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7 pags., 4 figs., Situated in the so-called "island of inversion," the nucleus $^{32}$Mg is considered as an archetypal example of the disappearance of magicity at $N=20$. We report on high statistics in-beam spectroscopy of $^{32}$Mg with a unique approach, in that two direct reaction probes with different sensitivities to the underlying nuclear structure are employed at the same time. More specifically, states in $^{32}$Mg were populated by knockout reactions starting from $^{33}$Mg and $^{34}$Si, lying inside and outside the island of inversion, respectively. The momentum distributions of the reaction residues and the cross sections leading to the individual final states were confronted with eikonal-based reaction calculations, yielding a significantly updated level scheme for $^{32}$Mg and spin-parity assignments. By fully exploiting observables obtained in this measurement, a variety of structures coexisting in 32Mg was unraveled. Comparisons with theoretical predictions based on shell-model overlaps allowed for clear discrimination between different structural models, revealing that the complete theoretical description of this key nucleus is yet to be achieved., N.K. acknowledges support of the Grant-in-Aid for JSPS Fellows (18J12542) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. K.W. acknowledges support from the Ministerio de Ciencia e Innovación (Spain) through the Ramón y Cajal program RYC-2017-22007. A.P. is supported in part by the Ministerio de Ciencia, Innovación y Universidades (Spain), Severo Ochoa program SEV-2016-0597 and grant PGC-2018-94583. The SDPF-M calculations were enabled by the CNS-RIKEN joint project for large-scale nuclear structure calculations and were performed mainly on the Oakforest-PACS supercomputer. N.S. acknowledges support from “Priority Issue on post-K computer” (hp190160) and “Program for Promoting Researches on the Supercomputer Fugaku” (hp200130) by JICFuS and MEXT, Japan. J.A.T. acknowledges support from the U.K. Science and Technology Facilities Council Grant No. ST/L005743/1. This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Nuclear Physics, under Grant No. DE-SC0020451 and by the U.S. National Science Foundation (NSF) under Grant No. PHY-1306297. GRETINA was funded by the U.S. DOE, Office of Science. Operation of the array at NSCL was supported by the U.S. NSF under Cooperative Agreement No. PHY-1102511 (NSCL) and DOE under Grant No. DE-AC02-05CH11231 (LBNL).

Background: In the "island of inversion", ground states of neutron-rich $sd$-shell nuclei exhibit strong admixtures of intruder configurations from the $fp$ shell. The nucleus $^{30}$Mg, located at the boundary of the island of inversion, serves as a cornerstone to track the structural evolution as one approaches this region. Purpose: Spin-parity assignments for excited states in $^{30}$Mg, especially negative-parity levels, have yet to be established. In the present work, the nuclear structure of $^{30}$Mg was investigated by in-beam $\gamma$-ray spectroscopy mainly focusing on firm spin-parity determinations. Method: High-intensity rare-isotope beams of $^{31}$Mg, $^{32}$Mg, $^{34}$Si, and $^{35}$P bombarded a Be target to induce nucleon removal reactions populating states in $^{30}$Mg. $\gamma$ rays were detected by the state-of-the-art $\gamma$-ray tracking array GRETINA. For the direct one-neutron removal reaction, final-state exclusive cross sections and parallel momentum distributions were deduced. Multi-nucleon removal reactions from different projectiles were exploited to gain complementary information. Results: With the aid of the parallel momentum distributions, an updated level scheme with revised spin-parity assignments was constructed. Spectroscopic factors associated with each state were also deduced. Conclusions: Results were confronted with large-scale shell-model calculations using two different effective interactions, showing excellent agreement with the present level scheme. However, a marked difference in the spectroscopic factors indicates that the full delineation of the transition into the island of inversion remains a challenge for theoretical models.

Self-conjugate nuclei in the A ≈ 70 – 80 region have attracted a great deal of attention due to phenomena such as shape coexistence and increasing collectivity along the N = Z line. We investigate the structure of nuclei in this region through lifetime measurements using the GRETINA array. The first implementation of the Differential Recoil Distance Doppler Shift technique with fast radioactive beams is demonstrated and verified through a measurement of the well-known B ( E 2 ; 2 1 + → 0 1 + ) transition strength in 74Kr. The method is then applied to determine the B ( E 2 ; 2 1 + → 0 1 + ) transition strength in 74Rb, the heaviest odd–odd N = Z nucleus for which this quantity has been determined. This result and extended systematics along N = Z suggest the dominance of the isoscalar part of the quadrupole transition strengths in self-conjugate nuclei, as well as the possible presence of shape coexistence in 74Rb.

