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Understanding Becquerels, Sieverts, Roentgens, Etc
By Ace Hoffman
What are Curies, Becquerels, Rems, Rads, Grays, Sieverts, Roentgens, Q, RBE etc.?
Here are some answers (quotes are taken from my book, The Code Killers (URL for free download: www.acehoffman.org ).
Let's start with a Curie: "An amount of radioactivity defined as 3.7 *10^18 decays per second... about equal to the radioactivity of one gram of pure radium.  Replaced by the Becquerel (Bq)."
Becquerel: "Exactly one radioactive decay per second.  Abbreviated Bq."
So those are just different measurements for the same thing:  Radioactive decays per unit of time, regardless of strength or type of radioactive emission.
A Curie is a lot of radiation.  A single Becquerel... not so much.
One Bq is equal to 27 picocuries, which makes sense because a picocurie (a millionth of a millionth of a Curie) is 0.037 disintegrations per second, and mathematically 0.037 times 27 equals (approximately) one.  Radioactive disintegrations, of course, don't actually happen in fractional amounts.  They either happen or they don't.  WHEN they are likely to happen can be guessed at by the isotope's half-life, but it's only a guess.
But knowing the disintegrations per second doesn't tell you very much, really. To guess at the damage a given amount of radiation causes, you still need to know the average energy of the disintegrations.  And of course, you need to know the type of emission: alpha, beta, gamma, x-ray, etc.. Each type has different properties, and each isotope's type(s) of emissions have average energy levels.  Some occur together -- a gamma ray and an alpha emission.  Some follow in short sequence:  A beta emission followed by a gamma ray shortly thereafter.
Sometimes the decay product is also radioactive.  This can go on for dozens of steps.
Gamma rays are very penetrating but have no mass and no charge.  They are pure energy,  traveling at the speed of light.
X-rays are less penetrating than gamma rays, having less energy, but are still damaging or "ionizing".
Alpha particles (also sometimes called alpha rays) are relatively massive (the size of helium atoms minus their two electrons) and don't travel very far before they've collided with so many things that they've slowed down, and become a helium atom out of place, grabbing two electrons and floating away.  It's said that a single alpha decay has enough energy to visibly reposition a grain of sand on the beach.
Alpha particles travel at "only" about 98% if the speed of light when they are first emitted during a radioactive decay.  Compared to beta particles, gamma rays and x-rays, that's slow!
Alpha particles are not much of an external radiation hazard because they can be blocked by a sheet of newspaper or dead layers of your skin (mucus membranes, eyes, and a few other exposed areas can be damaged by external alpha radiation).
But alpha particles released inside your body can do a lot of damage to molecules they collide with, and they have a double positive charge, which is also very damaging as they pass by many thousands of molecules before they slow down and capture two electrons.
Beta particles (also known as beta rays) are negatively charged particles which are ejected from the nucleus of an atom at 99.7% the speed of light or even faster.  Beta particles are tiny: They are only as big as electrons, which is what they are once they slow down.  Beta particles do most of their damage as their negative charge passes by other charged things -- protons and electrons.
When beta particles are traveling very quickly, their charge is not near any particular thing long enough to have any significant effect.  Most of the damage occurs when they've slowed down most of the way.  For this reason, the health effects for the exact same TOTAL energy "dump" per kilogram of body tissue for beta particles with low energy emission values, such as tritium, are HIGHER than for isotopes of elements with higher beta energy emission values.
But knowing the decays per second and the type of emissions, and their average energy levels, is still only a small part of understanding the potential damage from any particular radioactive release such as Fukushima Daiichi.
You also need to know the isotopic composition of the sample.  Otherwise, you won't be able to estimate what the Bqs or Curies will be in a minute, or a day, or a year, or a thousand years.  You need to know the half-lives of the isotopes that have been released, and the ratios of each isotope and each element.
A sample of plutonium-239 giving off one curie of radiation per hour (wow! that's a lot!) will give off about 99.999...% as much radiation tomorrow, or next year.  But a sample of Iodine-131 giving off the same amount of radiation today, will give off half as much radiation in just eight days, and half as much as that -- a quarter curie per hour-- eight days after that.  In a few months it will be gone completely.
But even knowing all THAT isn't nearly enough.
The next step is to estimate the absorbed dose.  One measure of this is the Radiation Absorbed Dose or RAD. Grays are another way to measure absorbed dose.
But, absorbed dose still doesn't provide an estimate of the damage the radiation may do.  For that, there is effective dose, which is measured in REM ("roentgen equivalent man") or sieverts.  Background radiation varies greatly by location and other factors, but is usually given as almost a third of a REM per year, expressed as "320 millirem" for instance.  How much that will go up because of Fukushima Daiichi is hard to estimate, but will surely be the subject of a future newsletter and much debate.
One additional, traditional, measurement of radiation is the roentgen (pronounced rent-gen (like rent again without the "a")) which is defined as 0.876 RADs "in air".
All of these yardsticks are blunderbuss attempts to estimate the potential damage from radiation as a function of energy dumped into the body.  One rad equals an absorbed dose of 0.01 joules of energy per kilogram of body tissue.  For ongoing radiation assaults, a time factor needs to be included: "1000 milli-sieverts per hour" or something like that.  They might call that "one sievert per hour" too.  Same thing.  (About 6 sieverts or 6 grays, or about 600 rem or 600 rads, is considered a fatal dose, the slow and painful death coming within a few weeks of exposure.  400 to 450 rem received over a short time will kill about half the population that receives it within about 30 days.)
What is really happening when radiation damages the body, in large or small doses, is a very complex microscopic assault on living tissue.  Certain elements concentrate in certain organs: Iodine in the thyroid, strontium in bones, astatine in the brain, etc..  If the percentage of radioactive strontium isotopes goes up compared to non-radioactive strontium isotopes (as it is in Japan today), the radioactive strontium will concentrate in bones and teeth.  And, sometime in the future, the incidence of bone cancer and leukemia will increase.
So simply averaging the assault across "whole bodies" can miss things and is improper.  Another adjustment factor is needed.
That's expressed by assigning each isotope of each element a Q (Quality factor) or RBE (relative biological effectiveness value), or the more modern "radiation weighting factor" (which works better with computers).
Analysts use these numbers to try to compare apples to oranges, or, more specifically, for example, tritium exposure in drinking water to an xray of your knee after you blow it out on the tennis court.
None of these values consider the effects of bioaccumulation:  Radioactive isotopes build up in the edible portions of one living thing (strontium concentrates in beans, for instance) and are then eaten by another (beans concentrate in Mexicans, for instance) up the food chain to us, at the "top".  When that happens, a dose that had been dispersed into the environment becomes concentrated again.
It's all a very inexact science, and that inexactitude is used by the nuclear industry to hide what is really nothing short of premeditated murder.
Ace Hoffman
Carlsbad, CA
The author has written extensively about nuclear power and is the author of several computer tutorials as well.  His book, The Code Killers, is available online at his web site:  AceHoffman.org
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