Threshold Photoneutron Cross Section forBe9

The ${\mathrm{Be}}^9 (\gamma ,n) {\mathrm{Be}}^8$ cross section was measured as a function of photon energy from less than 1 to about 40 keV threshold, by the threshold photoneutron technique. The cross section rises sharply to a maximum of 1.6 mb at 6 keV above threshold and then decreases slowly to 1.2 mb at 40 keV. An attempt was made to fit these data, and earlier data obtained from monoenergetic $\gamma$-ray source measurements, with a curve of the Breit-Wigner form in order to examine the possibility that the shape of the cross section is influenced primarily by a single level near threshold. The results show that such a curve is inadequate to explain the data, and thus indicate that a more complex theory and even better data are needed to understand fully the nature of the cross section near threshold for the ${\mathrm{Be}}^9(\gamma ,n)$ reaction.