The cross section for the ($n, 2n$) reaction in ${\mathrm{Be}}^9$ has been measured for neutron energies between 2.6 and 3.25 Mev. It was found that the reaction sets in sharply at approximately 2.7 Mev, from which energy the cross section rises rapidly to 0.70 barn at 3.25 Mev. This experimental result strongly suggests that the reaction proceeds primarily by means of an inelastic interaction which leaves the excited ${\mathrm{Be}}^9$ nucleus in its 2.43-Mev level before emission of the second neutron upon breakup. There is no peak in the ($n, 2n$) cross section corresponding to the broad peak at 2.73 Mev in the total neutron cross section.

• Received 14 March 1957

DOI:https://doi.org/10.1103/PhysRev.108.99