Photonuclear reactions and astrophysics B. S. Ishkhanov∗ Department of Physics, Lomonosov Moscow State University, Moscow, 119991 Russia and Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, 119991 Russia V. N. Orlin, K. A. Stopani, and V. V. Varlamov Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, 119991 Russia (Dated: Compiled on March 7, 2013) Photonuclear reactions play a prominent role in the nucleosynthesis processes in stars and in the Early Universe. Traditional and modern methods of studying photonuclear reactions are considered. Different factors which determine accuracy of photonuclear data are discussed. Cross sections of photonuclear reactions relevant to astrophysics are given. Contents Introduction 2 I. Main sources of photons in cosmic space and stars 3 II. Experimental study of photonuclear reactions 4 III. Analysis of discrepancies in photonuclear data 6 IV. Photonuclear reactions description in a combined model 8 V. Evaluation of photonuclear data 11 VI. Interaction of photons with atomic nuclei 12 VII. Primordial nucleosynthesis 15 VIII. e-process 16 IX. Photodisintegration of the iron peak nuclei 19 X. p-nuclei 22 Photodisintegration of Ta isotopes 23 Photodisintegration of Hg isotopes 26 XI. Photofission 29 Conclusions 31 Acknowledgments 31 References 31 ∗Electronic address: [email protected] 2 Introduction The world is composed of different chemical elements and their compounds. One of the fundamental questions of modern natural science is to determine the mechanism of production of chemical elements. This problem lies at the border between astrophysics and nuclear physics. Important information about the origin of chemical elements can be extracted from analysis of their abundances (Fig. 1). FIG. 1: Schematic representation of the chemical element abundance curve in the Solar system. Eight principal processes responsible for production of chemical elements were discussed in [1]. 1. Hydrogen burning. This is the main process that provides long-term energy release in stars. During burning four hydrogen nuclei combine producing one 4He nucleus. This process occurs either in the pp-chain of nuclear reactions or in the CNO cycle of nuclear reactions involving heavier nuclei: C, N, O, Ne, etc., acting as catalysts. Also certain light nuclei are produced in reactions that involve protons. 2. Helium burning. Once helium is accumulated in the star, due to gravitational force the helium core shrinks and becomes sufficiently hot, initiating the process of helium burning. The 12C, 16O, 20Ne nuclei are formed during helium burning. 3. α-process. This is a process of successive captures of α-particles by a 20Ne nucleus, resulting in the formation of 24Mg, 28Si, 32S, 36Ar, 40Ca. The α-process is responsible for high abundances of the Nα-type nuclei, where α is the 4He nucleus, N is an integer. 4. e-process. This is a process of production of the iron peak nuclei at very high temperatures and densities under thermodynamic equilibrium conditions. 5. s-process. A process of slow successive neutron captures, that is responsible for production of nuclei heavier than iron. The rate of the s-process is commensurate with the rate of beta decays of unstable nuclei produced by the neutron capture reactions. The time scale of the s-process is 102|105 yr. The s-process is responsible for the nuclear abundance peaks at A ∼ 98, 138, and 208. 3 6. r-process. This is a process, in which nuclei heavier than iron are formed in a series of rapid successive neutron captures at a rate significantly faster than the rate of beta decays of resulting unstable nuclei. A typical length of the r-process is 0:01|100 sec. The maxima at A = 80, 130, and 195 in the nuclear abundance curve are a result of the r-process. 7. p-process. Production of a number of proton-rich isotopes with low abundances as compared to the nearby normal and neutron-rich isotopes. The likely mechanism of p-nuclei production involves the (p,γ) and (γ,n) reactions. 8. X-process. This is a nucleosynthesis process responsible for production of the 6;7Li, 9Be, 10;11B isotopes. It is believed that these elements are produced in disintegration reactions triggered by cosmic ray particles. The set of nuclear reactions that take place in a star at thermodynamic equilibrium is governed by the stellar mass (table I). The temperature and density of a M = 25M star at different stages of nuclear processes are shown in Fig. 2. TABLE I: Allowed nuclear reactions in stars Mass, M Reactions 0.08 No 0.3 Hydrogen burning 0.7 Hydrogen and helium burning 5.0 Hydrogen, helium, and carbon burning 25.0 All fusion reactions with energy release FIG. 2: Schematic diagram of chemical element production in a star. The most important reactions of nucleosynthesis are induced by protons, neutrons, and α-particles. However, an important role is also played by the reactions triggered by non-thermalized photons, or photonuclear reactions. This review concentrates on the modern status of research in the field of photon-nucleus interactions relevant to the subject of stellar nuclear