ENERGY SPREAD OF ION-BEAMS PASSING A GAS STRIPPER

Since the energy spread of accelerated particle beams is not well know n for tandem-type machines, we have measured current-energy distributi ons for a variety of ion beams delivered by our 3 MV tandem at varying stripper gas pressure. The energy widths of light ions produced from injected negatively charged atoms, such as H- and He-, are mainly due to the accelerating voltage ripple, whereas for heavier ions, such as C- and F-, energy straggling in the stripper gas dominates. In case of injected negatively charged molecules, such as NH2- and CN-, Coulomb explosion in the gas stripper produces satellite peaks on both sides o f the unshifted central line. These deviations from Gaussian line shap e complicate precise lineshape analyses, as, for example, required in nuclear reaction Doppler spectrometry of vibrational states in target materials. The most relevant charge-exchange processes in the stripper gas will be discussed in some detail.