There are three aspects of radiation that are measured by various units: what is happening in the radioactive material; what exposure does a nearby object experience; and if that object is a living organism what biological effects does that exposure produce.
The material is undergoing radioactive decay. Unstable isotopes are decaying and emitting alpha particles, beta particles and gamma rays (there are other forms of ionizing radiation but these are the ones of concern with regard to nuclear fuels and their by-products).
The most basic measurement is the total number of disintegrations occurring per unit time in the material:
1 becquerel (SI unit) = 1 disintegration/second
1 curie (traditional unit) = 37 billion disintegrations/second
The emissions are in all directions. At a detector the most basic measurement is Counts Per Minute (CPM): the total number of disintegrations (alpha particles, beta particles and gamma rays) arriving at the detector. Keep in mind that the detector does not typically completely surround the source, so CPM is only the emissions that reach the detector.
Emissions encountering an object accumulate in that object and add up to "exposure". Exposure units have a complicated history and taxonomy.
A roentgen (traditional unit) is "the amount of radiation required to liberate positive and negative charges of one electrostatic unit of charge (esu) in 1 cm³ of dry air at standard temperature and pressure (STP). This corresponds to the generation of approximately 2.08×109 ion pairs." Use of roentgen units is now discouraged.
A rad (traditional unit) is "the dose causing 0.01 joule of energy to be absorbed per kilogram of matter"
1 roentgen = 1 rad, approximately.
A gray (SI unit) is "the absorption of 1 joule of radiation energy per kilogram of absorbing material". 1 Gy is approximately equal to 115 roentgens.
The effect of the radiation exposure on the object depends on the composition of the radiation (the proportion of alpha, beta and gamma radiation) and the object's material. So subsequent units use a conversion factor ("quality factor", Q) that reflects the biological effects on humans. Q ranges from 1 for x-rays, gamma rays and beta particles to 10 for neutrons to 20 for alpha particles.
A rem ("roentgen equivalent in man", traditional unit) is a measure of "radiation dose equivalent" and is the product of the quality factor times rads (or roentgens).
rems = Q x rads
The SI unit of radiation dose equivalent is the sievert. 1 sievert (SI unit) = 100 rems.
But that's just a measurement of energy absorbed adjusted for biological effect on human tissue without regard to duration of exposure. One could absorb a given amount of energy in a minute or in a lifetime. The Wikipedia sievert article lists doses for single events like dental x-rays as well as dose/time examples such as average background radiation exposure for an American resident per year.
The sievert article also gives symptom benchmarks:
1 becquerel (SI unit) = 1 disintegration/second
1 curie (traditional unit) = 37 billion disintegrations/second
The emissions are in all directions. At a detector the most basic measurement is Counts Per Minute (CPM): the total number of disintegrations (alpha particles, beta particles and gamma rays) arriving at the detector. Keep in mind that the detector does not typically completely surround the source, so CPM is only the emissions that reach the detector.
Emissions encountering an object accumulate in that object and add up to "exposure". Exposure units have a complicated history and taxonomy.
A roentgen (traditional unit) is "the amount of radiation required to liberate positive and negative charges of one electrostatic unit of charge (esu) in 1 cm³ of dry air at standard temperature and pressure (STP). This corresponds to the generation of approximately 2.08×109 ion pairs." Use of roentgen units is now discouraged.
A rad (traditional unit) is "the dose causing 0.01 joule of energy to be absorbed per kilogram of matter"
1 roentgen = 1 rad, approximately.
A gray (SI unit) is "the absorption of 1 joule of radiation energy per kilogram of absorbing material". 1 Gy is approximately equal to 115 roentgens.
The effect of the radiation exposure on the object depends on the composition of the radiation (the proportion of alpha, beta and gamma radiation) and the object's material. So subsequent units use a conversion factor ("quality factor", Q) that reflects the biological effects on humans. Q ranges from 1 for x-rays, gamma rays and beta particles to 10 for neutrons to 20 for alpha particles.
A rem ("roentgen equivalent in man", traditional unit) is a measure of "radiation dose equivalent" and is the product of the quality factor times rads (or roentgens).
rems = Q x rads
The SI unit of radiation dose equivalent is the sievert. 1 sievert (SI unit) = 100 rems.
But that's just a measurement of energy absorbed adjusted for biological effect on human tissue without regard to duration of exposure. One could absorb a given amount of energy in a minute or in a lifetime. The Wikipedia sievert article lists doses for single events like dental x-rays as well as dose/time examples such as average background radiation exposure for an American resident per year.
In most countries the current maximum permissible dose to radiation workers is 20 mSv per year averaged over five years, with a maximum of 50 mSv in any one year. This is over and above background exposure, and excludes medical exposure.
The sievert article also gives symptom benchmarks:
Symptoms of acute radiation (dose received within one day):[20]
- 0 – 0.25 Sv (0 – 250 mSv): None
- 0.25 – 1 Sv (250 – 1000 mSv): Some people feel nausea and loss of appetite; bone marrow, lymph nodes, spleendamaged.
- 1 – 3 Sv (1000 – 3000 mSv): Mild to severe nausea, loss of appetite, infection; more severe bone marrow, lymph node, spleen damage; recovery probable, not assured.
- 3 – 6 Sv (3000 – 6000 mSv): Severe nausea, loss of appetite; hemorrhaging, infection, diarrhea, peeling of skin,sterility; death if untreated.
- 6 – 10 Sv (6000 – 10000 mSv): Above symptoms plus central nervous system impairment; death expected.
- Above 10 Sv (10000 mSv): Incapacitation and death.
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