Search

Experimental knowledge of nuclear masses of exotic nuclei is important for understanding nuclear structure far from the valley of β‐stability, and as a direct input into astrophysical models. In the case of astrophysical processes involving neutron rich nuclei, such as nucleosynthesis during the r‐process and the evolution of matter in the crust of an accreting neutron star, we are mostly limited to using theoretical mass models. The time of flight (TOF) mass measurement technique allows measuring very short‐lived nuclei. It has been effectively applied using the fast fragment beams produced at the A1900 fragment separator at the National Superconducting Cyclotron Lab (NSCL) to reach masses very far from stability. We describe a recent mass measurement experiment in the neutron rich Fe region performed at the NSCL, and present preliminary results.

The nuclear potential and resulting shell structure are well established for the valley of stability, however, dramatic modifications to the familiar ordering of single-particle orbitals in rare isotopes have been found: new shell gaps emerge, some conventional magic numbers are no longer valid, and theoretical models that extrapolate from stability to describe observables in exotic nuclei can fail. Current efforts in nuclear structure physics are aimed at unraveling the driving forces behind this structural evolution, which was found most dramatic in neutron-rich species. This manuscript will outline some of the recent efforts at NSCL aimed at shedding light on the evolution of nuclear structure around neutron number N = 40 in neutron-rich Ti isotopes and in neutron-deficient Sn nuclei approaching 100Sn.

Direct reaction techniques are powerful tools to study the single-particle nature of nuclei. Performing direct reactions on short-lived nuclei requires radioactive ion beams produced either via fragmentation or the Isotope Separation OnLine (ISOL) method. Some of the most interesting regions to study with direct reactions are close to the magic numbers where changes in shell structure can be tracked. These changes can impact the final abundances of explosive nucleosynthesis. The structure of the chain of tin isotopes is strongly influenced by the Z=50 proton shell closure, as well as the neutron shell closures lying in the neutron-rich, N=82, and neutron-deficient, N=50, regions. Here we present two examples of direct reactions on exotic tin isotopes. The first uses a one-neutron transfer reaction and a low-energy reaccelerated ISOL beam to study states in 131Sn from across the N=82 shell closure. The second example utilizes a one-neutron knockout reaction on fragmentation beams of neutron-deficient 106,108Sn. In both cases, measurements of gamma rays in coincidence with charged particles proved to be invaluable., 11 pages, 5 figures, Zakopane Conference on Nuclear Physics "Extremes of the Nuclear Landscape", Zakopane, Poland, August 31 - September 7, 2014

The shell structure underlying shape changes in neutron-rich nuclei between $N=20$ and $N=28$ has been investigated by a novel application of the transient field technique to measure the first-excited state $g$ factors in $^{38}\mathrm{S}$ and $^{40}\mathrm{S}$ produced as fast radioactive beams. Details of the new methodology are presented. In both $^{38}\mathrm{S}$ and $^{40}\mathrm{S}$ there is a fine balance between the proton and neutron contributions to the magnetic moments. Shell-model calculations that describe the level schemes and quadrupole properties of these nuclei also give a satisfactory explanation of the $g$ factors. In $^{38}\mathrm{S}$ the $g$ factor is extremely sensitive to the occupation of the neutron ${p}_{3/2}$ orbit above the $N=28$ shell gap as occupation of this orbit strongly affects the proton configuration. The $g$ factor of deformed $^{40}\mathrm{S}$ does not resemble that of a conventional collective nucleus because spin contributions are more important than usual.

The one-neutron knockout reaction $^{9}\mathrm{Be}$($^{57}\mathrm{Cr}$,$^{56}\mathrm{Cr}$+\ensuremath{\gamma})X has been measured in inverse kinematics with an intermediate-energy beam. Cross sections to individual states in $^{56}\mathrm{Cr}$ were partially untangled through the detection of the characteristic \ensuremath{\gamma}-ray transitions in coincidence with the reaction residues. The experimental inclusive longitudinal momentum distribution and the yields to individual states are compared to calculations that combine spectroscopic factors from the full $\mathit{fp}$ shell model and nucleon-removal cross sections computed in a few-body eikonal approach.

