Speaker
Description
The recommended cross-section value for $^{235}$U neutron-capture at thermal energies is largely based on the difference from total and competing cross-sections of $^{235}$U. Despite its importance as a thermal neutron constant and its high value (100 barn), direct capture measurements are rare (only two exist for thermal energies) and exhibit large uncertainties. The reason is the difficulty to measure the characteristic radiation of the reaction product $^{236}$U within a dominant fission background and with $^{236}$U having a long half-life of 23.4 Myr. Importantly, capture of $^{235}$U may exhibit a significant deviation from a pure 1/v-behaviour around and below thermal energies.
Here, we present highly accurate capture data by applying a multi-isotope spike method to minimize systematic effects: we used the combination of neutron activation and subsequent accelerator-mass-spectrometry (AMS) for direct atom counting of the reaction product $^{236}$U. By utilizing different neutron fields we mapped the energy dependence of the capture cross section in six activations from thermal to ultra-cold energies: with an almost pure Maxwellian spectrum at room temperature at BR1 (Mol, Belgium), with cold neutrons at MLZ (Munich, Germany) and ILL (Grenoble, France) as well as with very cold neutrons also at ILL. AMS was performed at the VERA laboratory in Vienna. Using AMS, the capture cross-section is deduced simply by the measured isotope ratio $^{236}$/$^{235}$U divided by the neutron fluence.
By irradiating Uranium samples of natural U composition, we measured also $^{239}$Pu, the decay product of $^{239}$U produced from $^{238}$U(n,γ) in the same samples. I.e. we deduced simultaneously the $^{236}$U/$^{239}$Pu ratio, giving directly the cross-section ratio of $^{235}$U(n,γ) relative to $^{238}$U(n,γ). The latter ratio is therefore completely independent of the neutron fluence.
Activation with AMS allowed for an accurate determination of the $^{235}$U(n,γ) and $^{238}$U(n,γ) cross sections with an uncertainty of ~2% across the whole energy range. We will present our new experimental results and will also demonstrate the potential of this method for independent and accurate nuclear reaction cross section measurements.
