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The characteristics of 126,128Xe were investigated in Coulomb excitation measurements performed at the National Superconducting Cyclotron Laboratory (NSCL) Re-accelerator facility, ReA3, Michigan State University (MSU). The Xe nuclei were accelerated to sub-barrier energies and were impinged on 196Pt and 208Pb targets in separate experimental runs. The scattered nuclei and the de-excitation γ-rays were detected using the JANUS setup. Electromagnetic matrix elements were extracted from the experimental data with the help of the GOSIA/GOSIA2 codes. The results were compared to schematic Davydov-Filippov γ-rigid rotor theoretical calculations and large-scale calculations within a newly-established microscopic shell model (called PMMU model). The experimental results agree well with the theoretical predictions, except for the quadrupole moments of the second 2+ states in both nuclei, therefore challenging the interpretation of the γ-bands structure.

A sequence of excited states has been established for the first time in the proton-rich nucleus 48Fe (Z=26, N=22). The technique of mirrored (i.e. analogue) one-nucleon knockout reactions was applied, in which the T z = ±2 mirror pair, 48Fe/48Ti were populated via one-neutron/one-proton knockout from the secondary beams 49Fe/49V, respectively. The analogue properties of the reactions were used to help establish the new level scheme of 48Fe. The inclusive and exclusive cross sections were determined for the populated states. Large differences between the cross sections for the two mirrored reactions were observed and have been interpreted in terms of different degrees of binding of the mirror nuclei and in the context of the recent observations of suppression of spectroscopic strength as a function of nuclear binding, for knockout reactions on light solid targets. Mirror energy differences (MED) have been determined between the analogue T = 2 states and compared with the shell model predictions. MED for this mirror pair, due to their location in the shell, are especially sensitive to excitations out of the f 7 / 2 shell, and present a stringent test of the shell-model prescription.

Excited states in the neutron-rich nucleus Fe 68 were populated using a Be 9 ( Co 68 , 68 Fe + γ ) X charge-exchange reaction at 95 MeV/u. The new γ -ray transitions reported here for the first time complement data from β -decay studies and nucleon knockout reactions. In comparison to shell-model calculations with the LNPS effective interaction, two candidate states for the 6 1 + level emerge. The distinct population pattern of excited states and the magnitude of the cross section, σ inc = 0.51 ( 6 ) mb, make this reaction a promising one for future in-beam γ -ray spectroscopy. Reaction calculations with nuclear structure input from a new, locally optimized Hamiltonian, f 7 j 4 a , together with general considerations for heavy-ion-induced charge-exchange reactions appear consistent with most of the observations, although challenges remain.

The signatures of inversion between normal and intruder configurations of particle-hole excitations across the $N=20$ shell gap in the neutron-rich isotope $^{32}\mathrm{Mg}$ have long been of keen interest. Electromagnetic transition rates in the ground-state band are key quantities that provide insights into collective properties associated with the contributions of the 2p2h and 4p4h intruder configurations. The combination of TRIPLEX, GRETINA, and the S800 spectrograph enables model-independent lifetime measurements to determine electromagnetic transition rates in rare isotopes. The reduced $E2$ transition rates in $^{32}\mathrm{Mg}$ between the ${2}_{1}^{+}$ and ${0}_{1}^{+}$ states and between the ${4}_{1}^{+}$ and ${2}_{1}^{+}$ states have been measured, the latter representing the first experimental $B(E2)$ value for this transition. The $B(E2)$ strengths indicate large collectivity and strong contributions from the 2p2h and 4p4h intruder configurations that may change with spin in the ground-state band of $^{32}\mathrm{Mg}$.

The level schemes of the neutron-rich $^{37--42}\mathrm{P}$ isotopes are investigated via in-beam $\ensuremath{\gamma}$-ray spectroscopy following the fragmentation of a $^{45}\mathrm{Cl}$ projectile beam at intermediate beam energies. Information on $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidence relationships complemented by comparisons to shell-model calculations in the $sd\text{\ensuremath{-}}pf$ model space were used to construct excitation level schemes for these neutron-rich nuclei. For the odd-mass $^{37,39}\mathrm{P}$ isotopes, a level scheme is presented that appears essentially complete at low energies and exhausts the states predicted by the SDPF-MU shell-model Hamiltonian. Simple Nilsson configurations are proposed for the low-lying excited states of $^{38,39,40,41}\mathrm{P}$ from an analysis of the $E2$ transition matrix elements and moments calculated within the shell model.