The energy of the first excited state in the neutron-rich $N=28$ nucleus $^{45}\mathrm{Cl}$ has been established via in-beam \ensuremath{\gamma}-ray spectroscopy following proton removal. This energy value completes the systematics of the $E(1/{2}_{1}^{+})\ensuremath{-}E(3/{2}_{1}^{+})$ level spacing in odd-mass K, Cl, and P isotopes for $N=20\text{\ensuremath{-}}28$. The results are discussed in the framework of shell-model calculations in the $\mathit{sd}\text{\ensuremath{-}}\mathit{fp}$ model space. The contribution of the central, spin-orbit, and tensor components is discussed from a calculation based on a proton single-hole spectrum from $G$-matrix and $\ensuremath{\pi}+\ensuremath{\rho}$ meson exchange potentials. A composite model for the proton $0{d}_{3/2}\ensuremath{-}1{s}_{1/2}$ single-particle energy shift is presented.

Measurements of the $N=28$ isotones $^{42}\mathrm{Si}$, $^{43}\mathrm{P}$, and $^{44}\mathrm{S}$ using one- and two-proton knockout reactions from the radioactive beam nuclei $^{44}\mathrm{S}$ and $^{46}\mathrm{Ar}$ are reported. The knockout reaction cross sections for populating $^{42}\mathrm{Si}$ and $^{43}\mathrm{P}$ and a 184 keV \ensuremath{\gamma}-ray observed in $^{43}\mathrm{P}$ establish that the ${d}_{3/2}$ and ${s}_{1/2}$ proton orbits are nearly degenerate in these nuclei and that there is a substantial $Z=14$ subshell closure separating these two orbits from the ${d}_{5/2}$ orbit. The increase in the inclusive two-proton knockout cross section from $^{42}\mathrm{Si}$ to $^{44}\mathrm{S}$ demonstrates the importance of the availability of valence protons for determining the cross section. New calculations of the two-proton knockout reactions that include diffractive effects are presented. In addition, it is proposed that a search for the ${d}_{5/2}$ proton strength in $^{43}\mathrm{P}$ via a higher statistics one-proton knockout experiment could help determine the size of the $Z=14$ closure.

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

The inclusive cross sections for 38 different reaction products produced in the interaction of {sup 48}Ca, {sup 40}S, and {sup 42}S beams around 100 MeV/nucleon with a liquid deuterium target are reported. The cross sections for the {sup 48}Ca +{sup 2}H products are compared to those with {sup 48}Ca incident on the commonly used fragmentation targets {sup 9}Be and {sup 181}Ta and also to global calculations for fragmentation reaction cross sections based on the EPAX parameterization. The sizes of the measured reaction cross sections for the deuterium target were comparable to those of the cross sections measured on the heavier targets, indicating that nucleon removal from a deuterium target can be carried out for single- and multiple-nucleon knockout reaction studies. It was also found that the charge exchange cross sections were large enough that it should be possible to obtain nuclear structure information from these reactions.

Two-neutron knockout reactions from nuclei in the proximity of the proton dripline have been studied using intermediate-energy beams of neutron-deficient $^{34}\mathrm{Ar}$, $^{30}\mathrm{S}$, and $^{26}\mathrm{Si}$. The inclusive cross sections, and also the partial cross sections for the population of individual bound final states of the $^{32}\mathrm{Ar}$, $^{28}\mathrm{S}$ and $^{24}\mathrm{Si}$ knockout residues, have been determined using the combination of particle and \ensuremath{\gamma}-ray spectroscopy. Similar to the two-proton knockout mechanism on the neutron-rich side of the nuclear chart, these two-neutron removal reactions from already neutron-deficient nuclei are also shown to be consistent with a direct reaction mechanism.

The two-proton knockout reaction $^{9}\mathrm{Be}$($^{54}\mathrm{Ti}$,$^{52}\mathrm{Ca}+\ensuremath{\gamma}$) has been studied at 72 MeV/nucleon. Besides the strong feeding of the $^{52}\mathrm{Ca}$ ground state, the only other sizeable cross section proceeds to a ${3}^{\ensuremath{-}}$ level at 3.9 MeV. There is no measurable direct yield to the first excited ${2}^{+}$ state at 2.6 MeV. The results illustrate the potential of such direct reactions for exploring cross-shell proton excitations in neutron-rich nuclei and confirms the doubly-magic nature of $^{52}\mathrm{Ca}$.

