IONIZING RADIATION IN SPACE
The principal ionizing radiations to be found in space are summarized in table 2-2. Ionizing radiation endangers humans because it is capable of breaking chemical bonds in tissue. The damaging power depends upon the amount of energy deposited per unit volume, the rapidity with which the energy is transferred, and its concentration along the track of the particle of radiation. Radiation which deposits 100 ergs of energy (3) per g is said to deliver a dose of 1 rad. Because different forms of radiation may deposit this energy at different rates and with different intensities along the track, the biological damage of a dose of 1 rad varies with the type of radiation. To correct for this effect the radiation dose in rads is multiplied by the "relative biological effectiveness" (RBE) of the particular kind of radiation. The product is then a measure of danger of the particular kind of radiation, and that product is described in units of rems. Thus, 1 rem of neutrons and 1 rem of X-rays represent the same amount of biological danger. (For X-rays 1 rem results from the exposure of 1 roentgen.) The RBEs of most of the common kinds of radiation are given in the table.
TABLE 2-2. (gif format)
| Name | Charge (Z) | RBE | Location | |
|---|---|---|---|---|
| X-rays Gamma Rays | 0 0 | 1 1 | Radiation Belts, solar radiation and in the secondaries made by nuclear reactions, and by stopping electrons |
|
| Electrons=1.0 MeV 0.1MeV | 1 1 | 1 1.08 | Radiation belts | |
| Protons=100MeV 1.5 MeV 0.1 MeV | 1 1 | 1 - 2 8.5 10 |
Cosmic rays, inner radiation belts, solar cosmic rays |
|
| Neutrons=0.05 eV(thermal) .0001 Mev .005 MeV .02 MeV .5 MeV 1.0 MeV 10.0 MeV | 0 0 0 0 0 0 0 | 2.8 2.2 2.4 5 10.2 10.5 6.4 | Produced by nuclear interactions; found near the planets and the Sun and other matter |
|
| Alpha particles=5.0 MeV 1.0 MeV | 2 2 | 15 20 | Cosmic rays | |
| Heavy primaries | >= 3 | (see text) | Cosmic rays |
The damaging power of heavy charged particles with charge numbers equal to or greater than 3 is most conveniently described in terms of their ionizing power. This measures how many chemical bonds per unit of body mass are broken and thereby gives a rough measure of the tissue damage sustained.
Figure 2-8 plots the ionizing power of protons in silicon dioxide as a function of proton energy. Since the units of ionizing power are in units of mass traversed, the same values are reasonably accurate for all matter with a low charge number (Z), for example, human tissue. This basic curve holds for any ion species when the vertical axis is multiplied by the ion's charge number squared (Z^2).
Essentially the result is that the ionizing power increases as the particle energy decreases, so as to cause the more slowly moving particles to be the most damaging. In the extreme relativistic energy region the damage effects are basically constant - at a level which is termed the ionization minimum. At the lowest velocities the charged particles are finally neutralized by picking up electrons.
