Production of Low Energy Neutrons by Filtering through Graphite

Neutrons of energy much lower than thermal were produced by filtering a beam of thermal neutrons through a block of graphite 23 cm long. In such a block, Bragg scattering removes the neutrons whose wave-length is less than the largest Bragg wave-length in graphite, 6.69 angstroms. Measurement of the boron absorption of the filtered neutrons showed that they had an effective wave-lengh of 7.15 angstroms which corresponds to neutron temperatures around 18° Kelvin. The cross section of graphite for the filtered neutrons is 0.70×${10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$. That a part of this is caused by the incoherence due to thermal agitation of the atoms of the crystal was demonstrated by heating the crystal and observing the increase in this cross section. The filtered neutrons were used to show interference effects in other substances such as Be, Bi, and S. In water a fourfold increase in the scattering cross section of hydrogen due to chemical binding was observed.