Search

The level structure of neutron-deficient 58 Zn has been extracted experimen- tally. This nucleus is important for the rapid proton-capture process. 58 Zn was produced by using a (d,n)-type transfer reaction on 57 Cu in inverse kinematics at beam energies of 75 MeV/u. Several -ray transitions have been identified. The experiment utilized the state-of-the-art GRETINA -ray energy tracking array in conjunction with the large- acceptance spectrometer S800 at NSCL. The excitation energies of the identified low- lying states in 58 Zn are important for constraining the 57 Cu(p, ) 58 Zn reaction rate under X-ray burst conditions.

The 9Be(28Mg,27Na) one-proton removal reaction with a large proton separation energy of Sp(28Mg)=16.79 MeV is studied at intermediate beam energy. Coincidences of the bound 27Na residues with protons and other light charged particles are measured. These data are analyzed to determine the percentage contributions to the proton removal cross section from the elastic and inelastic nucleon removal mechanisms. These deduced contributions are compared with the eikonal reaction model predictions and with the previously measured data for reactions involving the re- moval of more weakly-bound protons from lighter nuclei. The role of transitions of the proton between different bound single-particle configurations upon the elastic breakup cross section is also quantified in this well-bound case. The measured and calculated elastic breakup fractions are found to be in good agreement., Comment: Phys. Rev. C 2014 (accepted)

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 nonyrast states of 124 Xe have been investigated with the 123 Te ( 3 He , 2 n ) reaction by means of in-beam γ -spectroscopy using a setup of 6 Compton-suppressed high-purity-germanium (HPGe) detectors. More than 65 new transitions, establishing 34 new levels, were observed. γγ -angular correlations were analyzed to assign spins and parities to the low-lying states, and to determine the multipolarities of the γ -transitions. In addition, the γ -excitation-function method was used for spin assignments. With these methods 10 (18) spin and parity assignments were made (some of them tentative) and 7 multipole mixing ratios could be determined. The excitation energies and B (E2) ratios of low-lying states are compared with predictions of the sd-IBM-1, giving rise to the identification of a 0 + state with O(6) quantum number σ=N−2 . Quadrupole shape invariants for the ground state and effective deformation parameters are calculated, showing an admixture of U(5) symmetry to the ground-state wave function.

A two-Q-phonon mixed-symmetry Jπ=22,ms+ state was identified in the nucleus 94Mo. This identification is based on lifetime information for the first nine 2+ states. The proton–neutron version of the interacting boson model (IBM-2) suggests the interpretation of the 26+ state of 94Mo as a two-Q-phonon 22,ms+ state. Recent microscopic calculations support this interpretation.

The non-yrast states of 123 Xe have been investigated with the reaction 123 Te( 3 He, 3n) by means of in-beam γ -spectroscopy using a setup of 6 Compton-suppressed high-purity-germanium (HPGe) detectors. More than 130 new transitions establishing over 35 new levels were observed. γγ angular correlations were analyzed to assign spins and parities to the low-lying states, and to determine the multipole mixing ratios of the γ -transitions. With this method 9 spin and parity assignments were made and 19 multipole mixing ratios could be determined. Excitation energies of negative-parity states and the branching ratios of their decays are compared with predictions of the Rigid Triaxial Rotor plus Particle model (RTRP).

The applicability of Coulomb dissociation reactions to determine the cross section for the inverse neutron capture reaction was explored using the reaction ${}^{8}$Li($\ensuremath{\gamma}$,$n$)${}^{7}$Li. A 69.5 MeV/nucleon ${}^{8}$Li beam was incident on a Pb target, and the outgoing neutron and ${}^{7}$Li nucleus were measured in coincidence. The deduced ($n$,$\ensuremath{\gamma}$) excitation function is consistent with data for the direct capture reaction ${}^{7}$Li($n$,$\ensuremath{\gamma}$)${}^{8}$Li and with low-energy effective field theory calculations.

