The neutron transformation of helium-3 has been applied to neutron spectroscopy in the medium energy range in order to overcome the disadvantages of previous methods which have usually been based on neutron scattering. By observing the pulse-height distribution when a proportional counter filled with helium-3, krypton, and carbon dioxide is irradiated with neutrons, the spectrum can be obtained. Details of the counter construction and vacuum equipment for filling the counter are described. In the range of energy from thermal to 1 Mev the spread (relative standard deviation of a peak) is about 3% and the wall effect agrees with theoretical predictions. The pulse height varies linearly with energy and some evidence suggesting that the mean energy required to produce an ion pair in krypton for helium-3 nuclei is about 6% greater than that for protons is presented. Since the neutron reaction cross section of helium-3, as a function of energy, is required to evaluate neutron spectra from observed distributions, it has been remeasured in the range 120 kev to 1 Mev with greater accuracy than previous measurements made by J. H. Coon [Phys. Rev. 80, 448 (1950)].

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