Nuclear Reaction Energies with an Absolute Ion Velocity Gauge

A time-of-flight technique was used to find the ${\mathrm{Li}}^7(p, n){\mathrm{Be}}^7$ threshold energy and an ${\mathrm{Al}}^{27}(p, \gamma ){\mathrm{Si}}^{28}$ resonant energy. Results place the ${\mathrm{Li}}^7(p, n)$ threshold at 1879.8±0.3 keV and the ${\mathrm{Al}}^{27}(p, \gamma )$ resonance at 991.6±0.2 keV. The lithium threshold was determined by use of a ${\left(\mathrm{yield}\right)}^{\frac{2}{3}}$ extrapolation. The aluminum resonant energy is taken as the half-height energy of a thick-target yield. Except for the earliest electrostatic analyzer results, there was good agreement with previous determinations of these energies. The proton beam was modulated at approximately 50 Mc/sec by use of an Einzel lens driven by a crystal controlled oscillator-amplifier at the ion source of an electrostatic accelerator. The time-of-flight equipment consisted of a drift tube of adjustable length, a "phase meter" employing a variable delay line, and two pickups consisting of tuned cylindrical tubes through which the proton beam passed. Frequency was measured by zero beating a variable-frequency crystal oscillator against the signal picked off the beam and counting the crystal frequency with a frequency counter standardized to WWV. The lithium and aluminum targets were protected from organic vapors by a concentric liquid nitrogen trap.