Nucleus Optical Potential Research Articles

Global density-dependent α–nucleon interaction for α–nucleus elastic scattering

Abstract We provide a global density-dependent 4He–nucleon (DD-αN) interaction to construct the α–nucleus optical model potential (OMP) across a wide range of incident energies. The global parametrization for the DD-αN interaction is obtained based on the proton–4He OMP, which reproduces the elastic scattering cross-section data very well in the incident energies of 12.04–500 MeV per nucleon. Using the present DD-αN interaction, we derive the α–nucleus potential by a folding procedure with the point-nucleon density obtained by a microscopic mean-field model. The density dependence of the DD-αN interaction is fixed phenomenologically to reproduce the α–nucleus elastic scattering cross-section data obtained from 16O, 40Ca, 58Ni, 90Zr, and 208Pb targets at E/A = 10–342.5 MeV. We also show the total reaction cross sections, which are helpful in fixing one free parameter, the renormalization factor for the imaginary part of the α–nucleus potential. Lastly, we show some examples, which clearly demonstrate the validity and power of the present DD-αN approach.

Production spectra with a Σ− hyperon in (π−,K+) reactions on light to heavy nuclei

We study theoretically production spectra with a ${\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}$ hyperon via $({\ensuremath{\pi}}^{\ensuremath{-}},{K}^{+})$ reactions on $^{12}\mathrm{C}$, $^{28}\mathrm{Si}$, $^{58}\mathrm{Ni}$, $^{115}\mathrm{In}$, and $^{209}\mathrm{Bi}$ targets, using the Green's function method in the framework of a distorted-wave impulse approximation with the optimal Fermi averaging for an elementary ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{K}^{+}{\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}\phantom{\rule{4pt}{0ex}}t$ matrix. Adopting distorted waves obtained by solving a Klein-Gordon equation for ${\ensuremath{\pi}}^{\ensuremath{-}}$ and ${K}^{+}$ mesons, we improve and update the calculated spectra of ${\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}$ production cross sections comprehensively in comparison with the data of the KEK-E438 experiment. We use several $\mathrm{\ensuremath{\Sigma}}$-nucleus (optical) potentials that are determined by fits to the ${\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}$ atomic data and that take into account the energy dependence arising from the nuclear excitation via $\mathrm{\ensuremath{\Sigma}}N\ensuremath{\rightarrow}\mathrm{\ensuremath{\Sigma}}N$ scatterings with a $\mathrm{\ensuremath{\Sigma}}$ hyperon effective mass. The results show that the absolute values and the shapes of these calculated spectra are in excellent agreement with those of the data, so that a mass-number dependence of integrated cross sections on the light-to-heavy targets is well reproduced in our calculations. It confirms that the $\mathrm{\ensuremath{\Sigma}}$-nucleus potentials having a repulsion inside the nuclear surface and an attraction outside the nucleus with a sizable absorption are favored in reproducing the data of the nuclear $({\ensuremath{\pi}}^{\ensuremath{-}},{K}^{+})$ spectra and the ${\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}$ atomic x ray simultaneously, whereas it is still difficult to determine the radial distribution of the $\mathrm{\ensuremath{\Sigma}}$-nucleus potential inside the nucleus and its strength at the center.

Study of6,8He+p elastic scattering using the BHF formalism with three-body forces

In the present work, we have analyzed the elastic scattering data of6He+p at 25.6, 38.6, 40.9, and 71 MeV/A and8He+p at 15.6, 25.6, 32.5, 66, and 72 MeV/A, using the microscopic local optical potential calculated within the framework of the Brueckner–Hartree–Fock (BHF) formalism. The calculation requires mainly two inputs: (1) the nucleon–nucleon (NN) interaction and (2) the nucleon distributions in target nuclei. In the present work, realistic internucleon (NN) potential from Argonne v18 (AV18) and the Urbana IX (UVIX) model of three-body forces with several nucleon density distributions are used for generating the nucleon–nucleus optical potential. We have used the exact method for calculating both the direct and the exchange parts of the spin–orbit potential. We confirm the earlier results that the spin–orbit potential for these neutron-rich nuclei is diffused and extended. Our results show that the different density distributions reproduce rather well the experimental differential cross sections for both isotopes, while the phenomenological density with a two-neutron halo gives satisfactory results for6He-p analyzing power data. None of the densities used for8He can reproduce the experimentally analyzed power data. Our analysis reveals that the calculated microscopic optical potentials, with and without three-body forces using the BHF approach, provide satisfactory agreement with the elastic scattering data for6,8He+p.