The timing and energy resolution properties of LaBr 3 (Ce) scintillators are well suited for use in γ γ fast-timing experiments. Using standard 60Co, 88Y, and 152Eu sources, the 16-element array of 1.5 inch × 1.5 inch right-cylindrical LaBr 3 (Ce) detectors and the associated readout electronics at the National Superconducting Cyclotron Laboratory were commissioned by measuring the lifetimes of excited states and the angular correlations of γ rays emitted in cascade. Excited-state lifetimes in 152Sm and 152Gd were measured using the exponential slope of the decay and a method based on the centroids of time-difference spectra. Results for the lifetimes are consistent with the well-known literature values which cover a range from a few picoseconds to several nanoseconds. Furthermore, the γ γ angular correlations for the 4 + → 2 + → 0 + cascade in 60Ni and the 3 − → 2 + → 0 + cascade in 88Sr were measured and shown to agree with GEANT4 simulations that take into account the expected angular correlations within the cascades as well as the geometry of the array.

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.

Electromagnetic transition strengths in the even-even neutron-rich sulfur isotopes $^{38,40,42,44}\mathrm{S}$ were measured using intermediate-energy Coulomb excitation at the National Superconducting Cyclotron Laboratory. By utilizing the sensitivity of the experimental technique to $E2$ excitations from the ground state, the evolution of the pattern of $B(E2)$ strengths to several low-lying ${2}^{+}$ states was investigated at $Z=16$ from near stability to the $N=28$ island of inversion. The experimental results allowed a detailed comparison with predictions from shell-model calculations using the SDPF-MU Hamiltonian, which was designed to describe collectivity in this region of the nuclear chart. While the shell-model calculations succeeded in modeling transition strengths at $N=22,24,26$, the experimental $B(E2;{0}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ at $N=28$ was smaller than the predicted value by about a factor of 2, similar to previous observations for chlorine and argon isotopes around $N=28$. The dependence of this overprediction by theory on the choice of effective charges was explored.

Discovering unexplored high-spin states in neutron-rich nuclei can open up a new direction to study band structure and the associated shell structure in isospin-asymmetric many-body systems. However, experimental reach has so far been limited to neutron-deficient or stable nuclei which are preferentially produced in fusion reactions used in such studies. Here, we report the first $\ensuremath{\gamma}$-ray spectroscopy with fusion reactions using a reaccelerated rare-isotope beam of $^{45}\mathrm{K}$ performed at the ReA3 facility of the National Superconducting Cyclotron Laboratory. Using particle and $\ensuremath{\gamma}$-ray coincidence techniques, three new higher-lying states around 6 MeV and five new $\ensuremath{\gamma}$-ray transitions were identified for $^{46}\mathrm{Ca}$, suggesting three independent band structures formed from different particle-hole configurations. The rotational-like band built on the ${0}_{2}^{+}$ state is established up to the tentatively assigned ${6}_{2}^{+}$ state. New results are compared to large-scale shell model calculations, confirming the validity of the effective interaction describing particle-hole excitations across the $Z=20$ and $N=28$ shell gaps in the vicinity of doubly magic $^{48}\mathrm{Ca}$.

The shape and collectivity of $^{106}\mathrm{Cd}$ was investigated via a sub-barrier-energy Coulomb excitation experiment performed at the National Superconducting Cyclotron Laboratory Re-accelerator facility using the JANUS setup. Transition matrix elements between low-lying states were found to agree with adopted values, and information on the shape and collectivity of higher-lying states was extracted for the first time. Locally optimized large-scale shell-model calculations were found to describe well the $B(E2)$ transition strengths but failed to reproduce the spectroscopic quadrupole moments ${Q}_{s}$. An analysis of the $E2$ rotational invariants and the normalized quadrupole moment ${q}_{s}$ indicates that this may be due to a significant degree of triaxiality in $^{106}\mathrm{Cd}$ which is not captured by the present shell-model calculations. Analogous calculations for the Fe isotopes (two protons below the $Z=28$ magic number) reveal the critical role of high-$j$ neutron configurations for the description of quadrupole moments in the heavy Fe and Cd isotopes (two protons below magic $Z=50$), but this effect is insufficient to explain the shape of $^{106}\mathrm{Cd}$, posing a puzzle for the understanding of nuclear structure towards $N=50$.

The discrepancy between observations from γ-ray astronomy of the ^{60}Fe/^{26}Al γ-ray flux ratio and recent calculations is an unresolved puzzle in nuclear astrophysics. The stellar β-decay rate of ^{59}Fe is one of the major nuclear uncertainties impeding us from a precise prediction. The important Gamow-Teller strengths from the low-lying states in ^{59}Fe to the ^{59}Co ground state are measured for the first time using the exclusive measurement of the ^{59}Co(t,^{3}He+γ)^{59}Fe charge-exchange reaction. The new stellar decay rate of ^{59}Fe is a factor of 3.5±1.1 larger than the currently adopted rate at T=1.2 GK. Stellar evolution calculations show that the ^{60}Fe production yield of an 18 solar mass star is decreased significantly by 40% when using the new rate. Our result eliminates one of the major nuclear uncertainties in the predicted yield of ^{60}Fe and alleviates the existing discrepancy of the ^{60}Fe/^{26}Al ratio.

