1. The problem statement, all variables and given/known data The energy required to ionize a helium atom is 24.6 eV. If helium gas is bombarded with charged particles, at what incident energy will ionization of helium just be observed if the projectiles are (a) alpha particles (b) protons (c) electrons? 2. Relevant equations Most of the exercises in this chapter have to do with conservation of momentum and energy, but otherwise I really don't know what equations would be relevant. 3. The attempt at a solution I need help knowing even where to begin with this. It seems like the entire kinetic energy (which I assume = "incident energy") of the incoming particle should go into the ionization, with the result that the total kinetic energy of the system after the collision = 0 and the helium atom is ionized. This would imply that the incident energy required is 24.6 eV and that the same amount is required regardless of what sort of particle is the projectile. But this is wrong. Hint please?