Low spin states of the nucleus 126Xe were studied by means of γ-spectroscopy using the fusion evaporation reaction 123Te(α,n)126Xe performed at the Cologne fn tandem accelerator. The γγ coincidence and singles spectra were measured with the osiris -cube spectrometer. Beside the ground and quasi-gamma bands many other low lying states were observed. γγ directional correlations from oriented states (DCO) were analyzed to determine the multipolarities of the γ-transitions. We observed dominant E2-transitions in the quasi-gamma band and from the gamma- to the ground band but also a large M1-fraction in the 6+2→61+ transition. We additionally found a strong M1-decay depopulating the 2+4-state at 2064 keV. The lifetime of this state was measured using the Doppler shift attenuation technique. The experimental data are compared with a calculation using the Proton–Neutron Interacting Boson Model (IBM-2). Good agreement is reached in the vicinity of the O(6)-limit. The observed M1-transition of the 2+4 state is interpreted as due to a fragment of the lowest 2+ mixed-symmetry state predicted by the IBM-2.

A fundamental question in nuclear physics is what combinations of neutrons and protons can make up a nucleus. Many hundreds of exotic neutron-rich isotopes have never been observed; the limit of how many neutrons a given number of protons can bind is unknown for all but the lightest elements, owing to the delicate interplay between single particle and collective quantum effects in the nucleus. This limit, known as the neutron drip line, provides a benchmark for models of the atomic nucleus. Here we report a significant advance in the determination of this limit: the discovery of two new neutron-rich isotopes--40Mg and 42Al--that are predicted to be drip-line nuclei. In the past, several attempts to observe 40Mg were unsuccessful; moreover, the observation of 42Al provides an experimental indication that the neutron drip line may be located further towards heavier isotopes in this mass region than is currently believed. In stable nuclei, attractive pairing forces enhance the stability of isotopes with even numbers of protons and neutrons. In contrast, the present work shows that nuclei at the drip line gain stability from an unpaired proton, which narrows the shell gaps and provides the opportunity to bind many more neutrons.

Breakthrough measurements of a rare decay process in an excited barium nucleus pave the way for the development of techniques that probe the structure and decay modes of atomic nuclei. See Letter p.406 Most decays of quantum states involve the emission of one photon, but in processes first predicted by Nobel laureate Maria Goeppert-Mayer more than 75 years ago, exotic decays emitting two photons can occur. An analogous exotic decay process in atomic nuclei is the double-gamma decay, which has been observed in cases where the usual single-gamma decay is forbidden. In an experimental tour de force, Heiko Scheit and colleagues observe a double-gamma decay which is in competition with a single-gamma decay. Through decay of caesium-137, the first excited state of barium-137 is populated. Monitoring the emissions for more than 50 days and carefully excluding alternative explanations for their data, the authors are able to unambiguously measure the competitive double-gamma decay. Precise measurements of the double-gamma decay rate promise to give access to nuclear structure information that is elusive by other methods.

Non-yrast low spin states of the nucleus 134 Ce were studied by means of γ -spectroscopy following the β -decay of 134 Pr. The activity was produced with the reaction 117 Sn( 19 F, 4 n ) 134 Pr at the Cologne fn tandem accelerator in the center of the osiris -cube spectrometer. The β -decay of the ground and medium spin isomeric states in 134 Pr populates states with spins up to 8 ℏ and excitation energies up to 4.1 MeV in 134 Ce. Besides ground and quasi-gamma bands, an excited band based on the 0 + 2 -state (“ K=0 ”-band), the “ K=4 ”-band and many other low lying states were observed. The γγ angular correlations were analyzed to assign spins and parities to the excited states, and to determine the multipolarities of the γ -transitions. We found dominant E2-transitions in the quasi-gamma band and from the gamma to the ground band. The experimental data are compared with calculations using the Interacting Boson Model (IBM). Good agreement is reached in the vicinity of the O(6)-limit.

