Calorimetric Determination of Beam Energy

A high-precision calorimetric system has been developed for determining beam energy. The beam is square-wave (on-off) modulated and the time derivative of the calorimeter temperature is determined at a time late in the modulation half-cycle. This avoids sensitivity variations associated with beam size and location on the calorimeter. The data are collected and analyzed in a small on-line computer. The use of beryllium, with its very small backscatter coefficient for electrons, permits the use of the method for electron accelerators. Using such a calorimeter with a high-stability electron accelerator, standard deviations of the order of 0.03 percent for series of 20 measurements were achieved for energies from 0.025 to 1 MeV over several hours time period. The calorimeter system is usually calibrated with a low energy (~ 20 keV) positive ion beam whose energy is well established. Where lower accuracy is acceptable, the sensitivity may be determined from first principles to within 1 percent. The method will be described and the optimization of the various parameters discussed.