International audience; Neutron-rich isotopes are an important source of new information on nuclear physics. Specifically, the spin-isospin components in the nucleon-nucleon (NN) interaction, e.g., the proton-neutron tensor force, are expected to modify shell structure in exotic nuclei. These potential changes in the intrinsic shell structure are of fundamental interest. The study of the excitation energy of states corresponding to specific configurations in even-even isotopes, together with the single-particle character of the first excited states of odd-A, neutron-rich Ni isotopes, probes the evolution of the neutron orbitals around the Fermi surface as a function of the neutron number a step forward in the understanding of the region and the nature of the NN interaction at large N/Z ratios. In an experiment carried out at the National Superconducting Cyclotron Laboratory [1], new spectroscopic information was obtained for 68Ni and the distribution of single-particle strengths in 67,69Ni was characterized by means of single-neutron knockout from 68,70Ni secondary beams. The spectroscopic strengths, deduced from the measured partial cross sections to the individual states tagged by their de-exciting gamma rays, is used to identify and quantify configurations that involve neutron excitations across the N = 40 harmonic oscillator shell closure. The de-excitation γ rays were measured with the GRETINA tracking array [2]. The results challenge the validity of the most current shell-model Hamiltonians and effective interactions, highlighting shortcomings that cannot yet be explained. These results suggest that our understanding of the low-energy states in such nuclei is not complete and requires further investigation.

The low-energy excitation level schemes of the neutron-rich $^{38--42}\mathrm{S}$ isotopes are investigated via in-beam $\ensuremath{\gamma}$-ray spectroscopy following the fragmentation of $^{48}\mathrm{Ca}$ and $^{46}\mathrm{Ar}$ projectiles on a $^{12}\mathrm{C}$ target at intermediate beam energies. Information on $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidences complemented by comparisons to shell-model calculations were used to construct level schemes for these neutron-rich nuclei. The experimental data are discussed in the context of large-scale shell-model calculations with the SDPF-MU effective interaction in the $sd\text{\ensuremath{-}}pf$ shell. For the even-mass S isotopes, the evolution of the yrast sequence is explored as well as a peculiar change in decay pattern of the second ${2}^{+}$ states at $N=26$. For the odd-mass $^{41}\mathrm{S}$, a level scheme is presented that seems complete below 2.2 MeV and consistent with the predictions by the SDPF-MU shell-model Hamiltonian; this is a remarkable benchmark given the rapid shell and shape evolution at play in the S isotopes as the broken-down $N=28$ magic number is approached. Furthermore, the population of excited final states in projectile fragmentation is discussed.

The distribution of single-particle strength in 67,69Ni was characterized with one-neutron knockout reactions from intermediate-energy 68,70Ni secondary beams, selectively populating neutron-hole configurations at N = 39 and 41, respectively. The spectroscopic strengths deduced from the measured partial cross sections to the individual final states, as tagged by their γ-ray decays, are used to identify and quantify neutron configurations in the wave functions. While 69Ni compares well to shell-model predictions, the results for 67Ni challenge the validity of current effective shell-model Hamiltonians by revealing discrepancies that cannot be explained so far. These results suggest that our understanding of the low-lying states in the neutron-rich, semi-magic Ni isotopes may be incomplete and requires further investigation on both the experimental and theoretical sides.

The quadrupole transition rate for the 41+→21+ transition of Ni58 was determined from an application of the recoil distance method with the Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA). The present result of the B(E2;41+→21+) was found to be 50-6+11e2fm4, which is about three times smaller than the literature value, indicating substantially less collectivity than previously believed. Shell model calculations performed with the GXPF1A effective interaction agree with the present data and the validity of the standard effective charges in understanding collectivity in the nickel isotopes is discussed.

International audience; One-neutron knockout reactions have been performed on a beam of radioactive 53Co in a high-spinisomeric state. The analysis is shown to yield a highly selective population of high-spin states in an exoticnucleus with a significant cross section, and hence represents a technique that is applicable to the plannednew generation of fragmentation-based radioactive beam facilities. Additionally, the relative cross sectionsamong the excited states can be predicted to a high level of accuracy when reliable shell-model input isavailable. The work has resulted in a new level scheme, up to the 11þ band-termination state, of the protonrichnucleus 52Co (Z ¼ 27, N ¼ 25). This has in turn enabled a study of mirror energy differences inthe A ¼ 52 odd-odd mirror nuclei, interpreted in terms of isospin-nonconserving (INC) forces in nuclei.The analysis demonstrates the importance of using a full set of J-dependent INC terms to explain theexperimental observations.

Comprehensive spectroscopy of the N = 29 nucleus 47Ar is presented, based on two complemen-tary direct reaction mechanisms: one-neutron pickup onto 46Ar projectiles and one-proton removal from the 1− ground state of 48K. The results are compared to shell-model calculations that use the state-of-the-art SDPF-U and SDPF-MU effective interactions. Also, from the 9Be(46Cl,45S+γ)X one-proton removal reaction, we report the first γ-ray transitions observed from 45S. Using compar-isons with shell-model calculations, and from the observed intensities and energy sums, we propose a first tentative level scheme for 45S.

