EXTERNAL-BEAM RBS IN AN UNENCLOSED HELIUM ENVIRONMENT

The external beam facility installed at the KN-3000 accelerator in Flo rence, formerly used for PIXE measurements, has been exploited also fo r RBS analysis. A particle detector has been placed at 135-degrees in a setup structure which also includes the two Si(Li) detectors simulta neously used for PIXE. A helium flow is kept in front of the particle detector and of the one used for low-energy X rays, as well as in the region between the beam exit window and the target; no chamber is used for gas containment, in order to allow us the most complete freedom f or target size and for its handling. Gas leaks upwards are limited by a soft rubber lid which is kept in contact with the target itself. Hel ium saturation of the volume in front of the target and of the detecto rs is attained with a gas flow of less than 1 l atm/min. All the ''use r-friendly'' features of the existing PIXE facility have been maintain ed: in particular, video-monitoring the sample with a remotely control led camera, checking beam position with laser aiming and sampling beam current with a rotating chopper are still possible and easy to perfor m. Particular care has been taken of particle-detector mounting and de tection geometry in order to obtain the best possible energy resolutio n. The present setup gives an overall system resolution of about 28 ke V FWHM, when bombarding ultra thin targets with 1.5 to 3 MeV protons. Further improvements are in progress. Preliminary applications have be en performed on samples which may not be placed under vacuum; in parti cular, combined PIXE-RBS measurements have been done to clarify some p roblems connected with ancient inks analysis (thickness determination, penetration within the underlying paper or parchment), and to check c omposition of thin standards evaporated onto polymeric supports.