Parity conservation in strong interactions: The 7Be(n, α)4he reaction

The abundance of evidence for lack of conservation of parity in weak interactions ralses the question as to whether parity is conserved in strong interactions. The spin of the Be/sup 7/ nucleus is 3/2 and the parity odd. Since the centrifugal barrier inhibits thermal neutron interactions with orbital angular momentum greater than zero, the parity of the initial system formed by the absorption of a thermal neutron must be odd. The symmetry and parity operators are identical for identical, spinless partioles and so a Be/sup 8/ state of odd parity cannot break up into two alpha particles if purity is conserved. The Q value of the Be/sup 7/(n, gamma )He/sup 4/ reaction (from the known masses) is 18.98 Mev. The disintegration of Be/sup 7/ by slow neutrons into two 9.5 Mev alpha particles was therefore sought as an indication of violation of Parity conservation. An upper limit of 25 mb was set for the (n, alpha ) cross section. An upper limit of 1 x 10/sup -7/ is also set for the square of the amplitude of the parity non-conserving component of the wave function. The results are not sensitive to whether the parity nonconserving forces conserve isotopic spin. (A.C.)