Neutron slowing-down time in finite water systems

The influence of the size of a moderator system on the neutron slowing-down time has been investigated. The experimental part of the study was performed on six cubes of water with side lengths from 8 to 30 cm. Neutrons generated in pulses of about 1 ns width were slowed down from 14 MeV to the indium resonance at 1.46 eV and were detected through the capture γ-radiation. In a separate measurement series (on the 12.5 cm cube) the space dependence of the slowing-down time at positions close to the source was studied. The observed behavior was similar to that reported in previous works concerned with infinite moderators. The experimental results for the six cubes were compared to the slowing-down times obtained from various theoretical approaches and from Monte Carlo calculations (in both cases space independent). All the methods show a decrease of the slowing-down time with decreasing size of the moderator. This effect was least pronounced in the experimental results. The agreement between the Monte Carlo results and those obtained using the diffusion approximation or the age-diffusion theory is surprisingly good, especially for large systems. The P1 approximation, on the other hand, leads to an overestimation of the effect of the finite size on the slowing-down time. (author)