No abstract

A dedicated setup for the in-beam measurement of absolute cross sections of astrophysically relevant charged-particle induced reactions is presented. These, usually very low, cross sections at energies of astrophysical interest are important to improve the modeling of the nucleosynthesis processes of heavy nuclei. Particular emphasis is put on the production of the p nuclei during the astrophysical γ process. The recently developed setup utilizes the highefficiency γ-ray spectrometer HORUS, which is located at the 10 MV FN tandem ion accelerator of the Institute for Nuclear Physics in Cologne.The design of this setup will be presented and results of the recently measured 89 Y(p,γ) 90 Zr reaction will be discussed. The excellent agreement with existing data shows, that the HORUS spectrometer is a powerful tool to determine total and partial cross sections using the in-beam method with high-purity germanium detectors.

Partial cross sections of the 89 Y(p, γ ) 90 Zr reaction have been measured to investigate the γ -ray strength function in the neutron-magic nucleus 90 Zr. For five proton energies between E p = 3.65 MeV and E p = 4.70 MeV, partial cross sections for the population of seven discrete states in 90 Zr have been determined by means of in-beam γ -ray spectroscopy. Since these γ -ray transitions are dominantly of E1 character, the present measurement allows an access to the low-lying dipole strength in 90 Zr.A γ -ray strength function based on the experimental data could be extracted, which is used to describe the total and partial cross sections of this reaction by Hauser-Feshbach calculations successfully. Significant differences with respect to previously measured strength functions from photoabsorption data point towards deviations from the Brink-Axel hypothesis relating the photo-excitation and de-excitation strength functions.

Background:The nucleosynthesis of the neutron-deficient p nuclei remains an open question in nuclear astrophysics. Beside uncertainties on the astrophysical side, the nuclear-physics input parameters entering Hauser-Feshbach calculations for the nucleosynthesis of the p nuclei must be put on a firm basis. Purpose: An extended database of experimental data is needed to address uncertainties of the nuclear-physics input parameters for Hauser-Feshbach calculations. Especially a-(-nucleus optical model potentials at low energies are not well known. The in-beam technique with an array of high-purity germanium (HPGe) detectors was successfully applied to the measurement of absolute cross sections of an (a,y ) reaction on a heavy nucleus at sub-Coulomb energies. Method: The total and partial cross-section values were measured by means of in-beam y -ray spectroscopy. For this purpose, the absolute reaction yield was measured using the HPGe detector array HORUS at the FN tandem accelerator at the University of Cologne. Total and partial cross sections were measured at four different a-particle energies from Ea -10.5 MeV to Ea = 12 MeV. Results: The measured total cross-section values are in excellent agreement with previous results obtained with the activation technique, which proves the validity of the applied method. With the present measurement, the discrepancy between two older data sets is removed. The experimental data was compared to Hauser-Feshbach calculations using the nuclear reaction code TALYS. With a modification of the semi-microscopic a + nucleus optical model potential OMP 3, the measured cross-section values are reproduced well. Moreover, partial cross sections could be measured for the first time for an (a , y ) reaction. Conclusions: A modified version of the semimicroscopic a + nucleus optical model potential OMP3, as well as modified proton and y widths, are needed in order to obtain a good agreement between experimental data and theory. It is found that a model using a local modification of the nuclear-physics input parameters simultaneously reproduces total cross sections of the ll2Sn(a,y) and ll2Sn(a,p) reactions. The measurement of partial cross sections turns out to be very important in this case in order to apply the correct y-ray strength function in the Hauser-Feshbach calculations. The model also reproduces cross-section values of a-induced reactions on l06Cd, as well as of (a.n) reactions on 1 l5 ll6Sn, hinting at a more global character of the obtained nuclear-physics input.

Background: Deviations between experimental data of charged-particle-induced reactions and calculations within the statistical model are frequently found. An extended data base is needed to address the uncertainties regarding the nuclear-physics input parameters in order to understand the nucleosynthesis of the neutron-deficient p nuclei. Purpose: A measurement of total cross-section values of the 130 Ba(p,γ ) 131 La reaction at low proton energies allows a stringent test of statistical model predictions with different proton+nucleus optical model potentials. Since no experimental data are available for proton-capture reactions in this mass region around A ≈ 130, this measurement can be an important input to test the global applicability of proton+nucleus optical model potentials. Method: The total reaction cross-section values were measured by means of the activation method. After the irradiation with protons, the reaction yield was determined by use of γ -ray spectroscopy using two clover-type high-purity germanium detectors. In total, cross-section values for eight different proton energies could be determined in the energy range between 3.6 MeV E p 5.0 MeV, thus, inside the astrophysically relevant energy region. Results: The measured cross-section values were compared to Hauser-Feshbach calculations using the statistical model codes TALYS and SMARAGD with different proton+nucleus optical model potentials. With the semimicroscopic JLM proton+nucleus optical model potential used in the SMARAGD code, the absolute cross-section values are reproduced well, but the energy dependence is too steep at the lowest energies. The best description is given by a TALYS calculation using the semimicroscopic Bauge proton+nucleus optical model potential using a constant renormalization factor. Conclusions: The statistical model calculation using the Bauge semimicroscopic proton+nucleus optical model potential deviates by a constant factor of 2.1 from the experimental data. Using this model, an experimentally supported stellar reaction rate for proton capture on the p nucleus 130 Ba was calculated. At astrophysical temperatures, an increase in the stellar reaction rate of 68% compared to rates obtained from the widely used NON-SMOKER code is found. This measurement extends the scarce experimental data base for charged-particle-induced reactions, which can be helpful to derive a more globally applicable proton+nucleus optical model potential.

