Experimental simulation of a stellar photon bath by bremsstrahlung: the astrophysical γ-process

Abstract

The nucleosynthesis of heavy proton-rich nuclei in a stellar photon bath at temperatures of the astrophysical γ-process was investigated where the photon bath was simulated by the superposition of bremsstrahlung spectra with different endpoint energies. The method was applied to derive (γ,n) cross sections and reaction rates for several platinum isotopes.PACS numbers: 26.30.+k, 98.80.Ft, 26.45.+h The trans-iron nuclei have been synthesized by neutron capture in the s-and r-processes, except the p-nuclei (p for proton-rich), with relative abundances of the order of 0.01 to 1% [1]. The main production mechanism of the p-nuclei is assumed to be photodisintegration in the γ-process, i.e. by (γ,n), (γ,p), and (γ,α) reactions induced on heavier seed nuclei synthesized in the s-and r-processes. Typical parameters for the γ-process are temperatures of 2 ≤ T 9 ≤ 3 (T 9 is the temperature in units of 10 9 K), densities ρ ≈ 10 6 g/cm 3 , and time scales τ in the order of seconds. Several astrophysical sites for the γ-process have been proposed, whereby the oxygenand neon-rich layers of type II supernovae seem to be good candidates. However, no definite conclusions have been reached yet [1], predominantly due to the lack of experimental data for the cross sections and reaction rates of the γ-induced reactions at astrophysically relevant energies. All reaction rates have been derived theoretically using statistical model calculations [1-6].The energy distribution of a thermal photon bath at a temperature T is given by the Planck distributionwhere n γ (E, T ) is the number of γ-rays at energy E per unit of volume and energy interval. In a photon-induced reaction B(γ,x)A the distribution leads to a temperature dependent decay rate λ(T ) of the initial nucleus Bwith the speed of light c and the cross section of the γ-induced reaction σ (γ,x) (E). Obviously, λ is also the production rate of the residual nucleus A. In the following we will focus on photodisintegration by the (γ,n) reactions. A large number of (γ,n) cross sections has been measured over the years [7,8]. However, most of the data have been obtained around the giant dipole resonance (GDR), i.e. at energies much higher than those in stars, and practically no data exist for the p-nuclei. The integrand in Eq. (2) is given by the product of the γ flux c n γ (E, T ), which decreases steeply with increasing energy E, and the cross section σ (γ,x) (E), which increases with E approaching the GDR region. The product leads then to a window at an effective energy E eff with a width ∆ similar to the Gamow window for charged-particle-induced reactions. If one assumes a typical threshold behavior of the (γ,n) cross section close to the threshold energy E thr , the effective energy is approximately given by E eff = E thr + 1 2 kT , and the typical width ∆ is in the order of 1 MeV (Fig. 1). 191 Pt reaction (E thr = 8676 keV) in a thermal photon bath with temperature T9 = 2.5. The Planck distribution nγ (E, T ) (dotted line) and the (γ,n) cross section σ(E) (dashed line) are giv...

Photoneutron cross sections for Mo isotopes: A step toward a unified understanding of(γ,n)and(n,γ)reactions

“…As discussed in [1,2], the γ SF plays an important role in both nuclear astrophysics and nuclear engineering. Photoneutron cross sections, hence the γ SF, immediately above neutron threshold is of direct relevance to the p-process nucleosynthesis which transforms pre-existing seed nuclei produced by the s-and r-process nucleosynthesis into neutron-deficient nuclei classified as p-process nuclei [3][4][5]. In contrast, the γ SF below the neutron threshold plays a key role in defining the electromagnetic de-excitation taking place after the neutron capture which, along with the β decay, drives the s-and r-process nucleosynthesis [6,7].…”

Predicted cross-sections for photon-induced particle emission

“…In general, nuclear reaction networks contain terms for both the forward and the reverse rate, in order to account for the varying relative importance of these reaction directions depending on temperature [4]. Recently, it has been suggested to use Bremsstrahlung radiation [7,8,9] or Compton back-scattered laser photons [10] to measure (γ,n), (γ,p), and (γ,α) reactions relevant for astrophysics. Such measurements can either be used to test cross section predictions or to derive capture cross sections for unstable, experimentally unaccessible targets [11,12,13,14,15].…”

Explosive Nucleosynthesis Study Using Laser Driven γ-ray Pulses

“…The 3 H(p, n) and 18 O(p, n) reactions are also used for kT = 52 keV [24] and 5 keV [25], respectively. For the study of the γ process, Mohr et al proposed a similar method using bremsstrahlung γ-rays [21,26], although the energy distribution of the bremsstrahlung γ-ray differs from the Planck distribution. To simulate the stellar Planck distribution, they measured the reaction rate for different maximum energy bremsstrahlung γ-rays and integrated them after the measurements.…”