Radiation types and units


in Science, Security, The environment

Types of radiation

Radiation is categorized in several different ways. One is on the basis of energy levels: ionizing radiation is sufficiently energetic that it can cause an atom or molecule to be stripped of an electron, turning it into an ion. This depends on the energy level of the individual particles or waves and has nothing to do with the total number of them. Non-ionizing radiation is simply that which doesn’t have enough energy to liberate an electron.

Another way to classify radiation is in terms of whether it is electromagnetic (consisting of photons) or particle radiation. There are three types of particle radiation: alpha decay, based on the emission of two protons and neutrons bound together in a helium nucleus, beta decay, wherein the particle emitted is an electron, and neutron radiation, where atoms release neutrons. Alpha particles are not generally very dangerous, because they are unable to penetrate much of substance. Even a few centimetres of air can have a strong protective effect. That said, ingestion can still be highly dangerous. The Polonium-210 that killed Alexander Litvinenko is an alpha emitter. Beta particles can usually be shielded from using a few milimetres of lead. Neutron radiation is unusual insofar as it is capable of producing radioactivity in the atoms it encounters. Shielding consists of a large mass of hydrogen rich materials.

Electromagnetic radiation with sufficient energy to be ionizing cosists of x-rays and gamma rays. Both consist of high-energy photons (those with short wavelengths), with gamma rays having shorter wavelengths than x-rays (10^(-12)m rather than 10^(-10)m). Shielding, especially for gamma rays, must be dense and fairly extensive.

Measuring radiation

Radiation is also measured in a variety of ways: important ones being Roentgens, rads, rems (Roentgen equivalent in man), Curies, Becquerels, and Sieverts.

Becquerels are a unit of radioactive decay based only on the number of decays per second. A Curie is equal to 3.7 x 10^10 Becquerels, and is approximately equivalent to the activity of 1 gram of Radium isotope. These units reflect the number of emissions only – not their physical or biological effects.

A Roentgen is a measure of ionizing radiation based on the ratio between charge and unit mass. Rads are a largely obselete unit of radiation dose, equal to 100 ergs of energy being absorbed by one gram of matter. Rems are the product of the number of Roentgens absorbed, multiplied by the biological efficiency of the radiation. Rems are also considered highly dated as a measure of radiation. 450 rems is an approximate lethal dose (LD50), for those who do not receive prompt treatment.

Sieverts are the recommended replacemend, “found by multiplying the absorbed dose, in grays, by a dimensionless “quality factor” Q, dependent upon radiation type, and by another dimensionless factor N, dependent on all other pertinent factors.” The LD50 for ionizing radiation is about 5 grays or about 3-5 Sieverts. If the biological efficiency used to calculate rems equals one, one Sievert is 100 rems.

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{ 4 comments… read them below or add one }

. February 11, 2008 at 11:23 am

The grisly effects of large doses of radiation are now well understood. Death can come in hours for those who suffer the very highest doses, and the relationship between sizeable exposures and long-term cancer risk is clear. But, largely due to a lack of data, the consequences of smaller doses are more controversial.

Much of the information on the health effects of radiation comes from studies on survivors of the Hiroshima and Nagasaki nuclear-bomb attacks, most of whom received fairly high doses. Those studies showed a clear relationship between cancer rates and radiation exposure; lacking data for lower doses, scientists extrapolated the relationship down to zero. The result—that no level of radiation could be considered safe, and that health risks increased linearly with exposure—was adopted as the official model, and remains the dominant theory today.

But not everyone is sure that it works. In 2005 The World Health Organisation (WHO) published a report into the aftermath of the 1986 Chernobyl meltdown in Ukraine. Although the explosion released more radioactivity than the Hiroshima bomb, the average exposure was much lower.

In contrast to predictions made at the time that tens of thousands of people could die, the WHO put the death toll at the time of the report at less than 50. Nor did the WHO find much evidence of increased rates of fertility problems or malformed children as a result of the accident. Its revised estimates predicted an eventual total of around 9,000 deaths—still a tragedy, although much smaller than first feared. Indeed, the scientists argued that the fall in the quality of health care resulting from the collapse of the Soviet Union had likely done far more harm to the citizens of Belarus, Ukraine and Russia than the nuclear meltdown.

. February 11, 2008 at 11:23 am

That healthy glow

Feb 11th 2008
From Economist.com
How much radiation is safe?

. February 11, 2008 at 11:24 am

Everyone on the planet is constantly exposed to low levels of background radiation, mostly from naturally-occurring radon gas.

Doses vary widely from place to place, from a global average of around 3 milliSieverts (mSv) a year (mostly from exposure to radon, a naturally-occurring gas) up to 260 mSv in Ramsar, an Iranian town whose streams contain large quantities of naturally occurring radium.

. September 2, 2009 at 11:19 pm

U.K.: The Puzzling Polonium-210 Attack
November 28, 2006

Polonium, also known as radium F, was discovered in 1897 by Marie Curie and her husband, Pierre. It is an alpha emitter, meaning that although it is highly radioactive, it cannot penetrate human skin or a sheet of paper. Although the element is common in nature (it is found in such things as dirt and tobacco), it does not naturally occur in lethal concentrations. Only about 100 micrograms of the polonium-210 isotope can be found in one metric ton of dirt, for example. Once concentrated, however, it is lethal. Polonium-210 emits 5,000 times more alpha particles than radium, and an amount the size of the period at the end of this sentence would contain about 3,400 times the lethal dose. A dose like that which killed Litvinenko would probably have been manufactured at a nuclear facility.

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