One-neutron knockout at intermediate beam energies, an experimental approach sensitive to the single-particle structure of exotic nuclei, has been applied to the well-bound N = 16 isotones 34Ar, 33Cl and 32S as well as to the N = 14 nucleus 32Ar where the knockout residue 31Ar is located at the proton drip line. The reduction of single-particle strength compared to USD shell-model calculations is discussed in the framework of correlation effects beyond the effective-interaction theory employed in the shell-model approach.

Dramatic advances are being made in nuclear structure physics because of the availability of facilities producing beams of exotic radioisotopes. Of these, silicon-42 is of particular interest for study of the effects of packing extra neutrons into a nucleus. Neutrons and protons in atomic nuclei are arranged in a series of energy levels or ‘shells’. When a shell has a full quota of particles, the nucleus is particularly stable: the ‘magic numbers’ for stability are 2, 8, 20, 28, 50, 82 and 126. Silicon-42 owes its existence to its 28 neutrons, but is still a short-lived atom. This short lifetime has been explained as a consequence of strong deformation but this is contradicted in a new study that suggests that silicon-42 is almost spherical. Nuclear shell structures—the distribution of the quantum states of individual protons and neutrons—provide one of our most important guides for understanding the stability of atomic nuclei. Nuclei with ‘magic numbers’ of protons and/or neutrons (corresponding to closed shells of strongly bound nucleons) are particularly stable1,2. Whether the major shell closures and magic numbers change in very neutron-rich nuclei (potentially causing shape deformations) is a fundamental, and at present open, question3,4. A unique opportunity to study these shell effects is offered by the 42Si nucleus, which has 28 neutrons—a magic number in stable nuclei—and 14 protons. This nucleus has a 12-neutron excess over the heaviest stable silicon nuclide, and has only one neutron fewer than the heaviest silicon nuclide observed so far5. Here we report measurements of 42Si and two neighbouring nuclei using a technique involving one- and two-nucleon knockout from beams of exotic nuclei6,7. We present strong evidence for a well-developed proton subshell closure at Z = 14 (14 protons), the near degeneracy of two different (s1/2 and d3/2) proton orbits in the vicinity of 42Si, and a nearly spherical shape for 42Si.

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 ground state of neutron-rich unbound $^{13}$Li was observed for the first time in a one-proton removal reaction from $^{14}$Be at a beam energy of 53.6 MeV/u. The $^{13}$Li ground state was reconstructed from $^{11}$Li and two neutrons giving a resonance energy of 120$^{+60}_{-80}$ keV. All events involving single and double neutron interactions in the Modular Neutron Array (MoNA) were analyzed, simulated, and fitted self-consistently. The three-body ($^{11}$Li+$n+n$) correlations within Jacobi coordinates showed strong dineutron characteristics. The decay energy spectrum of the intermediate $^{12}$Li system ($^{11}$Li+$n$) was described with an s-wave scattering length of greater than -4 fm, which is a smaller absolute value than reported in a previous measurement., Accepted for publication in Phys. Rev. C as a Rapid Communication

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

The ground state of Li-11 is the preeminent example of a two-neutron-halo nucleus and is part of an isobaric sextet. Another member of this sextet, the double isobaric analog of Li-11(g.s). in B-11, has been identified in the Be-12(p, 2n)B-11 reaction. The state was discovered through its two-proton decay branch. From detected 2p + Li-9 events, its excitation energy was determined to be 33.57(8) MeV using the invariant mass method. With the known masses of Li-11 and its isobaric analog state in Be-11, three members of the same sextet have measured masses for the first time, permitting the masses of the remaining members to be extrapolated using the isobaric multiplet mass equation. All members of this multiplet are expected to have a two-nucleon halo structure and this was found consistent with the evolution of the mass across the sextet. The momentum correlations of the detected protons were found to have both "diproton" and "cigar" components.

