(p, γ) Resonance-Curve Shapes and Measurements of Resonance Energies withH2+Beams

The observation of an apparent nonlinearity in the NRL 2-m radius electrostatic analyzer has led to an exhaustive investigation of ($p, \gamma$) resonance-curve shapes induced by the hydrogen molecular-ion beam. The midpoint of the rise in the thick-target yield curve obtained with the ${\mathrm{H}}_2^{+}$ beam, ${\mathrm{Al}}^{27}(p, \gamma )$ reaction at 992 keV, was observed to be 0.05% lower than that predicted from the corresponding observation with the ${\mathrm{H}}_1^{+}$ beam. When extremely thin targets were used, the energy coordinates of the peaks of the ${\mathrm{H}}_1^{+}$ and ${\mathrm{H}}_2^{+}$ yield curves agreed within 0.01%. Further studies with the ${\mathrm{H}}_2^{+}$ beam revealed additional deviations from expected behavior. The thick-target yield curve was seen to be asymmetric about the midpoint; there was a hump or peak near the top of the thick-target yield curve. The peaks of moderately thin-target yield curves were not shifted from resonance energy by as much as half the target thickness in energy loss units; the full widths at half-height of these moderately thin-target yield curves were greater than predicted from the full width at half-height of an extremely thin-target ${\mathrm{H}}_2^{+}$ beam yield curve and the target thicknesses in energy loss units obtained with the ${\mathrm{H}}_1^{+}$ beam on the same targets. The energy shifts and broadening effects of inert coatings of copper over the aluminum targets were significantly different for ${\mathrm{H}}_1^{+}$ and ${\mathrm{H}}_2^{+}$ beams with both beams referred to the same energy scale; and even greater broadening, but not midpoint displacement, was observed with ${\mathrm{H}}_1^{+}$ and ${\mathrm{H}}_1^0$ components stripped from the ${\mathrm{H}}_2^{+}$ beam in a gas cell. These deviations from expected behavior are all explained on the basis of the mechanism of dissociation of the ${\mathrm{H}}_2^{+}$ molecule, and the results are applied to calibration of beam deflection analyzers.