Plastic scintillators were exposed in the external 920-MeV $\alpha$-particle beam at the 184-in. cyclotron at Lawrence Radiation Laboratory. The beam-flux monitor was Ilford 100-μ L4 pellicles. The cross section for ${\mathrm{C}}^{12}(\alpha ,\alpha n){\mathrm{C}}^{11}$ was found to be 49.4 ± 1.8 mb.

• Received 30 October 1969

DOI:https://doi.org/10.1103/PhysRevC.2.793