Search

Knowledge of the low-lying monopole strength in $\mathrm{^{12}C}$ $-$ the Hoyle state in particular $-$ is crucial for our understanding of both the astrophysically important $3\alpha$ reaction and of $\alpha$-particle clustering. Multiple theoretical models have predicted a breathing mode of the Hoyle State at $E_{x} \approx 9$ MeV, corresponding to a radial in-phase oscillation of the underlying $\alpha$ clusters. The $\mathrm{^{12}C}(\alpha, \alpha^{\prime})\mathrm{^{12}C}$ and $\mathrm{^{14}C}(p, t)\mathrm{^{12}C}$ reactions were employed to populate states in $^{12}$C in order to search for this predicted breathing mode. A self-consistent, simultaneous analysis of the inclusive spectra with R-matrix lineshapes, together with angular distributions of charged-particle decay, yielded clear evidence for excess monopole strength at $E_{x} \approx 9$ MeV which is highly collective. Reproduction of the experimentally observed inclusive yields using a fit, with consistent population ratios for the various broad states, required an additional source of monopole strength. The interpretation of this additional monopole resonance as the breathing-mode excitation of the Hoyle state would provide evidence supporting a $\mathcal{D}_{3h}$ symmetry for the Hoyle state itself. The excess monopole strength may complicate analysis of the properties of the Hoyle state, modifying the temperature dependence of the $3\alpha$ rate at $T_{9} \gtrsim 2$ and ultimately, the predicted nucleosynthesis in explosive stars., Comment: 8 pages, 5 figures

The status of a project to measure spin-isospin responses of neutron drip-line nuclei using a new low-energy neutron detector, PANDORA (Particle Analyzer Neutron Detector Of Real-time Acquisition), is reported. The performance of PANDORA was characterized by the 6He(p,n)6Li reaction in inverse kinematics at the HIMAC facility in Chiba. Observation of the strong transition to the ground state in 6Li is discussed. Preliminary results of 11Li(p,n)11Be and 14Be(p,n)14B experiments in inverse kinematics at RI Beam Factory (RIBF) of RIKEN Nishina Center are also presented including the exotic decay channel of 11Be→9Li + d. Details of the experimental setup based on PANDORA and the SAMURAI large-acceptance magnetic spectrometer, as well as the combined data-acquisition system are described. The neutron-gamma discrimination capability of PANDORA was evaluated, Figure-of-Merit (FoM) values higher than those found in the literature for similar materials were derived from experimental data.

International audience; “Stretched” states are examples of the simplest nuclear excitations in the continuum, thus offering an excellent testing ground for various theoretical approaches. The decay of the stretched single-particle state in 13C, located at 21.47 MeV, was investigated in an experiment performed recently at the Cyclotron Centre Bronowice (CCB) at IFJ PAN in Kraków. First experimental information on the proton and neutron decay channels of this resonance was obtained by employing coincidence measurement of protons inelastically scattered on the 13C target and γ rays from daughter nuclei. The new experimental findings will be used for testing predictions obtained by the Gamow Shell Model calculations.

Background: Mg 24 is a strongly deformed nucleus in the ground state. Deformation effects can be observed in the structure of the isoscalar giant monopole and quadrupole resonances. Mg 24 is also a nucleus that is well known to present different types of cluster-oscillation modes. Both giant resonances and cluster states are strongly populated by isoscalar transitions. Purpose: To extract the E 0 , E 1 , and E 2 transition strengths via Li 6 scattering. The Li 6 probe is a powerful tool for investigating the isoscalar nuclear response with a very favorable ratio of resonance-to-continuum background. Method: Double-differential cross sections of Li 6 inelastic scattering, at the beam energy of 100 MeV/u, were measured in the excitation-energy range 10 – 40 MeV and scattering angles 0 − 3 ∘ . A multipole-decomposition analysis was performed for extracting the isoscalar E 0 , E 1 , and E 2 strength distributions. Results: The extracted multipole strengths were compared with predictions from consistent quasiparticle random phase approximation calculations. The theoretical predictions are in fair agreement with the experimental data. The E 0 strength was also compared with results from antisymmetrized molecular dynamics calculations found in the literature. A few peaks in the experimental data might be associated with clustering in Mg 24 . Conclusions: Ground-state deformation effects were observed in the isoscalar giant monopole resonance (ISGMR) and isoscalar giant quadrupole resonance (ISGQR) distributions. The ISGMR strength is split in two peaks around 19 and 28 MeV. The ISGQR exhibits a pronounced peak at 20 MeV with a broadening at the low-energy region, similar to predictions from microscopic calculations. Signatures of excitation of cluster states were observed in the E 0 response. Further studies including particle-decay measurements will be required to confirm the nature of the observed peaks.

