efficiency GRIFFIN $\gamma$-ray spectrometer. In this paper, recent highlights from the superallowed Fermi $\beta$ decay program at TRIUMF will be summarized, including high-precision half-life measurements for all three of the lightest superallowed emitters, $^{10}$C, $^{14}$O, and $^{18}$Ne, with the greatest sensitivity to a potential weak scalar current contribution, as well as high-precision branching-ratio measurements for the heavy superallowed emitters $^{62}$Ga and $^{74}$Rb.

A detailed, universal simulation tool for the Recoil Distance Method lifetime measurements at the National Superconducting Cyclotron Laboratory (NSCL) is presented. The measurements are made with the Koln-NSCL plunger-SeGA setup and utilize fast, secondary beams of exotic nuclei provided by the Coupled Cyclotron Facility and the A1900 fragment separator. The simulation tool, built upon GEANT4 and Root toolkits, incorporates all relevant aspects of a plunger experiment. The tool is applied in two ways. First, to aid in the design of new experiments by facilitating the choice of optimal experimental conditions. Second, to aid in off-line analysis of the experimental data and in particular to deduce the lifetime by comparison of experimental and simulated peak shapes in the γ -ray energy spectra. All the important features of the simulation tool are described, followed by examples of the application of the tool in the cases of real experiments completed or proposed at NSCL.

The 113Ru and 113Rh nuclei, produced in the spontaneous fission of 248Cm and in proton-induced fission of 238U , were studied by means of prompt gamma spectroscopy using the EUROGAM 2 array of Anti-Compton spectrometers and the IGISOL mass separator, respectively. For the 0.5s isomer, found in 113Ru in our recent study we propose the 7/2-[523] configuration. The present data indicate spin and parity 1/2+ for the ground state in 113Ru . This and some new data on the excited levels in 113Rh allows reinterpretation and a better understanding of the β- {} decay scheme of 113Ru . The properties of 113Ru were studied in the frame of the Core-Quasiparticle Coupling model, which has well reproduced the experimental data.

Quadrupole deformation parameters, | β 2 , ( p , p ′ ) | , have been deduced for 36,38,40Si from measured inelastic proton-scattering cross sections. Due to the strong Z = 14 subshell gap, low-lying quadrupole collectivity in these nuclei is attributed to the excitation of valence neutrons. Enhanced collectivity at N = 26 indicates a reduced N = 28 shell gap at large neutron excess in this chain of isotopes. Data are compared to large-scale shell-model calculations and prior Coulomb excitation measurements on 36,38Si.

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.

International audience; A program of high-precision half-life and branching-ratio measurements for superallowed Fermi emitters is being carried out at TRIUMF's Isotope Separator and Accelerator (ISAC) radioactive ion beam facility. Recent half-life measurements for the superallowed decays of 14O, 18Ne, and 26Alm, as well as branching-ratio measurements for 26Alm and 74Rb are reported. These results provide demanding tests of the Standard Model and the theoretical isospin symmetry breaking (ISB) corrections in superallowed Fermi decays.

A program of high-precision superallowed Fermi β decay studies is being carried out at the Isotope Separator and Accelerator (ISAC) radioactive ion beam facility at TRIUMF. Recent high-precision branching ratio measurements for the superallowed decays of 74Rb and 26Alm, as well as a half-life measurement for 26Alm that is the most precise half-life measurement for any superallowed emitter to date, are reported. These results provide demanding tests of the theoretical isospin symmetry breaking corrections in superallowed Fermi β decays.

Excited states have been identified in the very neutron-deficient $Z=58$ nucleus ${}^{123}$Ce. This is the most neutron-deficient odd-$A$ cerium isotope in which excited states have been identified. The states have been unambiguously assigned to ${}^{123}$Ce by detecting de-excitation $\ensuremath{\gamma}$ rays in coincidence with evaporated charged particles and neutrons. Three rotational bands, each consisting of at least eight $E2$ transitions, have been observed. The bands have tentatively been assigned to be based on neutrons in ${g}_{7/2}$ and ${h}_{11/2}$ orbitals. Two of the bands have been assigned to be signature partners, although no interband transitions have been observed. The aligned angular momenta of the bands have been studied in comparison with neighboring nuclei and with the results of Woods-Saxon cranking calculations. Observation of the deformation-dependent $\ensuremath{\pi}{({h}_{11/2})}^{2}$ quasiparticle alignment at 0.36 MeV/$\ensuremath{\hbar}$ in each of the bands suggests a quadrupole deformation of ${\ensuremath{\beta}}_{2}\ensuremath{\simeq}0.3$, in good agreement with theoretical predictions for the suggested possible configuration assignments.

