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Measuring
radiation doses
Despite the
difficulty of assesing damage done by radiation,
measurements of radiation doses are useful and necessary
to quantify radiation dangers. The definitions and units
of doses are unfortunately complicated.
| unit |
Equivalent
unit |
Description |
| R (Roentgen) |
1.0 R
= 2.58 E-4 C/kg |
This is a
measue of ionization charge per mass of the
sample being radiated. If a sample of mass m =
1.0 kg absorbs radiation which ionizes
2.58 E-4 C, the dose is one Roentgen.
The same ionization in a smaller sample,
corresponds to a larger exposure. |
| Gy
(Grey) |
1.0 Gy
= 1.0 J/kg |
This is a
measure of energy deposited in a sample. If a 1.0
kg sample absorbs 1.0 J of
energy, the sample had a dose of 1.0 Gy. |
| rad |
1.0 rad
= 0.01 Gy |
This is a
scaled down version of a Grey. Only 0.01 J
of energy in a 1.0 kg sample is
necessary for a dose of 1.0 rads. |
| rem |
Dose in rems
= RBE times Dose in rads |
This unit
accounts for the relative harmfulness of
different kinds of radiation. The RBE factor can
be found in tables and depends on the sort of
radiation and the energy of the individual
quanta. For instance, the RBE of alpha particle
ranges from 10-20 depending on the energy of the
alphas. |
Measurements of ionization per unit
mass (e.g. Roentgens) are called exposures
while measurements of energy per mass are called doses
(e.g. Greys, rads and rems).The
definitions are all "per mass", thus they are
different from one's usual idea of a dose which is an
amount not an amount-per-mass. The RBE factors of various
doses, which are used to calculate biological
equivalent doses in rems are shown in
the table below.
| Type of
Radiation |
RBE |
| 200-keV
X-rays |
1 |
| b particles (electrons) |
1 |
| protons |
10 |
| a particles |
10-20 |
| neutrons
(slow) |
2 |
| neutrons
(fast) |
10 |
Typically, a person
absorbs 0.3-0.4 rems of radiation yearly. A
sudden dose of a few hundred rems is surely
fatal.
Examples Radioactivity's
index
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