RBS CONTROL OF THE PROTON FLUENCE IN EXTERNAL PIXE ANALYSIS USING A HE ATMOSPHERE

Rutherford backscattering spectrometry (RES) has been implemented for on-line detection of the instantaneous as well as of the integrated pr oton flux in He based external PIXE analyses. The potential problems e ncountered with an added scattering foil were circumvented by making u se of the signal provided by scattering of protons from the He atoms. Two alternative RES geometries have been explored, one with the detect or being directed at the beam immediately behind the exit window of th e accelerator and another one viewing the gas and a reference foil (co vered with a thin Au film), the latter being inserted in the beam line at some distance behind a PIXE standard (Mn on an 8-mu m polypropylen e backing foil). For absolute calibration the analysis chamber was eva cuated to 6 X 10(-4) mbar and the primary ion current was measured wit h a Faraday cup, in parallel with the signals due to the Au reference film and the Mn standard. The sensitivity achieved in backscattering f rom He, 5 counts/(mbar mu C), was sufficient to allow a measurement an d control of the He pressure down to about 1 mbar (with a constant bea m current of typically 30 nA). Somewhat surprisingly it was found that the background level in the PIXE spectra of samples deposited on poly propylene backings decreases monotonically with increasing He pressure (background reduction by up to a factor of 5 at X-ray energies >10 ke V).