Neutron Multiplication in Bulk Beryllium

Neutron multiplication occurs in beryllium because of the high (n, 2n) cross section. On the basis of calculations made using microscopic nuclear data, multiplication in a beryllium blanket should improve the efficiency of a tritium breeder. Previous experiments have indicated that the net multiplication is too low for beryllium to be an effective neutron multiplier. It seemed appropriate to make a further study of the multiplication of 14-MeV neutrons in bulk beryllium, utilizing the superior isotropy and flat energy response of the manganese bath. In the manganese bath method a 14-MeV neutron source is placed at the center of a large tank containing an aqueous solution of MnSO₄. With a beryllium sample surrounding the neutron source in the sample chamber, the neutrons first multiply in beryllium and produce in the manganese bath an activity proportional to the source rate times the multiplication factor. The ratio of the “sample-in” and the “open beam” activities is the raw value of the multiplication. Several systematic corrections must then be applied to deduce the true multiplication in beryllium. Uncorrected values of the multiplication have been obtained for beryllium samples of four thicknesses. For beryllium thicknesses of 4.6, 12.0, 15.6, and 19.9 cm the multiplication values are 1.399, 1.928, 2.072, and 2.126, respectively. These values are affected by several systematic effects characteristic of the manganese bath. The values of these systematic corrections are established by a combination of calculation and experimental parameterization. The detailed calculations use the Monte Carlo program MCNP. The experimental values are in good agreement with those calculated from microscopic cross sections.