Nuclear reactions induced by photons play a vital role for very different aspects of basic research and applications in physics. They are a key ingredient for the synthesis of nuclei in the Universe ...and provide, due to the selectivity and the model-independence of the reaction mechanism, an extremely valuable probe for researchers. The penetrability of photons in the MeV energy range makes them, in addition, an ideal tool for meeting various societal challenges. The last two decades saw a rapid development of advanced photon sources and detection methods for photonuclear reaction products. Bremsstrahlung and quasi-monoenergetic photon beams with unprecedented intensity and quality combined with state-of-the-art detector technology paved the way for new scientific discoveries and technological applications.
This review focuses on a comprehensive overview of the most important developments since the turn of the millennium restricted to the energy range between atomic and hadronic degrees of freedom. This includes a description of the formalism of photonuclear reactions below and above the particle-separation threshold. The most important techniques used to generate photon beams in the MeV energy range are presented along with selected facilities and instrumentation for diagnostics and for the analysis of photonuclear reactions. The power of photons to probe the atomic nucleus is exemplified in a number of selected examples from fundamental and applied science. New developments, facilities, and ideas promise a vivid future for photonuclear physics.
The experimental E1 strength distribution below 4 MeV in rare-earth nuclei suggests a local breaking of isospin symmetry. In addition to the octupole states, additional J^{π}=1^{-} states with ...enhanced E1 strength have been observed in rare-earth nuclei by means of (γ,γ') experiments. By reproducing the experimental results, the spdf interacting boson model calculations provide further evidence for the formation of an α cluster in medium-mass nuclei and might provide a new understanding of the origin of low-lying E1 strength.
Several experimental studies using electromagnetic probes have found an enhancement of electric dipole strength between about 5 and 10 MeV. This phenomenon is usually denoted as Pygmy Dipole ...Resonance (PDR). The detailed structure of these excitations is still under debate. This manuscript will concentrate on the results of complementary experiments using hadronic probes to populate the PDR. These studies allow a first insight into the origin of the PDR. Finally, the manuscript will shortly discuss plans for future experiments.
Most theoretical approaches used in nuclear astrophysics to model the nucleosynthesis of heavy elements incorporate the so-called statistical model in order to describe the excitation and decay ...properties of atomic nuclei. One of the basic assumptions of this model is the validity of the Brink–Axel hypothesis and the related concept of so-called photon strength functions to describe γ-ray transition probabilities. We present a novel experimental approach that allows for the first time to experimentally determine the photon strength function simultaneously in two independent ways by a unique combination of quasi-monochromatic photon beams and a newly implemented γ–γ coincidence setup. This technique does not assume a priori the validity of the Brink–Axel hypothesis and sets a benchmark in terms of the detection sensitivity for measuring decay properties of photo-excited states below the neutron separation energy. The data for the spherical off-shell nucleus 128Te were obtained for γ-ray beam-energy settings between 3 MeV and 9 MeV in steps of 130 keV for the lower beam energies and in steps of up to 280 keV for the highest beam settings. We present a quantitative analysis on the consistency of the derived photon strength function with the Brink–Axel hypothesis. The data clearly demonstrate a discrepancy of up to a factor of two between the photon strength functions extracted from the photoabsorption and photon emission process, respectively. In addition, we observe that the photon strength functions are not independent of the excitation energy, as usually assumed. Thus, we conclude, that the Brink–Axel hypothesis is not strictly fulfilled in the excitation-energy region below the neutron separation threshold (Sn=8.78MeV) for the studied case of 128Te.
The purpose of this article is to describe the method used to determine the amount of 148Gd produced in the (α,γ) activation of enriched 144Sm samples. Advanced simulations are fitted to data to help ...constrain systematic uncertainties. The simulations are complimented with RBS measurements of the samples.
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Cross section measurements of (
p
,
γ
) reactions on the Mo isotopes have been performed at beam energies from 2 to 6.2 MeV that are relevant to the
p
-process. Partial cross sections and isomeric ...ratios were also determined for the
92
Mo case. Astrophysical S factors as well as reaction rates were derived from the experimental cross sections. Statistical model calculations were performed using the latest version (1.9) of the statistical model code TALYS and were compared with the new data. An overall good agreement between theory and experiment was found. In addition, the effect of different combinations of the nuclear input parameters entering the stellar reaction-rate calculations was investigated. It was found that, for certain combinations of optical-model potentials, nuclear level densities and
γ
-ray strength functions, the nuclear uncertainties propagated through the Hauser-Feshbach calculations are less than a factor of 2 which is well below the average discrepancies of the calculated
p
-nuclei abundances and the observations.
CologneAMS is a new centre for accelerator mass spectrometry (AMS) at the University of Cologne. It has been funded by the German Research Foundation (DFG) to improve the experimental conditions ...especially for those German scientists that apply the AMS technique for their geologic, environmental, nuclear chemical, and nuclear astrophysical research. The new AMS-device has been built by High Voltage Engineering Europe (HVEE) and has been installed in the existing accelerator area of the Institute of Nuclear Physics. The AMS-facility is designed for the spectrometry of 10Be, 14C, 26Al, 36Cl, 41Ca, 129I in and heavy ions up to 236U and 244Pu. The central part of the AMS-facility is a 6MV Tandetron™ accelerator. Downstream of the high energy mass spectrometer an additional switching magnet is used as a further filter element which supplies also additional ports for future extensions of the detector systems. The current status of CologneAMS and the results of the first test measurements will be presented.
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 high-efficiency γ-ray spectrometer HORUS, which is located at the 10MV 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 89Y(p,γ)90Zr 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.
The electric dipole strength distribution in 44Ca has been measured up to 10 MeV in high resolution photon scattering experiments for the first time. The data obtained have been compared to earlier ...measurements on (40,48)Ca in order to view the evolution of the electric pygmy dipole resonance (PDR). Calculations that were performed within the framework of the microscopic extended theory of finite Fermi systems, which adds contributions of the quasiparticle-phonon coupling to random phase approximation calculations, give a qualitative agreement with the experimental data for all three isotopes. We have shown that it is necessary to include this coupling to describe the PDR.
The generation of the collective nuclear E1 response is intimately connected to the breaking of proton-neutron symmetry. In this contribution, we present our recent results on a possible α-cluster ...dipole mode, which were obtained in the framework of the spdf interacting boson model. These extended studies of the low-lying E1 response support the general occurence of α-cluster dipole states in atomic nuclei.
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