Journal of physics / Conference series 1643, 012181 (2020). doi:10.1088/1742-6596/1643/1/012181, Published by IOP Publ., Bristol

Spin–isospin excitations in Nb100 were studied via the Mo100(t,He3) charge-exchange reaction at 115 MeV/u with the goal to constrain theoretical models used to describe the isovector spin response of nuclei. The experiment was performed with a secondary beam of tritons, and 3He particles were analyzed in the S800 magnetic spectrometer. Decay by neutron emission from excited states in Nb100 was observed by using plastic and liquid scintillator arrays. Differential cross sections were analyzed and monopole excitations were revealed by using a multipole decomposition analysis. The Gamow–Teller transition strength observed at low excitation energies, which is important for estimating the electron-capture rate in astrophysical scenarios, was strongly fragmented and reduced compared to single-particle and spherical mean-field models. The consideration of deformation in the theoretical estimates was found to be important to better describe the fragmentation and strengths. A strong excitation of the isovector spin giant monopole resonance was observed, and well reproduced by the mean-field models. Its presence makes the extraction of Gamow–Teller strengths at high excitation energies difficult. The branches for statistical and direct decay by neutron emission were identified in the spectra. The upper limit for the branching ratio by direct decay (integrated over all observed excitations) was determined to be 20±6%. Even though the statistical uncertainties in the neutron-coincident data were too large to perform detailed studies of the decay by neutron emission from individual states and resonances, the experiment demonstrates the feasibility of the method.

Spin–isospin excitations in Nb 100 were studied via the Mo 100 ( t , He 3 ) charge-exchange reaction at 115 MeV/u with the goal to constrain theoretical models used to describe the isovector spin response of nuclei. The experiment was performed with a secondary beam of tritons, and 3He particles were analyzed in the S800 magnetic spectrometer. Decay by neutron emission from excited states in Nb 100 was observed by using plastic and liquid scintillator arrays. Differential cross sections were analyzed and monopole excitations were revealed by using a multipole decomposition analysis. The Gamow–Teller transition strength observed at low excitation energies, which is important for estimating the electron-capture rate in astrophysical scenarios, was strongly fragmented and reduced compared to single-particle and spherical mean-field models. The consideration of deformation in the theoretical estimates was found to be important to better describe the fragmentation and strengths. A strong excitation of the isovector spin giant monopole resonance was observed, and well reproduced by the mean-field models. Its presence makes the extraction of Gamow–Teller strengths at high excitation energies difficult. The branches for statistical and direct decay by neutron emission were identified in the spectra. The upper limit for the branching ratio by direct decay (integrated over all observed excitations) was determined to be 20 ± 6 % . Even though the statistical uncertainties in the neutron-coincident data were too large to perform detailed studies of the decay by neutron emission from individual states and resonances, the experiment demonstrates the feasibility of the method.

Physics letters / B 763, 16 - 19 (2016). doi:10.1016/j.physletb.2016.10.015, Published by North-Holland Publ., Amsterdam

"Why are the tin isotopes soft?" has remained, for the past decade, an open problem in nuclear structure physics: models which reproduce the isoscalar giant monopole resonance (ISGMR) in the "doubly-closed shell" nuclei, $^{90}$Zr and $^{208}$Pb, overestimate the ISGMR energies of the open-shell tin and cadmium nuclei, by as much as 1 MeV. In an effort to shed some light onto this problem, we present results of detailed studies of the ISGMR in the molybdenum nuclei, with the goal of elucidating where--and how--the softness manifests itself between $^{90}$Zr and the cadmium and tin isotopes. The experiment was conducted using the $^{94,96,98,100}$Mo($\alpha,\alpha^\prime$) reaction at $E_\alpha = 386$ MeV. A comparison of the results with relativistic, self-consistent Random-Phase Approximation calculations indicates that the ISGMR response begins to show softness in the molybdenum isotopes beginning with $A=92$., Comment: Accepted for publication to Physics Letters B

International audience; An experiment aiming at measuring lifetimes of the second 2+ excited states in neutron-rich C and O isotopes was recently performed at the GANIL laboratory, in France. In these nuclei, recent ab initio calculations provide a detailed description of the nuclear excited states, predicting a strong sensitivity of the electromagnetic transition probabilities to the details of the nucleon–nucleon interactions, especially in connection with the role played by the three-body (NNN) forces. Thus, the measurement of 2 + states lifetimes in this region will be a stringent test of the importance of including the NNN forces in describing the electromagnetic properties of selected nuclear states. This contribution reports on the status of the experimental analysis for the case of lifetimes in 20O.

The Isoscalar Giant Monopole Resonance (ISGMR) and the Isoscalar Giant Dipole Resonance (ISGDR) compression modes have been studied in the doubly-magic unstable nucleus 56Ni. They were measured by inelastic α-particle scattering in inverse kinematics at 50MeV/u with the MAYA active target at the GANIL facility. The centroid of the ISGMR has been obtained at Ex=19.1 ±0.5MeV. Evidence for the low-lying part of the ISGDR has been found at Ex=17.4 ±0.7MeV. The strength distribution for the dipole mode shows similarity with the prediction from the Hartree–Fock (HF) based random-phase approximation (RPA)[1]. These measurements confirm inelastic α-particle scattering as a suitable probe for exciting the ISGMR and the ISGDR modes in radioactive isotopes in inverse kinematics This work was supported by the European Commission within the Seventh Framework Programme through IA-ENSAR (contract No. RII3-CT-2010-262010) and GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany, and the United States National Science Foundation (Grants Number PHY-1068192 and PHY-1419765) SI