The highly selective, intermediate-energy heavy-ion-induced neutron-pickup reaction, in combination with γ-ray spectroscopy using the γ-ray energy-tracking in-beam nuclear array (GRETINA), is shown to provide reliable relative spectroscopic strengths for high-l orbitals in nuclei more neutron rich than the projectile. The reaction mechanism gives a significant final-state-spin alignment that is validated through γ-ray angular-distribution measurements enabled by the position sensitivity of GRETINA. This is the first time that γ-ray angular distributions could be extracted from a high-luminosity, fast-beam reaction other than inelastic scattering. This holds great promise for the restriction and assignment of Jπ quantum numbers in exotic nuclei. We advance this approach to study the crucial N=28 shell closure and extract the ratio g9/2:f5/2 of bound neutron single-particle strengths in Ca49, a benchmark for emerging multi-shell ab initio and configuration-interaction theories that are applicable along the Ca isotopic chain.

We report results from an experiment on the decay of the high-K isomers in 254No. We have been able to establish the decay from the known high-lying four-quasiparticle isomer, which we assign as a K π = 16 + state at an excitation energy of E x = 2.928 ( 3 ) MeV . The decay of this state passes through a rotational band based on a previously unobserved state at E x = 2.012 ( 2 ) MeV , which we suggest is based on a two-quasineutron configuration with K π = 10 + . This state in turn decays to a rotational band based on the known K π = 8 − isomer, which we infer must also have a two quasineutron configuration. We are able to assign many new gamma-rays associated with the decay of the K π = 8 − isomer, including the identification of a highly K-forbidden Δ K = 8 E1 transition to the ground-state band. These results provide valuable new information on the orbitals close to the Fermi surface, pairing correlations, deformation and rotational response, and K-conservation in nuclei of the deformed trans-fermium region.

Background: The study of excited states in mirror nuclei allows us to extract information on charge-dependent (i.e., isospin-nonconserving) interactions in nuclei. Purpose: To extend previous studies of mirror nuclei in the f7/2 region, investigating charge symmetry breaking of the strong nuclear force. Methods: γ-ray spectroscopy has been performed for the mirror (Tz= ±3/2) pair 53Ni and 53Mn, produced via mirrored one-nucleon knockout reactions. Results: Several new transitions have been identified in 53Ni from which a new level scheme has been constructed. Cross sections for knockout have been analyzed and compared with reaction model calculations where evidence is found for knockout from high-spin isomeric states. Mirror energy differences between isobaric analog states have been computed, compared to large scale shell-model calculations, and interpreted in terms of isospin-nonconserving effects. In addition, lifetimes for the long-lived J π = 5 /2 − 1 analog states in both 53Mn and 53Ni have been extracted through lineshape analysis, giving half-lives of t1/2 = 120 (14) ps and t1/2 = 198 (12) ps,respectively. Conclusions: The inclusion of a set of isovector isospin-nonconserving matrix elements to the shell-model calculations gave the best agreement with the experimental data.

The neutron-deficient nucleus Sn 107 has been studied by using the one-neutron knockout reaction. By measuring the decay γ rays and momentum distributions of reaction residues, the spins of the ground, 5/2 + , and first-excited, 7/2 + , states of Sn 107 have been assigned by comparisons to eikonal-model reaction calculations. Limits on the inclusive and exclusive cross sections have been measured and transitions due to neutron removals from below the N=50 closed shell have been observed. New excited states up to 5.5 MeV in Sn 107 have been identified.

Author(s): Smalley, D; Iwasaki, H; Navratil, P; Roth, R; Langhammer, J; Bader, VM; Bazin, D; Berryman, JS; Campbell, CM; Dohet-Eraly, J; Fallon, P; Gade, A; Langer, C; Lemasson, A; Loelius, C; Macchiavelli, AO; Morse, C; Parker, J; Quaglioni, S; Recchia, F; Stroberg, SR; Weisshaar, D; Whitmore, K; Wimmer, K | Abstract: We studied transition rates for the lowest 1/2+ and 5/2+ excited states of C17 through lifetime measurements with the GRETINA array using the recoil-distance method. The present measurements provide a model-independent determination of transition strengths giving the values of B(M1;1/2+→3/2g.s.+)=1.04-0.12+0.03×10-2μN2 and B(M1;5/2+→3/2g.s.+)=7.12-0.96+1.27×10-2μN2. The quenched M1 transition strength for the 1/2+→3/2g.s.+ transition, with respect to the 5/2+→3/2g.s.+ transition, has been confirmed with greater precision. The current data are compared to importance-truncated no-core shell model calculations addressing effects due to continuum and three-body forces.