The single-particle structure of 57 Ni and level structure of 56 Ni were investigated with the 9 Be ( 57 Ni, 56 Ni+γ)X reaction at 73 MeV/nucleon. An inclusive cross section of 41.4(12) mb was obtained for the reaction, compared to a theoretical prediction of 85.4 mb, hence only 48(2)% of the theoretical cross section is exhausted. This reduction in the observed spectroscopic strength is consistent with that found for lighter well-bound nuclei. One-neutron removal spectroscopic factors of 0.58(11) to the ground state and 3.7(2) to all excited states of 56 Ni were deduced.

Fragmentation reactions with intermediate-energy heavy-ion beams exhibit a wide range of reaction mechanisms, ranging from direct reactions to statistical processes. We examine this transition by measuring the relative population of excited states in several sd-shell nuclei produced by fragmentation with the number of removed nucleons ranging from two to sixteen. The two-nucleon removal is consistent with a non-dissipative process whereas the removal of more than five nucleons appears to be mainly statistical., Comment: 5 pages, 6 figures

The shell structure underlying shape changes in neutron-rich nuclei near N=28 has been investigated by a novel application of the transient-field technique to measure the first-excited-state g factors in {sup 38}S and {sup 40}S produced as fast radioactive beams. There is a fine balance between proton and neutron contributions to the magnetic moments in both nuclei. The g factor of deformed {sup 40}S does not resemble that of a conventional collective nucleus because spin contributions are more important than usual.

The odd-odd fp-shell nucleus $^{52}\mathrm{Sc}$ was investigated using in-beam \ensuremath{\gamma}-ray spectroscopy following secondary fragmentation of a $^{55}\mathrm{V}$ and $^{57}\mathrm{Cr}$ cocktail beam. Aside from the known \ensuremath{\gamma}-ray transition at 674(5) keV, a new decay at ${E}_{\ensuremath{\gamma}}=212(3)$ keV was observed. It is attributed to the depopulation of a low-lying excited level. This new state is discussed in the framework of shell-model calculations with the GXPF1, GXPF1A, and KB3G effective interactions. These calculations are found to be fairly robust for the low-lying level scheme of $^{52}\mathrm{Sc}$ irrespective of the choice of the effective interaction. In addition, the frequency of spin values predicted by the shell model is successfully modeled by a spin distribution formulated in a statistical approach with an empirical, energy-independent spin-cutoff parameter.

The method of intermediate-energy Coulomb excitation has been widely used to determine absolute B(E2;0{sub 1}{sup +}{yields}2{sub 1}{sup +}) quadrupole excitation strengths in exotic nuclei with even numbers of protons and neutrons. Transition rates measured with intermediate-energy Coulomb excitation are compared to their respective adopted values and for the example of {sup 26}Mg to the B(E2;0{sub 1}{sup +}{yields}2{sub 1}{sup +}) values obtained with a variety of standard methods. Intermediate-energy Coulomb excitation is found to have an accuracy comparable to those of long-established experimental techniques.

$B(E3;{0}_{1}^{+}\ensuremath{\rightarrow}{3}_{1}^{\ensuremath{-}})$ transition matrix elements have been measured for even-mass $^{124\ensuremath{-}134}\mathrm{Xe}$ nuclei using subbarrier Coulomb excitation in inverse kinematics. The trends in energy $E({3}^{\ensuremath{-}})$ and $B(E3;{0}_{1}^{+}\ensuremath{\rightarrow}{3}_{1}^{\ensuremath{-}})$ excitation strengths are well reproduced using phenomenological models based on a strong coupling picture with a soft quadrupole mode and an increasing occupation of the intruder ${h}_{11/2}$ orbital.