Excitation-energy spectra of $^{11}\mathrm{C}$ nuclei near the ${\ensuremath{\eta}}^{\ensuremath{'}}$-meson production threshold have been measured by missing-mass spectroscopy using the $^{12}\mathrm{C}(p,d)$ reaction. A carbon target has been irradiated with a 2.5 GeV proton beam supplied by the synchrotron SIS-18 at GSI to produce ${\ensuremath{\eta}}^{\ensuremath{'}}$-meson bound states in $^{11}\mathrm{C}$ nuclei. Deuterons emitted at ${0}^{\ensuremath{\circ}}$ in the reaction have been momentum analyzed by the fragment separator (FRS), used as a high-resolution spectrometer. No distinct structure due to the formation of ${\ensuremath{\eta}}^{\ensuremath{'}}$-mesic states is observed although a high statistical sensitivity is achieved in the experimental spectra. Upper limits on the formation cross sections of ${\ensuremath{\eta}}^{\ensuremath{'}}$-mesic states are determined, and thereby a constraint imposed on the ${\ensuremath{\eta}}^{\ensuremath{'}}$-nucleus interaction is discussed.

The (10Be,10B [1.74 MeV]) charge-exchange reaction at 100 AMeV is presented as a new probe for isolating the isovector (ΔT = 1) non-spin-transfer (ΔS = 0) response of nuclei, with 28Si being the rst nucleus studied. By using a secondary 10Be beam produced by fast fragmentation of 18O nuclei at the NSCL Coupled Cyclotron Facility, applying the dispersion-matching technique with the S800 magnetic spectrometer to determine the excitation energy in 28Al, and performing high- resolution -ray tracking with the Gamma-Ray Energy Tracking Array (GRETINA) to identify the 1022-keV ray associated with the decay from the 1.74-MeV T = 1 isobaric analog state in 10B, a ΔS = 0 excitation-energy spectrum in 28Al was extracted. Monopole and dipole contributions were determined through a multipole-decomposition analysis, and the isovector giant dipole (IVGDR) and monopole (IVGMR) resonances were identi ed. The results show that this probe is a powerful tool for studying the elusive IVGMR, which is of interest for performing stringent tests of modern density functional theories at high excitation energies and for constraining the bulk properties of nuclei and nuclear matter. The extracted distributions were compared with theoretical calculations based on the normal-modes formalism and the proton-neutron relativistic time-blocking approximation. Calculated cross sections based on these strengths underestimate the data by about a factor of two, which likely indicates de ciencies in the reaction calculations based on the distorted wave Born approximation.

Excitation spectra of ^{11}C are measured in the ^{12}C(p,d) reaction near the η^{'} emission threshold. A proton beam extracted from the synchrotron SIS-18 at GSI with an incident energy of 2.5 GeV impinges on a carbon target. The momenta of deuterons emitted at 0° are precisely measured with the fragment separator (FRS) operated as a spectrometer. In contrast to theoretical predictions on the possible existence of deeply bound η^{'}-mesic states in carbon nuclei, no distinct structures are observed associated with the formation of bound states. The spectra are analyzed to set stringent constraints on the formation cross section and on the hitherto barely known η^{'}-nucleus interaction.

"Background-free" spectra of inelastic $\alpha$-particle scattering have been measured at a beam energy of 385 MeV in $^{90, 92}$Zr and $^{92}$Mo at extremely forward angles, including 0$^{\circ}$. The ISGMR strength distributions for the three nuclei coincide with each other, establishing clearly that nuclear incompressibility is not influenced by nuclear shell structure near $A\sim$90 as was claimed in recent measurements., Comment: 5 pages, 4 figures; accepted for publication in Phys. Lett. B

The possible existence of \eta'-nucleus bound states has been put forward through theoretical and experimental studies. It is strongly related to the \eta' mass at finite density, which is expected to be reduced because of the interplay between the $U_A(1)$ anomaly and partial restoration of chiral symmetry. The investigation of the C(p,d) reaction at GSI and FAIR, as well as an overview of the experimental program at GSI and future plans at FAIR are discussed., Comment: 7 pages, 3 figures; talk at the International Conference on Exotic Atoms and Related Topics (EXA2014), Vienna, Austria, 15-19 September 2014. in Hyperfine Interactions (2015)

The direct particle decay of the isoscalar giant dipole resonance (ISGDR) has been studied in Pb-208 using the (alpha,alpha'p) and (alpha,alpha'n) reactions at a bombarding energy of 200 MeV. The direct-decay channels of proton and neutron emissions were observed in coincidence with the scattered a-particles in order to study the population of proton-hole states in Tl-207 and neutron-hole states in Pb-207. The results of the partial and total branching ratios allowed us to test recently performed continuum-RPA (CRPA) calculations for the ISGDR and thereby to learn about its microscopic structure.

Spin-isospin excitations of the medium-mass nucleus Co-58 were studied using the (n, p)-type (d,He-2) charge-exchange reaction at an incident energy E-d = 170 MeV and small scattering angles. The achieved energy resolution of 130 keV, full width at half maximum, allowed the extraction of Gamow-Teller strength to 13 peaks at excitation energies lower than 4.1 MeV in the residual nucleus Co-58 and is consistent with the results of a multipole-decomposition analysis. The identification of Gamow-Teller transitions is possible by comparison of the measured cross sections with DWBA calculations. The results are used to discuss the relevance of the Gamow-Teller strength distribution at low excitation energy with regard to electron-capture rates used in calculations for the final stage of a massive star. (C) 2003 Elsevier B.V. All rights reserved.

