[en] Our current knowledge of the nucleosynthesis of nature's rarest naturally occurring isotope ¹⁸⁰Ta^m is reviewed. Production in the s-process seems possible via a weak branch of thermally β-unstable ¹⁷⁹Hf with subsequent neutron capture on the radioactive ¹⁷⁹Ta isotope. The effective life time of the long-lived isomer is strongly affected under s-process conditions by thermal coupling to the short-lived ground state through intermediate states. Nevertheless, based on the present knowledge of the relevant nuclear quantities, quantities, close to 100% of the solar abundance can be produced in dynamical s-process models. On the other hand, p-process simulations seem to be able to explain all of the solar abundance. Furthermore, recent improved studies of the ν-process also predict considerable contributions where - contrary to earlier work - the charged current reaction ¹⁸⁰Hf(ν, e^-) rather than the neutral current reaction ¹⁸¹Ta(ν, ν'n) is most important

[en] The nuclei ⁴⁸Ca and ⁹⁰Zr were investigated in 180 deg. high-resolution inelastic electron scattering for momentum transfers q≅0.35-0.8 fm^-1. Complete M2 strength distributions could be extracted in both nuclei up to excitation energies of about 15 MeV utilizing a fluctuation analysis technique. Second-RPA calculations successfully describe the experimentally observed strong fragmentation of the M2 mode. The quenching of the spin part is found to be comparable to the M1 case, contrary to previous claims suggesting a stronger reduction. A quantitative reproduction of the data requires the presence of appreciable orbital strength which can be interpreted as a torsional elastic vibration (the so-called twist mode)

[en] Magnetic dipole transitions in the self-conjugate nucleus ³²S up to an excitation energy of 12 MeV have been investigated in inelastic electron scattering at Θ_e = 180 at the S-DALINAC. Transition strengths have been determined from a plane-wave Born approximation analysis including Coulomb distortion. For the two strongest M1 transitions, where a discrepancy of a factor of about two was observed in previous (e, e^') experiments, values intermediate between the two extremes are deduced from the present work. The resulting strength distribution is well described by shell-model calculations using the unified sd-shell (USD) interaction and an effective M1 operator. The shell-model wave functions also provide a reasonable description of the form factors. A quasiparticle random phase approximation (QRPA) calculation is less successful. The present results allow for the first time studies of the form factor of extremely weak l-forbidden and isoscalar M1 excitations in ³²S. The l-forbidden transition allows a sensitive test of tensor corrections to the M1 operator. A combined analysis with the isospin-analog Gamow-Teller (GT) transitions in the A = 32 triplet reveals a situation similar to previous studies in A = 39 nuclei: microscopic calculations reasonably account for the GT strengths, but fail in the case of M1 strengths. A possible explanation may be found in the non-relativistic treatment of the latter. Some examples of the role of the relativistic corrections are discussed. A consistent description of the reduced transition strength and the form factor of the isoscalar M1 excitation requires isospin mixing with the close-lying isovector transitions. The extracted Coulomb matrix elements are roughly within the limits set by the approximate constancy of the spreading width derived from the analysis of compound-nucleus reactions. (orig.)

[en] The ⁵⁷Fe(γ, γ') reaction has been studied up to excitation energies of about 6.5 MeV. The resulting strength distribution below 4 MeV is compared to shell-model calculations with various effective interactions. The shell-model results suggest dominance of M1 excitations. A good description of the ⁵⁷Fe low-energy spectrum, moments and electromagnetic transitions is achieved with the FPD6 interaction. The model results suggest that the coupling of an unpaired nucleon in ⁵⁷Fe does not alter the orbital scissors mode strength compared to the even-mass neighbor ⁵⁶Fe, but leads to a fragmentation and a shift towards lower energies

[en] With the development of continuous wave accelerators electron induced coincidence experiments have become a powerful tool for giant resonance spectroscopy. The present work reports on some results of a recent study of the ⁴⁰Ca(e,e'x) reaction. The experiments were performed at the MAMI A microton in Mainz and at the superconducting accelerator S-DALINAC in Darmstadt. Data were taken at four momentum transfers in the range q = 0.25 - 0.66 fm^-1. The main decay channels (protons and alpha particles) were detected with a set of up to 10 ΔE-E telescopes covering an angular range of Θ_x -O^o - 200^o relative to the recoiling excited nucleus. (author)

