Scattering of Thermal Neutrons by Crystals

The effect of crystal interference on the scattering coefficient for thermal neutrons was investigated under different conditions. Calcite shows a large interference effect, the effective molecular cross section for scattering by a single crystal being about three times smaller than for a microcrystalline aggregate. It is also shown that a single crystal becomes more transparent when the velocity of the neutrons is decreased. Scattering has also been measured for bismuth and lead, where the difference between a single crystal and the microcrystalline material is less pronounced. The scattering in the latter case is the same as found for the liquid. Cooling the crystal to liquid nitrogen temperature does not produce any considerable change in scattering. The cross section for 90 percent ${\mathrm{Pb}}^{206}$ was measured and found to be the same as for the ordinary isotope mixture.