Excited states in C-18 were populated by the one-proton knockout reaction of an intermediate energy radioactive N-19 beam. The lifetime of the first 2(+) state was measured with the Koln/NSCL plunger via the recoil distance method to be tau (2(1)(+)) = 22.4 +/- 0.9(stat)(-2.2)(+3.3)(syst) ps, which corresponds to a reduced quadrupole transition strength of B(E2; 2(1)(+) -> 0(1)(+)) = 3.64(-0.14)(+ 0.15)(stat)(-0.47)(+0.40)(syst) e(2)fm(4). In addition, an upper limit on the lifetime of a higher-lying state feeding the 2(1)(+) state was measured to be tau < 4.6 ps. The results are compared to large-scale ab initio no-core shell model calculations using two accurate nucleon-nucleon interactions and the importance-truncation scheme. The comparison provides strong evidence that the inclusion of three-body forces is needed to describe the low-lying excited-state properties of this A = 18 system.

The high-spin structure of [sup 139]Sm has been investigated with the [sup 110]Pd([sup 34]S,5[ital n]) reaction at a beam energy of 159 MeV at the Stony Brook Tandem/Superconducting LINAC facility. The TESSA3 spectrometer array was used to collect [gamma]-[gamma] coincidence data from which an extended decoupled band structure has been found. It is proposed that this band is based on the [ital i][sub 13/2] ([Omega]=1/2) neutron intruder orbital. The decay pattern, enhanced dynamic moments of inertia, and aligned spins are consistent with the [nu][ital i][sub 13/2] bands in neighboring odd-neutron nuclides. Total Routhian surface and cranked shell model calculations reproduce qualitative features of these nuclides, further supporting the proposed interpretation.

High-spin states have been studied in Pr-135(59), populated through the Cd-116(Na-23,4n) reaction at 115 MeV, using the Gammasphere gamma-ray spectrometer. The negative-parity yrast band has been significantly extended to spin similar to 45 (h) over bar and excitation energy 21.5 MeV, showing evidence for several rotational alignments. The positive-parity yrast band of Ce-135(58), populated through the p4n channel of this reaction, was also populated to spin similar to 38 (h) over bar and excitation energy 18 MeV. Cranking calculations indicate that these nuclei are soft with respect to the triaxiality parameter gamma and that several competing nuclear shapes occur at high spin.

The transition rates for the 2{sub 1}{sup +} states in {sup 62,64,66}Fe were studied using the recoil distance Doppler-shift technique applied to projectile Coulomb excitation reactions. The deduced E2 strengths illustrate the enhanced collectivity of the neutron-rich Fe isotopes up to N=40. The results are interpreted using the generalized concept of valence proton symmetry which describes the evolution of nuclear structure around N=40 as governed by the number of valence protons with respect to Z{approx_equal}30. The trend of collectivity suggested by the experimental data is described by state-of-the-art shell-model calculations with a new effective interaction developed for the fpgd valence space.

Probing shell structure at a large neutron excess has been of particular interest in recent times. Neutron‐rich nuclei between the proton shell closures Z = 20 and Z = 28 offer an exotic testing ground for shell evolution. The development of the N = 40 gap between neutron fp and lg9/2 shells gives rise to highly interesting variations of collectivity for nuclei in this region. While 68Ni shows doubly magic properties in level energies and transition strengths, this was not observed in neighbouring nuclei. Especially neutron‐rich Fe isotopes proved particularly resistant to calculational approaches using the canonical valence space (fpg) resulting in important deviations of the predicted collectivity. Only an inclusion of the d5/2‐orbital could solve the problem [1]. Hitherto no transition strengths for 66Fe have been reported. We determined B(E2,21+→01+) values from lifetimes measured with the recoil distance Doppler‐shift method using the Cologne plunger for radioactive beams at National Superconducting ...

