Neutron Total Cross Sections in the 7- to 14-Mev Region

The neutron total cross sections for 35 elements and isotopes were measured at several energies between 7 and 14 Mev using monoenergetic neutrons produced by the Livermore variable-energy cyclotron through the $\mathrm{D}(d, n){\mathrm{He}}^3$ reaction. The effects of beam contamination by gamma rays and low-energy "breakup" neutrons were evaluated through time-of-flight measurements and were kept small by suitably high bias on the plastic scintillator detector. The accuracy of the cross sections is typically ±1 to 2%. Where comparisons with previous data are possible, the agreement ranges from excellent to poor (10% discrepancy). The data form a smooth cross-section surface when plotted versus energy and mass number, as expected on the basis of the optical model. The functional dependence on energy and mass is not simple but is in good agreement with the predictions of the optical model of Bjorklund and Fernbach.