We explore the limits of nuclear stability and the consequences on nuclear structure theory by measuring masses of neutron‐unbound nuclei and level energies above the neutron separation energy such as for the first excited state in 24O. Open problems in reaction theory are addressed by, among others, measurements of the Coulomb breakup of 8Li on a Pb and C target at 40 and 70 MeV/u.

The degree of collectivity in the neutron-rich nuclei {sup 32}Mg and {sup 34}Mg has been determined via intermediate-energy Coulomb excitation in inverse kinematics. Measured energies of the first excited 2{sup +} states and reduced electric quadrupole transition probabilities B(E2;0{sub g.s.}{sup +}{yields}2{sub 1}{sup +}) are presented for {sup 32}Mg and {sup 34}Mg. The results agree with previous measurements and confirm the placement of both nuclei within the ''island of inversion.''.

High-resolution nuclear resonance fluorescence experiments (NRF) were performed on $^{110,111,112,114,116}\mathrm{Cd}$ at the bremsstrahlung facility of the 4.3-MV Dynamitron accelerator in Stuttgart to study the low-lying dipole strength distributions in these vibrational nuclei. Numerous excited states, most of them previously unknown, were observed in the excitation energy range up to 4 MeV. Detailed spectroscopic information has been obtained on excitation energies, spins, decay widths, decay branchings, and transition probabilities. For states in the even-even isotopes $^{110,112,114,116}\mathrm{Cd}$, parities could be assigned from linear polarization measurements. Together with our previous results for $^{108,112,113,114}\mathrm{Cd}$ from NRF studies without polarization measurements, systematics was established for the dipole strength distributions of the stable nuclei within the Cd isotopic chain. The results are discussed with respect to the systematics of $E1$ two-phonon excitations and mixed-symmetry states in even-even nuclei near the $Z=50$ shell closure and the fragmentation of these excitation modes in the odd-mass Cd isotopes.

Low-lying excited states of $^{46}\mathrm{Ar}$ have been studied via inverse-kinematics proton scattering with a thick target. Coupled-channels calculations have been used to extract the deformation length of the ${2}_{1}^{+}$ state. This result, combined with existing Coulomb excitation data, yields a ratio of the neutron-to-proton transition matrix elements of ${M}_{n}/{M}_{p}=1.19(25)N/Z$, showing a departure from the proton dominance observed in the $N=28$ isotones above $^{48}\mathrm{Ca}$. The status of the $N=28$ shell below $^{48}\mathrm{Ca}$ is discussed.

We report on the first determination of the absolute B(E2;0+(1)-->2+(1)) excitation strength in the N=Z nucleus 72Kr. 72Kr is the heaviest N=Z nucleus for which this quantity has been measured and provides a benchmark in a region of the nuclear chart dominated by rapidly changing deformations and shapes mediated by the interplay of strongly oblate and prolate-driving orbitals. The deduced quadrupole deformation strength is in agreement with a variety of self-consistent models that predict an oblate shape for the ground state of 72Kr. Large-scale shell-model Monte Carlo calculations reproduce the experimental B(E2) value and link the result to the occupation of the deformation-driving g9/2 orbit.

The $^{9}\mathrm{Be}(^{46}\mathrm{Ar},^{45}\mathrm{Ar}+\ensuremath{\gamma})X$ one-neutron removal reaction has been studied in inverse kinematics at 70 MeV/nucleon. Coincidences with \ensuremath{\gamma} rays served to disentangle knockout events leading to the $^{45}\mathrm{Ar}$ ground state. The measured partial cross section corresponds to a spectroscopic factor of 4.9(7). The residue momentum distribution is compared with new calculations based on eikonal theory and represents the first case of an $\ensuremath{\ell}=3$ neutron removal, as is expected for populating a $0{f}_{7/2}$ hole in the $N=28$ projectile. However, the measured $^{45}\mathrm{Ar}$ momentum distribution has a marked low-momentum tail suggestive of dissipative effects whereas the eikonal model predictions are symmetric. The angular distribution of the residues confirms that there is a deviation from the model.

