Absolute Velocity Gauge for Nuclear Particle Beams

An instrument for determining the energies of accelerator particle beams in the 0–9 MeV range has been built in the form of a coaxial cylindrical cavity. It has been used to establish data points in the vicinity of the Coulomb‐nuclear interference minimum in p‐p scattering at 382.43 keV, with an accuracy of better than one part in 104 in energy. This is an order‐of‐magnitude higher precision than has been achieved previously with this technique. The collimated particle beam passes through 0.46‐mm gaps separated by the 125‐cm length of a hollow inner cylinder made of Invar. The beam is intensity modulated at 72 Mc, and the cavity is tuned to the modulation frequency. As the beam energy is adjusted until the particle transit time is an odd half‐multiple of the cavity period, and the shunt capacitors at opposite ends are adjusted for equal and opposite electromagnetic signals across the two gaps, a sharp null occurs in the signal picked up by an antenna in the cavity. The Q was 4000. The 19F(p,αγ)16O resonance was relocated at 340.45±0.04 keV, with an order‐of‐magnitude increase in precision.