Pion–nucleon sigma term σπNand deeply bound pionic atoms

AbstractWe investigate the possibility of determining the value of the pion–nucleon sigma term σπN precisely by the experimental observables of deeply bound pionic atoms. We discuss the sensitivity of the observables to σπN and take into account the typical errors of up-to-date experiments on deeply bound pionic atoms. We find that the gap in the binding energies and the width of the deeply bound pionic states are good observables for σπN value determination with experimental data. We also discuss the expected difficulties for the accurate determination of the value of σπN due to the correlation between the σ term and the potential parameter in the pion–nucleus optical potential.

Improved version of the α -nucleus optical model potential for reactions relevant to the γ process

The present paper proposes an improved version of the $\ensuremath{\alpha}$-nucleus optical model potential ($\ensuremath{\alpha}$-OMP), in which the real part of the potential is determined by the double-folding calculation using the density-dependent CDM3Y1 interaction plus a repulsive potential. The imaginary part of the $\ensuremath{\alpha}$-OMP is treated by a Woods-Saxon form with energy-dependent depth, while its real part is corrected by including the contribution of the dispersion relation derived from the strong variation of the potential at energies in the vicinity of the Coulomb barrier. The proposed potential is validated by calculating the $(\ensuremath{\alpha},n)$ and $(\ensuremath{\alpha},\ensuremath{\gamma})$ cross sections on different target nuclei and the obtained results are compared with those predicted by the recent $\ensuremath{\alpha}$-OMP suggested by Avrigeanu, as well as the available experimental data. The impacts of the energy-dependent imaginary part and the real dispersive term on the $\ensuremath{\alpha}$-induced cross sections are also investigated. The results obtained show that the proposed $\ensuremath{\alpha}$-OMP together with that of Avrigeanu describe reasonably well the experimental cross sections of all $(\ensuremath{\alpha},n)$ reactions being considered. In addition, the imaginary part of the proposed potential has a strong energy-dependent effect on the $\ensuremath{\alpha}$-induced cross section in the energy region below the Coulomb barrier, in particular for radiative $\ensuremath{\alpha}$-capture reactions. Two potentials also describe well the cross sections of $(\ensuremath{\alpha},\ensuremath{\gamma})$ reactions after rescaling the radiative $\ensuremath{\gamma}$ width. This leads to our conclusion that the present $\ensuremath{\alpha}$-OMP can provide precise $\ensuremath{\alpha}$ widths for the study of reactions relevant to the production of $p$ nuclei, and therefore more intensive theoretical research on the radiative strength functions is required.

Activation thick target yield measurement of Mo100(α,n)Ru103 for studying the weak r -process nucleosynthesis

Background: Light ($30\ensuremath{\le}Z\ensuremath{\le}45$) neutron-rich isotopes are thought to be synthesized in the neutrino-driven ejecta of core-collapse supernovae explosions via the weak $r$ process. Recent nucleosynthesis studies have demonstrated that $(\ensuremath{\alpha},xn)$ reactions play a particularly important role in the production of these isotopes. $\ensuremath{\alpha}$-nucleus optical model potentials ($\ensuremath{\alpha}$-OMPs) are used to model this nucleosynthesis scenario.Purpose: The different $\ensuremath{\alpha}$-OMP model parameters can affect the calculated cross sections by more than an order of magnitude in the relevant energy regions, which affects the production of light neutron-rich isotopes. Consequently, to constrain the astrophysical conditions characterizing the supernovae ejecta, the uncertainty of the nuclear physics input has to be reduced.Methods: The cross section of the $^{100}\mathrm{Mo}(\ensuremath{\alpha},n)^{103}\mathrm{Ru}$ reaction was measured by means of the activation method. 0.5 mm thick molybdenum disks were irradiated with ${E}_{\ensuremath{\alpha}}$ = 7.0 to ${E}_{\ensuremath{\alpha}}$ = 13.0 MeV $\ensuremath{\alpha}$ beams. Thick target yields and reaction cross sections were determined via $\ensuremath{\gamma}$-ray spectroscopy.Results: Cross sections at several energies below the Coulomb barrier were measured, reaching the astrophysically relevant energy region. Large discrepancies between the experimental values and statistical model predictions calculated using the well-known $\ensuremath{\alpha}$-OMPs were found. The measured cross section data could be excellently described by the Atomki-V2 potential. Therefore, this $\ensuremath{\alpha}$-OMP was used to derive the astrophysical reaction rates as a function of temperature.Conclusions: The successful reproduction of the measured cross sections in a wide energy region confirm the reliability of the Atomki-V2 potential. The usage of the new $^{100}\mathrm{Mo}(\ensuremath{\alpha},n)^{103}\mathrm{Ru}$ experimental data along with the Atomki-V2 potential reduces the nuclear uncertainties of the weak $r$-process production yields of nuclei with $36\ensuremath{\le}Z\ensuremath{\le}50$ to a marginal level.