The region of neutron-rich Cr isotopes has garnered much attention in recent years due to a rapid onset of collectivity near neutron number $N=40$. We report here on the first $\ensuremath{\gamma}$-ray spectroscopy beyond the $({4}_{1}^{+})$ state in $^{62,64}\mathrm{Cr}$, using nucleon removal reactions from several projectiles within a rare-isotope beam cocktail. A candidate for the ${6}^{+}$ state in $^{64}\mathrm{Cr}$ is presented as well as one for, possibly, the second excited ${0}^{+}$ state in $^{62}\mathrm{Cr}$. The results are discussed in comparison to the LNPS shell-model predictions that allow for neutron excitations across the $N=40$ harmonic oscillator gap into the ${g}_{9/2}$ and ${d}_{5/2}$ orbitals. The calculated level schemes for $^{62,64}\mathrm{Cr}$ reveal intriguing collective structures. From the predicted neutron particle-hole character of the low-lying states in these Cr isotopes, $^{62}\mathrm{Cr}$ emerges as a transitional system on the path to the center of the $N=40$ island of inversion.

The body of experimental measurements of intermediate-energy reactions that remove a single nucleon from a secondary beam of neutron- or proton-rich nuclei continues to grow. These data have been analysed consistently using an approximate, eikonal-model treatment of the reaction dynamics combined with appropriate shell-model descriptions of the projectile initial state, the bound final states spectrum of the reaction residue and single-particle removal strengths computed from their wave-function overlaps. The systematics of the ratio $R_s$ of the measured inclusive cross-section to all bound final states and the calculated cross-section to bound shell-model states -- in different regions of the nuclear chart and involving both very weakly-bound and strongly-bound valence nucleons -- is important in relating the empirically deduced orbital occupancies to those from the best available shell-model predictions. Importantly, several new higher-energy measurements, for which the sudden-approximation aspect of the dynamical description is placed on an even stronger footing, now supplement the previously-analysed measurements. These additional data sets are discussed. Their $R_s$ values are shown to conform to and reinforce the earlier-observed systematics, with no indication that the approximately linear reduction in $R_s$ with increasing nucleon separation energy is a consequence of a breakdown of the sudden approximation., Comment: 8 pages, 2 figures

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).

In this review article we discuss the present status of direct nuclear reactions and the nuclear structure aspects one can study with them. We discuss the spectroscopic information we can assess in experiments involving transfer reactions, heavy-ion-induced knockout reactions and quasifree scattering with (p,2p), (p,pn), and (e,e'p) reactions. In particular, we focus on the proton-to-neutron asymmetry of the quenching of the spectroscopic strength., 138 pages, 63 figures

The neutron-rich nuclei in the $N=28$ island of inversion have attracted considerable experimental and theoretical attention, providing great insight into the evolution of shell structure and nuclear shape in exotic nuclei. In this work, for the first time, quadrupole collectivity is assessed simultaneously on top of the $3/{2}^{\ensuremath{-}}$ ground state and the $7/{2}^{\ensuremath{-}}$ shape-coexisting isomer of $^{43}\mathrm{S}$, putting the unique interpretation of shape and configuration coexistence at $N=27$ and 28 in the sulfur isotopic chain to the test. From an analysis of the electromagnetic transition strengths and quadrupole moments predicted within the shell model, it is shown that the onset of shape coexistence and the emergence of a simple collective structure appear suddenly in $^{43}\mathrm{S}$ with no indication of such patterns in the $N=27$ isotone $^{45}\mathrm{Ar}$.

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.

Proton capture on the excited isomeric state of ^{26}Al strongly influences the abundance of ^{26}Mg ejected in explosive astronomical events and, as such, plays a critical role in determining the initial content of radiogenic ^{26}Al in presolar grains. This reaction also affects the temperature range for thermal equilibrium between the ground and isomeric levels. We present a novel technique, which exploits the isospin symmetry of the nuclear force, to address the long-standing challenge of determining proton-capture rates on excited nuclear levels. Such a technique has in-built tests that strongly support its veracity and, for the first time, we have experimentally constrained the strengths of resonances that dominate the astrophysical ^{26m}Al(p,γ)^{27}Si reaction. These constraints demonstrate that the rate is at least a factor ∼8 lower than previously expected, indicating an increase in the stellar production of ^{26}Mg and a possible need to reinvestigate sensitivity studies involving the thermal equilibration of ^{26}Al.

