In a typical HEBT line with a Xe48+ beam @ 200 MeV/u and of about 40 mm footwidth, profiles width measured with the Secondary Electron Monitor (SEM)-Grid (standard method) agree with the BIF profile data within 10%:

48-wire SEM-grid profile (green) and BIF-profile (blue) as a 20 shot average of a 2,3*10^9 Xe48+ beam @ 200 MeV/u extracted in 1 us (fast extraction mode).

However, to measure beam profiles of sub cm or even sub mm beams (close to targets) with tolarable errors, gas dynamics and lifetimes of the relevant states have to be investigated. The particle displacement can be estimated as in the terms below. All free parameters as the lifetime (Tau), particle mass (M) and the mean ionization state are specific for a certain gas species. Further information you find here...