POSSIBILITIES TO INVESTIGATE ASTROPHYSICAL PHOTONUCLEAR REACTIONS IN UKRAINE Ye. Skakun1, I. Semisalov1, V. Kasilov1, V. Popov1, S. Kochetov1, N. Avramenko1, V. Maslyuk2, V. Mazur2, O. Parlag2, D. Simochko2, I. Gajnish2 1NSC KIPT, Institute of High Energy and Nuclear Physics, Kharkiv, Ukraine 2 Institute of Electron Physics, National Academy of Sciences of Ukraine, Uzhgorod, Ukraine Reactions of proton capture (rp-process) and sequences of photodisintegrations of the (γ,n), (γ,α) and (γ,p) types (γ-process) play the key role in stellar nucleosynthesis of the so-called p-nuclei − a group of stable proton rich nuclides which could be created by none of slow (s) and rapid (r) radiative neutron capture reactions. There is need of knowledge of thousands of reaction rates to simulate natural abundances of the p-nuclei. Using bremsstrahlung beams from thin tantalum converters of the electron linear accelerator of NSC KIPT (Kharkiv) and the microtron of IEP (Uzhgorod) and conventional activation technique applying high resolution gamma-spectrometry we measured the integral cross sections of the (γ,n)-reactions on the nuclei of the 96Ru, 98Ru, 104Ru, 102Pd, and 110Pd isotopes the first two of which and palladium-102 are p-nuclei, and determined the reaction rates by a procedure of superposition of several bremsstrahlung spectra with different endpoints in the range from the thresholds to 14 MeV. The experimental reaction cross sections were compared to available data in overlapping energy range and the derived reaction rates to the predictions of the Hauser - Feshbach statistical model of nuclear reactions. In most cases theory underestimates the observed reaction rates in not great extent. 1. Introduction Occurring in nature chemical elements and their isotopes were synthesized in stars via different type low-energy nuclear reactions from pre-existing nucleons. The lightest nuclei were created through the process called the Big Bang nucleosynthesis while nuclei having masses up to iron (Z = 26) were produced through the successive fast nuclear fusion mechanisms in the cores of stars. The majority of trans-iron nuclei were synthesized by rapid (r) and slow (s) (compared to the intervening beta-minus decays) neutron capture processes, i.e. (n, γ)-reactions [1]. However there are 35 nuclei between 74Se (Z = 34) and 196Hg (Z = 80) which cannot be formed through these scenarios since they are screened from the r-process by another stable isotope in the same isobar chain and bypassed by the s-process. They were called by p-nuclei since they are located at the proton-rich side of the valley of stability and the scenario of their synthesis by p-process complemented by β+, electron capture and (n, γ)-reactions. Several isotopes of p-type are placed in the A = 90÷110 mass number region. The chains of the ruthenium and palladium isotopes and the ways of their star synthesis are shown in Fig. 1. The most of the natural ruthenium (having masses 99 and more) and palladium (masses 104 and more) isotopes were synthesized in the s (the thick broken arrow line in Fig. 1) and r (the individual thin solid arrows) processes. However both of these scenarios are bypassing the 96Ru, 98Ru and 102Pd p-nuclei. The 96Ru (98Ru) nuclide cannot be formed because of the 95Ru (97Ru) isotope radioactivity and the 96Mo (98Mo) isotope stability. The 102Pd nuclide is blocked by the radioactive 101Pd and stable 102Ru isotopes. A possible chance of these nuclei creation is p-process representing a combination of proton induced reactions (rp-process) and (γ, n), (γ, p) or (γ, α) photonuclear reactions (γ-process) on pre-existing s- and r-nuclei and the νp as an alternative process [2]. So if r-process widens the valley of stability and s-process elongates it then p-process moves backward from heavier nuclei to lighter ones. Ag p-process 102 108 110 10298 104 105 106 Pd p s s,r s,r s,r r Rh 103 p-process s,r r-process 96 9898 99 100 101 102 104 Ru ppp s,r s s,r s,r s,r Tc s-process r-process Fig. 1. The main nucleosynthesis mechanisms of the ruthenium and palladium isotopes production. The widely believed candidate site for the p-process are the deep O-Ne-rich layers of massive stars of SNe-II type 9 exploding as core-collapse supernovae at the stellar plasma temperatures of T9 = 1.8 ÷ 3.3 (T9 is the temperature in 10 Kelvin units). The primary stimulus to the cosmos nucleosynthesis theory development is to understand the shape of the abundance curve when plotted on a graph as a function of atomic mass and the strongly differing (by factors up to ten millions) abundances of the chemical elements and their several isotopes. The p-nuclei represent only a small fraction 570 (usually 0.01÷1 %) of the isotopic content of corresponding elements in the solar system. This