Excited states in C-16 were populated via the Be-9(N-17,C-16+gamma)X one-proton knockout reaction. The lifetime of the 2(1)(+) state in C-16 was measured using the recoil distance method. The extracted lifetime of tau(+)(21) = 11.4(-0.9)(+0.8)(stat) +/- 0.7(syst(B rho))(-1.5)(+0.0)(syst(feeding)) ps yields a deduced B(E2;2(1)(+) -> 0(1)(+)) = 4.21(-0.26)(+0.34)(stat)(-0.24)(+0.28)(syst(B rho))(-0.00)(+0.64)(syst(feeding)) e(2)fm(4) value in good agreement with a previous measurement. The one-proton knockout cross section is used to extract the proton amplitude of the C-16 2(1)(+) state, which confirms the neutron dominant character of this state. Gamma-ray branching ratios between the 2(2)(+) state and the 2(1)(+) and ground states were also determined. The results are compared with p-sd shell model and no-core shell model (with NN and NN + NNN) calculations. The inclusion of three-body forces are essential in order for the no-core shell model calculations to reproduce the experimental findings on the gamma-ray branching ratios.

The role of proton shell effects in the structure of the N=28 isotones 45Cl and 44S has been studied via one-proton knockout from 45Cl. We compare measured γ-ray intensities, inclusive and partial knockout cross sections, and the inclusive momentum distribution of outgoing 44S particles with shell-model and reaction-theory predictions. The strong population in this reaction of the recently identified 41+ state in 44S, identified through its subsequent γ-ray decay energy, makes a compelling case for a Jπ=3/2+ ground state in 45Cl.

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)

Expérience au NSCL; A systematic investigation of the inclusive cross sections for single-nucleon knockout reactions from p-shell nuclei has been performed. A total of seven reactions were studied for projectiles with masses between A = 7 and 10, having a wide range of nucleon separation energies. Results were obtained for a range of incident beam energies and targets. These differences were found to have a minimal impact on the deduced cross sections. Experimental results were compared to theoretical predictions based on variational Monte Carlo (VMC) nuclear structure calculations, whose radial overlap functions and neutron and proton densities were included in the reaction description. These results are compared with the conventional model, developed for heavier nuclei, that uses shell-model and Hartree-Fock structure inputs. The VMC-based calculations agreed with the experimental data for several reactions where deeply bound nucleons are removed but does not describe some of the more weakly bound nucleon removal cases with comparable accuracy.

Intermediate-energy Coulomb excitation measurements were performed on the neutron-rich isotopes ${}^{58,60,62}$Cr. The electric quadrupole excitation strengths, $B$($E$2; 0${}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+}$), of ${}^{60,62}$Cr are determined for the first time. The results quantify the trend of increasing quadrupole collectivity in the Cr isotopes approaching neutron number $N=40$. The results are confronted with large-scale shell-model calculations in the $fpgd$ shell using the state-of-the-art LNPS effective interaction. Different sets of effective charges are discussed that provide an improved and robust description of the $B(E2)$ values of the neutron-rich Fe and Cr isotopes in this region of rapid shell evolution. The ratio of the neutron and proton transition matrix elements, $|{M}_{n}/{M}_{p}|$, is proposed as an effective tool to discriminate between the various choices of effective charges.

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.

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 cross sections for single-nucleon knockout from Ca-36 on a Be-9 target at 70 MeV/nucleon were measured to be sigma(exp)(-p) = 51.1 +/- 2.6 mb for proton knockout and sigma(exp)(-n) = 5.03 +/- 0.46 mb for neutron knockout. The spectroscopic factors and orbital angular momenta of the neutrons and protons removed from Ca-36, leading to bound A = 35 residues, were deduced by comparison of the experimental cross sections and longitudinal- momentum distributions to those calculated in an eikonal reaction theory, and found to be S(p, 1d(3/2)) = 0.79 +/- 0.04 and S(n, 2s(1/2)) = 0.23 +/- 0.02 (relative to independent-particle-model values and only including experimental contributions to the uncertainties). As found in previous knockout studies, the spectroscopic factor deduced for the deeply bound neutron was significantly reduced relative to shell-model calculations, a result at variance with dispersive optical model (DOM) extrapolations, which suggest a spectroscopic factor closer to 60% of the independent-particle-model value.