The structure of exotic nuclei provides valuable tests for state-of-the-art nuclear theory. In particular electromagnetic transition rates are more sensitive to aspects of nuclear forces and many-body physics than excitation energies alone. We report the first lifetime measurement of excited states in $^{21}$O, finding $\tau_{1/2^+}=420^{+35}_{-32}\text{(stat)}^{+34}_{-12}\text{(sys)}$\,ps. This result together with the deduced level scheme and branching ratio of several $\gamma$-ray decays are compared to both phenomenological shell-model and ab initio calculations based on two- and three-nucleon forces derived from chiral effective field theory. We find that the electric quadrupole reduced transition probability of $\rm B(E2;1/2^+ \rightarrow 5/2^+_{g.s.}) = 0.71^{+0.07\ +0.02}_{-0.06\ -0.06}$~e$^2$fm$^4$, derived from the lifetime of the $1/2^+$ state, is smaller than the phenomenological result where standard effective charges are employed, suggesting the need for modifications of the latter in neutron-rich oxygen isotopes. We compare this result to both large-space and valence-space ab initio calculations, and by using multiple input interactions we explore the sensitivity of this observable to underlying details of nuclear forces., Comment: 23 pages, 3 figures

The single-particle structure of the N = 27 isotones provides insights into the shell evolution of neutron-rich nuclei from the doubly-magic 48Ca toward the drip line. 43S was studied employing the one-neutron knockout reaction from a radioactive 44S beam. Using a combination of prompt and delayed γ-ray spectroscopy the level structure of 43S was clarified. Momentum distributions were analyzed and allowed for spin and parity assignments. The deduced spectroscopic factors show that the 44S ground-state configuration has a strong intruder component. The results were confronted with shell model calculations using two effective interactions. General agreement was found between the calculations, but strong population of states originating from the removal of neutrons from the 2p3/2 orbital in the experiment indicates that the breakdown of the N = 28 magic number is more rapid than the theoretical calculations suggest.

The structure of ³³Si was studied by a one-neutron knockout reaction from a ³⁴Si beam at 98.5 MeV/u incident on a 9 Be target. The prompt γ rays following the de-excitation of ³³Si were detected using the GRETINA γ -ray tracking array while the reaction residues were identified on an event-by-event basis in the focal plane of the S800 spectrometer at the National Superconducting Cyclotron Laboratory. The presently derived spectroscopic factor values, C 2 S , for the 3 / 2 + and 1 / 2 + states, corresponding to a neutron removal from the 0 d 3 / 2 and 1 s 1 / 2 orbitals, agree with shell model calculations and point to a strong N = 20 shell closure. Three states arising from the more bound 0 d 5 / 2 orbital are proposed, one of which is unbound by about 930 keV. The sensitivity of this experiment has also confirmed a weak population of 9 / 2 − and 11 / 2 − 1 , 2 final states, which originate from a higher-order process. This mechanism may also have populated, to some fraction, the 3 / 2 − and 7 / 2 − negative-parity states, which hinders a determination of the C 2 S values for knockout from the normally unoccupied 1 p 3 / 2 and 0 f 7 / 2 orbits.

We present mass excesses (ME) of neutron-rich isotopes of Ar through Fe, obtained via TOF-$B\rho$ mass spectrometry at the National Superconducting Cyclotron Laboratory. Our new results have significantly reduced systematic uncertainties relative to a prior analysis, enabling the first determination of ME for $^{58,59}{\rm Ti}$, $^{62}{\rm V}$, $^{65}{\rm Cr}$, $^{67,68}{\rm Mn}$, and $^{69,70}{\rm Fe}$. Our results show the $N=34$ subshell weaken at Sc and vanish at Ti, along with the absence of an $N=40$ subshell at Mn. This leads to a cooler accreted neutron star crust, highlighting the connection between the structure of nuclei and neutron stars., Comment: Accepted for publication as a Rapid Communication in Physical Review C

The lifetimes of the first excited 2^{+} states in the N=Z nuclei ^{80}Zr, ^{78}Y, and ^{76}Sr have been measured using the γ-ray line shape method following population via nucleon-knockout reactions from intermediate-energy rare-isotope beams. The extracted reduced electromagnetic transition strengths yield new information on where the collectivity is maximized and provide evidence for a significant, and as yet unexplained, odd-odd vs even-even staggering in the observed values. The experimental results are analyzed in the context of state-of-the-art nuclear density-functional model calculations.

