Neutron Scattering Cross Sections of Para- and Orthohydrogen, and of N2, O2, and H2O

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Abstract
Measurements were made of the transmission of liquid para- and normal hydrogen for the slow neutrons obtained from a Rn-Be source by scattering in paraffin. In some of the experiments the paraffin was at room temperatures and in others it was cooled with liquid air. Teller, and Schwinger and Teller, have shown how the spin dependence of the interaction between neutrons and protons can be investigated by such measurements. As the neutrons captured by the protons were less than 1 percent of the scattered neutrons, the scattering by the thin layers of liquid hydrogen used was determined from the transmission measurements. The scattering cross section of liquid hydrogen for slow neutrons was found to depend upon the velocity of the neutrons and on the ortho-para composition of the hydrogen. The following values were obtained for the neutron scattering cross sections, σ, of liquid ortho- and parahydrogen: for ∼300°K neutrons (paraffin at room temperatures), σ (ortho)=56×10−24 cm2 per molecule, σ (para)=29×10−24; for ∼120°K neutrons (paraffin cooled with liquid air), σ(ortho)=79×10−24 and σ(para)=18×10−24. We were able to prove conclusively: (1) that the interaction between neutrons and protons is dependent upon the relative alignment of their spins, and (2) that the energy of the singlet state of the deuteron, in which the spins of the proton and neutron are antiparallel, is greater than the energy of these particles when far apart, that is, the singlet state of the deuteron is virtual. The scattering+capture cross sections of liquid oxygen and liquid nitrogen were measured for ∼300°K neutrons, and of water for ∼300° and ∼120°K neutrons. The cross sections for the 300°K neutrons were: σ(O)=4×10−24 cm2 per atom, σ(N)=13×10−24 cm2 per atom and σ(H2O)=91.2×10−24 cm2 per molecule. For the ∼120°K neutrons σ(H2O)=116×10−24 cm2 per molecule. The scattering cross sections of the protons in water, (12)[σ(H2O)−σ(O)], were 43.6×10−24 cm2 per proton for ∼300°K neutrons and 56×

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