Low-energy Measurement of the 96Zr(α,n)99Mo Reaction Cross Section and Its Impact on Weak r-process Nucleosynthesis

Abstract Lighter heavy elements beyond iron and up to around silver can form in neutrino-driven ejecta in core-collapse supernovae and neutron star mergers. Slightly neutron-rich conditions favor a weak r-process that follows a path close to stability. Therefore, the beta decays are slow compared to the expansion timescales, and (α,n) reactions become critical to move matter toward heavier nuclei. The rates of these reactions are calculated with the statistical model and their main uncertainty, at energies relevant for the weak r-process, is the α+nucleus optical potential. There are several sets of parameters to calculate the α+nucleus optical potential leading to large deviations for the reaction rates, exceeding even one order of magnitude. Recently the 96Zr(α,n)99Mo reaction has been identified as a key reaction that impacts the production of elements from Ru to Cd. Here, we present the first cross section measurement of this reaction at energies (6.22 MeV ≤ Ec.m. ≤ 12.47 MeV) relevant for the weak r-process. The new data provide a stringent test of various model predictions which is necessary to improve the precision of the weak r-process network calculations. The strongly reduced reaction rate uncertainty leads to very well-constrained nucleosynthesis yields for Z = 44–48 isotopes under different neutrino-driven wind conditions.

An event excess observed in the deeply bound region of the 12C (K−, p) missing-mass spectrum

Abstract We have measured, for the first time, the inclusive missing-mass spectrum of the $^{12}$C$(K^-, p)$ reaction at an incident kaon momentum of 1.8 GeV/$c$ at the J-PARC K1.8 beamline. We observed a prominent quasi-elastic peak ($K^-p \rightarrow K^-p$) in this spectrum. In the quasi-elastic peak region, the effect of secondary interaction is apparently observed as a peak shift, and the peak exhibits a tail in the bound region. We compared the spectrum with a theoretical calculation based on the Green’s function method by assuming different values of the parameters for the $\bar{K}$–nucleus optical potential. We found that the spectrum shape in the binding-energy region $-300 \, \text{MeV} < B_{K} < 40$ MeV is best reproduced with the potential depths $V_0 = -80$ MeV (real part) and $W_0 = -40$ MeV (imaginary part). On the other hand, we observed a significant event excess in the deeply bound region around $B_{K} \sim 100$ MeV, where the major decay channel of $K^- NN \to \pi\Sigma N$ is energetically closed, and the non-mesonic decay modes ($K^- NN \to \Lambda N$ and $\Sigma N$) should mainly contribute. The enhancement is fitted well by a Breit–Wigner function with a kaon-binding energy of 90 MeV and width 100 MeV. A possible interpretation is a deeply bound state of a $Y^{*}$-nucleus system.

Different folding models for 6Li+28Si elastic scattering

In this work, the elastic scattering of 6Li+[Formula: see text]Si system at wide range energies from 76 to 318[Formula: see text]MeV is analyzed. The analysis is carried out in the framework of the optical model (OM). Two different methods are adopted for nuclear optical potential of this system. The first method is the double folding cluster (DFC) for the real part supplied with an imaginary part in the Woods–Saxon (WS) form. In the second one, the double folding (DF) model based upon São Paulo potential (SPP) is used as real and imaginary parts each multiplied by a corresponding normalization factor. For [Formula: see text]Si, the full [Formula: see text]-cluster density is considered while the [Formula: see text]-deuteron ([Formula: see text]–[Formula: see text]) structure is considered for 6Li. Therefore, the DFC real central part is calculated by folding both [Formula: see text]–[Formula: see text] and [Formula: see text]–[Formula: see text] effective interaction between target and nuclei over the cluster densities of the target and projectile. The derived renormalized potentials give a successful description of the data. The present results are in good agreement with the previous work. This agreement confirms the validity of the present methods to generate nucleus–nucleus optical potentials.

