Photoneutron reactions in astrophysics
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CLEI Experiment
Photoneutron Reactions in Astrophysics V. V. Varlamov1)* , B. S. Ishkhanov1), 2) , V. N. Orlin1), N. N. Peskov1) , and K. A. Stopani1) Received January 13, 2014; in final form, May 19, 2014
Abstract—Among key problems in nuclear astrophysics, that of obtaining deeper insight into the mechanism of synthesis of chemical elements is of paramount importance. The majority of heavy elements existing in nature are produced in stars via radiative neutron capture in so-called s- and r processes, which are, respectively, slow and fast, in relation to competing β − -decay processes. At the same time, we know 35 neutron-deficient so-called bypassed p-nuclei that lie between 74 Se and 196 Hg and which cannot originate from the aforementioned s- and r-processes. Their production is possible in (γ, n), (γ, p), or (γ, α) photonuclear reactions. In view of this, data on photoneutron reactions play an important role in predicting and describing processes leading to the production of p-nuclei. Interest in determining cross sections for photoneutron reactions in the threshold energy region, which is of particular importance for astrophysics, has grown substantially in recent years. The use of modern sources of quasimonoenergetic photons obtained in processes of inverse Compton laser-radiation scattering on relativistic electrons makes it possible to reveal rather interesting special features of respective cross sections, manifestations of pygmy E1 and M 1 resonances, or the production of nuclei in isomeric states, on one hand, and to revisit the problem of systematic discrepancies between data on reaction cross sections from experiments of different types, on the other hand. Data obtained on the basis of our new experimental–theoretical approach to evaluating cross sections for partial photoneutron reactions are invoked in considering these problems. DOI: 10.1134/S1063778814110088
1. INTRODUCTION A determination of the mechanisms of chemicalelement production is one of the fundamental problems in modern natural sciences, which lies on the border of nuclear physics and astrophysics. Important information about the nature of elements is contained in the well-known A dependence of their abundances. The abundance in question decreases fast as the mass number increases, and the whole A dependence admits a clear-cut partition into two regions: that of relatively light and medium-mass elements, A < 70, and that of heavy elements, A ∼ 70–200 [1]. In the region of A < 70, nuclei are produced in the following processes: hydrogen burning, which leads to the formation of nuclei of up to helium; helium burning, which leads to the formation of carbon, oxygen, and neon nuclei; alpha-particle-capture processes leading to the formation of nuclei of up to calcium; and the e-process, which leads to the formation of the iron-peak nuclei. The so-called X-process in which lithium, beryllium, and boron isotopes are produced via the spallation of heavier isotopes under the 1)
Skobeltsyn Institute of Nuclear Physics, Moscow State University, Mo
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