Simpson and Tostevin proposed that the width and shape of exclusive parallel momentum distributions of the A

Electron-capture reactions play important roles in the late evolution of core-collapse supernovae. The electron-capture rates used in astrophysical simulations rely on theoretical calculations which have to be tested against and guided by experimental data. We report on the measurement of the Gamow-Teller strength distribution of the odd-mass nucleus $^{93}\mathrm{Nb}$ via the ($t,^{3}\mathrm{He}$ + $\ensuremath{\gamma}$) charge-exchange reaction at a beam energy of 115 MeV/u. The Gamow-Teller strength distributions were extracted up to an excitation energy in $^{93}\mathrm{Zr}$ of 10 MeV. The results were compared with shell-model and quasiparticle random-phase approximation (QRPA) calculations. The theoretical calculations fail to describe the details of the strength distribution, but estimate reasonably well the integrated Gamow-Teller transition strength. Electron-capture rates derived from the measured and theoretical strength distributions match reasonably well, especially at the higher stellar densities of importance for deleptonization during the collapse of the stellar core, since the electron-capture $Q$ value is close to zero and the Fermi energy sufficiently high to ensure that the details of the strength distribution do not have a strong impact on the derived rates. At stellar densities in excess of ${10}^{9}$ g/${\mathrm{cm}}^{3}$, the electron-capture rate based on a single-state approximation used in astrophysical simulations is slightly higher than the rates based on the data and the shell-model and QRPA calculations, likely due to the fact that the approximation includes temperature-dependent effects, which increase the rates. However, the difference is much smaller than that observed in recent studies of nuclei with $Zl40$ near $N=50$, suggesting that the single-state approximation does not account for Pauli-blocking effects for nuclei with $Zl40$ that are much stronger than those for $^{93}\mathrm{Nb}$ with $Z=41$.

New experimental data obtained from γ-ray tagged one-neutron and one-proton knockout from 55Co is presented. A candidate for the sought-after T = 1, T z = 0, Jπ = 6+ state in 54Co is proposed based on a comparison to the new data on 54Fe, the corresponding observables predicted by large-scale-shell-model (LSSM) calculations in the full fp-model space employing charge-dependent contributions, and isospin-symmetry arguments. Furthermore, possible isospin-symmetry breaking in the A = 54, T = 1 triplet is studied by calculating the experimental c coefficients of the isobaric mass multiplet equation (IMME) up to the maximum possible spin J = 6 expected for the (1f7/2)-2 two-hole configuration relative to the doubly-magic nucleus 56Ni. The experimental quantities are compared to the theoretically predicted c coefficients from LSSM calculations using two-body matrix elements obtained from a realistic chiral effective field theory potential at next-to-next-to-next-to leading order (N3LO).

Electromagnetic quadrupole transition strength is a sensitive probe of the evolution of the structure of nuclei, particularly the competition between collectivity and magicity. We have performed a new lifetime measurement of the 2 1 + state of 12Be to study the interplay of these phenomena. The lifetime was measured with the Doppler Shift Attenuation Method using the γ-ray detector GRETINA. Excited states of 12Be were produced via inelastic scattering at 55 MeV/nucleon, using several different targets to control for systematic uncertainties in the stopping powers. The lifetime is determined to be τ = 1.38 ± 0.10 (stat) ± 0.19 (sys) ps, which is about half the previously reported value at twice the precision. The reduced transition strength deduced from this result is B ( E 2 ; 2 1 + → 0 1 + ) = 14.2 ± 1.0 (stat) ± 2.0 (sys) e 2 fm 4 , which supports the quenching of the N = 8 shell gap in 12Be.

A new experimental setup for low-energy Coulomb excitation experiments was constructed in a collaboration between the National Superconducting Cyclotron Laboratory (NSCL), Lawrence Livermore National Laboratory (LLNL), and the University of Rochester and was commissioned at the general purpose beam line of NSCL’s ReA3 reaccelerator facility. The so-called JANUS setup combines γ -ray detection with the Se gmented G e A rray (SeGA) and scattered particle detection using a pair of segmented double-sided Si detectors (Bambino 2). The low-energy Coulomb excitation program that JANUS enables will complement intermediate-energy Coulomb excitation studies that have long been performed at NSCL by providing access to observables that quantify collectivity beyond the first excited state, including the sign and magnitude of excited-state quadrupole moments. In this work, the setup and its performance will be described based on the commissioning run that used stable 78Kr impinging onto a 1.09 mg/cm 2 208Pb target at a beam energy of 3.9 MeV/u.

The structure of 71Zn was investigated by one-neutron transfer and heavy-ion induced complex (deep-inelastic) reactions using the GRETINA-CHICO2 and the Gammasphere setups, respectively. The observed inversion between the 9/2+ and 1/2− states is explained in terms of the role of neutron pairing correlations. Non-collective sequences of levels were delineated above the 9/2+ isomeric state. These are interpreted as being associated with a modest oblate deformation in the framework of Monte-Carlo shell-model calculations carried out with the A3DA-m Hamiltonian in the p f g 9 / 2 d 5 / 2 valence space. Similarities with the structure of Ni 40 28 68 were observed and the shape-coexistence mechanism in the N = 40 region of neutron-rich nuclei is discussed in terms of the so-called Type-II shell evolution, with an emphasis on proton–neutron correlations between valence nucleons, especially those involving the shape-driving g 9 / 2 neutron orbital.

