The Harwell 500 kV Cockcroft-Walton accelerator and its use as a general research facility

Abstract

A 500 kV Cockcroft-Walton accelerator is described which can be operated with either a Harwell sputtering ion source or an rf ion source. Using the sputtering source, ions up to mass 209 have been accelerated with beam currents ranging from 1 to 80 μA while the rf source provides larger beams (typically 300 μA) of certain light ions. The heavy ion beams have been used mainly for ion implantation studies, range and energy loss measurements in silicon crystals and the generation of characteristic X-rays by heavy particle irradiation. The ion implantation studies have included the doping of silicon crystals with 11B and 31P for device manufacture and to investigate the profiles of the electrically active carriers; the doping of compound semiconductors for electroluminescence studies; and bombardment of semiconductors to investigate radiation damage. Provision is made in the target stage of the implantation chamber for heating and cooling the specimens and orienting crystal axes with respect to the beam direction. The rf ion source is used mainly to provide large beams of deuterons for neutron production. Neutrons generated by the D-T reaction have been used in activation analysis of flowing systems; in the development of a neutron shielding window for use in neutron therapy; and for the irradiation of phantoms and human subjects for radiobiological research. Typical experimental results taken from the implantation work, range measurements, characteristic X-ray studies and neutron activation experiments are shown in order to illustrate the wide application of the accelerator. Its usefulness indicated the need for a lower energy machine to complement the facility. We now have an 80 kV accelerator under construction. This machine, fitted with a Harwell sputtering ion source, is being sited so that it can be coupled with the 500 kV machine to provide two beams simultaneously at certain target positions.