The ``island of the inversion'' isotope $^{31}\mathrm{Mg}$ was investigated using an intermediate energy proton knockout reaction from $^{32}\mathrm{Al}$ at 90 MeV/nucleon at the National Superconducting Cyclotron Laboratory. A negligible cross section to the ground state supports an intruder-dominant configuration in $^{31}\mathrm{Mg}$. The partial cross sections to excited final states preferentially populate configurations in the $^{31}\mathrm{Mg}$ residues which have no neutron excitations across the $N=20$ shell closure. There is strong evidence that the reaction populates the $3/{2}^{+}$ and $5/{2}^{+}$ states which correspond with the neutron-hole states predicted by the shell model with the model space truncated above the $\mathit{sd}$-shell.

Absolute transition probabilities of the first 2+ state in 110,114Pd were remeasured using the recoil distance Doppler shift technique following projectile Coulomb excitation at intermediate beam energies for the first time. The 110Pd experiment served to check the novel technique as well as the method used for the data analysis which is based on the examination of γ‐ray lineshapes. Whereas the measured B(E2) value for 110Pd agrees very well with the literature, the value obtained for 114Pd 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.

Absolute transition probabilities for the first 2{sup +} states in {sup 110,114}Pd were measured using the recoil distance Doppler shift technique following projectile Coulomb excitation at intermediate beam energy. This technique was applied for the first time to projectiles produced in a primary fragmentation reaction, which get Coulomb excited in a secondary reaction. The {sup 110}Pd data was used to check the novel experimental technique as well as the data analysis procedure, which is based on the examination of {gamma}-ray line shapes. 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 new experimental data are compared to calculations in the framework of the Interacting Boson Model. The data are also used to test the novel concept of valence proton symmetry, which allows for extrapolation of nuclear properties to very neutron-rich nuclei.

High-spin states have been populated in Te-110(52) via Ni-58(Ni-58,alpha 2p gamma) reactions at 240 and 250 MeV. The Gammasphere gamma-ray spectrometer was used in conjunction with the Microball charged-particle detector. The high-spin (I>30) collective level scheme of Te-110, up to similar to 45h, is discussed in this paper. Four new decoupled (Delta I=2) high-spin structures have been observed for the first time, together with two strongly coupled (Delta I=1) bands. These bands all show the characteristics of smooth band termination, and are discussed within the framework of the cranked Nilsson-Strutinsky approach.

Transition rate measurements are reported for the ${2}_{1}^{+}$ and ${2}_{2}^{+}$ states in $N=Z$ $^{64}\mathrm{Ge}$. The experimental results are in excellent agreement with large-scale shell-model calculations applying the recently developed GXPF1A interactions. 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 knockout reaction. RDM studies of knockout 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.

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)

In applications with segmented high purity Ge detectors or other detector arrays with tens or hundreds of channels, where the high development cost and limited flexibility of application specific integrated circuits outweigh their benefits of low power and small size, the readout electronics typically consist of multi-channel data acquisition modules in a common chassis for power, clock and trigger distribution, and data readout. As arrays become larger and reach several hundred channels, the readout electronics have to be divided over several chassis, but still must maintain precise synchronization of clocks and trigger signals across all channels. This division becomes necessary not only because of limits given by the instrumentation standards on module size and chassis slot numbers, but also because data readout times increase when more modules share the same data bus and because power requirements approach the limits of readily available power supplies. In this paper, we present a method for distributing clocks and triggers between 4 PXI chassis containing DGF Pixie-16 modules with up to 226 acquisition channels per chassis in a data acquisition system intended to instrument the over 600 channels of the SeGA detector array at the National Superconducting Cyclotron Laboratory. Our solution is designed to achieve synchronous acquisition of detector waveforms from all channels with a jitter of less then 1 ns, and can be extended to a larger number of chassis if desired., Comment: CAARI 2006

The 8 pi spectrometer, installed at the TRIUMF-ISAC facility, was the world's most sensitive gamma-ray spectrometer dedicated to beta-decay studies. A description is given of the 8 pi spectrometer and its auxiliary detectors including the plastic scintillator array SCEPTAR used for beta-particle tagging and the Si(Li) array PACES for conversion electron measurements, its moving tape collector, and its data acquisition system. The recent investigation of the decay of Cs-124 to study the nuclear structure of Xe-124, and how the beta-decay measurements complemented previous Coulomb excitation studies, is highlighted, including the extraction of the deformation parameters for the excited 0(+) bands in Xe-124. As a by-product, the decay scheme of the (7(+)) Cs-124 isomeric state, for which the data from the PACES detectors were vital, was studied. Finally, a description of the new GRIFFIN spectrometer, which uses the same auxiliary detectors as the 8 pi spectrometer, is given.