We report the first experimental constraints on spectroscopic factors and strengths of key resonances in the 30P(p, γ)31Sreaction critical for determining the production of intermediate-mass elements up to Ca in nova ejecta. The 30P(d, n)31Sreaction was studied in inverse kinematics using the GRETINA γ-ray array to measure the angle-integrated cross-sections of states above the proton threshold. In general, negative-parity states are found to be most strongly produced but the absolute values of spectroscopic factors are typically an order of magnitude lower than predicted by the shell-model calculations employing WBP Hamiltonian for the negative-parity states. The results clearly indicate the dominance of a single 3/2−resonance state at 196 keV in the region of nova burning T≈0.10–0.17GK, well within the region of interest for nova nucleosynthesis. Hydrodynamic simulations of nova explosions have been performed to demonstrate the effect on the composition of nova ejecta.

Over the last decade, the Low-Energy Nuclear Physics (LENP) and Nuclear Astrophysics (NAP) communities have increasingly organized themselves in order to take a coherent approach to resolving the challenges they face. As a result, there is a high level of optimism in view of the unprecedented opportunities for substantial progress. In preparation of the 2015 US Nuclear Science Long Range Plan (LRP), the two American Physical Society Division of Nuclear Physics town meetings on LENP and NAP were held jointly on August 21–23, 2014, at Texas A&M, College Station, in Texas. These meetings were co-organized to take advantage of the strong synergy between the two fields. The present White Paper attempts to communicate the sense of great anticipation and enthusiasm that came out of these meetings. A unanimously endorsed set of joint resolutions condensed from the individual recommendations of the two town meetings were agreed upon. The present LENP White Paper discusses the above and summarizes in detail for each of the sub-fields within low-energy nuclear physics, the major accomplishments since the last LRP, the compelling near-term and long-term scientific opportunities plus the resources needed to achieve these goals, along with the scientific impact on, and interdisciplinary connections to, other fields.

The Facility for Rare Isotope Beams (FRIB) is a major new scientific user facility under construction in the United States for nuclear science research with beams of rare isotopes [1]. FRIB is funded by the United States Department of Energy Office of Science (DOE-SC) and is located on the campus of Michigan State University (MSU), in East Lansing, Michigan, USA. FRIB will support the mission of the Office of Nuclear Physics in the DOE-SC. The total FRIB project cost is $730 million and the facility is scheduled to be completed by 2022. The project has passed the 75% complete mark and is managed toward early completion in 2021. FRIB is estimated to provide access to 80% off all isotopes of elements lighter than uranium.

The dense soup of matter in the core of neutron stars is hard to model, but particle-accelerator experiments in which energetic electrons scatter off atomic nuclei could help to explore this high-density regime. A test of effective nucleon–nucleon interactions at short separations.

Author(s): Titus, R; Ney, EM; Zegers, RGT; Bazin, D; Belarge, J; Bender, PC; Brown, BA; Campbell, CM; Elman, B; Engel, J; Gade, A; Gao, B; Kwan, E; Lipschutz, S; Longfellow, B; Lunderberg, E; Mijatovic, T; Noji, S; Pereira, J; Schmitt, J; Sullivan, C; Weisshaar, D; Zamora, JC | Abstract: Background: In the late stages of stellar core collapse just prior to core bounce, electron captures on medium-heavy nuclei drive deleptonization. Therefore, simulations require the use of accurate reaction rates. Nuclei with neutron number near N=50 above atomic number Z=28 play an important role. Rates presently used in astrophysical simulations rely primarily on a relatively simple single-state approximation. In order to improve the accuracy of the astrophysical simulations, experimental data are needed to test the electron-capture rates and to guide the development of better theoretical models and astrophysical simulations. Purpose: The purpose of the present work was to measure the Gamow-Teller transition strength from Kr86 to Br86, to derive the stellar electron-capture rates based on the extracted strengths, and to compare the derived rates with rates based on shell-model and quasiparticle random-phase approximation (QRPA) Gamow-Teller strengths calculations, as well as the single-state approximation. An additional purpose was to test the impact of using improved electron-capture rates on the late evolution of core-collapse supernovae. Method: The Gamow-Teller strengths from Kr86 were extracted from the Kr86(t,He3+γ) charge-exchange reaction at 115MeV/u. The electron-capture rates were calculated as a function of stellar density and temperature. Besides the case of Kr86, the electron-capture rates based on the QRPA calculations were calculated for 78 additional isotopes near N=50 above Z=28. The impact of using these rates instead of those based on the single-state approximation is studied in a spherically symmetrical simulation of core collapse just prior to bounce. Results: The derived electron-capture rates on Kr86 from the experimental Gamow-Teller strength distribution are much smaller than the rates estimated based on the single-state approximation. Rates based on Gamow-Teller strengths estimated in shell-model and QRPA calculations are more accurate. The core-collapse supernova simulation with electron-capture rates based on the QRPA calculations indicate a significant reduction in the deleptonization during the collapse phase. Conclusions: It is important to utilize microscopic theoretical models that are tested by experimental data to constrain and estimate Gamow-Teller strengths and derived electron-capture rates for nuclei near N=50 that are inputs for astrophysical simulations of core-collapse supernovae and their multimessenger signals, such as the emission of neutrinos and gravitational waves.

