I want to fully understand what happens to the alpha particle as it flies away from the radioactive atom into air and how it interacts with gases that make up air.

  1. Can it collide with neutral gas atom,knock away one or more of its electrons without absorbing those electrons?

  2. Does it stop after hitting just one atom,transfering all its kinetic energy into it or does it crash into multiple atoms along its path,losing speed gradualy? How many collisions will happen before it stops?

  3. Imagine closed system,sort of chamber with impenetrable walls,there is single radioactive atom and neutral atoms.When the radioactive atom undergoes alpha decay,it creates the helium ion which will seek to absorb two electrons to become neutral.Problem I see is that there is now not enough electrons to make all atoms neutral,since there is +2 charged helium nucleus and all other atoms are neutral,so if it "sucked" two electrons to become neutral from some other atom,the victim neutral atom would then be left with +2 charge so the closed system will end up with permanent +2 charge.Is that assumption correct?

  • 2
    $\begingroup$ The overall change is not changed by alpha emission. The protons in the alpha aren't new. They were in the mother nucleus in the first place. And you might start by reading the chapter of the Particle Data Book on "The passage of Radiation Through Matter". $\endgroup$ Jan 27, 2017 at 7:17
  • $\begingroup$ I didnt think new protons are created,I think that the electrons disappear! If they dont,then what happens to them? Since they arent on the alpha particle,do they stay in the radioactive atom that ejected alpha particle? So while the alpha particle is flying away with -2 charge,the radioactive atom that ejected it is left with +2 charge? $\endgroup$ Jan 27, 2017 at 7:29

1 Answer 1


Let me try to answer your questions:

  1. Yes it will collide with neutral gas particles (atoms and molecules). Especially in the beginning, when the alpha particle has still a high kinetic energy, it will dissociate molecules and ionize atoms.
  2. It will likely do a lot of collisions before being slowed down. A way to describe the probability for a collision is the concept of the cross section which allows you to calculate a mean stopping range for an alpha particle in atmospheric pressure. In this question you find an answer that shows the mean stopping range as a function of the alpha particle's energy in air (on the order of centimetres).
  3. It is true that the alpha particle will eventually recombine with 2 electrons and those electrons might be missing somewhere else. Keep in mind that during the emission of the alpha particle, a neutron might have been created as well and free neutrons experience the beta decay where an electron is created (together with a proton and an electron-antineutrino). Also note that you always have a small amount of free electrons due to, for example, background radiation. Finally, if you have a positively charged particle left it will probably just stick to another atom/molecule and form some compound.
  • 1
    $\begingroup$ Parts 1 and 2 are accurate, part 3 is not. Generally alpha decay doesn't create a neutron - it just expels some charge from the mother nucleus, which will be left with two extra electrons. $\endgroup$ Jun 21, 2018 at 11:32
  • $\begingroup$ @EmilioPisanty true, I just wanted to mention that there exists other/additional mechanisms resulting in free electrons. Also, I was probably thinking of nuclear fusion... (D+T->He+n) $\endgroup$
    – Alf
    Jun 21, 2018 at 13:24

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