Radiation by Rick Maltese
There are two opposing views on the effects of radiation. One theory is based on a long held view that many substances, normally considered toxic, are benign in small doses – not harmful. In small amounts these poisons can be healthful. Called Hormesis, This theory also applies to radioactive substances.
The other theory is called the Linear No Threshold (LNT) model which assumes any radiation is dangerous. Many nuclear advocates are convinced that fear plays a factor; and extreme attention to erring on the side of caution is an attitude that is preventing the growth of the nuclear energy industry.
Radiation is not a new phenomenon. It has been with us from the beginning of time. In fact the universe has become less radioactive over time! One reason that we can cope with radiation is that all creatures, including humans, evolved in a time when natural radiation was much higher than it is now. Radiation comes from the largest atoms in the periodic table. These atoms are less stable because of their size and show up towards the end of the periodic table.
Radioactive decay gives off either alpha or beta radiation. These particle emissions are the results of the change of the original atom to another element; either two atomic numbers smaller (alpha) or one higher (beta). Alpha and Beta particles (sometimes called rays) are the activity that occurs naturally when a radioactive substance changes to another element – which also confirms E = mc2.
Alpha Particles (Alpha Rays)
These are slow moving and have only a short range emission of a few centimeters if in the air. The energy alpha particles emit varies; with higher energy alpha particles being emitted from larger nuclei, but most alpha particles have energies of between 3 and 7 MeV (mega-electron-volts), corresponding from extremely long to extremely short half-lives of alpha-emitting nuclides, respectively. This energy is a substantial amount of energy for a single particle, but their high mass means alpha particles have a lower speed (with a typical kinetic energy of 5 MeV, that speed is 15,000 km/s – which is 5% of the speed of light) than any other common type of radiation (Beta particles – β, neutrons, etc.). Because of their charge and large mass, alpha particles are easily absorbed by materials; and they can travel only a few centimeters in air. They can be absorbed by tissue paper or the outer layers of human skin (about 40 micrometers, equivalent to a few cells deep). Alphas are not, in general, dangerous to life unless the source is ingested or inhaled, in which case they become extremely dangerous.
Alpha emitters: radium, radon, uranium, and thorium. Alpha radiation is the most destructive form of ionizing radiation. It is the most strongly ionizing, and with large enough doses can cause any or all of the symptoms of radiation poisoning. It is estimated that chromosome damage from alpha particles is anywhere from 10 to 1000 times greater than that caused by an equivalent amount of gamma or beta radiation, with the average being set at 20 times.
Transmutation of elements from one to another had been understood since 1901 as a result of natural radioactive decay, but when Rutherford projected alpha particles from alpha decay into air, he discovered this produced a new type of radiation which proved to be hydrogen nuclei (Rutherford named these protons). Further experimentation showed the protons to be coming from the nitrogen component of air, and the reaction was deduced to be a transmutation of nitrogen into oxygen in the reaction:
14N + α → 17O + proton
Beta Particles (Beta Rays)
Of the three common types of radiation given off by radioactive materials (alpha, beta and gamma) beta has the medium penetrating power and the medium ionizing power. It can travel several feet in air. Although the beta particles given off by different radioactive materials vary in energy, most beta particles can be stopped by a few millimeters of aluminum. Being composed of charged particles, beta radiation is more strongly ionizing than gamma radiation.
Beta emitters: strontium-90, carbon-14, tritium, and sulfur-35.
Example: Cs-137 decays by beta radiation to become Ba-137M. The M means meta-stable which means that the nucleus has excess energy. This excess energy is dissipated by the release of a gamma ray almost immediately.
Gamma radiation is electromagnetic, has no mass, and does not change the original atom to another element. Gamma radiation travels the farthest and is analogous to fluorescence; where excess energy is emitted in the form of a photon. Gamma rays are the most intense (smallest) wavelength in the electromagnetic spectrum – so small the wavelengths can pass through mirrors! Gamma rays are also used to irradiate food which kills harmful bacteria. These are created within super nova explosions, nuclear reactions, the plasma of lightning strikes, and the more humdrum process of radioactive decay. Gamma rays can be dangerous if the strength and length of exposure is high. Gamma ray photons are emitted from the nucleus and are a lot more energetic than fluoresced photons, which are emitted from the electron cloud.
Gamma emitters: iodine-131, cesium-137, cobalt-60, radium-226, and technetium-99m
There are two measurement standards by which to list units of radiation.
The conventional (U.S.) units are curie, rem, rad, and roentgen.
The standard international units are bequerel, sievert, gray, and coulomb per kilogram.
Curies and bequerels denote amounts of radiation being given off by an object.
Rem and sievert denote biological risk of exposure to radiation.
Rads and greys denote radiation absorbed by an object – including people.
Roentgens and c/kg denote a quantity of ionizing radiation.
It should be noted that when calculating exposure rates several factors must be considered: type and strength of radiation emitted, volume (size) of radioactive object, distance from object, length of time near the object, type and thickness of any shielding, as well as the absorbing material being irradiated.
According to the United States Nuclear Regulatory Commission, a person would receive a dose equivalent of 1 millirem from any one of the following activities:
- 3 days of living in Atlanta
- 2 days of living in Denver
- 1 year of watching television (on average)
- 1 year of wearing a watch with a luminous dial
- 1 coast-to-coast airline flight
- 1 year living next door to a normally operating nuclear power plant
All organic matter (both plant and animal) contains minute amounts of radiation from radioactive potassium-40, radium-226 and other isotopes. In addition, all water (ice, snow, vapour) on Earth contains measurable amounts of dissolved uranium and thorium! As a result, the average person receives an average internal dose of about 30 millirem of these materials per year from what we eat and drink, as illustrated by the following table. (Amounts are shown in picocuries per kilogram.)
|Natural Radioactivity in Food
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