Excited states of the neutron-rich isotopes $^{42,44}\mathrm{S}$ and $^{41,43}\mathrm{P}$ have been studied via inverse-kinematics proton scattering from a liquid hydrogen target, using the GRETINA $\ensuremath{\gamma}$-ray tracking array to extract inelastic-scattering cross sections. Deformation lengths of the ${2}_{1}^{+}$ excitations in $^{42,44}\mathrm{S}$ have been determined and, when combined with deformation lengths determined with electromagnetic probes, yield the ratio of neutron-to-proton matrix elements ${M}_{n}/{M}_{p}$ for the ${2}_{1}^{+}$ excitations in these nuclei. The present results for $^{41,43}\mathrm{P}(p,{p}^{\ensuremath{'}})$ are used to compare two shell-model interactions, SDPF-U and SDPF-MU. As in a recent study of $^{42}\mathrm{Si}$, the present results on $^{41,43}\mathrm{P}$ favor the SDPF-MU interaction.

The development of advanced γ-ray tracking arrays allows for a sensitive new technique to investigate elusive states of exotic nuclei with fast rare-isotope beams. By taking advantage of the excellent energy and position resolution of the Gamma-Ray Energy Tracking In-beam Nuclear Array, we developed a novel technique to identify in-flight isomeric decays of the 0⁺₂ state in 32Mg populated in a two-proton removal reaction. We confirm the 0⁺₂→2+1γ-ray transition of ³²Mg and constrain the 0⁺₂ decay lifetime, suggesting a large collectivity. The small partial cross section populating the 0⁺₂ state in this reaction provides experimental evidence for the reduced occupancy of the normal configuration of the 0⁺₂ state, indicating the intruder dominance of this state.

Angle-integrated cross-section measurements of the $^{56}$Ni(d,n) and (d,p) stripping reactions have been performed to determine the single-particle strengths of low-lying excited states in the mirror nuclei pair $^{57}$Cu-$^{57}$Ni situated adjacent to the doubly magic nucleus $^{56}$Ni. The reactions were studied in inverse kinematics utilizing a beam of radioactive $^{56}$Ni ions in conjunction with the GRETINA $\gamma$-array. Spectroscopic factors are compared with new shell-model calculations using a full $pf$ model space with the GPFX1A Hamiltonian for the isospin-conserving strong interaction plus Coulomb and charge-dependent Hamiltonians. These results were used to set new constraints on the $^{56}$Ni(p,$\gamma$)$^{57}$Cu reaction rate for explosive burning conditions in x-ray bursts, where $^{56}$Ni represents a key waiting point in the astrophysical rp-process., Comment: Final version accepted and published. 5 pages, 2 figures

The Gamow-Teller strength distribution from ${}^{88}$Sr was extracted from a $(t,{}^{3}\text{He}+\gamma)$ experiment at 115 MeV/$u$ to constrain estimates for the electron-capture rates on nuclei around $N=50$, between and including $^{78}$Ni and $^{88}$Sr, which are important for the late evolution of core-collapse supernovae. The observed strength below an excitation energy of 8 MeV was consistent with zero and below 10 MeV amounted to $0.1\pm0.05$. Except for a very-weak transition that could come from the 2.231-MeV $1^{+}$ state, no $\gamma$ lines that could be associated with the decay of known $1^{+}$ states were identified. The derived electron-capture rate from the measured strength distribution is more than an order of magnitude smaller than rates based on the single-state approximation presently used in astrophysical simulations for most nuclei near $N=50$. Rates based on shell-model and quasiparticle random-phase approximation calculations that account for Pauli blocking and core-polarization effects provide better estimates than the single-state approximation, although a relatively strong transition to the first $1^{+}$ state in $^{88}$Rb is not observed in the data. Pauli unblocking effects due to high stellar temperatures could partially counter the low electron-capture rates. The new data serves as a zero-temperature benchmark for constraining models used to estimate such effects.

