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I am told that, when a heavy charged particle passes through a substance, ionization occurs. I am then told that the most probable occurrence is collision with an atomic electron. Finally, I am told that, when this occurs, a relatively slow secondary electron, with kinetic energy not exceeding the ionization energy, is knocked out.

I find this confusing, and have the following questions:

  1. How is collision with an atomic electron the most probable occurrence? Aren't the electrons extremely small compared to the nucleus?

  2. How does the collision of the heavy charged particle with the primary electron have an affect on the secondary electron? I don't understand how the secondary electron is affected by this collision.

  3. Why must the kinetic energy not exceed the ionization energy?

I would greatly appreciate it if people would please take the time to clarify these points.

Related: https://en.wikipedia.org/wiki/Delta_ray

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How is collision with an atomic electron the most probable occurrence? Aren't the electrons extremely small compared to the nucleus?

Nope. Stop thinking of electrons as particles and start thinking of them as being delocalized over atomic orbitals (as "electron clouds" rather than electrons). The size of a nucleus is on the order of $1-10$ fm, while the smallest electron cloud is around $30$ pm, which is a thousand times bigger.

How does the collision of the heavy charged particle with the primary electron have an affect on the secondary electron? I don't understand how the secondary electron is affected by this collision.

There might be some confusion of terminology here. Usually the "secondary electron" refers to what you're calling the "primary electron", that is, the electron that is liberated by the collision with a heavier particle.

Why must the kinetic energy not exceed the ionization energy?

This seems incorrect. It would be helpful to have a reference for these claims, but the maximum kinetic energy of the electron liberated by ionization can, for high collision energies, far exceed the ionization energy of the atom.

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  • $\begingroup$ This is a great clarifying answer! Thank you! $\endgroup$ May 22, 2020 at 1:54
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Any charge passing through a material would first encounter electrons since they are the most outer layer of an atom. Also they are light and the bounded but with a very small binding energy. So, moving them is a lot easier than moving a nucleus, they are heavy and far apart compared to electrons (if a nucleus were a beach ball the electrons would be kilometers away). Therefore it is most likely that the firsts interactions would be with electrons. Once they are moved from their orbits these electrons hit other electrons (thus "secondary"), the incoming particle is long gone, since they have been removed from their orbits but not from the material the energy carried might be less than the energy needed for ionization. It would be better to have a reference from where you are reading.

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  • $\begingroup$ But since electrons are so small, there is no "layer" -- the area surrounding the nucleus is nearly entirely empty space. I don't think this explanation satisfies the question of why a collision with an atomic electron is the most probable occurrence. $\endgroup$ May 22, 2020 at 1:41
  • $\begingroup$ "small" is not really a good term. They are light but highly interactive. And they are all around the nucleus in a cloud, you don't need to hit a static "small" electron but knock out just one of many which exist in a that cloud surrounding the nucleus and around 20.000 times larger. The question here as in quantum mechanics is the probability of knocking out one electron and that's higher than hitting a nucleus which is electrically hidden by the surrounding electrons (the incoming particle will see no charge from afar) and small compared to the size of the electronic cloud. $\endgroup$ May 22, 2020 at 3:08
  • $\begingroup$ That makes sense. Thanks for the clarification. $\endgroup$ May 22, 2020 at 3:19

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