| Particle | Weight | Charge |
|---|---|---|
| proton | 1 | 1 |
| alpha | 4 | 2 |
| electron beta rays | 1/1840 | -1 |
| positron ” | 1/1840 | 1 |
| deuteron | 2 | 1 |
| triton | 3 | 1 |
| meson | 1/8 | 1, -1 |
| average light fission fragment | 97 | 20 |
| average heavy fission fragment | 138 | 22 |
If the fission fragments were completely stripped of their orbital electrons, they would have charges even greater than the values indicated in the table. The reader will recall that the average charge of the nucleus of the light fission fragment is 38, and of the heavy, 54. But such highly positively charged particles exert an enormous attraction on electrons. Some of these remain attached even during the fission process itself. As the fission products lose their speed during passage through matter, they pick up more electrons and gradually lose their charge.
When any of these energetic charged particles moves through matter, it interacts with electrons in the atoms. As a result of this interaction, the electrons may be dislodged from their usual states of motion. If the interaction is gentle—either because the charged particle passes the atom at a considerable distance or else because the particle is moving so rapidly that the interaction lasts for only a short time—the electron may be left undisturbed. If the interaction is more violent, however, the electron may be excited to a more energetic state of motion while still remaining in the same atom or molecule; or it may actually be ejected, ending up at some other atomic site. In this latter event the original atom is left with a residual positive charge and is said to be ionized. At the same time the displaced electron is apt to unite with whatever atom or molecule happens to be nearby, creating in this way a negative ion. The whole process may be described as forming an ion pair. In the wake of the charged particle one finds, therefore, ionized and excited atoms and molecules. A rearrangement of atoms will now ensue which leads to new chemical compounds. The important thing for us is, however, that these chemical changes do not depend very much on the type of particle which produced the ionization; the proportion between ionization, excitation, and eventual chemical reaction remains more or less the same. Roughly speaking, the more ion pairs that are formed in living cells, the greater is the extent of biological damage.
To make an ion pair requires the expenditure of a certain amount of energy. It might seem as though this amount should depend crucially on the weight, charge, and energy of the particle, and also on the medium through which the particle is moving. This is not so. There is some dependence, of course, but only slight. Any charged particle, irrespective of its energy, moving in any medium—air, water, soil, or living tissue—creates ion pairs at the rate of about one per 32 electron-volts. A one-million-electron-volt particle produces about 30,000 ion pairs before losing all of its energy. (When it does lose its energy, if it is a positively charged particle, it will pick up enough electrons to become neutral. An alpha particle, for example, will become an ordinary helium atom; a proton will become an atom of hydrogen.)
We have said that two charged particles having the same energy, produce the same total number of ionizations. There is an important respect, however, in which charged particles of the same energy may differ. That is, in the density of ionization along their paths. In particular, the more slowly the particle is moving and the greater its charge, the more ionization and damage it will produce in a given distance. At the same time it will lose energy at a greater rate. If we compare two charged particles of the same energy plowing into matter, the one which leaves the deeper furrow will be stopped more quickly.
For a greater charge it is easy to understand that the electrical interaction is increased and hence each atomic electron is more strongly disturbed. If, on the other hand, the particle moves more slowly (which is usually the case if it is heavy) it spends a longer time in the neighborhood of the atomic electrons. The electrical interaction thus has a longer duration and is more effective in ejecting an electron. For this same reason the density of ionization along the path of a particular charged particle should tend to become greater and greater as the particle slows down. Actually this tendency is opposed in the case of a fission fragment by the increased likelihood of the particle’s picking up electrons and reducing its charge. As a result, the ionization density for these fragments is rather uniform. If a heavily charged, slow particle moves through matter it leaves so many disturbed and disrupted molecules behind that now these molecules may react with each other. Therefore heavy ionization may lead to peculiar effects. Nevertheless all ionizing particles give rise to roughly similar chemical change and destruction.
Except for the beta rays, all the charged particles are very heavy compared to the electron. Consequently, as they move through matter and interact with the atomic electrons, their paths are not perceptibly deflected from the original direction. The beta rays, on the other hand, having the same weight as the atomic electrons, are appreciably affected by their encounters and are frequently forced to change direction. Their paths are thus winding and random.
Because the beta ray does not travel in a straight line, its ability to penetrate matter must not be measured by its total path length. As a rule of thumb, the range of a beta particle, being the distance it travels along the line of its original direction, is about one half of its total path length. For heavier charged particles, however, no distinction need be made between range and actual distance traveled.
The most important fact about the ranges of charged particles is that they are small. An alpha particle, for instance, with a typical radioactive energy of a few million electron-volts, has a range in water (or living tissue) of a few thousandths of an inch. Such a particle could not penetrate a sheet of paper. A fission fragment, despite its great energy, is even less penetrating than the alpha particle. The proton has a somewhat greater range than the alpha particle. But the beta ray, because of its low weight, has by far the greatest range of any of the charged particles. Even it, however, goes only a fraction of an inch in solid or liquid materials.
The following table shows the ranges (in inches) of some of the charged particles in air and water as a function of energy (in millions of electron-volts):