The measurement of ETA for 235U in the energy region below 1 eV

Measurements of the neutron energy dependence of eta for 235U in the region below 1 eV have been carried out on the Condensed Matter Target of the Harwell 136 MeV electron linear accelerator HELIOS, using a 10 m flight path. The measurements were carried out with a NE213 liquid scintillator in conjunction with a pulse shape discriminator to separate events due to fast neutrons and gamma-rays incident on the detector. The detector was used to detect both the fast neutrons from the thick 235U samples and gamma-rays emitted from 'thick' 10B, Cd and Gd samples used to measure the shape of the incident neutron spectrum. The ratio of the fast neutron counts from the 235U to the gamma-ray counts from the spectrum measurements gives the neutron energy dependence of eta after some small corrections for finite sample thickness effects. The accuracy of the measurement is ∼1/2 % in the energy region between 15 and 100 meV. Below 15 meV uncertainties in the background cause an increase in the errors which are ∼5 % at ∼3 meV and above 200 meV uncertainties in the cross-sections used in the corrections for the 235U sample size introduce errors of the order of 2 to 3 % in the eta values. (author)