Total Neutron Cross Section ofXe135as a Function of Energy

The total neutron cross section of ${\mathrm{Xe}}^{135}$ as a function of energy has been remeasured at Oak Ridge National Laboratory under more favorable conditions than obtained in earlier measurements. A sample thickness of 2.5×${10}^{18}$ atoms of ${\mathrm{Xe}}^{135}$ gas per ${\mathrm{cm}}^2$ was procured from the gases generated in a homogeneous reactor. A mechanical time-of-flight chopper was used to select neutrons in the energy range from 0.01 ev to several thousand ev. The number of ${\mathrm{Xe}}^{135}$ atoms in the sample was determined by means of mass spectrometer measurements on the long-lived daughter, ${\mathrm{Cs}}^{135}$. The data of the low-energy resonance were fitted to the single-level Breit-Wigner formula, taking into account Doppler corrections, equally well with the following two sets of parameters: statistical weight factor $g=\frac{3}{8}$; resonance energy ${\epsilon }_0=0.08472\mathrm{}\pm 0.00027$ ev; neutron width at energy ${\epsilon }_0$, ${{\Gamma }_n}^0=0.03477\mathrm{}\pm 0.00021$ ev; capture width, ${\Gamma }_{\alpha }=0.083303\mathrm{}\pm 0.00062$ ev; for $g=\frac{5}{8}$, ${\epsilon }_0=0.08415\mathrm{}\pm 0.00028$ ev; ${{\Gamma }_n}^0=0.02057\mathrm{}\pm 0.00012$ ev; ${\Gamma }_a=0.09493\mathrm{}\pm 0.00071$ ev. The errors quoted are the standard deviations derived from the statistics of the measurements. Systematic errors are discussed in the body of the paper. No evidence for resonances at energies greater than 0.085 ev was observed. The results described are interpreted in terms of recent considerations on the statistics of the properties of nuclear energy levels.