The even $^{52\ensuremath{-}56}\mathrm{Ti}$ isotopes have been studied with intermediate-energy Coulomb excitation and absolute $B(E2;{0}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ transition rates have been obtained. These data confirm the presence of a subshell closure at neutron number $N=32$ in neutron-rich nuclei above the doubly magic nucleus $^{48}\mathrm{Ca}$ and provide no direct evidence for the predicted $N=34$ closure. Large-scale shell model calculations with the most recent effective interactions are unable to reproduce the magnitude of the measured strengths in the semimagic Ti nuclei and their strong variation with neutron number.

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

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

The configurations of calcium nuclei make them good test cases for studies of nuclear properties. The measurement of the masses of two heavy calcium nuclei provides benchmarks for models of atomic nuclei. See Letter p.346 The calcium atom provides an ideal system for the study of nuclear shell evolution, from the valley of stability to the limits of existence. Although predictions for the masses of the neutron-rich isotopes 51Ca and 52Ca have been tested by direct measurements, it is an open question as to how nuclear masses evolve for heavier calcium isotopes. Frank Wienholtz and colleagues report the mass determination of the exotic calcium isotopes 53Ca and 54Ca, using a multi-reflection time-of-flight mass spectrometer. The results provide key information for theoretical models and show that a description of extreme neutron-rich nuclei can be closely connected to a deeper understanding of nuclear forces.

The collectivity of the odd-mass nucleus $^{55}\mathrm{Ni}$ was explored via intermediate-energy Coulomb excitation using a powerful combination of particle and $\ensuremath{\gamma}$-ray spectroscopy. A $\ensuremath{\gamma}$-ray at 2879(18) keV was observed and is interpreted to deexcite a member of the core-coupled quintuplet ${2}_{1}^{+}(^{56}\mathrm{Ni})\ensuremath{\bigotimes}\ensuremath{\nu}{f}_{7∕2}^{\ensuremath{-}1}$ at the same energy. By similarity with the mirror nucleus $^{55}\mathrm{Co}$, transition probabilities were calculated assuming ${J}^{\ensuremath{\pi}}=9∕{2}^{\ensuremath{-}}$ and ${J}^{\ensuremath{\pi}}=11∕{2}^{\ensuremath{-}}$ for this state. Both assumptions lead to a transition strength higher than predicted by a large-scale shell-model calculation using the GXPF1 effective interaction and exceed the value predicted within a simple weak-coupling approach.

The isotopes {sup 54}Ni, {sup 56}Ni, and {sup 58}Ni have been studied via intermediate-energy Coulomb excitation at the National Superconducting Cyclotron Laboratory. Absolute B(E2;0{sub 1}{sup +}{yields}2{sub 1}{sup +}) excitation strengths have been determined for all three nuclei using a consistent experimental approach, and a measure of quadrupole collectivity has been established in the vicinity of the much discussed doubly magic shell closure at N=Z=28. The energy of and excitation strength to the first 2{sup +} state of {sup 54}Ni have been observed for the first time. The results are compared to large-scale shell-model calculations that predict that the E2 strength in {sup 54}Ni should be divided between the first and second 2{sup +} states.

The low-lying dipole strength distributions in the odd-mass isotopes $^{135,137}\mathrm{Ba}$ were studied in nuclear resonance fluorescence experiments (NRF) performed at the Stuttgart Dynamitron facility using bremsstrahlung beams with end point energies of 4.1, 3.1, and $2.5\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. Numerous excited states, most of them unknown so far, were observed in the excitation energy range up to $4\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. Detailed spectroscopic information has been obtained on excitation energies, decay widths, decay branching ratios, and transition probabilities. The results for $^{137}\mathrm{Ba}$ are compared with calculations in the framework of the Quasiparticle-Phonon Model. The new data for $^{135,137}\mathrm{Ba}$ complete the systematics of low-lying dipole excitations as observed for the even Ba isotopes $^{134,136,138}\mathrm{Ba}$ in previous NRF experiments in Stuttgart. The complete systematics within the Ba isotopic chain, exhibiting a nuclear shape transition, is discussed with respect to $E1$ two-phonon excitations, $M1$ scissors mode excitations, and in regard to the new critical point symmetries.

