Internal target for proton extraction

A typical target used in ejected proton beams at the synchrotron. The target proper is the copper rod 1 x 2 X 100 mm3 , (the axel of the two wheels) supported by thin wires. When the proton beam (whose cross-section is of the order of 2 X 3 mm2 ) hits the target, a large amount of charge is ejected, mainly as 'knock-on' electrons, and this effect is used for 'burst-intensity monitoring'. The target is simply electrically insulated and connected to a charge measuring instrument. The amount of charge removed from the target gives information about the number of interactions (primary and secondary) that have taken place. External targets are usually set up in air and not in a vacuum enclosure. The free charges in the heavily-ionized air surrounding the target could therefore partially neutralize the required signal. However, this difficulty is largely overcome by coating the target surface with a thin layer of aluminium oxide, which prevents the low-energy charges in the air entering the target.