16O contamination in 4He analysis beams |
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Authors: | ST Picraux JA Borders RA Langley |
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Affiliation: | Sandia Laboratories, Albuquerque, N. Mex. 87115 U.S.A. |
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Abstract: | The 4He+ beam from a Van de Graaff accelerator is often accompanied by an 16O+ beam of the same energy. If, after acceleration and before magnetic analysis, one electron is stripped from the oxygen ion to form 16O2+, these ions will not be separated from 4He+ during magnetic analysis. The 16O2+ fraction in the 4He+ beam was measured by analyzing back-scattering spectra for a thin Au film and the O to He ion charge ratio of 2:1 was confirmed by electrostatic deflection. Ion source conditions and the pressure in the beam lines strongly affect the 16O2+ fraction and the dependence of the O2+/O+ ratio on pressure is found to be in approximate agreement with predicted values. A formula for estimating the 16O2+ beam intensity, based on charge exchange data and measurement of the primary 16O+ beam intensity, is NO2+] = 13 P(torr) l(cm) NO+], where P is the total gas pressure, l the length of the beam line between the accelerator and switching magnet and NO2+] and NO+] are the beam intensities of the respective species. The 16O2+ contamination of 4He+ beams is easily removed by electrostatic separation. |
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