We report on the first experimental study of quadrupole collectivity in the very neutron-rich nuclei $^{47,48}\mathrm{Ar}$ using intermediate-energy Coulomb excitation. These nuclei are located along the path from doubly magic Ca to collective S and Si isotopes, a critical region of shell evolution and structural change. The deduced $B(E2)$ transition strengths are confronted with large-scale shell-model calculations in the sdpf shell using the state-of-the-art SDPF-Uand EPQQM effective interactions. The comparison between experiment and theory indicates that a shell-model description of Ar isotopes around $N=28$ remains a challenge.

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.

The ground state of $^{28}$F has been observed as an unbound resonance $2\underline{2}0$ keV above the ground state of $^{27}$F. Comparison of this result with USDA/USDB shell model predictions leads to the conclusion that the $^{28}$F ground state is primarily dominated by $sd$-shell configurations. Here we present a detailed report on the experiment in which the ground state resonance of $^{28}$F was first observed. Additionally, we report the first observation of a neutron-unbound excited state in $^{27}$F at an excitation energy of $25\underline{0}0 (2\underline{2}0)$ keV., 10 pages, 11 figures, Accepted for publication in Phys. Rev. C

We present a novel technique for studying the quenching of shell gaps in exotic isotopes. The method is based on extracting Gamow-Teller ($\ensuremath{\Delta}L=0$, $\ensuremath{\Delta}S=1$) transition strengths [$B(\mathrm{GT})$] to low-lying states from charge-exchange reactions at intermediate beam energies. These Gamow-Teller strengths are very sensitive to configuration mixing between cross-shell orbitals, and this technique thus provides an important complement to other tools currently used to study cross-shell mixing. This work focuses on the $N=8$ shell gap. We populated the ground and 2.24 MeV ${0}^{+}$ states in $^{12}\mathrm{Be}$ using the $^{12}\mathrm{B}({1}^{+})$ ($^{7}\mathrm{Li}$, $^{7}\mathrm{Be}$) reaction at $80\text{ }\text{ }\mathrm{MeV}/u$ in inverse kinematics. Using the ground-state $B(\mathrm{GT})$ value from $\ensuremath{\beta}$-decay measurements ($0.184\ifmmode\pm\else\textpm\fi{}0.007$) as a calibration, the $B(\mathrm{GT})$ for the transition to the second ${0}^{+}$ state was determined to be $0.214\ifmmode\pm\else\textpm\fi{}0.051$. Comparing the extracted Gamow-Teller strengths with shell-model calculations, it was determined that the wave functions of the first and second ${0}^{+}$ states in $^{12}\mathrm{Be}$ are composed of $25\ifmmode\pm\else\textpm\fi{}5%$ and $60\ifmmode\pm\else\textpm\fi{}5%$ $(0s{)}^{4}(0p{)}^{8}$ configurations, respectively.

The technique of invariant mass spectroscopy has been used to measure, for the first time, the ground state energy of neutron-unbound $^{28}\mathrm{F}$, determined to be a resonance in the $^{27}\mathrm{F}+n$ continuum at $2\underline{2}0(\underline{5}0)\text{ }\text{ }\mathrm{keV}$. States in $^{28}\mathrm{F}$ were populated by the reactions of a $62\text{ }\text{ }\mathrm{MeV}/\mathrm{u}$ $^{29}\mathrm{Ne}$ beam impinging on a $288\text{ }\text{ }\mathrm{mg}/{\mathrm{cm}}^{2}$ beryllium target. The measured $^{28}\mathrm{F}$ ground state energy is in good agreement with USDA/USDB shell model predictions, indicating that $pf$ shell intruder configurations play only a small role in the ground state structure of $^{28}\mathrm{F}$ and establishing a low-$Z$ boundary of the island of inversion for $N=19$ isotones.

