Attenuation method for fast ion beams: metastable state population and cross section measurements

An attenuation method for a two-component singly charged ion beam, which enables the beam metastable fraction to be determined and nearly pure ground and metastable state beams to be produced, is described. The role of the collisionally induced excitation and de-excitation processes occurring during the beam attenuation process is analysed in detail. The possibility of determining the corresponding cross section sigma 11* and sigma 1*1 from the attenuation data is considered also. As an illustration of the method, the results of the analysis for the attenuation of a 10 keV N+ ion beam in argon are given. The electron capture cross sections for ground and metastable state ions, and the de-excitation cross section are obtained with an estimated uncertainty of about 20%: sigma 10=12.4*10-16 cm2, sigma 1*0=3.1*10-16 cm2 and sigma 1*1=3.5*10-16 cm2. The cross section for the excitation of the metastable state involved is estimated to be sigma 11* <2*10-17 cm2.