Absolute measurements of the

C^{12} (p, p n) C^{11}

cross sections were carried out in the external pencil beam of the Cosmotron at 2 and 3 Bev. The proton flux was measured with a counter telescope, and the

C^{11}

activity induced in 1-inch-thick plastic scintillators was determined by internal scintillation counting. Corrections for the formation of

C^{11}

by secondary particles produced in the thick targets were determined in separate experiments in which thin- and thick-target cross sections were compared directly. Other corrections for scattering and absorption effects and for deadtime losses in counters are discussed. Measurements were carried out both at proton fluxes sufficiently low to permit direct counting with the primary telescope and at higher fluxes which required scaling procedures with secondary telescopes. The measured cross sections for the

C^{12} (p, p n) C^{11}

reaction are 26.0±0.9 and 26.6±1.0 mb at 2.0 and 3.0 Bev, respectively. From these data and from previously published cross-section ratios, the cross section of the

{Al}^{27} (p, 3 p n) {Na}^{24}

reaction is found to be 10.4±0.6 and 10.0±0.6 mb at 2.0 and 3.0 Bev, respectively.

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