Secondary Emission Monitors at the Bevatron-Bevalac

Secondary Emission Monitors (SEM) are used for high intensity, high energy beam fluence monitoring of heavy ions. The improved electronics and autozeroing of background noise has extended the useful range of the SEM down to the region where the limiting factor is capacitive changes between the chamber foils, from acoustic and mechanical vibration. Usable levels are in the 104 particles per pulse range for C6 ions. The secondary electron yield is proportional to 1/ß2. This gives an increase in yield of about a factor of 25 for injection energies over peak energies at the Bevatron. This enhanced yield has been exploited in designing relative intensity and position monitors that give usable signal levels while intercepting only a small percentage of the injected beam. The detector in this case is a wire grid. The output can be: 1. Sum proportional to the beam intensity or as a time profile; 2. Split grids for a right-left monitor; 3. Single wire scan for a spatial profile. These monitors give usable signals down to a level of 0.01-0.1 ¿A of injected beam.