J. Hasper

Determination of 141Pr(alpha,n)144Pm cross sections at energies of relevance for the astrophysical p-process using the gamma-gamma coincidence method

The reaction 141 Pr(α,n) 144 Pm was investigated between Eα = 11 MeV and 15 MeV with the activation method using the γγ coincidence method with a segmented clover-type high-purity Germanium (HPGe) detector. Measurements with four other HPGe detectors were additionally made. The comparison proves that the γγ coincidence method is an excellent tool to investigate cross sections down to the microbarn range. The (α,n) reaction at low energy is especially suited to test α + nucleus optical-model potentials for application in the astrophysical p process. The experimentally determined cross sections were compared to Hauser-Feshbach statistical model calculations using different optical potentials and generally an unsatisfactory reproduction of the data was found. A local potential was constructed to improve the description of the data. The consequences of applying the same potential to calculate astrophysical ( γ,α ) rates for 145 Pm and 148 Gd were explored. In summary, the data and further results underline the problems in global predictions of α + nucleus optical potentials at astrophysically relevant energies.

Investigation of photoneutron reactions close to and above the neutron emission threshold in the rare earth region

We have investigated the photoneutron cross section of the isotopes {sup 148,150}Nd, {sup 154}Sm, and {sup 154,160}Gd close to the neutron emission threshold in photoactivation experiments at the superconducting Darmstadt electron linear accelerator S-DALINAC. Naturally composed targets were activated with a high-intensity bremsstrahlung beam at various energies and the reaction yields have been determined by measuring the activity of the produced radioactive isotopes with HPGe detectors. The results are compared to two different statistical model calculations.

Nuclear astrophysics with real photons—the data acquisition system of the NEPTUN tagger setup

Photodissociation reactions play an important role in p-process nucleosynthesis. A precise knowledge of the energy dependence of a cross section is mandatory to determine the reaction rates for astrophysical network calculations. The NEPTUN tagger setup constructed at the S-DALINAC will provide high resolution measurements of (γ,n), (γ,p) and (γ, α) reactions. Besides a general overview on this setup its data acquisition system will be explained in more detail.

Nuclear Astrophysics with Tagged Photons

For the modeling of the astrophysical s-process the knowledge of neutron capture cross sections is crucial. Where experimental data is not available for the (n,γ) cross-section like e.g. short-lived branching points, the observation of the inverse (γ ,n) reaction can provide useful information – in particular, if the cross section can be measured as a function of energy. The NEPTUN tagger setup at the S-DALINAC delivers a suited tagged photon beam (6–20MeV). In a first test run the resolution turned out to be 35 keV which is close to NEPTUN’s design specifications.

Systematics and fragmentation of low-lying electric dipole strength

We report on systematic investigations of low‐lying electric dipole (E1) strength in atomic nuclei. Using the method of real photon scattering the dipole response of all stable N = 82 isotones has been measured with high resolution. In all cases a resonance like structure of E1 strength is observed in the energy region 5–8 MeV, which shows a strong fragmentation. The total integrated strength increases towards the proton poorer isotones, indicating a dependence of the total strength on the neutron‐to‐proton ratio. The experimental results are compared to microscopic calculations within the Quasi‐particle Phonon Model (QPM). By including complex configurations of up to three phonons the calculation is able to reproduce also the fragmentation of the E1 strength and is in good agreement with the experimental observation.

Activation Experiments for Nuclear Astrophysics

The study of (γ,n) reactions can be used to constrain the theoretical predictions of the neutron capture cross sections of short‐lived branching points in the s process. The usability of the activation technique to study these (γ,n) reactions is discussed as one example of an activation experiment in nuclear astrophysics. Two photon sources using bremsstrahlung and laser‐Compton backscattered photons where such experiments were carried out are compared.

Determination of 141 Pr(α,n) 144 Pm cross sections at energies of relevance for the astrophysical p process using the γγ coincidence method

The reaction 141 Pr(α,n) 144 Pm was investigated between Eα = 11 MeV and 15 MeV with the activation method using the γγ coincidence method with a segmented clover-type high-purity Germanium (HPGe) detector. Measurements with four other HPGe detectors were additionally made. The comparison proves that the γγ coincidence method is an excellent tool to investigate cross sections down to the microbarn range. The (α,n) reaction at low energy is especially suited to test α + nucleus optical-model potentials for application in the astrophysical p process. The experimentally determined cross sections were compared to Hauser-Feshbach statistical model calculations using different optical potentials and generally an unsatisfactory reproduction of the data was found. A local potential was constructed to improve the description of the data. The consequences of applying the same potential to calculate astrophysical ( γ,α ) rates for 145 Pm and 148 Gd were explored. In summary, the data and further results underline the problems in global predictions of α + nucleus optical potentials at astrophysically relevant energies.

Determination of Pr-141(alpha, n)Pm-144 cross sections at energies of relevance for the astrophysical p process using the gamma gamma coincidence method

The reaction 141 Pr(α,n) 144 Pm was investigated between Eα = 11 MeV and 15 MeV with the activation method using the γγ coincidence method with a segmented clover-type high-purity Germanium (HPGe) detector. Measurements with four other HPGe detectors were additionally made. The comparison proves that the γγ coincidence method is an excellent tool to investigate cross sections down to the microbarn range. The (α,n) reaction at low energy is especially suited to test α + nucleus optical-model potentials for application in the astrophysical p process. The experimentally determined cross sections were compared to Hauser-Feshbach statistical model calculations using different optical potentials and generally an unsatisfactory reproduction of the data was found. A local potential was constructed to improve the description of the data. The consequences of applying the same potential to calculate astrophysical ( γ,α ) rates for 145 Pm and 148 Gd were explored. In summary, the data and further results underline the problems in global predictions of α + nucleus optical potentials at astrophysically relevant energies.

Experiments on photon-induced reactions for p-process nucleosynthesis

Photodesintegration rates – like (γ ,n), (γ ,p), and (γ ,α) – play an important role in the nucleosynthesis of the so-called p nuclei. These proton-rich, in general very low-abundant isotopes cannot be produced by neutron capture reactions. Experiments using continuous-energy bremsstrahlung and the activation technique are perfectly suited to carry out systematic surveys on (γ ,n) reactions. In addition, single cases can be studied in more detail using energy-resolved methods as provided by photon sources based on the principle of Laser-Compton backscattering or photon tagging. An overview about current results on photon-induced reactions and recent instrumental developments is given.

Probing astrophysical reaction-rate calculations in photoneutron experiments

Advanced astrophysical reaction-network calculations fo r the nucleosynthesis of heavy elements within the s, r, and p processes involve ten thousands of reactions. Most of the re action rates are adopted from calculations based on the Hauser-Feshbach theory. Experimental investigations of photoneutron reaction rates are well suited to test the re liability of these calculations and to constrain the nuclear-physics input relevant for statisti cal model codes. In this context, recent experimental results stemming from photoactivation exper im nts are discussed.