[en] The nuclei ⁴⁸Ca, ⁵⁸Ni and ⁹⁰Zr were investigated with high-resolution inelastic electron scattering at 180deg in a momentum transfer range q ≅ 0.35 - 0.8 fm^-1. Complete M2 strength distributions could be extracted up to excitation energies of about 15 MeV utilizing a fluctuation analysis technique. Microscopic calculations including the coupling to 2p-2h states successfully describe the experimentally observed strong fragmentation of the M2 mode. A quantitative reproduction of the data requires a quenching similar to the M1 case and the presence of appreciable orbital strength which can be interpreted as a torsional elastic vibration (the so-called twist mode). A comparison to proton scattering in kinematics which favor spin-isospin excitation of J^π = 2^- sates reveals direct evidence for dominantly orbital excitations in ⁵⁸Ni in line with microscopic predictions. (author)

[en] The giant resonance region in ⁴⁰Ca has been studied with inelastic electron scattering at the Mainz microton MAMI and the superconducting Darmstadt linear accelerator S-DALINAC for a momentum transfer range q=0.25-0.66 fm^-1. Complete angular correlation functions for the coincident α decay into the ground state (α₀) and first excited state (α₁) of ³⁶Ar have been measured. A model-independent multipole decomposition is presented for the α₀ channel under the assumption of a mixture of E0/E1/E2 strengths. It is demonstrated that this method can be extended to the α₁ channel assuming that the relative weight of the allowed decay angular momenta is determined by transmission coefficients and for a specific choice of relative phases between multipole amplitudes suggested by the data. The resulting distributions of isoscalar E0 and E2 strengths for the α₀ and α₁ channels are tested against microscopic calculations including coupling to the continuum and complex configurations of the 1p-1h x phonon type. Assuming statisitcal decay the total isoscalar E0 response can be reconstructed. An exhaustion of 44.2(8.8)% of the energy-weighted sum rule for the isoscalar giant monopole resonance is found in the excitation energy interval 11-19 MeV. The B(E0) strength distribution is in fair agreement with recent (α,α') measurements under extreme forward angles suggesting a centroid energy of about 17-18 MeV. (orig.)

[en] Two examples of recent work on the structure of low-energy electric dipole modes are presented. The first part discusses the systematics of the pygmy dipole resonance (PDR) in stable tin isotopes deduced from high-resolution (γ,γ') experiments. These help to distinguish between microscopic QRPA calculations based on either a relativistic or a nonrelativistic mean-field description, predicting significantly different properties of the PDR. The second part presents attempts to unravel the structure of dipoles modes at energies below the giant dipole resonance (GDR) in ²⁰⁸Pb with a high-resolution measurement of the (p-vector,p-vector') reaction under 0 deg.

[en] The excitation of the J^π;T=1⁺;0 and 1⁺;1 states at E_x=12.71 and 15.11 MeV in ¹²C has been studied at momentum transfers q<0.5 fm^-1 in inelastic electron scattering. The corresponding ground state radiation widths and transition radii for the isoscalar and isovector transition are deduced as Γ_γ0=0.32(2) eV, R_tr=2.95(9) fm and Γ_γ0=35.9(6) eV, R_tr=2.63(4) fm, respectively. For the evaluation of Γ_γ0 and R_tr for the transition to the 15.11 state (e,e'), data of the present work have been combined with older data from other low energy electron accelerators. The results support the strong isospin mixing of the two 1⁺ states, and in the frame of a two-state model a Coulomb matrix element < H_C>=118(8) keV is derived. This value agrees with other experiments using electromagnetic or pionic probes but is much more accurate. Furthermore, the measured difference in the transition radii for the isoscalar and isovector transitions allows for the first time a unique determination of the sign of < H_C>

No abstract available