Elastic Scattering Cross Section of Pion (.π+ ) with N, 16O and 28Si

Abstract The pion–nucleon and nucleus scattering at intermediate energies have been intensively studied in recent years on the basis of meson exchange theory. In this work the optical potentials of the pion-nucleon (π+ N) interactions are obtained on the basis of the single-boson-exchange (SBE). We used sets of boson parameters suggested by the Julich group, to get the radial forms of the interacting pion nucleon optical potential V(r), and we use the effective nuclear density of the target nuclei (oxygen and Silicon) to obtain the pion nucleus optical potential. The differential crosses sections for (π+ 16O) and (π+ 28Si) are calculated by using the first born approximation, the scattering angle in the frame of the center of mass system, within the framework of time independent Schrodinger equation, and in the π+ energy region Plab < 0.5 GeV. All results are found in a good agreement with the available experimental data.

Microscopic optical potential obtained from energy-density-functional approach for neutron–nucleus elastic scattering

Nucleon–nucleus (NA) optical potentials are microscopically generated from a fully self-consistent framework of the particle-vibration coupling (PVC), in which the nucleon–nucleon (NN) effective interaction of the Skyrme type is consistently used to describe the Hartree–Fock (HF) mean-field, the small amplitude collective motions of the target, and the particle-collective states coupling. For the first time, a systematic calculation of low-energy NA elastic scattering off a series of doubly closed-shell nuclei is carried out without ad hoc adjusted parameters. Angular distributions obtained using the present optical potentials are in good agreement with the experimental data. This will be a major step forward in the applications of the Skyrme energy-density-functional theory to build up the global microscopic optical potentials, which are expected to be a powerful tool for the study of unstable (exotic) nuclei at low-incident nucleon energies.

Optical potential for light nuclei and momentum-space eikonal phase function

The space radiation environment comprises all of the nuclei in the periodic table with energies that extend from a fraction of an MeV/n to TeV/n. The vast range of projectile–target and energy combinations necessitates highly efficient and accurate cross section codes for use in radiation transport codes. As particles in the space radiation environment impinge on shielding materials, nuclear reactions, such as nuclear fragmentation, occur. One way of estimating nuclear fragmentation cross sections is to use an abrasion–ablation model, which describes how nucleons are dislodged from the nuclei as a result of nuclear collisions and the mechanism by which excited pre-fragments decay via particle emission to more stable states. The well-known partial wave solution method cannot be used directly for the computation of abrasion cross sections. Instead, abrasion cross sections may be computed by slightly altering the Eikonal solution method, which is a high energy (small scattering angle) approximation that depends on the nucleus–nucleus optical potential. The aim of the current work is to present two efficient methods for the computation of the Eikonal phase shift function. Analytic formulas of the optical potential are presented in the position-space representation for nuclei that are well-represented by harmonic-well nuclear matter densities (A &lt; 20), which reduces the Eikonal phase factor to an integration over a single dimension. Next, the Eikonal phase function is presented in the momentum-space representation, which is particularly useful when the Fourier transform of the position-space optical potential is known. These new methods increase the computational efficiency by three orders of magnitude and allow for rapid prediction of elastic differential, total, elastic, and reaction cross sections in the Eikonal approximation.

α -induced reactions on In115 : Cross section measurements and statistical model analysis

[Background] Alpha-nucleus optical potentials are basic ingredients of statistical model calculations used in nucleosynthesis simulations. While the nucleon+nucleus optical potential is fairly well known, for the alpha+nucleus optical potential several different parameter sets exist and large deviations, reaching sometimes even an order of magnitude, are found between the cross section predictions calculated using different parameter sets. [Purpose] A measurement of the radiative alpha-capture and the alpha-induced reaction cross sections on the nucleus 115In at low energies allows a stringent test of statistical model predictions. Since experimental data is scarce in this mass region, this measurement can be an important input to test the global applicability of alpha+nucleus optical model potentials and further ingredients of the statistical model. [Methods] The reaction cross sections were measured by means of the activation method. The produced activities were determined by off-line detection of the gamma-rays and characteristic x-rays emitted during the electron capture decay of the produced Sb isotopes. The 115In(alpha,gamma)119Sb and 115In(alpha,n)118Sbm reaction cross sections were measured between Ec.m. = 8.83 MeV - 15.58 MeV, and the 115In(alpha,n)118Sbg reaction was studied between Ec.m. = 11.10 MeV - 15.58 MeV. The theoretical analysis was performed within the statistical model.

