I'm sure my understanding of an ionization chamber is incorrect, so please point out the error.
Suppose we are using an sealed ionization chamber to detect the energies (trajectories) of a particular particle. Based on what I've read online, how this is done is the particle comes wizzing through the ionization chamber and ionizes some of the gas particles (which in this case let's say is isobutane or some other non-air gas). The ionized gas particle and its newly freed electron will both travel to the cathode and anode respectively (unless the electron starts a Townsend Avalanche), and then that newly created signal can be read and interpreted.
My question is, how does the particle enter the ionization chamber if it is sealed? If it isn't sealed, then why doesn't that non-air gas within the chamber leak out?