Beam Energy Measurements at the Bevalac

There is an increasing need for experimenters to have a more exact value for the kinetic energy of the particles used in their experiments. Values for kinetic energy can be calculated within about two percent for particles in the Bevalac using nominal values for the magnetic field and the radial position of the circulating beam. In the Bevalac there are several problems that make it very difficult to determine a more precise value for magnetic field. The radial field shape enclosed within the B dot integrating loop on the poletips changes as a function of field strength. The effective magnetic quadrant length also changes as a function of field strength. This causes a major perturbation in the radial position of the equilibrium orbit as well as some uncertainty in the value of the magnetic field. The details of these effects are discussed in an internal report. In addition to the magnetic field value, we must have adequate information about the radial position of the closed orbit to determine a precise value for the kinetic energy. If we have sufficient information about the closed orbit, we have a known effective path length for the particle. If we can measure the transit time of the particle on that path we have a time-of-flight measurement. In a circular machine, this time measurement is a frequency measurement which is one of the most precise measurements we can make. A Hewlett Packard 5360A frequency counter can read to 1 part in 106 for a 0.