Health, Safety and Environment
www.hse.ubc.ca
Radioactive Decay
The rate of radioactive decay is
referred to as the
activity:
measure of the number of
radionuclide decays per unit of time
UNITS OF ACTIVITY
Becquerel (Bq) = 1 disintegration per second (dps)
1 Bq
= 27 picoCuries
1 kiloBq (kBq)
= 27 nanoCuries
1 MegaBq (MBq) = 27 microCuries (µCi)
1 GigaBq (GBq) = 27 milliCuries (mCi)
1 TeraBq (TBq) = 27 Curies
*The rate of decay of a radionuclide is
exponential with time.
At time T, activity A is given by:
A = Aoe
–λT
A =initial activity
o
λ = decay constant
Activity of 1 mg of U-238 is 10 Bq
Activity of 1 mg of Am-241 is 1 x 108 Bq
?????
The difference is related to a unique property
of all radionuclides called the half-life.
Half-Life
*The half-life of a radionuclide is the
time required for it to lose 50% of its
activity by radioactive decay.
*Each radionuclide has its own unique
half-life regardless of the quantity or
form – solid,gas,liquid.
* The half-life is an unalterable property
of the radioisotope.
* Half-lives of radioisotopes vary:
Uranium-238 4.5 x 109 years
Americium-241 4.32 x102 years
Polonium-212 0.3 microseconds
•After 7 half-lives the activity decreases to
about 1% of its original value.
•After 10 half-lives the activity reduces
to one thousandth of its original value.
Mathematically the concept of half-life
Can be defined by :
A
=
Ao
____
n= #half-lives
2n
The decay constant and half life are related :
t1/2 = ln(2)
λ
= 0.693
λ
t1/2 = 0.693
λ
A = Aoe
–λT
Combine to form:
A=
/
-0.693
t
T1/2
Aoe
A researcher has 500 MBq of
use it for 35 days.
32P
but cannot
What is the activity when she wants to use
the material?
For
32P
T ½ = 14.3 days
A = Ao
2n
After 1 half-life
(n=1)
A = 500
2
= 250 MBq
After 2 half-lives
(n=2)
A = 500
4
= 125 MBq
After 3 half-lives
(n=3)
A = 500
8
= 62.5 MBq
A=
/
-0.693
t
T1/2
Aoe
A0  500 MBq , t  35 d, T1/2  14.3 days
A  500MBq x e
-.693 x 35/14.3
 500MBq x e
A = 91.7 MBq
-1.696
*History
*Radiation : types
alpha, beta, positron, gamma, x-rays
neutron activation
*Energy levels: ionizing radiation.
*Radioactive decay
For EXAM!
Learn characteristics of two isotopes
•Type of radiation : alpha, beta, gamma
•Energy of radiation: eV
•Shielding required: material and thickness
•Half-life
(Tables 1, 4 of manual)