Background: Thick-target-induced nucleon-adding transfer reactions onto energetic rare-isotope beams are an emerging spectroscopic tool. Their sensitivity to single-particle structure complements one-nucleon removal reaction capabilities in the quest to reveal the evolution of nuclear shell structure in very exotic nuclei. Purpose: To add intermediate-energy, carbon-target-induced one-proton pickup reactions to the arsenal of $\gamma$-ray tagged direct reactions applicable in the regime of low beam intensities and to apply these for the first time to $fp$-shell nuclei. Methods: Inclusive and partial cross sections were measured for the $\nuc{12}{C}(\nuc{48}{Cr},\nuc{49}{Mn}+\gamma)$X and $\nuc{12}{C}(\nuc{50}{Fe},\nuc{51}{Co}+\gamma)$X proton pickup reactions at 56.7 and 61.2 MeV/nucleon, respectively, using coincident particle-$\gamma$ spectroscopy at the NSCL. The results are compared to reaction theory calculations using $fp$-shell-model nuclear structure input. For comparison with our previous work, the same reactions were measured on \nuc{9}{Be} targets. Results: The measured partial cross sections confirm the specific population pattern predicted by theory, with pickup into high-$\ell$ orbitals being strongly favored; driven by linear and angular momentum matching. Conclusion: Carbon target-induced pickup reactions are well-suited, in the regime of modest beam intensity, to study the evolution of nuclear structure, with specific sensitivities that are well described by theory., Comment: 9 pages, 5 figures, Physical Review C, Final accepted version

We report final-state-exclusive measurements of the light charged fragments in coincidence with 26Ne residual nuclei following the direct two-proton removal from a neutron-rich 28Mg secondary beam. A Dalitz-plot analysis and comparisons with simulations show that a majority of the triple- coincidence events with two protons display phase-space correlations consistent with the (two-body) kinematics of a spatially-correlated pair-removal mechanism. The fraction of such correlated events, 56(12) %, is consistent with the fraction of the calculated cross section, 64 %, arising from spin S = 0 two-proton configurations in the entrance-channel (shell-model) 28Mg ground state wave function. This result promises access to an additional and more specific probe of the spin and spatial correlations of valence nucleon pairs in exotic nuclei produced as fast secondary beams., Comment: accepted for publication in Physical Review Letters

The location of electron capture heat sources in the crust of accreting neutron stars depends on the masses of extremely neutron-rich nuclei. We present first results from a new implementation of the time-of-flight technique to measure nuclear masses of rare isotopes at the National Superconducting Cyclotron Laboratory. The masses of 16 neutron-rich nuclei in the scandium -- nickel range were determined simultaneously, improving the accuracy compared to previous data in 12 cases. The masses of $^{61}${V}, $^{63}${Cr}, $^{66}${Mn}, and $^{74}${Ni} were measured for the first time with mass excesses of $-30.510(890)$ MeV, $-35.280(650)$ MeV, $-36.900(790)$ MeV, and $-49.210(990)$ MeV, respectively. With the measurement of the $^{66}$Mn mass, the locations of the two dominant electron capture heat sources in the outer crust of accreting neutron stars that exhibit superbursts are now experimentally constrained. We find that the location of the $^{66}$Fe$\rightarrow^{66}$Mn electron capture transition occurs significantly closer to the surface than previously assumed because our new experimental Q-value is 2.1 MeV (2.6$\sigma$) smaller than predicted by the FRDM mass model., Comment: 4 pages, 3 figures

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.

Neutron-unbound states in ${}^{25}$F and ${}^{26}$F were populated via the reactions ${}^{9}$Be(${}^{26}$Ne,${}^{24}$F + n) and ${}^{9}$Be(${}^{26}$Ne,${}^{25}$F + n), respectively. A resonance close to the neutron separation energy in ${}^{25}$F was identified with a decay energy of $28\ifmmode\pm\else\textpm\fi{}4$ keV. This resonance corresponds to an excited state in ${}^{25}$F at $4249\ifmmode\pm\else\textpm\fi{}116$ keV assuming it decays to the ground state of ${}^{24}$F. Guided by shell-model calculations, a spin and parity of 1/2${}^{\ensuremath{-}}$ can be assigned to this state. In the spectrum of ${}^{26}$F, which was produced in a nucleon-exchange reaction, there are indications for an excited state with a decay energy of $\ensuremath{\sim}$270 keV.