Gamow-Teller transitions in fp-shell nuclei have been studied using the 58 Ni( d , 2 He) charge exchange reaction at an incident energy of 170 MeV and forward angles. The measurements have been performed at AGOR using the Big-Bite Spectrometer and the EuroSuperNova focal-plane detection system. The unprecedented energy resolution of about 130 keV FWHM for excitations in the residual nucleus 58 Co allowed for the first time the extraction of single transitions in the β + direction up to about 4 MeV excitation. The impact on electron-capture rates in astrophysical environments has been evaluated.

A series of measurements have been performed at KVI to obtain the vector analyzing power Ay Of the H-2((p) over right arrow, pd) reaction as a function of incident beam energy at energies of 120, 135, 150, and 170 MeV. For all these measurements, a range of upsilon (c.m) from 30 degrees to 170 degrees has been covered. The purpose of these investigations is to observe possible spin-dependent effects beyond two-nucleon forces. When compared to the,predictions of Faddeev calculations, based on two-nucleon forces only, significant deviations are observed at all energies and at center-of-mass angles between 70 degrees and 130 degrees. The addition of present-day three-nucleon forces does not improve the description of the data, demonstrating the still insufficient understanding of the properties of three-nucleon systems.

Decay via proton emission of isobaric analog states (IAS’s) in 91 Nb was studied using the 90 Zr(α, t) reaction at E�=180 MeV. This study provides information about the damping mechanism of these states. Decay to the ground state and low-lying phonon states in 90 Zr was observed. The experimental data are compared with theoretical predictions wherein the IAS ‘single-particle’ proton escape widths are calculated in a continuum RPA approach. The branching ratios for decay to the phonon states are explained using a simple model.

Gamow-Teller (GT) transitions and $M1$ transitions from or to the ground states are analogous in the pair of $T=1/2$ mirror nuclei, if the excited states hold the mirror-symmetry properties. In order to study the mirror-symmetry structure of the $A=27$ mirror nuclei ${}^{27}\mathrm{Al}$ and ${}^{27}\mathrm{Si},$ experimental data on GT transition strengths obtained from ${}^{27}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{Si}}}^{27}\mathrm{Al}$ $\ensuremath{\beta}$ decay and from a good-resolution ${}^{27}\mathrm{Al}{(}^{3}\mathrm{He}{,t)}^{27}\mathrm{Si}$ reaction at 150 MeV/nucleon and 0\ifmmode^\circ\else\textdegree\fi{} have been compared with $M1$ transition strengths obtained from $\ensuremath{\gamma}$ decay in ${}^{27}\mathrm{Al}$ and ${}^{27}\mathrm{Al}(\ensuremath{\gamma},{\ensuremath{\gamma}}^{\ensuremath{'}})$ reaction. Good overall correspondence of transitions is observed for states up to nearly 9 MeV in excitation energy, where the particle-decay channel becomes important. The difference of excitation energies of analog states in the mirror nuclei are about 250 keV at this excitation energy. From a comparison of $B(M1)$ and $B(\mathrm{GT})$ strengths of the analogous transitions, the contribution of the isoscalar and the orbital terms in the $M1$ operator is found to become prominent in weak $M1$ transitions.

The isoscalar giant dipole resonance (ISGDR) in ${}^{208}\mathrm{Pb}$ has been investigated using inelastic scattering of 200 MeV $\ensuremath{\alpha}$ particles at and near 0\ifmmode^\circ\else\textdegree\fi{} where the angular distribution of the ISGDR can be clearly differentiated from those of other modes. The ``difference of spectra'' technique was employed to separate the ISGDR from the high-energy octupole resonance (HEOR). These data provide the clearest evidence yet for the ISGDR adjacent to the HEOR. With these results, all expected isoscalar $\ensuremath{\lambda}\ensuremath{\le}3$ resonances in ${}^{208}\mathrm{Pb}$ have been identified.

Using inelastic $\alpha$-scattering at extremely forward angles, including $0^\circ$, the strength distributions of the isoscalar giant monopole resonance (ISGMR) have been measured in the $^{204,206,208}$Pb isotopes in order to examine the proposed mutually enhanced magicity (MEM) effect on the nuclear incompressibility. The MEM effect had been suggested as a likely explanation of the "softness" of nuclear incompressibility observed in the ISGMR measurements in the Sn and Cd isotopes. Our experimental results rule out any manifestation of the MEM effect in nuclear incompressibility and leave the question of the softness of the open-shell nuclei unresolved still., Comment: Accepted for publication in Physics Letters B. Very minor changes in text

The energy of the charge-exchange Anti-analog Giant Dipole Resonance (AGDR) has been calculated for the 208Pb isotope using the state-of-the-art fully self-consistent relativistic proton-neutron quasiparticle random-phase approximation based on the Relativistic Hartree-Bogoliubov model. It is shown that the AGDR centroid energy is very sensitively related to the corresponding neutron-skin thickness. The neutron-skin thickness of 208Pb has been determined very precisely by comparing the theoretical results with the available experimental data on E(AGDR). The result DR(pn)= 0.161(42) agrees nicely with the previous experimental results., Comment: arXiv admin note: substantial text overlap with arXiv:1302.6007, arXiv:1205.2325

