NEUTRON THERMALIZATION MEASUREMENTS IN GRAPHITE

Using a pulsed source of 14 MeV neutrons, the rate of attainment of the asymptotic neutron spectrum was measured internally for 16 graphite blocks, and externally for 7 blocks, in the B2 range 2·99-25·30. 10-3 cm-2 using a silver transmission technique. Values of 36·2 ± 4·9. 105 cm4 sec-1 and 660 ± 115 μsec were obtained for the diffusion cooling coefficient and the infinite medium thermalization time constant, at a graphite density of 1·685 gcm-3. The former result agrees with that obtained from decay constant measurements, while the latter agrees with some recent similar work but not with earlier measurements. Reasons for this are suggested. The time required to attain an equilibrium spectrum in graphite was found to be about 2·1 msec., independent of B2, a result in good agreement with that of other authors. The B2 range investigated went to values higher than any reported previously for graphite and exceeded the theoretical (B2)*. The results suggested that an apparent asymptotic spectrum was attained over the whole range, although for B2 > 17·3. 10-3 cm-2 counting statistics were insufficient to establish this with certainty. © 1969.