Establishing how and when large N/Z values require modified or new theoretical tools is a major quest in nuclear physics. Here we report the first measurement of the lifetime of the 2{sup +}{sub 1} state in the near-dripline nucleus {sup 20}C. The deduced value of {tau}{sub #28;2{sup +}{sub 1}} = 9.8 ± 2.8(stat){sup +0.5}{sub −1.1}(syst) ps gives a reduced transition probability of B(E2;2{sup +}{sub 1}{yields}0{sup +}{sub g.s.}) = 7.5{sup +3.0}{sub −1.7}(stat){sup +1.0}{sub −0.4}(syst) e{sup 2}fm{sup 4} in good agreement with a shell model calculation using isospin-dependent effective charges.

The neutron-rich $N=28$ nucleus $^{44}\mathrm{S}$ was studied using the two-proton knockout reaction from $^{46}\mathrm{Ar}$ at intermediate beam energy. We report the observation of four new excited states, one of which is a strongly prolate deformed 4${}^{+}$ state, as indicated by a shell-model calculation. Its deformation originates in a neutron configuration which is fundamentally different from the ``intruder'' configuration responsible for the ground-state deformation. Consequently, we do not have three coexisting shapes in $^{44}\mathrm{S}$, but three coexisting configurations, corresponding to zero-, one-, and two-neutron particle-hole excitations.

We report on the first direct measurement of the proton separation energy for the proton-unbound nucleus {sup 69}Br. Bypassing the {sup 68}Se waiting point in the rp process is directly related to the 2p-capture rate through {sup 69}Br, which depends exponentially on the proton separation energy. We find a proton separation energy for {sup 69}Br of S{sub p}({sup 69}Br) = -785{sub -40}{sup +34} keV; this is less bound compared to previous predictions which have relied on uncertain theoretical calculations. The influence of the extracted proton separation energy on the rp process occurring in type I x-ray bursts is examined within the context of a one-zone burst model.

The proportionality between differential cross sections at vanishing linear momentum transfer and Gamow-Teller transition strength, expressed in terms of the unit cross section (${\mathrm{\ensuremath{\sigma}\ifmmode \hat{}\else \^{}\fi{}}}_{\mathit{GT}}$), was studied as a function of target mass number for ($t$,$^{3}\mathrm{He}$) and ($^{3}\mathrm{He}$,$t$) reactions at $115A$ MeV and $140A$ MeV, respectively. Existing ($^{3}\mathrm{He}$,$t$) and ($t$,$^{3}\mathrm{He}$) data on targets with mass number $12\ensuremath{\leqslant}A\ensuremath{\leqslant}120$ were complemented with new and reevaluated ($t$,$^{3}\mathrm{He}$) data on proton, deuteron, $^{6}\mathrm{Li}$, and $^{12}\mathrm{C}$ targets. It was found that in spite of the small difference in beam energies between the two probes, the unit cross sections have a nearly identical and simple dependence on target mass number $A$, for $A\ensuremath{\geqslant}12$: ${\mathrm{\ensuremath{\sigma}\ifmmode \hat{}\else \^{}\fi{}}}_{\mathit{GT}}=109/{A}^{0.65}$. The factorization of the unit cross sections in terms of a kinematical factor, a distortion factor, and the strength of the effective spin-isospin transfer nucleus-nucleus interaction was investigated. Simple phenomenological functions depending on mass number $A$ were extracted for the latter two. By comparison with plane and distorted-wave Born approximation calculations, it was found that the use of a short-range approximation for knock-on exchange contributions to the transition amplitude results in overestimated cross sections for reactions involving the composite ($^{3}\mathrm{He}$,$t$) and ($t$,$^{3}\mathrm{He}$) probes.