Antiproton–nucleus quasi-bound states within the 2009 version of the Paris N̅ N potential

We studied the p̅ interactions with the nuclear medium within the 2009 version of the Paris N̅ N potential model. We constructed the p̅–nucleus optical potential using the Paris S - and P-wave p̅ N scattering amplitudes and treated their strong energy and density dependence self-consistently. We considereda phenomenological P-wave term as well. We calculated p̅ binding energies and widths of the p̅ bound in various nuclei. The P-wave potential has very small effect on the calculated p̅ binding energies, however, it reduces the corresponding widths noticeably. Moreover, the S -wave potential based on the Paris amplitudes supplemented by a phenomenological P-wave term yields the p̅ binding energies and widths in very good agreement with those obtained within the RMF model consistent with p̅-atom data.

Cross section of α-induced reactions on iridium isotopes obtained from thick target yield measurement for the astrophysical γ process

The stellar reaction rates of radiative α-capture reactions on heavy isotopes are of crucial importance for the γ process network calculations. These rates are usually derived from statistical model calculations, which need to be validated, but the experimental database is very scarce. This paper presents the results of α-induced reaction cross section measurements on iridium isotopes carried out at first close to the astrophysically relevant energy region. Thick target yields of 191Ir(α,γ)195Au, 191Ir(α,n)194Au, 193Ir(α,n)196mAu, 193Ir(α,n)196Au reactions have been measured with the activation technique between Eα=13.4 MeV and 17 MeV. For the first time the thick target yield was determined with X-ray counting. This led to a previously unprecedented sensitivity. From the measured thick target yields, reaction cross sections are derived and compared with statistical model calculations. The recently suggested energy-dependent modification of the α+nucleus optical potential gives a good description of the experimental data.

Missing-mass spectroscopy with the Li6(π−,K+)X reaction to search for HΛ6

We searched for the bound state of the neutron-rich $\Lambda$-hypernucleus ${}^{6}_{\Lambda}$H, using the ${}^{6}$Li($\pi^{-}, K^{+}$)X double charge-exchange reaction at a $\pi^{-}$ beam momentum of 1.2 GeV/c at J-PARC. A total of $1.4 \times 10^{12}$ $\pi^{-}$ was driven onto a ${}^{6}$Li target of 3.5-g/$\rm cm^2$ thickness. No event was observed below the bound threshold, i.e., the mass of ${}^{4}_{\Lambda}$H + 2n, in the missing-mass spectrum of the ${}^{6}$Li($\pi^{-}, K^{+}$)X reaction in the $2^{\circ}$ < $\theta_{\pi K}$ < $20^{\circ}$ angular range. Furthermore, no event was found up to 2.8 MeV/$c^2$ above the bound threshold. We obtained the the double-differential cross section spectra of the ${}^{6}$Li($\pi^{-}, K^{+}$)X reaction in the angular range of $2^{\circ}$ < $\theta_{\pi K}$ < $14^{\circ}$. An upper limit of 0.56 nb/sr (90% C.L.) was obtained for the production cross section of the ${}^{6}_{\Lambda}$H hypernucleus bound state. In addition, not only the bound state region, but also the $\Lambda$ continuum region and part of the $\Sigma^{-}$ quasi-free production region of the ${}^{6}$Li($\pi^{-}, K^{+}$)X reaction, were obtained with high statistics. The present missing-mass spectrum will facilitate the investigation of the $\Sigma^{-}$-nucleus optical potential for $\Sigma^{-}$-${}^{5}$He through spectrum shape analysis.

K−N amplitudes below threshold constrained by multinucleon absorption

Six widely different subthreshold K−N scattering amplitudes obtained in SU(3) chiral-model EFT approaches by fitting to low-energy and threshold data are employed in optical-potential studies of kaonic atoms. Phenomenological terms representing K− multinucleon interactions are added to the EFT-inspired single-nucleon part of the K−-nucleus optical potential in order to obtain good fits to kaonic-atom strong-interaction level shifts and widths across the periodic table. Introducing as a further constraint the fractions of single-nucleon K− absorption at rest from old bubble-chamber experiments, it is found that only two of the models considered here reproduce these absorption fractions. Within these two models, the interplay between single-nucleon and multinucleon K− interactions explains features observed previously with fully phenomenological optical potentials. Radial sensitivities of kaonic atom observables are re-examined, and remarks are made on the role of ‘subthreshold kinematics’ in absorption-at-rest calculations.