Background: In neutron-rich nuclei around $N=40$ rapid changes in nuclear structure can be observed. While $^{68}\mathrm{Ni}$ exhibits signatures of a doubly magic nucleus, experimental data along the isotopic chains in even more exotic Fe and Cr isotopes---such as excitation energies and transition strengths---suggest a sudden rise in collectivity toward $N=40$.Purpose: Reduced quadrupole transition strengths for low-lying transitions in neutron-rich $^{58,60,62}\mathrm{Cr}$ are investigated. This gives quantitative new insights into the evolution of quadrupole collectivity in the neutron-rich region close to $N=40$.Method: The recoil distance Doppler-shift (RDDS) technique was applied to measure lifetimes of low-lying states in $^{58,60,62}\mathrm{Cr}$. The experiment was carried out at the National Superconducting Cyclotron Laboratory (NSCL) with the SeGA array in a plunger configuration coupled to the S800 magnetic spectrograph. The states of interest were populated by means of one-proton knockout reactions.Results: Data reveal a rapid increase in quadrupole collectivity for $^{58,60,62}\mathrm{Cr}$ toward $N=40$ and point to stronger quadrupole deformations compared to neighboring Fe isotopes. The experimental $B(E2$) values are reproduced well with state-of-the-art shell-model calculations using the LNPS effective interaction. A consideration of intrinsic quadrupole moments and ${\mathrm{B}}_{42}$ ratios suggest an evolution toward a rotational nature of the collective structures in $^{60,62}\mathrm{Cr}$. Compared to $^{58}\mathrm{Cr}$, experimental ${\mathrm{B}}_{42}$ and ${\mathrm{B}}_{62}$ values for $^{60}\mathrm{Cr}$ are in better agreement with the $E(5)$ limit.Conclusion: Our results indicate that collective excitations in neutron-rich Cr isotopes saturate at $N=38$, which is in agreement with theoretical predictions. More detailed experimental data of excited structures and interband transitions are needed for a comprehensive understanding of quadrupole collectivity close to $N=40$. This calls for additional measurements in neutron-rich Cr and neighboring Ti and Fe nuclei.

Author(s): Henry, TW; Bentley, MA; Clark, RM; Davies, PJ; Bader, VM; Baugher, T; Bazin, D; Beausang, CW; Berryman, JS; Bruce, AM; Campbell, CM; Crawford, HL; Cromaz, M; Fallon, P; Gade, A; Henderson, J; Iwasaki, H; Jenkins, DG; Lee, IY; Lemasson, A; Lenzi, SM; Macchiavelli, AO; Napoli, DR; Nichols, AJ; Paschalis, S; Petri, M; Recchia, F; Rissanen, J; Simpson, EC; Stroberg, SR; Wadsworth, R; Weisshaar, D; Wiens, A; Walz, C | Abstract: Background: Triplet energy differences (TED) can be studied to yield information on isospin-non-conserving interactions in nuclei. Purpose: The systematic behavior of triplet energy differences (TED) of T=1, Jπ=2+ states is examined. The A=62 isobar is identified as having a TED value that deviates significantly from an otherwise very consistent trend. This deviation can be attributed to the tentative assignments of the pertinent states in Ga62 and Ge62. Methods: An in-beam γ-ray spectroscopy experiment was performed to identify excited states in Ga62 using Gamma-Ray Energy Tracking In-Beam Nuclear Array with the S800 spectrometer at NSCL using a two-nucleon knockout approach. Cross-section calculations for the knockout process and shell-model calculations have been performed to interpret the population and decay properties observed. Results: Using the systematics as a guide, a candidate for the transition from the T=1, 2+ state is identified. However, previous work has identified similar states with different Jπ assignments. Cross-section calculations indicate that the relevant T=1, 2+ state should be one of the states directly populated in this reaction. Conclusions: As spins and parities were not measurable, it is concluded that an unambiguous identification of the first T=1, 2+ state is required to reconcile our understanding of TED systematics.

International audience; Analyses of data from both deep inelastic reactions at Argonne National Laboratory and single-and multiple-particle knockout reactions at Michigan State University revealed new gamma-ray transitions in even-even Ni-66,68,70,72(38,40,42,44) and in Co-67(40) that provide strong evidence for multiple shape coexistence at N = 38 and 40 and deep prolate minima in Ni-70(42) and isotonic Cu-71(42). A new transition at 642 keV is proposed for Ni-66 as the prolate 2(+) to 0(+) transition. Two new transitions in Ni-72 at 915 and 1225 keV were identified in the knock-out reaction study and could represent de-population of prolate states. Taken together with recent theoretical work using the Monte Carlo shell model, a well defined region of shape coexistence can be seen existing precisely between 38

The first lifetime measurement used to study the magnetic response of halo nuclei is presented. The lifetime of the first excited state of the one-neutron halo nucleus 19C has been measured by two complementary Doppler-shift techniques with the Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA). The B(M1; 3/2+ →1/2+g.s.) strength of 3.21(25)×10−3μ2 N determined for this decay represents a strongly hindered M1 transition among light nuclei. Shell-model calculations predict a strong hindrance due to the near-degeneracy of the s1/2 and d5/2 orbitals among neutron-rich carbon isotopes, while tensor corrections and loosely bound effects are necessary to account for the remaining strength.

