Geoscience Reference
In-Depth Information
15.9.6.1 Gamma Detection Instrumentation
The types of equipment used to evaluate gamma radiation in the workplace include:
• Ion chambers
• Gas proportional devices
• Geiger-Mueller counter
15.9.6.2 Shielding for Gamma and X-Rays
Shielding gamma and x-radiation depends on energy level. Protection follows an exponential func-
tion of shield thickness. At low energies, absorption can be achieved with millimeters of lead. At
high energies, shielding can attenuate gamma radiation.
15.9.7 r
adioaCtive
d
eCay
e
quations
Radioactive materials emit alpha particles, beta particles, and photon energy, and lose a proportion
of their radioactivity with a characteristic half-life. This is known as radioactive decay. To calculate
the amount of radioactivity remaining after a given period of time, use the following basic formulae
for decay calculations:
−
λ
Later activity Earlier activity)
=
×
e
×
(Elap
sedtime)
(15.25)
−λ
AAe
=× ×
t
where
λ = Decay constant (probability of an atom decaying in a unit time) = ln 2/
T
.
A
= New or later radioactivity level
A
i
= Initial radioactivity level
t
= Time.
ln 2 = 0.693.
T
= Radioactive half-life (time period in which half of a radioactive isotope decays)
Use the following to determine the time required for a radioactive material to decay (
A
o
to
A
):
t
= (-ln
A
/
A
i
)(
T
/ln 2)
(15.26)
where
t
= Time.
A
= New or later radioactivity level.
A
i
= Initial radioactivity level.
T
= Radioactive half-life (time period in which half of a radioactive isotope decays).
ln 2 = 0.693.
Basic rule of thumb:
In 7 half-lives, radioactivity is reduced to <1%; in 10 half-lives, to <0.1%.
To determine the rate of radioactive decay, keep in mind that radioactive disintegration is directly
proportional to the number of nuclei present. Thus, the radioactive decay rate is expressed in nuclei
disintegrated per unit time.
A
i
= (0.693/
T
)(
N
i
)
(15.27)
where
A
i
= Initial rate of decay.
N
i
= Initial number of radionuclei.
T
= Half life.
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