High spin states in 126Ce and 127Pr were populated via heavy ion reactions with 182-MeV 35Cl projectiles on a thick 96Ru target. Prompt γ–γ coincidences were measured. New states in 126Ce and 127Pr were found. The results are discussed in the framework of the core–quasiparticle coupling model.

Excited states in $^{40}\mathrm{Si}$ have been established by detecting $\ensuremath{\gamma}$ rays coincident with inelastic scattering and nucleon removal reactions on a liquid hydrogen target. The low excitation energy, 986(5) keV, of the ${2}_{1}^{+}$ state provides evidence of a weakening in the $N=28$ shell closure in a neutron-rich nucleus devoid of deformation-driving proton collectivity.

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., Comment: 24 pages, 10 figures

Parity is a key observable in nuclear spectroscopy. Linear polarization measurements of $\gamma$-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 $\alpha$-$\gamma$ correlations from a \nuc{249}{Cf} source., Comment: 7 pages, 9 figures

The level structure of $^{197}\mathrm{Au}$ has been studied using the ($n,{n}^{'}\ensuremath{\gamma}$) reaction. A germanium detector array (GEANIE) for \ensuremath{\gamma}-ray detection and the ``white'' neutron source at LANSCE/WNR were used for the measurement. The energy of the incident neutrons was determined using the time-of-flight technique. Partial \ensuremath{\gamma}-ray cross sections were measured for a total of 90 transitions of $^{197}\mathrm{Au}$. A total of 52 new \ensuremath{\gamma} rays were assigned to $^{197}\mathrm{Au}$ and placed on the level scheme based on the partial \ensuremath{\gamma}-ray excitation functions and the $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence data. The level scheme of $^{197}\mathrm{Au}$ is now more complete up to ~2 MeV excitation energy. The first excited states of the decoupled band built on the proton-hole ${h}_{11/2}$ configuration were established.

High-spin states in the nucleus $^{102}\mathrm{Ru}$ have been investigated via the $^{96}\mathrm{Zr}$($^{13}\mathrm{C}$,\ensuremath{\alpha}3n) reaction at beam energies of 51 and 58 MeV, using the euroball IV \ensuremath{\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 ${E}_{x}~12$ MeV with ${I}^{\ensuremath{\pi}}=({26}^{+}$); the previously published negative-parity bands have been extended up to ${E}_{x}~11$ and \ensuremath{\sim} 9 MeV with ${I}^{\ensuremath{\pi}}=({23}^{\ensuremath{-}}$) and $({20}^{\ensuremath{-}})$, respectively. The deduced high-spin structure has been compared with Woods-Saxon total Routhian surface calculations and, on the basis of the measured Routhians, aligned angular momenta, and $B(M1)/B(E2)$ ratios, $\ensuremath{\nu}{h}_{11/2}({g}_{7/2},{d}_{5/2}$) configurations are suggested for the negative-parity structures.

Excited states of the nucleus $^{102}\mathrm{Ru}$ have been studied using the $^{96}\mathrm{Zr}$($^{10}\mathrm{B}$,$p3n$) reaction at a beam energy of $42\phantom{\rule{0.3em}{0ex}}\text{MeV}$. The emitted \ensuremath{\gamma} rays were detected with the Gammasphere spectrometer. The known positive-parity structure built on the second ${2}^{+}$ state was extended up to the ${10}^{+}$ and ${11}^{+}$ states. This structure has been interpreted as a quasi-\ensuremath{\gamma} band. Its energy staggering as a function of spin has been analyzed in order to derive information on the type of triaxiality present in $^{102}\mathrm{Ru}$, a nucleus that serves as one of the cores for chiral structures in the mass $A\ensuremath{\approx}100$ region.

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.

To address the question of collectivity in neutron‐rich nuclei near N=28, an inverse kinematics inelastic proton scattering experiment has been performed. Confirmation of excited states in 36,38Si has been made. The path from excitation cross‐section to deformation parameter is outlined.

The possible occurrence of sub‐shell gaps at N = 32 and N = 34 in neutron‐rich titanium isotopes is discussed in light of new experimental results from (i) deep‐inelastic reactions measured with Gammasphere at the ATLAS facility at Argonne National Laboratory and from (ii) intermediate‐energy Coulomb excitation performed at the National Superconducting Cyclotron Laboratory at Michigan State University.