The structure of semi-magic Ni-70(28)42 was investigated following complementary multinucleon-transfer and secondary fragmentation reactions. Changes to the higher-spin, presumed negative-parity states based on observed gamma-ray coincidence relationships result in better agreement with shell-model calculations using effective interactions in the neutron f(5/2)pg(9/2) model space. The second 2(+) and (4(+)) states, however, can only be successfully described when proton excitations across the Z = 28 shell gap are included. Monte Carlo shell-model calculations suggest that the latter two states are part of a prolate-deformed intruder sequence, establishing an instance of shape coexistence at low excitation energies similar to that observed recently in neighboring Ni-68.

Lifetimes of low-lying excited states have been measured in $^{66}\mathrm{Ge},^{69}\mathrm{Se}$, and $^{65}\mathrm{Ga}$ using a $\ensuremath{\gamma}$-ray lineshape method. The results confirm the previously reported ${7}_{1}^{\ensuremath{-}}$ state lifetime in $^{66}\mathrm{Ge}$. The lifetime of the yrast $5/{2}^{\ensuremath{-}}$ state in $^{65}\mathrm{Ga}$ is measured for the first time. Lifetime measurements of two excited $3/{2}^{\ensuremath{-}}$ states in $^{69}\mathrm{Se}$ are also reported. Two previously unobserved $\ensuremath{\gamma}$ rays have been identified in $^{69}\mathrm{Se}$. $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence measurements have been used to place one of these in the level scheme. $^{69}\mathrm{Se}$ excited state populations are compared to shell-model calculations using the GXPF1A interaction in the $\mathit{fp}$ model space. Theoretical spectroscopic factors to excited states in $^{69}\mathrm{Se}$ have identified three candidate levels for the origin of one of the new transitions.

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.

Shell evolution is studied in the neutron-rich silicon isotopes 36,38,40 Si using neutron single-particle strengths deduced from one-neutron knockout reactions. Configurations involving neutron excita- tions across the N = 20 and N = 28 shell gaps are quantified experimentally in these rare isotopes. Comparisons with shell model calculations show that the tensor force, understood to drive the col- lective behavior in 42 Si with N = 28, is already important in determining the structure of 40 Si with N = 26. New data relating to cross-shell excitations provide the first quantitative support for repulsive contributions to the cross-shell T = 1 interaction arising from three-nucleon forces.

The reduced transition probability $B(E2;{0}^{+}\ensuremath{\rightarrow}{2}^{+})$ has been measured for the neutron-rich nucleus $^{74}\mathrm{Ni}$ in an intermediate energy Coulomb excitation experiment performed at the National Superconducting Cyclotron Laboratory at Michigan State University. The obtained $B(E2;{0}^{+}\ensuremath{\rightarrow}{2}^{+})=64{2}_{\ensuremath{-}226}^{+216}\text{ }\text{ }{e}^{2}\text{ }{\mathrm{fm}}^{4}$ value defines a trend which is unexpectedly small if referred to $^{70}\mathrm{Ni}$ and to a previous indirect determination of the transition strength in $^{74}\mathrm{Ni}$. This indicates a reduced polarization of the $Z=28$ core by the valence neutrons. Calculations in the $pfgd$ model space reproduce well the experimental result indicating that the $B(E2)$ strength predominantly corresponds to neutron excitations. The ratio of the neutron and proton multipole matrix elements supports such an interpretation.

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 $\ensuremath{\pi}{g}_{9/2}\ensuremath{\nu}{g}_{9/2}$ configuration.

The structure of the neutron-rich silicon isotopes Si36,38,40 was studied by one-neutron and one-proton knockout reactions at intermediate beam energies. We construct level schemes for the knockout residues Si35,37,39 and Al35,37,39 and compare knockout cross sections to the predictions of an eikonal model in conjunction with large-scale shell-model calculations. The agreement of these calculations with the present experiment lends support to the microscopic explanation of the enhanced collectivity in the region of Si42. We also present an empirical method for reproducing the observed low-momentum tails in the parallel momentum distributions of knockout residues.