fact is hopeful to believe in formation of the p-nuclei from pre-existing s and r seed nuclei. The (p,γ)-reactions require a high-temperatures of stellar plasma to overcome the proton Coulomb barrier. So photonuclear reactions can play larger role in the heavier p- nuclei synthesis. Available astrophysical nucleosynthesis simulation codes have probably their own uncertainties but they must be provided with such high precision nuclear data as reaction rates converted from cross sections, nuclear masses, decay data, structure features of many nuclei etc. p-Process involves thousands of photonuclear reactions on around 2000 nuclei-“targets” many of which are radioactive even in the ground state and often populated in excited states according to the Boltzman statistics of star gas at high temperatures. Impossibility to measure cross sections of nuclear reactions on unstable and excited nuclei increases the importance of theoretical calculations of nuclear reaction cross sections in the frame of the statistical Hauser - Feshbach model [3]. Therefore experimental values of nuclear reaction cross sections are both nuclear data for star nucleosynthesis simulation and to test predictive power of the statistical theory of nuclear reactions. Large efforts were applied in recent years to measure charged particle (see, e.g. [4] and references therein) and gamma induced ([5, 6] and references therein) reaction cross sections in astrophysically interesting energy range. However, current databases (cf. KADoNIS database [7]) show the scarce of available data in question. We present the results of our measurements of cross sections of (γ, n)-reactions on ruthenium (96Ru, 98Ru, 104Ru) and palladium (102Pd, 110Pd) isotopes: in the bremsstrahlung photon energy range from the respective threshold to 14 MeV, the proton rich 96Ru, 98Ru, and 102Pd isotopes of which are p-nuclei. The cross sections are transformed to the reaction rates by the superposition method proposed by Darmstadt group [8, 9] and compared to the prediction of the NON-SMOKER code [10] utilizing the statistical theory of nuclear reactions. 2. Generalities of photonuclear reactions We concentrate on the dominant (γ, n)-reaction. The reaction rate λ(γ,n)(T) for (γ, n)-process depending on temperature T of a thermal photon bath of a star is expressed by the equation: ∞ λ=()TcnETEdEPlanck (,) σ () , (1) (,)γγnn∫ (,) γ 0 Planck where c is the speed of light, nETγ (,) represents the blackbody Planck distribution, i. e. the number of photons of a star environment per unit energy and volume interval 2 Planck = 11 E nETγ (,) 2 3 π ()c= exp()E − 1 kT ] 96 95 and σ ()T corresponds to the cross section of the (γ, n)- -1 (,)γ n s Ru(γ,n) Ru P -1 la Planck -15 nc 25 reaction. The photon flux cnγ (of Eq. 1) decreases k keV -27 10 s 6x10 ] p ec n -3 o tr i σ(Ε) σ(Ε) exponentially with increasing energy (the dropping curve in u t 20 -20 m c fm e (E) γ 10 s -1 γ Fig. 2 marked as “Planck spectrum”) while the ( , n)-reaction -27 s cn 4x10 s 15 o cross section steeply increases from the threshold to the giant r keV -25 10 Gamow window C 10 dipole resonance curve maximum (the growing curve “Cross 2x10-27 [mb] Sigma -30 section” at the right panel in Fig. 2). The product of these (Ε,Τ) (Ε,Τ) γ 10 5 n quantities (i. e. the integrand of Eq.1) is a narrow peak located at about kT/2 above the reaction threshold which was called 1234567891011121314 by the Gamow window by analogy with the similar picture for E [MeV] γmax the charged particle reactions. Fig. 2. Astrophysically relevant energy region Bremsstrahlung spectra of our electron accelerators are (“Gamow window” peak in the right panel) approximately described by the Schiff distribution [11] for a determined as the product of the Planck spectrum and thin converters. Darmstadt group found [8, 9] that the Planck 96 95 the Ru(γ, n) Ru reaction cross section for the and bremsstrahlung distributions have rather similar shapes in stellar temperature T9 = 3. the limits of the narrow high energy region and showed that a superposition of several (6 or more) bremsstrahlung spectra with differing end point energies can reproduce the Planck spectrum over enough high-energy range: ≈ brems cnγγ(,) E T∑ aioi () T N ( E, ,), E i 571 brems where N γ represents the bremsstrahlung spectrum with the end point energy E0 and aTi () the strength coefficients adjusting for each temperature T. The experimentally determined bremsstrahlung spectrum photonuclear reaction yield brems σ Y is proportional to the integrated product of the NEEγ (,)oi, and the reaction cross section ()E : E0 96 γ 95 YENEEdE~()(,).σ brems Ru( ,n) Ru ∫ γ 0 S ] n -20 -3 10 fm -1 Combining the two last equations one can obtain the next -30 T =3.0 expression for the reaction rate instead of Eq.