A Cs vapor target is described that uses a surface ionization detector inside the target chamber for direct Cs density determination and whose length is variable, permitting variation of the target thickness at constant target density. By use of a variable length target, accurate cross sections are determined that do not require knowledge of the ’’effective’’ target length. Sample cross section data obtained using this target are shown for electron capture by H+and H0and electron loss by H−in Cs vapor at 1000 eV.