Results of decay spectroscopy on nuclei in the vicinity of the doubly magic $^{48}\mathrm{Ni}$ are presented. The measurements were performed with a time projection chamber with optical readout which records tracks of ions and protons in the gaseous volume. Six decays of $^{48}\mathrm{Ni}$, including four events of two-proton ground-state radioactivity, were recorded. An advanced reconstruction procedure yielded the $2p$ decay energy for $^{48}\mathrm{Ni}$ of ${Q}_{2p}=1.29(4)$ MeV. In addition, the energy spectra of $\ensuremath{\beta}$-delayed protons emitted in the decays of $^{44}\mathrm{Cr}$ and $^{46}\mathrm{Fe}$, as well as half-lives and branching ratios, were determined. The results were found to be consistent with the previous measurements made with Si detectors. A new proton line in the decay of $^{44}\mathrm{Cr}$ corresponding to the decay energy of 760 keV is reported. The first evidence for the $\ensuremath{\beta}2p$ decay of $^{46}\mathrm{Fe}$, based on one clear event, is shown.

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.

Transition strengths for decays from low-lying states in A ∼ 70 nuclei have been deduced from lifetime measurements using the recoil distance Doppler shift technique. The results confirm the collectivity previously reported for the 21+→0gs+ decay in 68 Se and reveal a relative decrease in collectivity in 70 Br. This trend is reproduced by shell model calculations using the GXPF1A interaction in an fp model space including the Coulomb, spin-orbit and isospin non-conserving interactions. The 31+→21+ decay in 70 Br is found to have a very small B(M1) value, which is consistent with the configuration of the state being dominated by the coupling of f52 protons and neutrons. The results suggest that the g92 orbit does not play an important role at low spin in these nuclei. The B(E2) values for the decays of the (T = 1) 21+ states in 70 Br and 70 Se are almost identical, suggesting there is no major shape change between the two nuclei at low spin.

The transition rates from the yrast 2+ and 4+ states in the self-conjugate 72Kr 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+→2+ transition suggests a prolate character of the excited states. The reduced collectivity previously reported for the 2+→0+ transition was confirmed. The irregular behavior of collectivity points to the occurrence of a rapid oblate-prolate shape transition in 72Kr, providing stringent tests for advanced theories to describe the shape coexistence and its evolution.

We report on the experimental study of quadrupole collectivity in the neutron-deficient nucleus \nuc{104}{Sn} using intermediate-energy Coulomb excitation. The $B(E2; 0^+_1 \rightarrow 2^+_1)$ value for the excitation of the first $2^+$ state in \nuc{104}{Sn} has been measured to be $0.180(37)~e^2$b$^2$ relative to the well-known $B(E2)$ value of \nuc{102}{Cd}. This result disagrees by more than one sigma with a recently published measurement \cite{Gua13}. Our result indicates that the most modern many-body calculations remain unable to describe the enhanced collectivity below mid-shell in Sn approaching $N=Z=50$. We attribute the enhanced collectivity to proton particle-hole configurations beyond the necessarily limited shell-model spaces and suggest the asymmetry of the $B(E2)$-value trend around mid-shell to originate from enhanced proton excitations across $Z=50$ as $N=Z$ is approached., Accepted for publication as rapid communication in Physical Review C

Production cross sections for neutron-rich nuclei from the fragmentation of a ${}^{82}$Se beam at 139 MeV/u were measured. The longitudinal momentum distributions of 126 neutron-rich isotopes of elements $11\ensuremath{\le}Z\ensuremath{\le}32$ were scanned using an experimental approach of varying the target thickness. Production cross sections with beryllium and tungsten targets were determined for a large number of nuclei including several isotopes first observed in this work. These are the most neutron-rich nuclides of the elements $22\ensuremath{\le}Z\ensuremath{\le}25$ (${}^{64}$Ti, ${}^{67}$V, ${}^{69}$Cr, and ${}^{72}$Mn). One event was registered consistent with ${}^{70}$Cr and another one with ${}^{75}$Fe. The production cross sections are correlated with ${Q}_{g}$ systematics to reveal trends in the data. The results presented here confirm our previous results from a similar measurement using a ${}^{76}$Ge beam and can be explained with a shell model that predicts a subshell closure at $N=34$ around $Z=20$. This is demonstrated by systematic trends and calculations with the abrasion-ablation model that are sensitive to separation energies.

The low-spin level scheme of ${}^{68}$Ni was investigated following two-neutron-knockout and multinucleon-transfer reactions. The energy of the first excited state was determined to be ${E}_{x}({0}_{2}^{+})=1603.5(3)$ keV. Relative $B(E2)$ transition probabilities were deduced and compared with shell-model calculations using several modern effective interactions. Theory reproduces the data well, but indicates substantial mixing of multi-particle, multi-hole configurations for the lowest observed ${0}^{+}$ and ${2}^{+}$ states.