Fragmentation and splitting of the Gamow-Teller (GT) strength has been observed in a systematic study of the ${(}^{3}$He,t) charge-exchange reaction at E${(}^{3}$He)=200 MeV over the entire range of stable Sn isotopes. Triton energy spectra were observed with a high-resolution magnetic spectrometer at angles near \ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{} where \ensuremath{\Delta}L=0 transitions are enhanced. Excitation energies, widths, 0\ifmmode^\circ\else\textdegree\fi{} cross sections, and strengths B(GT) were determined. A theoretically predicted configuration splitting of the main Gamow-Teller component into two components, expected to be dominant near A=118 at the onset of the filling of the 1${\mathit{h}}_{11/2}$ neutron orbital, could not be observed. This may be due to the fact that the total widths of the resonances of 5--6 MeV exceed the predicted splitting. A comparison of the 0\ifmmode^\circ\else\textdegree\fi{} cross sections for the transitions to the Gamow-Teller resonances and the isobaric analog states leads to strengths B(GT) for the main Gamow-Teller components of typically 65% of the sum-rule value of 3(N-Z). Three to four additional Gamow-Teller fragments (``pygmy resonances'') were observed in all final nuclei at lower excitation energies. The excellent energy resolution of the experiment made it possible to observe a pronounced fine structure in these low-lying resonances which is believed to be due to coupling to two-particle--two-hole doorway states. Also seen with all target nuclei was a systematic sequence of strong ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${1}^{+}$ states near the ground states in all Sb isotopes (${\mathit{E}}_{\mathit{x}}$=0 to 220 keV). In addition, strong \ensuremath{\Delta}L=1 resonances were observed in all nuclei at excitation energies of typically 20 MeV. Furthermore, nonresonant background from quasifree charge exchange was observed. An average of \ensuremath{\sim}85% of all excess neutrons seems to contribute to this background in approximate agreement with results from (e,e'p) experiments.

The microscopic structure of both the Gamow-Teller resonance (GTR) at Eχ = 15.6 MeV and the spin-flip ΔL=1 resonances at higher excitation energies in 208Bi has been investigated by observing their direct proton decay to the 3p12, 2f52, 3p32, 1i32 and 2f72 neutron-hole states in 207Pb, following their excitation via the 208Pb (3He,t) reaction at E(3He) = 450 MeV. Decay protons were measured at backward angles in coincidence with tritons detected at very forward angles including θ = 0°. The total widths of the resonances as well as total and partial proton escape widths were determined and compared to theoretical estimates for the GTR. Using the same experimental set-up and conditions proton decay of the spin-flip ΔL=1 resonances in 12C was also measured.

Spin-isospin excitations in nuclei have been studied by means of the ( 3 He,t) reactions at 450 MeV. The Gamow-Teller (GT) resonances are found to be strongly excited. A fine structure of the giant Gamow Teller resonances in medium-heavy nuclei is observed with an energy resolution of 130 keV. The volume integrals of the effective interactions J στ and J τ have been determined experimentally. The importance of high resolution observation of the ( 3 He,t) reactions at intermediate energies, both, to study the double β decay processes and to measure the detection sensitivities for solar neutrino experiments is discussed.

The statistical γ-ray decay of the compound nucleus 154 Dy ∗ formed at an excitation energy of 69 MeV is studied in three angular-momentum windows 〈 J 〉 = 31,42 and 50ℏ. The GDR strength function extracted from the data indicates a constant centroid energy for the resonance E GDR = 15.2 ± 0.5 MeV, a small deformation of the nuclear shape for all spin regions and an increase in the resonance width with increasing angular momentum. The latter might indicate larger shape fluctuations at higher angular momenta. A systematical study of the dependence of the extracted strength function with different methods of analysis is presented, and their discrepancies discussed.

The (Pb(He,t)Bi)-Pb-208-He-3-Bi-208 reaction has been studied with the RCNP ring cyclotron at E(He-3) = 450 MeV using the magnetic spectrometer Grand Raiden at theta = 0-degrees. The Gamow-Teller resonance is strongly excited at this bombarding energy. Its microscopic structure has been investigated by measuring the direct proton decay to the neutron-hole states in Pb-207. The branching ratio for proton emission of a few % is much smaller than that for the decay of the isobaric analog state. The total width of the Gamow-Teller resonance as well as its total and partial escape widths have been determined. Several recent theoretical estimates for the escape width differ from the measured values by factors of approximately 2.5.

Spin-isospin excitations in nuclei have been studied by means of the (3He,t) reactions at 450 MeV on the targets sBe, natC, *%ii, 58Ni, 62Ni, seZr, l18Sn, **OSn, 124Sn, and 154Sm. The new spectrometer “Grand Raiden” was employed for the (3He,t) study for the first time. Observed (3He,t) spectra at 0’ show remarkable similarity with those from the (p,n) results at intermediate energies, suggesting a simple direct reaction mechanism for the (3He,t) reaction at 450 MeV. The GamowTeller (GT) resonances and the spin-flip AL=1 resonances are excited very strongly. A fine structure of the GT resonances in medium-heavy nuclei is observed with an energy resolution of 210 keV. It is demonstrated that the (3He,t) reaction at 450 MeV is a powerful tool to study the spin-isospin excitations in nuclei.

