Posted on April 22, 2011 8:57 am

Becquerels, Sieverts, Roentgens

Understanding Becquerels, Sieverts, Roentgens,

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: ).

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).”

“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

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

Sometimes the
decay product is also radioactive.  This can go on for dozens of

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

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

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

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


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: