I was hoping somebody could help me out with this problem I'm stuck on: So in a mass spectrometer, an ionizing current of 2 pA causes ions of Cs+ to be produced. these ions have an energy of about 10 kev. I need to find the average distance (mean free path) between any two Cs+ ions at this current. Any help will be more than appreciated Here's what I'm thinking: dq/dt = i so i = dq/dx*dx/dt i can get velocity from the energy: 1/2mv^2 ( i use mass of Cs atom) if i use dq = charge of an electron: since i have a singly ionized Cs atom i get a distance of about 1.7 cm. now i know that at ultra high vacuum the distance between these ions is of the order of 10^5 m . that's all i have to go on. i cannot check if my answer is correct. hence this posting. alternatively i can use the mean free path equation but i don't know if can i use the same equation for mean free path of atoms as given my the kinetic theory of gases for this problem?? is there some factor that you need to put in to account for them being ions ???