Expérience GANIL, Spectrometre LISE; International audience; The goal of the present paper is to attempt to clarify the nuclear reaction mechanism leading to the production of fragments at zero degree with neutron number larger than that in the 48Ca projectile, at about 60 MeV per nucleon. The production cross sections of the extremely neutron-rich Si and P isotopes were measured. Concerning the nuclear reaction mechanism leading to the production of these isotopes, one should probably refer to a particular type of transfer mechanism, which results in low excitation energy for the fragments, rather than to the ‘genuine' fragmentation mechanism. An upper limit of about 0.05 pb was estimated for the production cross section for the 47P isotope for which no count was observed.

To study the Gamow-Teller (GT) transitions to the pf-shell nucleus Cu-64, the Ni-64(He-3,t)Cu-64 experiment was performed at the Research Center for Nuclear Physics (RCNP) Ring Cyclotron, Osaka, using a He-3 beam of 140 MeV/nucleon. The outgoing tritons were momentum analyzed by the Grand Raiden spectrometer at 0(degrees). A high energy resolution of 32 keV (full width at half-maximum) allowed the separation of individual levels in the excitation-energy region from 0 to 3.5 MeV. In addition to the ground state (gs), known to be a J(pi)=1(+) GT state, many low-lying states showed L=0 nature, suggesting that they are candidates for GT states. Because the GT strength B(GT) for the gs transition is known from the beta-decay measurement, the strengths for the excited states could be determined using the proportionality between the B(GT) and the reaction cross section extrapolated to q=0 momentum transfer. At higher excitation energies, the level density becomes high and the so-called GT giant resonance dominates the spectrum. The lower and the upper limits of the strength contained in this energy region were estimated. Our results show that less than 55% of the strength predicted by the Ikeda sum rule is located in the excitation-energy region from 0 to 17 MeV..

The 96Mo(d,2He)96Nb charge-exchange reaction was investigated at an incident energy of Ed=183.5 MeV. An excitation-energy resolution of 110 keV was achieved. The experiment was performed at KVI, Groningen, using the magnetic spectrometer BBS at three angular positions: 0°,2.5°, and 6°. We found that below 6 MeV almost the entire Gamow-Teller (GT+) strength is concentrated in a single state at 0.69 MeV excitation energy. As Mo96 is the daughter of the ?? decay nucleus Zr96, the present result provides information about the nuclear matrix elements active in the 2??? decay of Zr96.

The (d,He-2) charge-exchange reaction on the double-beta decay (beta beta) nucleus Zn-64 has been studied at an incident energy of 183 MeV. The two protons in the S-1(0) state (indicated as He-2) were both momentum analyzed and detected simultaneously by the BBS magnetic spectrometer and its position-sensitive detector. He-2 spectra with a resolution of about 115 keV (FWHM) have been obtained allowing identification of many levels in the residual nucleus Cu-64 with high precision. Zn-64 is one of the rare cases undergoing a beta beta decay in beta(+) direction. In the experiment presented here, Gamow-Teller (GT(+)) transition strengths have been extracted. Together with the GT(-) transition strengths from Ni-64(He-3,t) data to the same intermediate nucleus Cu-64, the nuclear matrix elements of the beta beta decay of Zn-64 have been evaluated. Finally, the GT(+/-) distributions are compared with shell-model calculations and a critical assessment is given of the various residual interactions presently employed for the pf shell.

The direct and statistical neutron decay of the isoscalar giant dipole resonance was studied in Zr-90, Sn-116, and Pb-208 using the (alpha,alpha(')n) reaction at a bombarding energy of 200 MeV. The spectra of fast decay neutrons populating valence hole-states of the Z,N-1 nuclei were analyzed, and estimates for the branching ratios were determined. The results were compared to recent continuum random-phase approximation calculations.

Electron capture and beta decay play important roles in the evolution of presupernovae stars and their eventual core collapse. These rates are normally predicted through shell-model calculations. Experimentally determined strength distributions from charge-exchange reactions are needed to test modern shell-model calculations. We report on the measurement of the Gamow-Teller strength distribution in Co-58 from the Ni-58(t,He-3) reaction with a secondary triton beam of an intensity of similar to 10(6) pps at 115 MeV/nucleon and a resolution of similar to 250 keV. Previous measurements with the Ni-58(n,p) and the Ni-58(d,He-2) reactions were inconsistent with each other. Our results support the latter. We also compare the results to predictions of large-scale shell-model calculations using the KB3G and GXPF1 interactions and investigate the impact of differences between the various experiments and theories in terms of the weak rates in the stellar environment. Finally, the systematic uncertainties in the normalization of the strength distribution extracted from Ni-58(He-3, t) are described and turn out to be nonnegligible due to large interferences between the Delta L=0,Delta S=1 Gamow-Teller amplitude and the Delta L=2,Delta S=1 amplitude.

We have used the 43 MeV/nucleon primary tritium beam of the AGOR facility with an intensity of 4x10(7) pps and the BBS experimental setup to study the (t,He-3) reaction between 0 degrees and 5 degrees lab angles on C-12, Ca-48, and Ni-58 targets. The standard ray-tracing procedure has allowed us to obtain excitation-energy spectra up to 30 MeV in six angular bins for each residual nucleus, with an average energy resolution of 350 keV. The reaction mechanism has been described in distorted-waves Born approximation (DWBA) using the DWBA98 code. In this approximation, the form factor is treated as a folding of an effective projectile-nucleon interaction with a transition density. The effective projectile-nucleon interaction has been adjusted to reproduce the 0(degrees) cross section of the 1(+) ground state of B-12 populated in the C-12(t,He-3) reaction. We have employed random-phase approximation (RPA) wave functions of excited states to construct the form factor instead of the normal modes wave functions used earlier. This new DWBA+RPA analysis is used to compare calculated and experimental cross sections directly and to discuss the giant resonance excitations in K-48 and Co-58 nuclei.

The S-32(d,He-2)P-32 charge-exchange reaction has been studied at forward angles and at an incident energy of E-d=170 MeV. The two protons in the S-1(0)(pp) state (indicated as He-2) were both momentum analyzed and detected by the same spectrometer and detector. High resolution He-2 spectra have been obtained allowing identification of many levels in the residual nucleus with high precision. The (d,He-2) reaction selectively excites spin-isospin-flip transitions, and at vanishing momentum transfer these are largely governed by the Gamow-Teller transition operator for the beta(+) direction. In the case of the self-conjugate nucleus S-32, the (d,He-2) probe is calibrated against the elementary (p,n) probe going into the mirror beta(-) direction. The deduced results are compared with those obtained from (p,p(')) and (e,e(')) reactions, which excite the same levels in the isospin multiplet. Angular distributions of cross sections for the (d,He-2) reaction are presented and compared with DWBA calculations. A level-by-level comparison with shell-model calculations using the USD interaction has been performed, and the structure of the l-forbidden ground state transition is discussed. From the comparison with (p,p(')), (e,e(')), and (p,n) data the isospin-analog J(pi)=1(+) levels in the triplet of the A=32 nuclei are deduced.

To investigate the importance of three-nucleon forces (3NF) systematically over a broad range of intermediate energies, the differential cross sections of elastic proton-deuteron scattering have been measured at proton bombarding energies of 108, 120, 135, 150, 170 and 190 MeV at center-of-mass angles between $30^\circ$ and $170^\circ$. Comparisons with Faddeev calculations show unambiguously the shortcomings of calculations employing only two-body forces and the necessity of including 3NF. They also show the limitations of the latest few-nucleon calculations at backward angles, especially at higher beam energies. Some of these discrepancies could be partially due to relativistic effects. Data at lowest energy are also compared with a recent calculation based on \chipt.

A 130 MeV primary tritium beam, newly developed at the superconducting cyclotron AGOR, has been used for the investigation of 1ħω and 2ħω isovector giant resonances in 58 Co via the τ + channel. The angular distributions of double-differential cross sections have been extracted between E x = 7 and 30 MeV from θ lab = 0° to 6°. A DWBA analysis with normal-mode (NM) wave functions and a multipole-decomposition analysis (MDA) indicate strength for the monopole resonances (dominated by the spin-flip transition) fragmented between 10 and 27 MeV excitation energy, in rather good agreement with preliminary RPA calculations.

Cross sections and analyzing powers for the ${}^{11}\mathrm{B}(\stackrel{\ensuremath{\rightarrow}}{p}{,p}^{\ensuremath{'}})$ reaction have been measured using a 150-MeV polarized proton beam from the AGOR cyclotron at KVI. For the stronger inelastic transitions, also spin-flip probabilities have been extracted. To predict cross sections and spin observables a microscopic (coordinate space) model of elastic scattering has been used, whereas for inelastic scattering calculations were performed in the distorted-wave approximation. In the model calculations a medium-modified two-nucleon effective interaction was used and nuclear structure information was obtained from shell-model calculations in a complete $(0+2)\ensuremath{\Elzxh}\ensuremath{\omega}$ model space for parity-conserving transitions and a $1\ensuremath{\Elzxh}\ensuremath{\omega}$ shell-model description in the case of parity-changing transitions. With the help of these model calculations, spin-isovector parts of the $M1$ strengths for the negative-parity states at excitation energies of 2.125 MeV ${(J}^{\ensuremath{\pi}}{=1/2}^{\ensuremath{-}}),$ 4.445 MeV ${(J}^{\ensuremath{\pi}}{=5/2}^{\ensuremath{-}}),$ 5.020 MeV ${(J}^{\ensuremath{\pi}}{=3/2}^{\ensuremath{-}}),$ and 8.920 MeV ${(J}^{\ensuremath{\pi}}{=5/2}^{\ensuremath{-}})$ have been extracted and compared to known Gamow-Teller strengths for the analog transitions to ${}^{11}\mathrm{C}.$

The neutron-skin thickness in Pb-208 was determined by measuring the cross section of the isovector giant dipole resonance (GDR) in inelastic scattering of 196 MeV alpha-particles. From the comparison of measured and calculated cross sections the relative difference of proton and neutron distribution radii (DeltaR(PN)/R-0) was deduced to be 2.2 +/- 1.2%.

It has been shown that the neutron 2d5=2 orbital has to be included in shell-model calculations to explain the large quadrupole collectivity observed in the Fe and Cr of N = 40 region, probably a new island of inversion similar to the one known for light nuclei around N = 20. Indeed, in these calculations using a large valence neutron space, the position, i.e. the singleparticle energy, of the 2d5=2 orbital is a crucial ingredient. The experiment discussed in this paper aims to determine the $\nu$1g9=2–$\nu$2d5=2 gap using the neutron stripping reaction d(68Ni; p)68Ni which has been performed at GANIL in inverse kinematics. Preliminary results are presented.