Background: Uncertainties in adopted models of particle + nucleus optical-model potentials directly influence the accuracy in the theoretical predictions of reaction rates as they are needed for reaction-network calculations in, for instance, y-process nucleosynthesis. The improvement of the a + nucleus optical-model potential is hampered by the lack of experimental data at astrophysically relevant energies especially for heavier nuclei. Purpose: Measuring the l87R e(a,n)190Ir reaction cross section at sub-Coulomb energies extends the scarce experimental data available in this mass region and helps understanding the energy dependence of the imaginary part of the a + nucleus optical-model potential at low energies. Method: Applying the activation method, after the irradiation of natural rhenium targets with a-particle energies of 12.4 to 14.1 MeV, the reaction yield and thus the reaction cross section were determined via y-ray spectroscopy by using the Cologne Clover Counting Setup and the method of y y coincidences. Results: Cross-section values at five energies close to the astrophysically relevant energy region were measured. Statistical model calculations revealed discrepancies between the experimental values and predictions based on widely used a+nucleus optical-model potentials. However, an excellent reproduction of the measured crosssection values could be achieved from calculations based on the so-called Sauerwein-Rauscher a + nucleus optical-model potential. Conclusion:The results obtained indicate that the energy dependence of the imaginary part of the a + nucleus optical-model potential can be described by an exponential decrease. Successful reproductions of measured cross sections at low energies for a-induced reactions in the mass range 141 < A < 187 confirm the global character of the Sauerwein-Rauscher potential.

The validity of the Brink-Axel hypothesis, which is especially important for numerous astrophysical calculations, is addressed for 116;120;124 Sn below the neutron separation energy by means of three independent experimental methods. The γ-ray strength functions (GSFs) extracted from primary γ-decay spectra following charged-particle reactions with the Oslo method and with the shape method demonstrate excellent agreement with those deduced from forward-angle inelastic proton scattering at relativistic beam energies. In addition, the GSFs are shown to be independent of excitation energies and spins of the initial and final states. The results provide a critical test of the generalized Brink-Axel hypothesis in heavy nuclei, demonstrating its applicability in the energy region of the pygmy dipole resonance.

Background: The semi-magic Sn (Z = 50) isotopes have been subject to many nuclear-structure studies. Signatures of shape coexistence have been observed and attributed to two-proton-two-hole (2p-2h) excitations across the Z = 50 shell closure. In addition, many low-lying nuclear-structure features have been observed which have effectively constrained theoretical models in the past. One example are so-called quadrupole-octupole coupled states (QOC) caused by the coupling of the collective quadrupole and octupole phonons. Purpose: Proton-scattering experiments followed by the coincident spectroscopy of γ rays have been performed at the Institute for Nuclear Physics of the University of Cologne to excite low-spin states in 112 Sn and 114 Sn, to determine their lifetimes and extract reduced transitions strengths B(ΠL). Methods: The combined spectroscopy setup SONIC@HORUS has been used to detect the scattered protons and the emitted γ rays of excited states in coincidence. The novel (p, p ′ γ) DSA coincidence technique was employed to measure sub-ps nuclear level lifetimes. Results: 74 level lifetimes τ of states with J = 0 − 6 were determined. In addition, branching ratios were deduced which allowed the investigation of the intruder configuration in both nuclei. Here, sd IBM-2 mixing calculations were added which support the coexistence of the two configurations. Furthermore, members of the expected QOC quintuplet are proposed in 114 Sn for the first time. The 1 − candidate in 114 Sn fits perfectly into the systematics observed for the other stable Sn isotopes. Conclusions: The E2 transition strengths observed for the low-spin members of the so-called intruder band support the existence of shape coexistence in 112,114 Sn. The collectivity in this configuration is comparable to the one observed in the Pd nuclei, i.e. the 0p-4h nuclei. Strong mixing between the 0 + states of the normal and intruder configuration might be observed in 114 Sn. The general existence of QOC states in 112,114 Sn is supported by the observation of QOC candidates with J = 1.