Rotational bands have been studied to high spins in the neutron-deficient $^{124,125}\mathrm{Ce}$ nuclei. The nuclei were populated using the $^{64}\mathrm{Zn}(^{64}\mathrm{Zn},2p2n)^{124}\mathrm{Ce}$ and $^{64}\mathrm{Zn}(^{64}\mathrm{Zn},2pn)^{125}\mathrm{Ce}$ reactions, with a beam energy of $260\phantom{\rule{0.3em}{0ex}}\text{MeV}$. High-fold $\ensuremath{\gamma}$-ray coincidence data were collected using the Gammasphere germanium-detector array. The Microball charged-particle detector array was used to provide channel selection. The previously established level structures of both $^{124,125}\mathrm{Ce}$ have been extended to high spins $(\ensuremath{\sim}30\ensuremath{\hbar})$. In addition, several new bands have been identified. The alignments of pairs of ${h}_{11∕2}$ neutrons and protons are observed in both nuclei. The alignments are compared to the predictions of Woods-Saxon cranked shell model calculations and to the systematics of ${h}_{11∕2}$ quasiparticle alignments in neighboring nuclei. The apparent ability of the cranked shell model to explain the ${h}_{11∕2}$ neutron alignments in $^{124,125}\mathrm{Ce}$ highlights the previously reported discrepancies between experiment and theory for the $^{128}\mathrm{Ce}$ isotope and, to a lesser extent, $^{126}\mathrm{Ce}$.

Chirality or handedness is a property that has im- portant consequences in fields of science as diverse as biology, chemistry, and physics. In nuclear physics, the coupling of three mutually perpendicular angular momenta induces structure effects due to chirality (1). Interesting chiral properties were observed for the �h 11=2 �h 11=2 configuration of proton (� ) and neutron (� ) single particle levels in triaxial odd-odd nuclei in the A 130 mass region of the chart of atomic nuclei. It is important to show that these chiral symmetry properties are of a general nature and not related only to a specific nuclear mass region. The purpose of this study is to investigate a different region of triaxial nuclei, which necessarily involves a different configuration, to examine the general aspects of chirality in nuclei. The best chiral properties observed to date were discovered in the 104 Rh nucleus involving theg 9=2 �h 11=2 configuration where the valence proton and neutron play opposite roles to those in the A 130 region. Doublet rotational bands related to nuclear chirality were observed in odd-odd nuclei having triaxial shapes. Nuclear chirality results when the angular momenta of the valence proton, the valence neutron, and the core rotation tend to be mutually perpendicular. This occurs when high-j particlelike and holelike orbitals align their angular momenta along the short and long axes of nuclear deformation, respectively, minimizing the interaction energy, and the core-rotation angular momentum is ori- ented along the intermediate axis because it has the largest (irrotational flow) moment of inertia. The result- ing aplanar total angular momentum can be arranged into a left- or a right-handed system, which differs by intrinsic chirality; the two systems are related by the chiral operator, a combination of time reversal and rota- tion by 180. When chiral symmetry is thus broken in the intrinsic frame, the necessary restoration of the symme- try in the laboratory frame manifests itself as degenerate doubletI 1 bands from the doubling of states. The merged states combine the left- and right-handed systems in a way that satisfies the laboratory chiral symmetry requirement. Effects of chirality for odd-odd nuclei were first ob- served in the A 130 region where triaxial deformations were expected. Two near degenerate bands in 134 Pr (2) were observed; microscopic calculations carried out us- ing 3D tilted axis cranking resulted in triaxial deforma- tions and chiral solutions over an extended frequency (spin) range for theh 11=2 �h 11=2 configuration (3). Subsequently, experiments on N 75 isotones (4) of 134 Pr and neighboring isotones (5) revealed near degen- erate partner bands of theh 11=2 �h 11=2 yrast bands, suggesting an island of chirality near A 130. Since the 134 Pr nucleus showed the smallest band separation, a GAMMASPHERE experiment (6) was performed which considerably extended the doublet bands showing the levelsE 50 keV apart at spin I 15 �

Evidence for chirality in nuclei has been found in N ∼ 75 isotopes in the A ∼ 130 mass region. This phenomenon is a signature of triaxiality in nuclei and there is a clear need to study other regions of the Segre chart to see if further examples can be found. Potential Energy Surface (PES) calculations suggest that the N ∼ 57–63, Z ∼ 43–45 region is another promising island of triaxiality. The present experimental study with the Euroball γ‐ray array, using the reaction 96Zr(13C, p2n) at 51 MeV is aimed to search for chirality in 106Rh. Channel selection of 106Rh was done using the charged‐particle array DIAMANT. The yrast level scheme, studied previously in fusion‐fission work, has been confirmed and extended in the present study. In addition, a new strongly coupled band, lying at an excitation energy of ∼ 300 keV above the yrast band, has been found. The two structures show the characteristic properties of the chiral phenomenon.

