The attachment of slow electrons in carbon dioxide

The formation of positive and negative ions in carbon dioxide has been investigated by means of a Lozier apparatus. The negative ion process was interpreted as CO 2 + e → CO( X 1 Σ + ) + O - (2 P 0 ). The (peak) cross-section for electron attachm ent was found to be 5·07 ± 0·5 x 10 -19 cm 2 at 7·8 eV, and the ionization cross-section reached a m axim um value of 6·80 x 10 -16 cm 2 at 85 eV. Measurements of the electron affinity of oxygen by the normal electron impact method yielded a value of 1·6 ± 0·2 eV for O - ions formed with an initial kinetic energy of 1·8 eV. It is shown that this apparent value of electron affinity must be corrected, because of the initial kinetic energy of the ions and the energy spread of the source electrons, and then yields a value of 1·2 ± 0·3 eV.

GAMMA–NEUTRON REACTIONS IN O16 AND N14

The cross sections for the reactions O16(γ, n)O15 and N14(γ, n)N13 have been determined by measurement of the induced positron activities. The oxygen cross-section curve has a slowly increasing initial portion after which it rises rapidly to a maximum peak cross section of 11.4 millibarns at 24.0 Mev. The nitrogen cross-section curve has an initial peak at about 13 Mev. Above 17 Mev., this curve increases sharply to a maximum value of 2.84 millibarns at 24 Mev. In order to explain the shape of the nitrogen curve, a relative total neutron yield curve has been determined from 12 to 20 Mev. by detecting the emitted neutrons with a rhodium foil. This neutron yield curve has the same shape as the nitrogen saturated specific activity curve. The anomalous shapes of the nitrogen and oxygen cross-section curves are explained in terms of the variation of gamma-ray absorption with energy.