The absolute differential cross section for the elastic scattering of (20.35±0.25)-Mev protons has been measured for enriched ${\mathrm{Zn}}^{64}$, ${\mathrm{Zn}}^{66}$, and ${\mathrm{Zn}}^{68}$ foils. In the angular range of 30°-160° about 50 measurements were made for each foil (spaced from 1° to 5°) to an estimated accuracy of about 5% standard deviation. Scattered protons were detected by nuclear emulsions wrapped around a 4-in. diameter scattering chamber, all angles being exposed simultaneously. Detector energy resolution is 2.5%, angular resolution is 1° standard deviation, and relative angular shifts are determined to 0.1°. Correction has been made for the finite sizes of beam and detector and for multiple scattering in the target and in the detector stopper. The ±0.25-Mev energy spread includes maximum and minimum energies due to beam drift, beam spread, and target foil thickness. Three minima are found for each isotope: ${\mathrm{Zn}}^{64}$ at 63°, 104°, 142°; ${\mathrm{Zn}}^{66}$ at 62°, 102.5°, 142°; and ${\mathrm{Zn}}^{68}$ at 61°, 101°, 142°. The absolute cross sections are approximately the same except at the third minima, where for ${\mathrm{Zn}}^{64}$, ${\mathrm{Zn}}^{66}$, and ${\mathrm{Zn}}^{68}$ they are respectively, 1.46, 1.07, and 0.61 mb/sterad.

• Received 27 April 1959

DOI:https://doi.org/10.1103/PhysRev.115.1700