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High-energy neutrons and protons in the range 20.4 MeV to more than 400 MeV can be detected in 1720-g plastic scintillators by means of (n, 2n) and (p, pn) reactions with carbon nuclei. Carbon-11 is formed which emits positrons of Emax = 0.98 MeV and has a half life of 20.4 min. With scintillators irradiated to saturation in a neutron flux density of 1 neutron cm -2 sec -1, a subsequent 5-min waiting period and a 38-min counting time, the standard deviation in the counting statistics is less than 15 per cent. Background considerations are discussed along with some experimental and theoretical values of the (n, 2n) reaction cross sections.

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