Dipole strength inLa139below the neutron-separation energy

Abstract

The γ -ray strength function is an important input quantity for the determination of the photoreaction rate and the neutron capture rate for astrophysics as well as for nuclear technologies. To test model predictions, the photoabsorption cross section of 139 La up to the neutron-separation energy was measured using bremsstrahlung produced at the electron accelerator ELBE of Forschungszentrum Dresden-Rossendorf with an electron beam of 11.5 MeV kinetic energy. The experimental data were analyzed by applying Monte Carlo simulations of γ -ray cascades to obtain the intensities of the ground-state transitions and their branching ratios. We found a significant enhancement of electric dipole strength in the energy range from 6 to 10 MeV that may be related with a pygmy dipole resonance. The present data are combined with photoneutron cross sections for 139 La and compared with results of calculations on the basis of a quasiparticle-random-phase approximation using an instantaneous-shape sampling.

Dipole strength inSe80forsprocess and nuclear transmutation ofSe79

“…We applied statistical methods to estimate the (6) 45 (10) 044304-5 intensities of branching transitions to low-lying excited levels and of the branching ratios of the ground-state transitions. These methods were also applied in earlier photon-scattering experiments at γ ELBE [11][12][13][14][15]17,18]. The intensity distribution contains ground-state transitions and, in addition, branching transitions to lower-lying excited states (inelastic transitions) as well as transitions from those states to the ground state (cascade transitions).…”

“…Simultaneously, the branching ratios b 0 of the ground-state transitions are deduced for each energy bin . In an individual nuclear realization, the branching ratio b 0 is calculated as the ratio of the sum of the intensities of the ground-state transitions from all levels in to the total intensity of all transitions depopulating those levels to any low-lying levels including the ground state [11][12][13][14][15]17,18]. By dividing the summed intensities in a bin of the experimental intensity distribution of the ground-state transitions with the corresponding branching ratio, we obtain the absorption cross section in each bin as σ γ = σ γ γ /b 0 for each nuclear realization.…”

“…The low-energy tail of the GDR below S n is of particular interest because of an extra enhancement of observed E1 strength. This pygmy dipole resonance (PDR) [7][8][9], has been found, for example, in 76 Se [10], 78 Se [11], nuclides around N = 50 [12][13][14], around N = 82 [15][16][17][18], and in 181 Ta [19].…”

Improvements and testing practical expressions for photon strength functions of E1 gamma-transitions

“…The Figs. 1, 2 show the comparisons of the experimental data with the calculations for the isotopes 130,132 Sn [13], 88 Sr and 139 La [14,15].…”

E1 and M1 strength functions at low energy

“…To determine the intensity in the nuclear quasicontinuum, the spectrum of γ rays scattered from the target in atomic processes is simulated by using the code GEANT4 and subtracted from the responsecorrected experimental spectrum. The contributions of unresolved strength to the spectra are demonstrated for the nuclides 139 La [16] and 208 Pb [17] in Figs. 1 and 2, respectively.…”