The nucleus 52 Fe with s N = Z =26 d has been investigated using intermediate-energy Coulomb excitation in inverse kinematics. A reduced transition probability of B s E 2;0 1 + ! 2 1 + d = 817 s 102 d e 2 fm 4 to the first excited 2 + state at 849.0 ( 5 ) keV was deduced. The increase in excitation strength B s E 2 " d with respect to the even-mass neighbor 54 Fe ( B s E 2 " d = 620 s 50 d e 2 fm 4 ) agrees with shell-model expectations as the magic number N =28 is approached. This measurement completes the systematics of reduced transition strengths to the first excited 2 + state for the even-even N = Z nuclei up to mass A = 56 ispartof: Physical Review C, Nuclear Physics vol:70 issue:3 status: published

The 9Be(32Ar, 31Ar)X reaction, leading to the 5/2+ ground state of a nucleus at the proton drip line, has a cross section of 10.4(13) mb at a beam energy of 65.1 MeV/nucleon. This translates into a spectroscopic factor that is only 24(3)% of that predicted by the many-body shell-model theory. We introduce refinements to the eikonal reaction theory used to extract the spectroscopic factor to clarify that this very strong reduction represents an effect of nuclear structure. We suggest that it reflects correlation effects linked to the high neutron separation energy (22.0 MeV) for this state.

A new experimental approach was developed that can reduce the uncertainties in astrophysical rapid proton capture ($rp$) process calculations due to nuclear data. This approach utilizes neutron removal from a radioactive ion beam to populate the nuclear states of interest. Excited states were deduced by the $\ensuremath{\gamma}$-decay spectra measured in a semiconductor Ge-detector array. In the first case studied, $^{33}\mathrm{Ar}$, excited states were measured with uncertainties of several keV. The 2 orders of magnitude improvement in the uncertainty of the level energies resulted in a 3 orders of magnitude improvement in the uncertainty of the calculated $^{32}\mathrm{Cl}(p,\ensuremath{\gamma})^{33}\mathrm{Ar}$ rate that is critical to the modeling of the $rp$ process. This approach has the potential to measure key properties of almost all interesting nuclei on the $rp$-process path.

One-neutron knockout reactions from the deeply bound N=16 isotones with Z=16,17, and 18 have been studied in inverse kinematics with intermediate-energy beams. gamma-ray spectroscopy in coincidence with the detection of knockout residues allowed for an investigation of the one-neutron removal leading to individual excited states. Spectroscopic factors are deduced in the framework of the sudden and eikonal approximations and are compared to USD shell-model predictions. The momentum distributions observed in the experiment are used to identify the angular momentum l carried by the knockedout neutron by comparing with calculations based on a black-disk reaction model. The systematics of reduced single-particle occupancies attributed to the effect of short-range correlations, observed so far for stable and near-magic nuclei in (e,e(')p) and (d,He-3) reactions and in one-nucleon knockout on light deeply bound systems, are extended in this work.

The Coupled Cyclotron Facility at the National Superconducting Cyclotron Laboratory at Michigan State University provides a large variety of new isotopes previously inaccessible and others at rates sufficienct for in‐beam spectroscopy. This talk presents some of our recent results elucidating the structure of exotic nuclei.After a general overview of scientific highlights from the first two years of operation particular results from several in‐beam gamma‐ray spectroscopy experiments in the vicinities of neutron numbers N=16, N=20, N=28 in the π(sd) shell around 56Ni will be discussed. Inelastic scattering experiments with gamma‐ray detection on light and heavy targets have determined specific transition matrix elements and excited state energies. One‐ and two‐particle nucleon knockout reactions were used to investigate the wave functions of specific states and to deduce corresponding spectroscopic factors.

Electric dipole transitions between ${3}_{1}^{\ensuremath{-}}$ octupole-phonon states and one-phonon ${2}_{1,\mathrm{ms}}^{+}$ mixed-symmetry states of ${}^{92}\mathrm{Zr},$ ${}^{94}\mathrm{Mo},$ ${}^{96}\mathrm{Ru},$ and ${}^{142}\mathrm{Ce}$ are observed and provide the first evidence for F-vector $E1$ transitions in nuclei. For effective boson models of the nuclear valence shell, F spin is the analog of isospin in the shell model. The ${3}_{1}^{\ensuremath{-}}\ensuremath{\rightarrow}{2}_{1,\mathrm{ms}}^{+}$ $E1$ transitions are stronger than those of the ${2}_{1}^{+}$ state. The effective $E1$ operator in the sdf-interacting boson model-2 is concluded to have F-vector character, in analogy with the well-known isovector character of the $E1$ operator in microscopic models.