reactions. I. MAIN SOURCES OF PHOTONS IN COSMIC SPACE AND STARS There are several principal sources of sufficiently high-energy photons in stars and in cosmic space: • annihilation of particles and antiparticles produced in the Big Bang; 4 • bremsstrahlung radiation from electrons and positrons; • inverse Compton scattering of photons on high-energy electrons; • the π0 ! γ + γ decay; • gamma transitions between excited nuclear states; • scattering of the cosmic microwave background photons on high-energy nuclei; • acceleration of electrons in non-homogeneities of interstellar plasma; • gamma radiation from supernovae and active galaxy kernels. The energies of the photons produced during these cosmic processes are distributed in a broad energy range from 10−1 to 107 keV. In thermodynamic equilibrium with stellar medium the energy of radiation is described by the Planck's law. The number of photons with kinetic energy E in a unit volume of phase space 1 3 E2 1 n = ; (1) c π2 E ~ e kT − 1 where T is the temperature of the stellar matter. II. EXPERIMENTAL STUDY OF PHOTONUCLEAR REACTIONS Systematic studies of photonuclear (primarily photoneutron) reactions started in the 50s. As the photon-nucleus interaction is purely electromagnetic and can be effectively calculated and explained, reactions of this kind provide definite advantages in what concerns their description and interpretation in comparison with charged particle reactions. But at the same time experimental studies of photon interaction with atomic nucleus pose many specific difficulties. The most important of them is the absence of intensive monoenergetic photons in nature, so experimentalists from the beginning tried to devise various techniques to get sources of nearly quasimonoenergetic photons. Historically the first were the experiments using bremsstrahlung beams produced by electron accelerators. The intensities of bremsstrahlung beams were sufficiently high and good statistical accuracy of results could be achieved. The energy spectrum of bremsstrahlung photons is continuous. That means that at a given end-point energy Emax one can measure directly only the reaction yield Emax α Z N(Emax) = σ(E)W (Emax;E) dE; (2) D(Emax)" 0 where σ(E) is the reaction cross section at the photon energy E, W (Emax;E) is the bremsstrahlung spectrum with end-point energy Emax, N(Emax) is the number of reaction events, D(Emax) is gamma-dose, is detector efficiency, α is a normalization constant. To determine σ(E) several specially developed methods for unfolding were conventionally applied. The most widely used were the inverse matrix method and the regularization method. In such an approach one needs to solve a mathematically complicated inverse problem to obtain reaction cross section information from reaction yields, so in the 1960s an alternative method was introduced. It is based on the phenomenon of relativistic positron annihilation in flight. The spectrum of quasimonoenergetic annihilation photons is nearly Gaussian, with sufficiently small width (approximately several hundred keV). Due to the fact that positron annihilation photons are mixed with positron bremsstrahlung, a three-stage difference experiment strategy was needed. At the first stage the yield Y (Emax)e+ of the reaction induced by both annihilation and bremsstrahlung photons is measured. At the second stage the yield Y (Emax)e− of the reaction is measured using bremsstrahlung radiation from the electron beam. At the third stage the results are normalized, and under assumption that bremsstrahlung spectra for electrons and positrons are identical the cross section of the reaction triggered by quasimonoenergetic photons is obtained as a result of subtracting the latter measurement from the first one σ(E) = Y (Emax)e+ − Y (Emax)e− : (3) As the intensity of quasimonoenergetic photon beams was very low statistical accuracy of the measurements was poor. 5 Only few measurements of total photoneutron reaction cross section measurements were performed using actual monoenergetic photons, that is, tagged photons obtained in very complex and low intensity experiments using coin- cidences of outgoing photons and scattered electrons. Recently more advanced methods have become to be actively used in different laboratories to study photonuclear reactions at the energies close to the nucleon separation energy. Experiments using continuous-energy bremsstrahlung combined with the induced activity measurement technique are perfectly suited to carry out systematic surveys on various types of photonuclear reactions. The complete sys- tematics of groundstate reaction rates of numerous isotopes were studied at the High Intensity Photon Setup (HIPS) of the superconducting linear electron accelerator S-DALINAC facility at the Technishe Universitat Darmstadt [2, 3]. Continuous-energy bremsstrahlung was produced by fully stopping a monoenergetic electron beam in a thick copper radiator target. Due to the high currents of up to 40 µA, the photon intensity reaches maximum values of about 7 −1 −1 −2 10 keV s cm at 80% of the maximum emergy Emax.