When a radioactive isotope decays, it emits one or more of three forms of ionizing radiation: alpha particles, beta particles or gamma rays. One problem in discussing the effects of radiation is the lack of understanding of these different types and how they affect the body.
Alpha-particles are actually helium nuclei, which can be seen from the periodic table to have an atomic number of two and an atomic weight of four. [In alpha-decay, the radioactive isotope is changed to the isotope of an element with an atomic number two less and an atomic weight four less than the original element.]
In the subatomic world, the emission of an alpha-particle is like shooting shot put with a sling shot: there is a lot of mass involved, but it doesn't travel very far. This particle gives up its considerable energy in one-half to two inches of air. It can't penetrate the skin and, consequently, isn't dangerous when outside the body. Because of its mass, however, it is considered to be quite dangerous when inside the body - particularly when inhaled, where it may remain in the lungs in close proximity to lung tissue cells for extended periods. [It is common knowledge that plutonium, an alpha emitter, is deadly when inhaled. You will see in chapter 16 that common knowledge may be terribly mistaken.] Fortunately, there are some very good data on this subject that we'll look into.
Beta-particles are high energy electrons that can penetrate up to three feet of air, or the first layer of skin cells, and can cause a burn not unlike that from falling asleep in the tanning bed. Beta-burns were a particular problem for workers after the Chernobyl chemical explosion and for technicians involved in some phases of the weapons-testing program in Nevada. With a mass some 1/7344 that of an alpha-particle, its energy is derived from its speed, which can approach 99.8% of the speed of light. Relatively speaking, beta "rays" are not considered much of a threat to human health although there can be complications arising from beta-burns.
A third possible product from the decay of a radionuclide (a fancy word for a radioactive isotope) is gamma radiation, which is considered to be the greatest danger from nuclear decay. It is very similar - in fact in some cases identical - to X-rays and can penetrate several feet of concrete or inches of steel. Like any other electromagnetic radiation, its intensity falls off as the square of the distance from the source. For instance, the exposure at 100 yards is 1/900 that at 10 feet; at a quarter mile, the radiation is reduced by a factor of 17,424 compared with the 10-foot value. [Luckey mentions another form of radiation, delta rays, with low energies and penetrating power, but - because of their abundance and proximity to cell structures - are important in radiobiology. See Radiation Hormesis, page 2.]
There are other forms of radiation that should be mentioned even though they are not the normal products of natural decay. The first is cosmic radiation. It consists of various types of particles, sub-particles, and high-energy photons arriving on Earth from every direction in the cosmos. Cosmic radiation, with both solar and galactic components, can have unbelievably high energies, but, fortunately, it poses no danger, since there is so little of it. For instance, protons that originated in far-off galaxies four billion years ago have energies 100 million times greater than can be created in our most advanced particle accelerators. But only about one of these per year is detected by the Akeno Giant Air Shower Array located just west of Tokyo. [Energies are in the range of 3x1020 electron-volts. For more information see Scientific American, January 1999, page 32.]
These cosmic sources make up less than 10% of the background radiation that the average U.S. citizen receives, yet we still receive about 1,500 cosmic "hits" per second, each of which - because of the penetrating nature of all high-energy particles - collides with about 10,000 of the hundred-trillion cells in the adult human body. That's 15 million "cellular events" per second. If the quotation from Dr. Gofman at the beginning of this chapter is accurate, then we are indeed in a heap of trouble.
It is, by the way, the action of cosmogenic (a fancy scientific word for "from outer space) neutrons on atoms of atmospheric nitrogen that produces the carbon 14 used for radiometric dating. [Carbon 14 dating, developed in 1947 by W.F. Libby, is based on the fact that 14C is continually produced by cosmic rays. When the high-energy ray collides with atoms in the atmosphere, free neutrons are produced which, when absorbed by a nitrogen (14N) atom, cause it to eject a proton, thus converting it to 14C. This radioisotope is taken in by plants and animals while they are alive but remains constant after death. By measuring the 14C in comparison with its decay products, the approximate age of the fossil can be determined.]
Other neutron sources - accelerators, nuclear reactors, and bombs - have great potential for danger, as the lack of charge on the neutron allows it to penetrate some eight to ten feet of packed earth - about 50% farther than gamma radiation. Strictly speaking, neutrons are not ionizing radiation, since they have no electrical charge with which to influence the electrical affinity between protons and electrons. But they can smack into light-weight atoms such as hydrogen and cause them to become ionizing projectiles.
Finally, we have X-rays, which are typically produced when an energetic electron is stopped in its tracks. Just as with gamma rays, these can travel long distances and have the potential for doing harm to the body. During the past several decades, the energy - and therefore potential harm - of diagnostic X-rays has greatly decreased because of the increased sensitivity of the film and the detectors being used. Similarly, therapeutic X-ray equipment is designed to focus energy on smaller areas with less effect on healthy tissue. Sadly, an unwarranted fear of radiation causes many people who could be benefited by the use of X-ray diagnosis and therapy to shun such treatment - and thereby become subjected to unnecessary real dangers.
A similar situation is found in the commercial/industrial environment where X-rays are subjected to such overprotective rules that their primary benefit - being able to detect flaws in welds and other material in order to protect human life - is rendered uneconomical. (No doubt there is also reluctance on the part of workers - who are victims of LNT theory fears - to use the equipment.)
All of the above forms of radiation are termed ionizing radiation because they have the ability to strip electrons from their orbits around nuclei, making the lone electron a negative ion and the "left-behind" proton a positive ion. Table 3 shows the electromagnetic spectrum illustrating the various kinds of ionizing and non-ionizing radiation. For all practical purposes, ultra-violet radiation is not ionizing, and it is the action of ionization that defines the beginning of the X-ray portion of the electro-magnetic spectrum. [While UV radiation cannot ionize an atom, it can dissociate a molecule. For example, cosmic UV can smack into an O2 molecule splitting it into two atomic oxygen atoms - which are very chemically active.]
Table 3 - The Electromagnetic Spectrum
- Radio waves (AM) - long wavelength ~100 meters*
- Shortwave radio - low frequence ~1 million Hz
- Television VHF - low energy ~10^-9 electron volts
- Television UHF
- Microwaves (including ovens)
- Infrared (heat) radiation
- Red-orange light
- Green-blue light
- Ultraviolet A
- Ultraviolet B
- Ultraviolet C
- Vacuum ultraviolet (absorbed by short path through air)
- Low energy X-rays
- Deep therapy X-rays - short wavelenght ~10^-14 meters
- Gamma rays (overlaps with X-rays) - high frequency ~10^22 Hz
- Cosmic photons - high energy ~100 million electron volts
* The ~ symbol indicates an approximate measure.