Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

The inelastic neutron cross section of 7Li has no sharp resonances and a fairly low threshold of 546 keV. Below the breakup threshold at 5291 keV only one gamma-ray is emitted at E = 477,6 keV. It is therefore suited as a reference cross section. Lithium has technical usage as a 3H-producer in future fusion reactors as well as in molten salt reactors. But there are recent measurements disagreeing with already evaluated data. To resolve this dissonance, an 170 h Experiment was carried out at the nELBE facility of the HZDR. A 4 mm thick LiF-disk was used as a target, the neutron flux was determined with a 235U parallel plate fission chamber. The flight path for the 7Li(n,n’ )7Li reaction was 8,3 m. As detectors four two-inch LaBr3-detectors as well as three three-inch LaBr3-detectors and two miniball-type HPGe detectors with three 60 % crystals each were used. The measurement of the cross section is solely a measurement of the de-excitation of the first exited 7Li-State at 477,6 keV. The second exited state at 4,63 MeV already decays via particle emission and thus does not contribute any gamma-radiation. The experiment benefits from the high neutron flux at nELBE (80 n/s/keV @ 1 MeV) as well as from the precise fission chamber of the PTB (H19) for the neutron flux calibration. A Geant4-Simulation is used to determine correction factors as the transmission from the H19 to the target as well as the multiple scattering correction and the self absorption of the 477,6 keV Gamma-Rays. The deduced cross section from both detector types are consistent, but they can’t reproduce the data from Nyman et al. The deviations are up to 20 %. The half life of the by means of bremsstrahlung in air produced positroniums in the experiment is 116(7) ns.