SETUP FOR LARGE AREA LOW-FLUENCE IRRADIATIONS WITH QUASI-MONOENERGETIC 0.1-5 MEV LIGHT IONS

Ion energy range from 100 keV to 5 MeV is troublesome if the goal is a low intensity, monoenergetic beam over large area. This type of beam could be used for detector calibration or low-fluence implantation. The energies available from radioactive αsources are of several MeVs, the same level as the minimum energy in well controlled beams from RADiation Effects Facility (RADEF)[1]. The right ion energy range can be achieved by means of small accelerators, but low and homogenous fluence over large area is very difficult to achieve. A versatile setup has been developed in Jyvaskyla for low-fluence, large area irradiations in the ion energy range of 0.1–5 MeV. In this setup, H, He or O ions are first accelerated to a certain energy (for He for example from 200 keV to 5 MeV) with 1.7 MV Pelletron accelerator. This beam is directed to a thin evaporated layer of heavy element on a self-supporting 50 nm thick carbon foil. The ions backscattered form this foil form the beam which is used in the application. The energy of the backscattered ions is determined by the scattering kinematics and energy loss in the scatterer and intensity is determined by incident beam current, scattering cross-section and distance from the scatterer. By having multiple scatterers with different elements evaporated on them, the energy of backscattered ions can be changed in seconds. Fine-tuning of the energy can be done in minutes by means of changing the terminal energy of the accelerator. In this paper we present the figures of merit of the setup in light of ion energy range and energy divergence, and intensity range and homogeneity. The performance of the setup in an application is demonstrated measuring the energy resolution and detection efficiency of silicon strip detectors to be used in future neutral particle analyzer (NPA) in Joint European Torus (JET).