Low-spin states in the odd-odd $N=Z$ nucleus ${}^{58}\mathrm{Cu}$ were investigated with the ${}^{58}\mathrm{Ni}(p,n\ensuremath{\gamma}{)}^{58}\mathrm{Cu}$ fusion evaporation reaction at the FN-TANDEM accelerator in Cologne. $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidences, $\ensuremath{\gamma}\ensuremath{\gamma}$-angular correlations, and signs of $\ensuremath{\gamma}$-ray polarizations were measured. Seventeen low-spin states below 3.6 MeV and 17 new transitions were observed. Ten multipole mixing ratios and 17 $\ensuremath{\gamma}$-branching ratios were determined for the first time. New detailed spectroscopic information on the ${2}_{2}^{+}$ state, the isobaric analogue state (IAS) of the ${2}_{1}^{+},T=1$ state of ${}^{58}\mathrm{Ni},$ makes ${}^{58}\mathrm{Cu}$ the heaviest odd-odd $N=Z$ nucleus with known ${B(E2;2}^{+},T=\stackrel{\ensuremath{\rightarrow}}{1}{0}^{+},T=1)$ value. The ${4}^{+}$ state at 2.751 MeV, observed here for the first time, is identified as the IAS of the ${4}_{1}^{+},T=1$ state in ${}^{58}\mathrm{Ni}.$ The new data are compared to full $\mathrm{pf}$-shell-model calculations with the GXPF1 residual interaction and to calculations within a ${\mathrm{pf}}_{5/2}$ configurational space with a residual surface $\ensuremath{\delta}$ interaction. The role of the ${}^{56}\mathrm{Ni}$ core excitations for the low-spin structure in ${}^{58}\mathrm{Cu}$ is discussed.

Intermediate-energy Coulomb excitation is a key method to investigate collectivity in exotic nuclei far from $\ensuremath{\beta}$ stability. We report on the measurement of the absolute ${B(E2;0}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ excitation strength in ${}^{46}\mathrm{Ar}$ for five different minimum impact parameters. Our findings underline the validity, feasibility, and perspective of this technique for the study of exotic nuclei also in the regime of higher beam energy.

The reaction $^{9}\mathrm{B}\mathrm{e}(^{28}\mathrm{M}\mathrm{g},^{26}\mathrm{N}\mathrm{e}+\ensuremath{\gamma})X$ has been studied at $82\text{ }\text{ }\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{n}\mathrm{u}\mathrm{c}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{n}$ together with two similar cases, $^{30}\mathrm{M}\mathrm{g}$ and $^{34}\mathrm{S}\mathrm{i}$. Strong evidence that the reactions are direct is offered by the parallel-momentum distributions of the reaction residues and by the inclusive cross sections. The pattern of the partial cross sections for $^{28}\mathrm{M}\mathrm{g}$ suggests the presence of correlations. A preliminary theoretical discussion based on eikonal reaction theory and the many-body shell model is presented. The reaction holds great promise for the study of neutron-rich nuclei.

Excited low-spin states in ${}^{94}\mathrm{Mo}$ have been studied extensively with a combination of $\ensuremath{\gamma}$-ray spectroscopic techniques. We have performed photon scattering experiments, $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidence measurements following the ${\ensuremath{\beta}}^{+}$ decay of ${}^{94}{\mathrm{Tc}}^{\mathrm{m}},$ in-beam $\ensuremath{\gamma}\ensuremath{\gamma}$ coincidence studies with the reaction ${}^{91}\mathrm{Zr}(\ensuremath{\alpha}{,n)}^{94}\mathrm{Mo},$ and ${}^{94}\mathrm{Mo}{(n,n}^{\ensuremath{'}}\ensuremath{\gamma})$ measurements. These experiments yielded detailed information about the low-spin level scheme of ${}^{94}\mathrm{Mo}.$ From measured lifetimes, branching ratios, and multipole mixing ratios, many absolute transition strengths were determined. These data permitted the interpretation of several low-spin states in terms of collective excitations, that are not fully symmetric with respect to the proton-neutron degree of freedom, the so-called mixed-symmetry (MS) states: The one-phonon ${2}^{+}$ MS state and three members of an expected multiplet of two-phonon MS states were identified from absolute $M1$ and $E2$ transition strengths. The data also enable us to identify several proton-neutron-symmetric multiphonon states.

