Answer to Question #12012 Submitted to "Ask the Experts"

What happens to the alpha or beta particle after it gives up all of its energy? I understand that the particle becomes stable and resides where it stopped (in the material or medium it stopped in). Beta particles (essentially electrons) should become part of the atom it's near or be thrown out again by the nucleus. The alpha particle should wind up in the air as helium gas, right? Can we account for the particle being without kinetic energy? Or does it become potential particle energy?

You are correct in your inference that the emitted alpha particle, after dissipating its kinetic energy, will become a helium atom by picking up a couple of electrons from atoms or molecules in the vicinity. While the alpha particle is born with kinetic energy it also has rest mass, based on its makeup of two protons and two neutrons, independent of its kinetic energy; therefore its existence does not depend on it having kinetic energy. We might also consider the overall conservation of electric charge that takes place in the decay process.

Assuming that the original atom that emits the alpha particle, as well as all the atoms and molecules in the system being considered, are charge neutral (that is, they have equal numbers of protons and electrons), the decaying atom, after emission of the alpha particle, will have an excess of two electrons (relative to the number of protons in the nucleus). Thus, when the alpha particle, which carries a positive charge of +2 because of its deficiency of electrons, picks up two electrons to become a helium atom, the entire system in which the decay event occurs remains charge neutral. Electrons in the system likely undergo an appropriate physical shuffle so as to eventually leave all atoms and molecules in a neutral state.

Your inference about the fate of a beta particle is partly correct. After the beta particle has lost its kinetic energy, it will eventually find its way into the electronic structure of an atom or molecule. It cannot again be emitted by a nucleus because it is impossible for such an electron to enter into a stable nucleus. While this may seem strange because the beta particle originated from the transformation of a neutron into a proton and an electron, the electron does not stay within the nucleus, rather it is immediately ejected as the beta particle. It is possible for some unstable nuclei to capture electrons from the inner electron shells of the respective atoms. This process is somewhat the opposite of beta decay in that the captured electron immediately combines with a proton to produce a neutron. This reaction, however, requires a significant amount of energy, which is not available in stable atoms; only some specific unstable (i.e., radioactive) atoms that can make sufficient energy available (at least 782 kiloelectronvolts [keV]) during their decay process are able to undergo this process, which is termed electron capture decay.

George Chabot, PhD, CHP