There are two principal forms of radioactive decay, which produce three types of radiation (α, β, γ).
An unstable atomic nucleus has essentially two options to overcome instability: either one or more nuclear particles can be expelled, or a particle inside the nucleus can undergo transformation. The expulsion of nuclear particles can involve the release of a proton, a neutron, or of a complete daughter nucleus. Most often, it is a helium nucleus (two protons and two neutrons) that is expelled (α-decay), since this fragment is particularly stable. First and foremost, the helium nucleus carries a double positive charge (which appears at the upper right of the element symbol):
α-Radiation can be stopped by a piece of cardboard, or will travel through the air for only a few centimeters. If isotopes that emit α-radiation are taken up into food, however, practically their full hazardous potential will be unleashed within the body.
In β-decay, one nuclear particle is transformed into another. For example, when a neutrally-charged neutron is transformed into a positively-charged proton, a fast electron is expelled ( a), below left ). This electron is designated as β-.
Incidentally, there are also processes that might seem rather more fantastic ( b), above right); thus, a proton can be converted into a neutron. This produces an anti-electron (a form of antimatter), which is designated with a β+ (i.e., a positron). This transformation requires an expenditure of energy, and the nucleus must “borrow” an electron where one previously did not exist. This deficit is then made up by the generation of antimatter. The calculation is then balanced for the system when a β+ (positron) encounters an electron, whereupon they react and are mutually annihilated, leaving only the residual energy that was carried by the β+ particle. The situation is a bit more complex since there are still other subatomic particles involved.
The nuclear radiation that arises via the β-decay of electrons (or positrons) can be stopped by a metal sheet, and penetrates only a few centimeters into solid material..
The following figure summarizes the nuclear processes as a function of the number of neutrons (N) and protons (P) in the nucleus:
We must now back up a bit to explain an additional type of nuclear radiation. Electromagnetic radiation, which we know in the form of light and heat (infrared), can carry different amounts of energy. Non-harmful radiation is powerful enough to raise the energy of electrons in their orbits slightly (recall that the mass of an electron is 2000 times smaller than that of a neutron or proton). The electrons continue in their orbits about the atoms, and sometimes themselves release the introduced energy in the form of soft radiation. This situation is comparable to when a brick is thrown and strikes the ground at some point. An atomic nucleus corresponds to about 10,000 of such bricks. When a nucleus decays, the residual energy can be present in the daughter fragments, which in this case emit significantly harder radiation that can penetrate through walls. As high-energy equivalents to simple light beams, this γ-radiation carries approximately 10,000 times more energy.
If an especially heavy nucleus decays, it might first produce two cleavage products of medium weight, and these can then decay further. This process is referred to as “fission”. Of particular interest is the case in which U-235 undergoes nuclear fission. When the uranium-235 nucleus takes up a neutron and thus becomes uranium-236, it cleaves into two fission products together with 2-3 neutrons. These fission products then emit nuclear radiation according to the processes described above. The typical cleavage into xenon-137 and strontium-96 serves to demonstrate:
Additional possible products from the nuclear fission of uranium-235, such as iodine-131, strontium-90, and cesium-137, will be described in the section on Hazardous Isotopes.
In addition to hard radiation, this process generates heat that can be used in power production. Control of the nuclear fission is closely linked with control of the neutrons. For one thing, not every neutron that is produced will trigger a new nuclear process. For another, the neutrons can be absorbed in principle as a way to make the nuclear fission controllable. To stop the process completely in case of emergency, a neutron poison such as boron can be used. A boron atom can capture a neutron and be transformed from boron-10 to boron-11. Both boron-10 and boron-11 are stable.
Now that the important nuclear processes have been explained, we can provide an informative legend for our isotope chart.