Search for Subthreshold Neutron Production in Be

We have searched for neutrons from the 3-body photon-induced reaction 9Be + γ → α + α + n using bremsstrahlung produced by electrons from a 2-MV Van de Graaff. The target was located within a block of beryllium surrounded by an array of 3He proportional counters embedded in paraffin. Based on energy and intensity calibrations of the accelerator and detector using the 9Be + γ → 8Be+n reaction, an upper limit of 93 nb (4σ) was placed on the cross section for neutron production between the 3-body and 2-body thresholds. This value is substantially below a previously experimental result using photoexcitation by a 142Pr gamma source and also below an earlier theoretical estimate. We suggest that bremsstrahlung due to beta rays between 1665 keV and the 2160-keV end-point of the 142Pr beta-ray spectrum could account for the photoneutron yield in the 3-body region that had previously been attributed to 142Pr gamma rays