Determination of the capture to the fission cross-sections ratio by measurements of the gamma-rays coincidence multiplicity spectra at neutron resonance absorption by 235U

The capture to the fission cross-sections ratio (alpha value) was investigated by the measurements of gamma-rays time-of-flight spectra from the first to fifteenth multiplicities on the 500 m flight path of the IBR-30 reactor. One has used a 16-section scintillation detector with NaI(Tl) crystals and two metal samples of 0.00137 and 0.00411 at/b thicknesses (the content 235U : 238U = 90% : 10%). The multiplicity spectra of coincidence and the alpha values α=σγ/σf were obtained for 212 resolved resonances in the neutron energy region E=19-1260 eV and for energy groups over the range 20-2000 eV. The experimental alpha values were compared with the similar values calculated on the basis of evaluated data from different libraries using the model of neutron cross sections description in multilevel R-matrix approximation