The main difficulty in the measurement of the U yield ratio ( = σ(capture)/ σ(fission) is the separation of γ rays associated with the reaction U(n,γ) from the ones that come from fission fragments and which represent parasitic radiation.
The prototype of an ionization chamber that would allow to identify fission fragments was constructed and tested on AIFIRA accelerator of CENBG. Detection efficiency for fission exceeds 90% and the time resolution of the setup is 2 ns.
Picture of the setup: the ionization chamber is placed on the beam axis and CD detector is located at 90°.
The CD detector served for counting the emitted γ-rays by the method explained in the part Radiative capture Pa(n,γ).
The final detector set-up will contain 2 mg of U distributed among 5 targets by 0.4 mg. These targets will be fabricated by radiochemists at IPN Orsay. The first series of measurements are scheduled on the time flight basis of GELINA accelerator of IRMM at Geel. The setup will consist of an ensemble of 10 detectors for γ rays of CD type surrounding the ionization chamber that will work in the anticoincidence in order to eliminate γ rays originating from fission fragments.