High-spin states have been studied in neutron-deficient {sup 124}{sub 57}La{sub 67}, populated through the {sup 64}Zn({sup 64}Zn,3pn) reaction at 260 MeV. The Gammasphere {gamma}-ray spectrometer has been used in conjunction with the Microball charged-particle detector, the Neutron Shell, and the Argonne Fragment Mass Analyzer, in order to select evaporation residues of interest. The known band structures have been extended and new bands found. Most of the bands are linked together, allowing more consistent spin and parity assignments. Comparison of band properties to cranking calculations has allowed configuration assignments to be made and includes the first identification of the g{sub 9/2} proton-hole in an odd-odd lanthanum isotope. Two bands have been assigned a {pi}h{sub 11/2}{circle_times}{nu}h{sub 11/2} structure; the yrast one exhibits a signature inversion in its level energies below I=18.5{Dirac_h}, while the excited one exhibits a signature inversion above I=18.5{Dirac_h}.

High-spin states have been studied in neutron-deficient Xe-114(54), populated through the Ni-58(Ni-58,2p) fusion-evaporation reaction at 230 MeV. The Gammasphere gamma-ray spectrometer has been used in conjunction with the Microball charged-particle detector in order to select evaporation residues of interest. The yrast band has been greatly extended to a tentative spin of 52 (h) over bar and shows features consistent with smooth band termination. This band represents the first evidence for a core-excited (six-particle, two-hole) proton configuration above Z = 53.

Chirality in triaxial odd-odd nuclei is discussed as a novel form of spontaneous symmetry breaking in nuclear structure physics. It is shown that the orthogonal coupling of angular momenta for the valence proton, valence neutron and the core rotation, which violates time reversal, results, for specific configurations, from the triaxial shape and irrotational flow moment of inertia. The model calculations are investigated as a study of the characteristics of chirality. Recent progress from experimental studies of the πh11/2νh11/2 partner bands in triaxial A∼130 nuclei is reviewed.Chirality in triaxial odd-odd nuclei is discussed as a novel form of spontaneous symmetry breaking in nuclear structure physics. It is shown that the orthogonal coupling of angular momenta for the valence proton, valence neutron and the core rotation, which violates time reversal, results, for specific configurations, from the triaxial shape and irrotational flow moment of inertia. The model calculations are investigated as a study of the characteristics of chirality. Recent progress from experimental studies of the πh11/2νh11/2 partner bands in triaxial A∼130 nuclei is reviewed.

Excited states in ${}^{126}\mathrm{Ce}$ have been observed with the GAMMASPHERE \ensuremath{\gamma}-ray detector array, used in conjunction with the MICROBALL charged-particle detector. The reaction ${}^{64}\mathrm{Zn}{(}^{64}\mathrm{Zn},\mathrm{xpn}\ensuremath{\alpha})$ (beam energy 260 MeV) was used to populate a wide range of nuclei in the neutron-deficient region with $A\ensuremath{\approx}120.$ ${}^{126}\mathrm{Ce}$ was populated via the $2p$ evaporation channel. The three previously observed bands have been extended to higher spins and some other structures have been identified. The yrast band shows evidence of a delayed neutron alignment occurring at a rotational frequency \ensuremath{\omega}\ensuremath{\approx}0.5 MeV/\ensuremath{\Elzxh} as observed in the neighboring odd-A nucleus ${}^{127}\mathrm{Pr}.$ One of the two excited bands shows evidence for a similar crossing at a slightly lower frequency and also exhibits a sudden gain in alignment at \ensuremath{\omega}=0.57 MeV/\ensuremath{\Elzxh}. The third band may involve the coupling of a \ensuremath{\gamma}-vibrational state. All three rotational bands are discussed in terms of standard and extended cranked shell model calculations.

