Re: Radiation Units

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

Hi Tina:
Let my try to make sense out of it. Historical approach is
probably the best in this case. I will not consult books for
numbers or conversion factors.

1) It started in 1920's when it was discovered that, on the
average, English radiologists died earlier than other doctors.
The unit of radiation exposure of dry air, called roentgen (r),
was introduced. It was based on how many ion pairs is
produced by X rays in one cubic centimeter of dry air at
STP. Ionization chambers were used to measure roentgens.

2) Then came two questions "why ionization?", "why
air?", "what kind of radiation?" etc. Some argued that
ionization is just one of several effects of X rays and
that it would be better to define the dose in terms of
"energy lost" rather that in terms of part of that energy
(producing ionization). The unit of dose, rad, was defined
in terms of ergs (units of energy used by physicists at
that time) per gram of dry air. It turned out that numerically
one rad is not very different from one roentgen (1R~1r).

3) In principle one can use a calorimeter to measure
rads in different materials. But this is not easy when
doses are small. For that reason rads were usually
calculated theoretically while in practice roentgens
were measured with ionization chambers. A dose for
a cancer patient, for example, could be 60 R per
treatment. An ionization chamber (often inserted
into a bag of rice to simulate human tissue) could be
used to determine how long a patient would have to
be irradiated under a specific x-ray machine.

4) Experiments on rats, and other mammals, established
that a single dose of about 500 R would result in (more
or less rapid) death of 50%. Most mammals would die
after a dose of 1000 R. But something on the order
of 100,000 R is needed to preserve food (killing
bacteria). The "permissible yearly dose," for radiologists
and other professional people, is now 5 R per year. It
used to be much larger. The dose most of us receive
from natural sources (cosmic rays, etc.) is typically
about 200 mR. For that reason the "legally permissible
yearly dose" for general population (from man-made
sources) is now close to 200 mR.

4) Are biological effects always proportional to the
number of rads? The answer is no. A dose of one rad
from x rays and the dose of one rad from penetrative
electrons, for example, were found to produce slightly
different effects. For that reason a new unit called rem
was introduced. By definition one rem is the same as
one rad multiplied by a number which is between 1 and
10, depending on the kind of radiation (10 is neutrons).
For x rays and gamma rays rads and rem are identical.
Legal doses, such as those above, are often expressed
in terms of rems rather than in terms of rads.

5) Then came the SI and a decision was made to define
the dose in terms of jouls per kilogram instead of ergs
per gram. That is the origin of the new dose unit Gy (gray).
It is not hard to show that 1 Gy=100 R. Likewise the
rem was replaced by sievert (Sv). By definition one
Sv is the product of Gy and of the so-called "effect
number (1 to 10, as before)" chosen by biologists.

6) The radioactivity is expressed in terms of number of
disintegration per second; this number is now called
one becquerel (Bq). The old unit of activity, curie (Ci),
is still used, it is defined as 3.7*10^10 decays per
second. The activity of one gram of radium is close to
1 Ci. I hope this will be useful. It is always important
to ask "how much" when debating dangers of radiations.
Ludwik Kowalski

Tina Fanetti wrote:

> Hi
> Okay so I have this information about radiation doses.  I've got unit=
> s running all over the place.
>
> There are rem, mrem (i assume are millirem), Greys, Sv, LD. =20
>
> Can someone tell me how these units connect or are related???
>
> Its not making any sense what so ever.  Why can't they all be in the =
> same units.