Momentum dependence of the imaginary part of the $ \omega$ - and $ \eta^{\prime}$ -nucleus optical potential

The photoproduction of $\omega$ and $\eta^\prime$ mesons off carbon and niobium nuclei has been measured as a function of the meson momentum for incident photon energies of 1.2-2.9 GeV at the electron accelerator ELSA. The mesons have been identified via the $\omega \rightarrow \pi^0 \gamma \rightarrow 3 \gamma$ and $\eta^\prime\rightarrow \pi^0 \pi^0\eta \rightarrow 6 \gamma$ decays, respectively, registered with the CBELSA/TAPS detector system. From the measured meson momentum distributions the momentum dependence of the transparency ratio has been determined for both mesons. Within a Glauber analysis the in-medium $\omega$ and $\eta^\prime$ widths and the corresponding absorption cross sections have been deduced as a function of the meson momentum. The results are compared to recent theoretical predictions for the in-medium $\omega$ width and $\eta^\prime$-N absorption cross sections. The energy dependence of the imaginary part of the $\omega$- and $\eta^\prime$-nucleus optical potential has been extracted. The finer binning of the present data compared to the existing data allows a more reliable extrapolation towards the production threshold. The modulus of the imaginary part of the $\eta^\prime$ nucleus potential is found to be about three times smaller than recently determined values of the real part of the $\eta^\prime$-nucleus potential, which makes the $\eta^\prime$ meson a suitable candidate for the search for meson-nucleus bound states. For the $\omega$ meson, the modulus of the imaginary part near threshold is comparable to the modulus of the real part of the potential. As a consequence, only broad structures can be expected which makes the observation of $\omega$ mesic states very difficult experimentally.

Partial cross sections of theMo92(p,γ)reaction and theγstrength inTc93

Background: Mo92 is the most abundant nucleus of the p nuclei, with an isotopic abundance of more than 14 %. The γ-process nucleosynthesis is believed to produce Mo92 but fails to explain its large abundance, especially with respect to the other p nuclei produced in the same stellar environment. Further studies require precise nuclear models for the calculation of reaction cross sections. Purpose: A measurement of the total and partial cross sections of the Mo92(p,γ)Tc93 reaction allows for a stringent test of statistical-model predictions. Not only different proton+nucleus optical model potentials, but also the γ-ray strength function of Tc93 can be investigated. In addition, high-resolution in-beam γ-ray spectroscopy enables the determination of new precise nuclear structure data for Tc93. Method: Total and partial cross-section values were measured by using the in-beam method. Prompt γ rays emitted during the irradiation of Mo92 with protons at seven different energies between 3.7 and 5.3 MeV were detected by using the high-purity germanium (HPGe) detector array HORUS at the Institute for Nuclear Physics, University of Cologne. The γγ-coincidence method was applied to correlate γ-ray cascades in Tc93 with their origin in the Mo92+p compound state. Results: The measured cross sections are compared to Hauser-Feshbach calculations by using the statistical-model code talys on the basis of different nuclear physics input models. Using default settings based on standard phenomenological models, the experimental values cannot be reproduced. A shell-model calculation was carried out to predict the low-energy M1 strength in Tc93. Together with Gogny-Hartree-Fock-Bogoliubov (Gogny-HFB) or Skyrme-HFB plus quasi-particle random-phase approximation (QRPA) models for the γ-ray strength function, the agreement between experimental data and theoretical predictions could be significantly improved. In addition, deviations from the adopted level scheme were found. Conclusions: By using Gogny- or Skyrme-HFB+QRPA E1 and shell-model M1 strength functions, statistical-model predictions can be significantly improved. Partial cross sections provide a valuable testing ground for γ-ray strength functions for nuclear astrophysics applications. In addition, they can be used to investigate nuclear-structure properties of the compound nucleus.

Determination of theω- andη′-nucleus optical potential

The ω and η ′-nucleus interaction has been studied in photoproduction reactions off C and Nb targets, using the CBELSA/TAPS detector system. Transparency ratio measurements provide information on the inelastic cross section and in-medium width of mesons and thereby on the imaginary part of the meson-nucleus potential. The real part of the optical potential can be deduced from measurements of the excitation function and momentum distribution which are sensitive to the sign and depth of the potential. Data taken on a C and Nb target have been analysed to determine the real and the imaginary part of the ω - and η ′-nucleus optical potential. The momentum dependence of the imaginary part of both mesons is presented and discussed. The results are compared to previous experimental results and to model calculations assuming different scenarios. The data are consistent with a weakly attractive potential for both mesons. The relatively small in-medium width of the η ′ meson encourages the search for η ′ bound states.