Intermediate-energy Coulomb excitation measurements are performed on the N≥40 neutron-rich nuclei Fe66,68 and Cr64. The reduced transition matrix elements providing a direct measure of the quadrupole collectivity B(E2;2+1→0+1) are determined for the first time in Fe4268 and Cr4064 and confirm a previous recoil distance method lifetime measurement in Fe4066. The results are compared to state-of-the-art large-scale shell-model calculations within the full fpgd neutron orbital model space using the Lenzi-Nowacki-Poves-Sieja effective interaction and confirm the results of the calculations that show these nuclei are well deformed, This work was supported by the National Science Foundation under Grant No. PHY11-02511 (NSCL), and by the U.S. Department of Energy under Awards No. DEAC02- 05CH11231 (LBNL), No. DE-FG02-08ER41556 (NSCL), No. DE-FG52-06NA26206, and No. DE-FG02- 05ER41379 (University of Richmond). Part of this work was performed under the auspices of the U.S. Department of Energy Lawrence Livermore National Laboratory (LLNL) under Award No. DE-AC52-07NA27344. A. P. recognizes MICINN (Spain) (Grant No. FPA2011-29854) and Comunidad de Madrid (Spain) (Project No. HEPHACOS S2009-ESP-147) for support. We would also like to thank the operations staff at NSCL for their outstanding work in beam delivery

Production cross sections for neutron-rich nuclei from the fragmentation of a 82Se beam at 139 MeV/u were measured. The longitudinal momentum distributions of 122 neutron-rich isotopes of elements $11 \le Z \le 32$ were determined by varying the target thickness. Production cross sections with beryllium and tungsten targets were determined for a large number of nuclei including several isotopes first observed in this work. These are the most neutron-rich nuclides of the elements $22 \le Z \le 25$ (64Ti, 67V, 69Cr, 72Mn). One event was registered consistent with 70Cr, and another one with 75Fe. A one-body Qg systematics is used to describe the production cross sections based on thermal evaporation from excited prefragments. The current results confirm those of our previous experiment with a 76Ge beam: enhanced production cross sections for neutron-rich fragments near Z=20., Talk given at the 11th International Conference on Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1, 2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference Series (JPCS)

Beta-delayed proton emission from nuclides in the neighborhood of 100Sn was studied at the National Superconducting Cyclotron Laboratory. The nuclei were produced by fragmentation of a 120 MeV/nucleon 112Sn primary beam on a Be target. Beam purification was provided by the A1900 Fragment Separator and the Radio Frequency Fragment Separator. The fragments of interest were identified and their decay was studied with the NSCL Beta Counting System (BCS) in conjunction with the Segmented Germanium Array (SeGA). The nuclei 96Cd, 98Ing, 98Inm and 99In were identified as beta-delayed proton emitters, with branching ratios bp = 5.5(40)%, 5.5+3 -2%, 19(2)% and 0.9(4)%, respectively. The bp for 89Ru, 91,92Rh, 93Pd and 95Ag were deduced for the first time with bp = 3+1.9 -1.7%, 1.3(5)%, 1.9(1)%, 7.5(5)% and 2.5(3)%, respectively. The bp = 22(1)% for 101Sn was deduced with higher precision than previously reported. The impact of the newly measured bp values on the composition of the type-I X-ray burst ashes was studied.

The lifetimes of the first 2${}^{+}$ states in the neutron-deficient ${}^{76,78}$Sr isotopes were measured using a unique combination of the $\ensuremath{\gamma}$-ray line-shape method and two-step nucleon exchange reactions at intermediate energies. The transition rates for the 2${}^{+}$ states were determined to be $B(\mathrm{E}2;{2}^{+}\ensuremath{\rightarrow}{0}^{+})=2220(270)$ ${e}^{2}$fm${}^{4}$ for ${}^{76}$Sr and 1800(250) ${e}^{2}$fm${}^{4}$ for ${}^{78}$Sr, corresponding to large deformation of ${\ensuremath{\beta}}_{2}$ $=$ 0.45(3) for ${}^{76}$Sr and 0.40(3) for ${}^{78}$Sr. The present data provide experimental evidence for mutually enhanced collectivity that occurs at $N$ $=$ $Z$ $=$ 38. The systematic behavior of the excitation energies and $B(\mathrm{E}2)$ values indicates a signature of shape coexistence in ${}^{76}$Sr, characterizing ${}^{76}$Sr as one of most deformed nuclei with an unusually reduced $E$(4${}^{+}$)$/E$(2${}^{+}$) ratio.

Shell gap at the magic number N = 82 is important to reproduce the 2nd peak of r-process abundance. If a spin-orbit force is reduced in a very neutron-rich region, a shell quenching at N = 82 and a new shell closure at N = 70 are predicted. A shell evolution by the spin-orbit-force reduction can be searched for through the shape evolution of Zr isotopes around an expected double magic nuclei, 110Zr(Z=40,N=70). We performed β-γ and isomer spectroscopy at RIBF to observe low-lying states in 106,108Zr. The present results indicate a well deformed shape for 106,108Zr. The drastic reduction of the spin-orbit force most likely does not occur around 110Zr on an r-process path. © 2012 American Institute of Physics.

