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The neutron-proton capture cross section at the neutron laboratory velocity of 2200 m/sec has been determined from the time decay of the thermalized neutron population following short bursts of fast neutrons in water samples of widely varying sizes. Use of an intense pulsed neutron beam enables elimination of many of the problems encountered in earlier experiments. The present result for this cross section is 332.6 + 0.7 mb, which is the most precise of any result obtained by this method and is comparable in accuracy and consistent with the most accurate values determined by any method.NUCLEAR REACTIONS p(n, y)d, E=0.0253 eV; measured Lifetimes of thermal neutrons in water samples of various sizes; deduced o,

The slow neutron cross sections of free atoms of O, N, and A have been determined to be; O: 3.73, N: 9.96, A: 0.68 barn. Between 1 and 15 ev the N2 curve shows an E~* slope of 0.34 barnX (ev)* per atom. Measurements on these elements have been extended to over 10,000 ev with no resonances observed. Dependence of cross section upon neutron energy from 1 ev to 0.003 ev is presented for O2, N2, H2, H 2 0, methane, ethane, propane, »-butane, cetane, ethylene, 1,3-butadiene, and liquid w-butane. At the lowest energies, the large observed H2 cross section is attributed largely to thermal translational motion of the molecule. The measurements on the hydrocarbons show the effect of the binding of H in various molecules. Comparison with the theories of Bethe and Arley is made. No difference has been detected in cross-section measurements on gaseous and liquid w-butane.

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