108Cd was investigated using the powerful combination of two complementary experimental techniques: γγ‐spectroscopy following the β‐decay of 108In and the non‐selective (α,n) fusion evaporation reaction. This resulted in the observation of 120 new states and more than 580 new transitions. 30 new spin assignments and more than 80 multipole mixing ratios were derived from γγ‐angular correlation analyses and eight effective lifetimes stem from a Doppler Shift Attenuation (DSA) analysis following the fusion evaporation reaction. The proton 2p‐2h excited intruder band, which is typical for near proton‐magic nuclei with a sizable imbalance between valence protons and neutrons, was established up to the 4+ member including lower limits for the absolute E2 transition strengths of interband and intraband decays. The heavily suppressed absolute E2 transition strength out of the proposed intruder band indicates pure bands with little mixing. These findings are compared to neighboring Cd isotopes and related six quas...

The Interacting Boson Model (IBM-2) predicts a class of low-lying states, which are nonsymmetric with respect to the proton-neutron degree of freedom [1,2]. Such collective mixed-symmetric (MS) states have been identified in the nucleus 94Mo [3–5], which lies two neutrons above the N = 50 shell closure, applying a powerful combination of experimental techniques: real photon scattering, fusionevaporation reactions and a β-decay study. Thus, the onephonon MS 2 ms + state has been identified, as well as members of the multiplet resulting from the coupling of the symmetric (2 1 + )and the MS (2 ms + ) one-phonon quadrupole excitations. The most prominent member of this multiplet is the two-phonon MS 1 ms + state, which is known as scissors mode [6] in axially deformed nuclei. The MS character of these states has also been confirmed by recent shell model calculations [7]. In the framework of a systematic study of MS states in nuclei around the N = 50 shell closure, the nucleus 96Mo has been of special interest for the investigation of the evolution of the 2 ms + state as a function of deformation. A photon scattering experiment has been performed at the Stuttgart Dynamitron accelerator.

The nucleus ${}^{66}\mathrm{Zn}$ is investigated using $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidence studies following the $\ensuremath{\beta}$ decay of the ${0}^{+}$ ground state of ${}^{66}\mathrm{Ga}.$ The data survey short-lived ${J}^{\ensuremath{\pi}}{=1}^{+}{,2}^{+}$ states, $E2/M1$ mixing ratios, and, together with the previously known lifetimes, also absolute transition strengths $B(\ensuremath{\sigma}L).$ The proton-neutron mixed-symmetry states ${2}_{\mathrm{ms}}^{+}$ (isovector quadrupole excitation) and ${1}_{\mathrm{ms}}^{+}$ (``scissors mode'') were identified from their decay patterns and absolute transition strengths. A $\ensuremath{\gamma}$ transition between these mixed-symmetry states was observed and its rate is discussed. We additionally present a candidate for the two-phonon ${2}_{2,ms}^{+}$ state, whose structure is closely related to the ${1}_{\mathrm{ms}}^{+}$ excitation. Fifteen $M1$ and $E2$ strengths involving mixed-symmetry states show a reasonable agreement with the prediction of the pure O(6) limit of the proton-neutron extension of the interacting boson model (IBM-2) using the effective quadrupole boson charges ${e}_{\ensuremath{\pi}}{+e}_{\ensuremath{\nu}}$ and the g-factor difference ${g}_{\ensuremath{\pi}}\ensuremath{-}{g}_{\ensuremath{\nu}}$ as the only free parameters.