Where Do Atoms Go When They Decay?

Where do atoms go when they decay?
The short answer to your question is that most of the time the atoms produced by radioactive decay remain close to where they were produced, but this is not always the case. Following is some information that might be helpful.

When a radioactive atom undergoes a nuclear decay event (the significant decay modes are alpha decay, beta decay, electron capture, and spontaneous fission), the decaying nucleus undergoes a transformation in identity associated with the change in the number of protons in the nucleus. All radioactive decay events are spontaneous and, therefore, are associated with an exoergic process. At the time of the decay event the decay energy is split between the residual atom (actually an ion at this point) and any particles and/or photon radiation produced at the time.

For example, when a radioactive atom undergoes conventional beta decay, a neutron in the nucleus is transformed into an electron (called the beta particle) and a proton plus a third particle called an antineutrino (an antiparticle that is nearly massless with zero charge). The beta particle and the antineutrino are immediately ejected from the nucleus and completely out of the atom. The residual nucleus now has an identity associated with the element in the periodic table that has one more nuclear proton than did the decaying nucleus. The residual nucleus has one fewer electron circulating about it than is required for electrical neutrality; thus the “atom” produced is initially an ion.

In the typical beta decay event almost all of the decay energy is split as kinetic energy between the beta particle and the antineutrino. The residual ion from the decay is much more massive than the beta particle and the antineutrino and consequently, because of the restrictions imposed by conservation of momentum, the residual ion carries away a tiny fraction of the decay energy. For example, when 60Co decays by beta decay, the beta particle has a maximum kinetic energy of about 0.3 million electron volts (0.3 MeV); the beta particle energy is at its maximum when the antineutrino energy is zero. The 60Ni product ion has a maximum kinetic energy of about 3.5 eV. This is a small amount of energy which is just about sufficient to break a chemical bond or two but incapable in most instances of freeing the residual atom from its surroundings. The ion quickly becomes electrically neutral by picking up an electron from its surroundings. It then resides as a “foreign” atom in the native material in which it was born.

The situation can be somewhat different for other decay processes. If we consider alpha decay, the products of the decay are an alpha particle and the residual product ion, which contains two fewer protons and two fewer neutrons than did the original decaying atom. The residual product ion has an initial charge of -2 (because it has two more electrons than required for the neutral product atom), but this charge is usually lost and even more electrons may be stripped from the residual atom as it recoils from the decay site. One important difference between alpha decay and beta decay is that the alpha particle is very massive, compared to the beta particle, being more than 7,000 times more massive. As a consequence of this relatively large mass, when the decay event occurs the alpha particle goes off in one direction propelled by the kinetic energy it has received from the decay event, and the residual ion recoils in the opposite direction with sufficient energy to conserve momentum. It turns out that alpha decay is confined largely to relatively heavy nuclides, typically with mass numbers in the range of 200 and greater. For typical alpha particle energies between 5 and 6 MeV, the recoiling ion will have a kinetic energy on the order of 100 keV (100,000 eV.)

Given chemical/physical bonding energies of 1 to 2 eV or so, this amount of energy is sufficient to break many bonds and overcome forces that might restrict the ion from moving significantly. The result has some important implications. For example, when workers are dealing with highly radioactive solutions or even solid materials with high concentrations of alpha emitters, it has been observed that the radioactive material manages to leave open containers and migrate to various other locations in the area, seemingly under its own power. This is a result of alpha decay events occurring close to the surface of the solution or the solid and the relatively large recoil kinetic energy of the residual ion being distributed among thousands of atoms in its immediate vicinity. This can result in a whole aggregate of atoms being stripped from the surface; this aggregate, which may contain significant numbers of radioactive atoms, may then get caught up in air and carried to a different location and/or an alpha decay event may occur in the aggregate and cause another piece of the aggregate to be removed and transported to another location. This kind of process is sometimes described as spontaneous “creep” of the radioactive material, and demands special confinement attention when workers are dealing with highly radioactive alpha emitting materials.

The same phenomenon of “aggregate recoil” has been found to be responsible for the transport of radioactive alpha emitters through high-efficiency air-filter materials that are normally associated with virtually no penetration by particulate materials. In this instance radioactive aggregates collect on the filter surface as contaminated air is drawn through the filter. When an alpha decay event occurs in the aggregate, the recoil energy sometimes tears free a smaller aggregate, which gets entrained in the moving airstream and gets transported deeper into the filter. Eventually, some radioactivity may penetrate the filter as a consequence of such sequential events occurring.  

Regardless of where the residual ion of an alpha decay event ends up it will achieve electrical neutrality and reside as a foreigner among its neighboring atoms. In the case of an aggregate particle that carries the atom that resulted from the original decay event, the atom’s immediate neighbors may be the same as those of the original decaying atom, but the aggregate could be in a very different location from its starting place.

The other decay modes will not be discussed here as most of the major considerations that apply to your question are covered by the cases considered above. There are some additional implications of the process of atom identity change and displacement that are important in specialized situations but they are beyond the intended scope of this discussion.

George Chabot, PhD
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