Interaction of particle beam with small amount of material - a thin target - is a base of many methods to measure beam parameters, mainly transverse beam size. Wire scanners, SEM grids, beam screens, stripping foils, Ionization Profile Monitors, beam gas vertex detectors, beam-induced fluorescence monitors - they all use a direct beam interaction with (usually) small amount of material. Clearly SOLID targets are quite diffferent then GASEOUS ones, but some aspects, for instance generation of delta electrons - are the same.

The thin targets do not perturb the beam going through. For instance in case of 30 um carbon fibre, only one every 10000 relativistic protons going through undergoes nuclear reaction and is removed from the beam. The Coulomb scattering affects trajectory of other protons in negligible way, and the effect on the beam emittance even in circulating relativistic beams can be neglected.

Another interesting effect is decrease of the energy deposit in a thin target much below the one expected from classical dE/dx effect due to emission of delta electrons, which escape the target effectively removing a significan part of the energy.

In case of gaseous thin (low pressure) targets the effect of the gas "burnout" by the beam can be neglected - replenishment of the gas atoms is faster then the "burnout" process. The electrons emitted due to ionization travel through beam vacuum without any interactions with other electrons or atoms. But before they leave the beam vicinity their movement is strongly affected by the beam electric field (magnetci field plays small role as most of the electrons are slow).

mariusz sapinski 2014