Excitation Function for theC12(π−,π−n)C11Reaction

The excitation function for the reaction ${\mathrm{C}}^{12}({\pi }^{-},{\pi }^{-}n){\mathrm{C}}^{11}$ was measured from 53 to 1610 MeV by bombarding targets of plastic scintillator with pions. The intensity of the pion beam was monitored with a two-counter telescope and 40-Mc/sec scaling system. The scintillator target was mounted on a phototube and became the detector for the ${\mathrm{C}}^{11}$ positron activity. Corrections were made for muon contamination in the beam, coincidence losses in the monitor system, ${\mathrm{C}}^{11}$ activity produced by stray background at the accelerator, ${\mathrm{C}}^{11}$ activity produced by secondaries in the target, and the efficiency of the ${\mathrm{C}}^{11}$ detection system. The ${\mathrm{C}}^{12}({\pi }^{-},{\pi }^{-}n){\mathrm{C}}^{11}$ cross sections rise from a threshold at about 50 MeV to a peak of about 70 mb at 190 MeV after which they decrease to 30 mb at 373 MeV and are relatively constant at higher energies. The (${\pi }^{-},{\pi }^{-}n$) peak occurs at the same energy as the resonance in free-particle ${\pi }^{-}n$ scattering at 190 MeV. Calculations based on a "knock-on collision" mechanism and "sharp-cutoff" nuclear density reproduce the shape of the experimental excitation function, but the magnitudes of the calculated cross sections are low by a factor of about 5 or 6. This simple model indicates that the ${\mathrm{C}}^{12}({\pi }^{-},{\pi }^{-}n){\mathrm{C}}^{11}$ reaction occurs in the nuclear surface region at all bombarding energies.