A level scheme is presented for ${}_{53}^{111}\mathrm{I},$ populated by the ${}^{58}\mathrm{Ni}{(}^{58}\mathrm{Ni},\ensuremath{\alpha}p)$ reaction. A backed-target experiment, at a low beam energy of 210 MeV, was performed using the JUROSPHERE spectrometer, while a thin-target experiment at 250 MeV was performed using the GAMMASPHERE spectrometer in conjunction with the MICROBALL charged-particle detector array. The new level scheme fits well the systematics of light iodine nuclei and provides evidence for a terminating band at the highest spins. Low-lying transitions previously assigned to ${}^{111}\mathrm{I}$ could not be confirmed.

The properties of exotic neutron-rich nuclei between the proton shell closures Z = 20 and Z = 28 are of particular interest for the understanding of the shell structure for large neutron excess. Effects related to the energy gap between the neutron fp and 1g9/2 shells lead to a strong variation of collectivity for nuclei around N = 40. Whereas 68Ni was found to have doubly magic properties, this was not observed in neighbouring nuclei. Recent shell model calculations for the neutron rich iron isotopes clearly reveal the difficulty to describe nuclei in this mass region and resulted in large deviations of the predicted collectivity depending on the valence space. However, no experimental data on the transition strength existed for the very exotic nucleus 66Fe at N = 40. Here we present the newest results on absolute transition strengths of the lowest excited states in 62,64,66Fe measured model independently using the recoil distance Doppler-shift (RDDS) method. The experiments were performed at NSCL at Michigan State University with the Cologne/NSCL plunger device using Coulomb excitation in inverse kinematics at energies of 80 MeV/u. Our results yield a much higher collectivity for 64,66Fe than expected and allow tests of new calculations.

Absolute transition probabilities are fundamental observables for nuclear structure. The recoil-distance-Doppler-shift (RDDS) technique, also called plunger technique, is a well established tool for the determination of these important experimental quantities via the measurement of lifetimes of excited nuclear states. Nowadays nuclear structure investigations are concentrated on exotic nuclei which are often produced with extremely small cross sections or with very low beam intensities. In order to use the RDDS technique also for the investigation of very exotic nuclei this method has to be adapted to the specific needs of these special reactions. This article gives an overview on recent RDDS measurements with the new differential plunger in combination with particle detectors and recoil spectrometers. These were done with projectile multistep Coulomb excitation at low beam energies (≈ 5 MeV/u) and at intermediate beam energies (≈ 100 MeV/u) using one step Coulomb excitations and knockout reactions.

International audience; 136 P. Joshi et al. / Physics Letters B 595 (2004) 135–142AbstractThe nucleus 106Rh was populated using the reaction 96Zr(13C, p2n) at a beam energy of 51 MeV. γ -ray transitions wereidentified using the EUROBALL-IV γ -ray spectrometer and the DIAMANT charged particle array. The yrast band, which isbased upon a πg−19/2 ⊗ νh11/2 configuration, has been extended to I π = (22−). A new I = 1 band has been identified whichresides ∼ 300 keV above the yrast band. Core–quasiparticle coupling model calculations show reasonably good agreement withthe data. The properties of the two pairs of strongly coupled bands are consistent with a chiral interpretation for these states. 2004 Elsevier B.V

The systematics of smoothly terminating rotational bands based on proton 2p-2h excitations in the A=110 mass region are presented. Terminating bands (or nearly so) based on this proton excitation have been found in nuclei ranging from 107In, up to 114Te, and possibly extending to 54Xe nuclei. The impressive agreement between experimental data and theoretical calculations is also presented. However, recently discovered structures based on proton 1p–1h excitations begin to show disagreement with theoretical calculations. These new bands are also discussed. The current and future directions of research into smooth band termination will be presented.

Odd-odd nuclei treated as core-particle-hole systems with various collective cores and various particle-hole configurations are investigated within the Core-Particle-Hole Coupling (CPHC) model. A new symmetry, called the S-symmetry, is identified as a combination of the α-parity of the collective core and the proton-neutron symmetry of the valence proton and neutron in particle-hole configurations involving single-particle states with the same quantum numbers. It is found that the S-symmetric odd-odd nuclei show signatures which are usually considered as fingerprints of nuclear chirality, namely doublet band structure with a particular pattern of electromagnetic transitions. Reported results imply that the rigid rotor with a symmetric valence proton-neutron configuration is only a special case of the system with the novel S-symmetry. Therefore, it is an open question whether the chiral fingerprints discussed so far identify uniquely the orthogonal coupling of angular momentum in the intrinsic system.