Fission Cross Section Of 235U Research Articles

Absence of structure in the fission cross section of 235U in the neutron energy range 380 − 620 keV

The 235U fission cross section is measured with high resolution by a time-of-flight method over the energy range 380 - 620 keV. In contrast to results recently obtained by another author, no evidence for intermediate structure is found.

The Ratio of 238 U Capture and 235U Fission Cross Sections in Fast Reactors

The Ratio of ²³⁸ U Capture and ²³⁵U Fission Cross Sections in Fast Reactors

Measurements of the 235U neutron fission cross section in the energy range 0·04–14 MeV

The neutron fission cross section of 235U has been measured relative to the n-p scattering cross section at a number of neutron energies in the range 0·04–4 MeV to an accuracy of about 3 per cent. Hydrogen and methane gas proportional counters and a Polyethylene radiator semiconductor counter were used as recoil proton flux measuring instruments in the appropriate parts of this neutron energy range. Good agreement was obtained between the shapes of the recoil proton spectra and those calculated. The effect of neutron scattering and the loss of fission fragments in the uranium foil were investigated experimentally. At 1·0, 2·25 and 14·1 MeV the present results agree reasonably well with a currently accepted best curve through previous measurements of this cross section (BNL-325). At energies less than 500 keV however the cross sections measured here are about 7 per cent below this curve and at an isolated measurement at 5·4 MeV the present result is 1 l per cent below previous measurements.

Interference effects in fission cross sections

An interference analysis has been carried out of the slow neutron fission cross sections of 235U and 239Pu. Use was made in this analysis of the results of η and total cross-section measurements made on a pulsed cyclotron beam neutron spectrometer. (1,2) Using the method of analysis developed by VOGT (3) the authors find that there is satisfactory agreement between the results of the calculations and the experimental data. The relative signs of the reduced width amplitudes are obtained and also the extent of the interference between the strongly interfering levels. Correlation is observed between the signs of the amplitudes. The numter of effectively open fission channels is found to be close to unity for 239Pu and to two for 235U. Spins of several of the plutonium levels and relative spins of the 235U resonances are deduced. The mean fission widths of levels with dissimilar spins are obtained.

Interference effects in fission cross sections

Abstract An interference analysis has been carried out of the slow neutron fission cross sections of 235U and 239Pu. Use was made in this analysis of the results of η and total cross-section measurements made on a pulsed cyclotron beam neutron spectrometer. (1,2) Using the method of analysis developed by VOGT(3) the authors find that there is satisfactory agreement between the results of the calculations and the experimental data. The relative signs of the reduced width amplitudes are obtained and also the extent of the interference between the strongly interfering levels. Correlation is observed between the signs of the amplitudes. The numter of effectively open fission channels is found to be close to unity for 239Pu and to two for 235U. Spins of several of the plutonium levels and relative spins of the 235U resonances are deduced. The mean fission widths of levels with dissimilar spins are obtained.

The fission cross section of 235U from 0·01 eV to 0·1 eV and its absolute value at 0·0253 eV

The absolute fission cross section of 235U at the reference neutron velocity of 2200 m/sec was measured. The counting rate of a 4π-fission chamber was compared with that of a 10B ionization chamber by alternate measurements in a neutron beam emergent from the thermal column of BRI. The energy selection was done with a slow chopper previously calibrated with respect to the sharp breaks of σ t for α Fe, Be, Bi, Al, C (graphite). The efficiency of the 1B chamber was compared to the well-known absorption cross section of 197Au by activation measurements with a (β-γ) coincidence technique. The thickness of the 235U layer was determined by comparison with α-standards of the same isotopic composition, and controlled by a mass-spectrometric method. Our determination yields: σ f 0 = (587 ± 6) barns Some information is also obtained about the nearby negative energy levels.

The fission cross-sections of 233U and 235U for neutrons having energies between 3 and 800 keV

The effective fission cross-sections of 233U and 235U are presented for neutrons of energy from 3 to 800 keV obtained from the T( p, n) 3He reaction in an electrostatic accelerator. The fission cross-section of 233U falls from 7·5 barn at 3·4 keV to 1·9 barn at 780 keV; the corresponding values for 233U are 4·8 and 1·05 barn, respectively.

Fluctuations in Slow Neutron Average Cross Sections

The wide distribution in the values of the neutron widths and spacings of slow neutron levels leads to fluctuations in the cross section averaged over several levels. A method of deriving information on the properties of these distributions from observations on the fluctuations is described. Fluctuations in total cross section measurements are analysed and agreement to order of magnitude is obtained with the predicted width and spacing distributions, but it does not appear that exact agreement is obtained at present The 235U fission cross section also is analysed by this method, but the results do not agree with the theory of the method. It is probable that these discrepancies are due to the effect of correlations in the distributions.

The Fission Cross Sections of233U,235U,238U and239Pu for Neutrons in the Energy Range 0.030 MeV to 3.0 MeV

The fission cross sections of 233U, 235U, 238U and 239Pu have been measured as a function of neutron energy for neutrons in the energy range 0.030 MeV to 3.0 MeV. In addition, measurements of the ratios of the fission cross sections as a function of neutron energy have been made. The fission cross sections and the ratios have been shown to be consistent and a set of best values of the cross sections has been computed. The accuracy over most of the range is to about 3%, falling to 5% at the extremes.

Validity of the inverse velocity law for the fission cross section of 235U

Synopsis Cadmium ratio measurements were used in order to check the validity of the 1/ v law for fission of 235 U in the resonance region. The fast fission effect was eliminated by measuring the transmission of epicadmium neutrons through nickel, which has different cross sections for resonance and fast neutrons. It was found that the 1/ v law does not hold quite good in the resonance region, the resonance integral being ∼ 20 barns.