Proton Total Reaction Cross Sections at 16.4 MeV

Total reaction cross sections for protons of a laboratory energy of 16.4 MeV at the center of foil targets of C, Mg, Al, Ni, Cu, and Pb have been measured by a beam attenuation method. The technique differs from other measurements with intermediate energy protons in that a double-focusing magnetic spectrometer is contained within the scintillation counter telescope which precedes the target. The magnet selects a beam free from slit-scattered protons, with a precisely determined momentum, while the focusing compensates for the beam divergence in the first detector so that all detectors see comparable counting rates. Solid-state circuitry with controlled recovery characteristics was developed to permit instantaneous rates in excess of ${10}^6$ protons/sec and to circumvent the problem of a low duty cycle. The measurements require several major corrections, and continuing effort to improve the evaluation of these corrections since this measurement was first described has led to the following values for reaction cross sections:

Total reaction cross sections have been predicted by optical-model analyses of proton elastic scattering at this energy with a variety of optical potentials. The measured values for Ni and Cu lie somewhat lower than the predictions of the optical model, while the values for Pb and C are higher than the predictions.