The neutron-rich nucleus 110Mo has been investigated by means of γ-ray spectroscopy following the β-decay of 110Nb, produced using in-flight fission of a 238U beam at 345 MeV/nucleon at the RIBF facility. In addition to the ground-band members reported previously, spectroscopic information on the low-lying levels of the quasi-γ band built on the second 2 + state at 494 keV has been obtained for the first time. The experimental finding of the second 2 + state being lower than the yrast 4 + level suggests that axially-asymmetric γ softness is substantially enhanced in this nucleus. The experimental results are compared with model calculations based on the general Bohr Hamiltonian method. The systematics of the low-lying levels in even–even A ≈ 110 nuclei is discussed in comparison with that in the neutron-rich A ≈ 190 region, by introducing the quantity E S / E ( 2 1 + ) , E S = E ( 2 2 + ) − E ( 4 1 + ) , as a global signature of the structural evolution involving axial asymmetry.

The isomeric and $\ensuremath{\beta}$ decays of the $N=Z+2$ nucleus ${}^{96}\phantom{\rule{-0.16em}{0ex}}$Ag were investigated at the National Superconducting Cyclotron Laboratory. A cascade of $\ensuremath{\gamma}$-ray transitions originating from the deexcitation of a microsecond isomer was observed for the first time and was found in coincidence with two previously known transitions with energies of 470 and 667 keV. The isomeric half-life was determined as 1.45(7) $\ensuremath{\mu}$s, more precise than previously reported. The existence of a second, longer-lived microsecond isomer, associated with a 743-keV $\ensuremath{\gamma}$ transition, is also proposed here. Shell model results within the $({p}_{3/2}{p}_{1/2}{f}_{5/2}{g}_{9/2})$ model space, using the jj44b interaction, reproduced level energies and isomeric decay half-lives reasonably well.

The decay of the extremely neutron-deficient $^{48}\mathrm{Ni}$ was studied by means of an imaging time-projection chamber, which allowed the recording of tracks of charged particles. The decays of six atoms were observed. Four of them clearly correspond to two-proton radioactivity, providing the first direct evidence for this decay mode in $^{48}\mathrm{Ni}$. Two decays represent $\ensuremath{\beta}$-delayed proton emission. The half-life of $^{48}\mathrm{Ni}$ is determined to be ${T}_{1/2}=2.{1}_{\ensuremath{-}0.4}^{+1.4}$ ms.

The low-lying states in ¹⁰⁶Zr and ¹⁰⁸Zr have been investigated by means of β-γ and isomer spectroscopy at the radioactive isotope beam factory (RIBF), respectively. A new isomer with a half-life of 620 ± 150 ns has been identified in ¹⁰⁸Zr. For the sequence of even-even Zr isotopes, the excitation energies of the first 2⁺ states reach a minimum at N = 64 and gradually increase as the neutron number increases up to N = 68, suggesting a deformed subshell closure at N = 64. The deformed ground state of ¹⁰⁸Zr indicates that a spherical subshell gap predicted at N = 70 is not large enough to change the ground state of ¹⁰⁸Zr to the spherical shape. The possibility of a tetrahedral shape isomer in ¹⁰⁸Zr is also discussed.

Decay studies of very neutron‐deficient nuclei 43Cr and 48Ni with the application of a time projection chamber with optical readout are presented.

The low-lying states in ¹⁰⁶Zr and ¹⁰⁸Zr have been investigated by means of β-γ and isomer spectroscopy at the radioactive isotope beam factory (RIBF), respectively. A new isomer with a half-life of 620 ± 150 ns has been identified in ¹⁰⁸Zr. For the sequence of even-even Zr isotopes, the excitation energies of the first 2⁺ states reach a minimum at N = 64 and gradually increase as the neutron number increases up to N = 68, suggesting a deformed subshell closure at N = 64. The deformed ground state of ¹⁰⁸Zr indicates that a spherical subshell gap predicted at N = 70 is not large enough to change the ground state of ¹⁰⁸Zr to the spherical shape. The possibility of a tetrahedral shape isomer in ¹⁰⁸Zr is also discussed.

The new, neutron-deficient, superheavy element isotope {sup 285}114 was produced in {sup 48}Ca irradiations of {sup 242}Pu targets at a center-of-target beam energy of 256 MeV (E* = 50 MeV). The {alpha} decay of {sup 285}114 was followed by the sequential {alpha} decay of four daughter nuclides, {sup 281}Cn, {sup 277}Ds, {sup 273}Hs, and {sup 269}Sg. {sup 265}Rf was observed to decay by spontaneous fission. The measured {alpha}-decay Q values were compared with those from a macroscopic-microscopic nuclear mass model to give insight into superheavy element shell effects. The {sup 242}Pu({sup 48}Ca,5n){sup 285}114 cross section was 0.6{sub -0.5}{sup +0.9} pb.