Proton and deuteron inelastic scattering experiments, performed with an energy resolution of 12–15 keV, have been used to study negative-parity states of vibrational and transitional nuclei with mass between 98 and 150. The analysis has been focussed on the isovector components, on the quadrupole-octupole two-phonon states and on the fragmentation of the octupole strength. This latter displays a regular dependence on the product of proton and neutron valence particle numbers and is satisfactorily reproduced by IBM-1 + f-boson calculations. Other features of the experimental spectra, as the relative positions of the 3− states, exhibit a dependence on the ratio of valence particle numbers and indicate that a IBM-2 approach might be more appropriate.

Single-neutron pick-up via the (d, t) reaction was studied at E d = 50 MeV on all stable, even- A tin isotopes with an emphasis on the pick-up from the first major shell below the valence shell. The deeply bound hole states thus observed exhibit themselves as a gross structure phenomenon with a systematic variation of its width as a function of neutron number. The most extensive study was done for the 116 Sn(d, t) 115 Sn reaction for which angular distributions were obtained not only for the deeply bound hole states but also for the pick-up of valence nucleons, and for the excitation of the isobaric analogue states. A DWBA analysis yielded the full sum-rule strength for the pick-up of the valence nucleons. It located in the gross structure about 45% of the combined 1g 9 2 , 2p 1 2 , 2p 3 2 and 1f 5 2 strength. Additional strength may be found in the continuous “background” on which the gross structure peaks are riding. A systematic study of the gross structure shapes and cross sections in all odd- A tin isotopes is presented. The observed variation in the spreading of the 1g 9 2 hole strength as a function of neutron number is compared with recent calculations.

The decay of the isoscalar giant monopole and quadrupole resonances in 90 Zr has been studied with the 90 Zr(α, α'n) 89 Zr and the 90 Zr(α, α'p) 89 reactions. For the giant monopole resonance (GMR), a branching ratio for (neutron : proton : alpha) decay of (85(20) : 10(2) : ( case9 2 + ground state of 90 Zr. The proton decay spectrum of the GMR is also in overall agreement with statistical model calculations. The proton as well as neutron decay from the region of the giant quadrupole resonance (GQR) indicate that in the singles (α, α') cross section the L >2 excitation amounts to 25–50% of the L = 2 cross section.

In coincidence with tritons from the $^{208}\mathrm{Pb}$ ($^{3}\mathrm{He}$, $t$)$^{208}\mathrm{Bi}$ reaction proton spectra corresponding to decay into definite final states in $^{207}\mathrm{Pb}$ have been measured. The coincidence yield is described as exclusively coming from the decay of two resonances, the isobaric analog state (IAS) and the Gamow-Teller (GT) resonance. An analysis in terms of a formation cross section times a branching ratio gives a good description for the IAS and for the shape of the GT resonance. The yield from the GT resonance is, however, an order of magnitude larger than calculated.

The nucleus 16F was studied via the 16O(3He, t) reaction at 81 MeV. Differential cross sections for many states were obtained and interpreted with DWBA calculations, using microscopic wave functions and an effective projectile-nucleon interaction. Proton decay to several states in 15O was observed and angular correlations for protons in coincidence with tritons detected at θ = 0° were measured. Several spin-parity assignments have been made. The distribution of isovector ΔL = 1 strength could be deduced. The analog of the giant dipole resonance ( E x ⋍ 9.5 MeV ) is strongly excited. The magnetic quadrupole strength has two strong components, one low, at Ex = 0.424 MeV, and one high, at E x ⋍ 7.5 MeV . Evidence is given for a proportionality between cross section and M2 strength for transitions to Jπ = 2− states, which possibly make the (3He, t) reaction a suitable tool for determining quantitatively isovector M2 (or Bij) strengths.

The 16O(α,α′)16O reaction was studied at Eα = 104 MeV in the region of the giant resonances (GR). A prominent structure which could be resolved into various components is observed between Ex = 17 and 25 MeV. The components of the structure above Ex = 20 MeV vary in shape with angle indicating the existence of various multipolarities. DWBA calculations were performed with the collective form factor prescription. For l = 0, 1 and 3, DWBA calculations were also performed using microscopic wave functions. The forward angular behaviour of the structure between Ex = 23 and 24 MeV is found to be consistent with l = 0 transfer, while for other components of the structure above E x = 20 MeV , l≧3 contributions could not be excluded. The fraction of the isoscalar quadrupole energy weighted sum rule (EWSR) exhausted between Ex = 17 and 25 MeV is 40−10%+20%. Our extracted isoscalar quadrupole strength distribution agrees qualitatively with theoretical results. Many states of well known spin and parity are excited below Ex = 17 MeV. Satisfactory fits could be obtained for the natural parity states with DWBA using the collective form factor prescription except for the 2+ state at 9.85 MeV and 4+ state at 10.35 MeV and the 1−, T = 0 states. For the 2+ and 4+ states, coupled channel calculations (CC) describe the experimental angular distributions correctly, while for the 11− states we obtain good fits using a microscopic wave function description of these states.