Author(s): Elman, B; Gade, A; Janssens, RVF; Ayangeakaa, AD; Bazin, D; Belarge, J; Bender, PC; Brown, BA; Campbell, CM; Carpenter, MP; Crawford, HL; Crider, BP; Fallon, P; Forney, AM; Harker, J; Liddick, SN; Longfellow, B; Lunderberg, E; Prokop, CJ; Sethi, J; Taniuchi, R; Walters, WB; Weisshaar, D; Zhu, S | Abstract: The neutron-rich Ni isotopes have attracted attention in recent years because of the occurrence of shape or configuration coexistence. We report on the difference in population of excited final states in Ni70 following γ-ray tagged one-proton, one-neutron, and two-proton knockout from Cu71, Ni71, and Zn72 rare-isotope beams, respectively. Using variations observed in the relative transition intensities, signaling the changed population of specific final states in the different reactions, the role of neutron and proton configurations in excited states of Ni70 is probed schematically, with the goal of identifying those that carry, as leading configuration, proton excitations across the Z=28 shell closure. Such states are suggested in the literature to form a collective structure associated with prolate deformation. Adding to the body of knowledge for Ni70, 29 new transitions are reported, of which 15 are placed in its level scheme.

An extensive, model-independent analysis of the nature of triaxial deformation in ^{76}Ge, a candidate for neutrinoless double-beta (0νββ) decay, was carried out following multistep Coulomb excitation. Shape parameters deduced on the basis of a rotational-invariant sum-rule analysis provided considerable insight into the underlying collectivity of the ground-state and γ bands. Both sequences were determined to be characterized by the same β and γ deformation parameter values. In addition, compelling evidence for low-spin, rigid triaxial deformation in ^{76}Ge was obtained for the first time from the analysis of the statistical fluctuations of the quadrupole asymmetry deduced from the measured E2 matrix elements. These newly determined shape parameters are important input and constraints for calculations aimed at providing, with suitable accuracy, the nuclear matrix elements relevant to 0νββ.

We report on the observation of excited states in the neutron-deficient phosphorus isotopes $^{26,27,28}\mathrm{P}$ via in-beam $\ensuremath{\gamma}$-ray spectroscopy with both high-efficiency and high-resolution detector arrays. In $^{26}\mathrm{P}$, a previously unobserved level has been identified at 244(3) keV, two new measurements of the astrophysically important $3/{2}^{+}$ resonance in $^{27}\mathrm{P}$ have been performed, $\ensuremath{\gamma}$ decays have been assigned to the proton-unbound levels at 2216 and 2483 keV in $^{28}\mathrm{P}$, and the $\ensuremath{\gamma}$-ray line-shape method has been used to make the first determination of the lifetimes of the two lowest-lying excited states in $^{28}\mathrm{P}$. The expected Thomas-Ehrman shifts were calculated and applied to levels in the mirror nuclei. The resulting level energies from this procedure were then compared with the energies of known states in $^{26,27,28}\mathrm{P}$.

The ^{23}Al(p,γ)^{24}Si reaction is among the most important reactions driving the energy generation in type-I x-ray bursts. However, the present reaction-rate uncertainty limits constraints on neutron star properties that can be achieved with burst model-observation comparisons. Here, we present a novel technique for constraining this important reaction by combining the GRETINA array with the neutron detector LENDA coupled to the S800 spectrograph at the National Superconducting Cyclotron Laboratory. The ^{23}Al(d,n) reaction was used to populate the astrophysically important states in ^{24}Si. This enables a measurement in complete kinematics for extracting all relevant inputs necessary to calculate the reaction rate. For the first time, a predicted close-lying doublet of a 2_{2}^{+} and (4_{1}^{+},0_{2}^{+}) state in ^{24}Si was disentangled, finally resolving conflicting results from two previous measurements. Moreover, it was possible to extract spectroscopic factors using GRETINA and LENDA simultaneously. This new technique may be used to constrain other important reaction rates for various astrophysical scenarios.

A more detailed test of the implementation of nuclear forces that drive shell evolution in the pivotal nucleus ^{42}Si-going beyond earlier comparisons of excited-state energies-is important. The two leading shell-model effective interactions, SDPF-MU and SDPF-U-Si, both of which reproduce the low-lying ^{42}Si(2_{1}^{+}) energy, but whose predictions for other observables differ significantly, are interrogated by the population of states in neutron-rich ^{42}Si with a one-proton removal reaction from ^{43}P projectiles at 81 MeV/nucleon. The measured cross sections to the individual ^{42}Si final states are compared to calculations that combine eikonal reaction dynamics with these shell-model nuclear structure overlaps. The differences in the two shell-model descriptions are examined and linked to predicted low-lying excited 0^{+} states and shape coexistence. Based on the present data, which are in better agreement with the SDPF-MU calculations, the state observed at 2150(13) keV in ^{42}Si is proposed to be the (0_{2}^{+}) level.