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Health, Safety and Environmentwww.hse.ubc.ca
The rate of radioactive decay is referred to as the
activity:
measure of the number of radionuclide decays per unit of time
Radioactive Decay
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
Ao =initial activity
λ = decay constant
The difference is related to a unique property of all radionuclides called the half-life.
Activity of 1 mg of U-238 is 10 BqActivity of 1 mg of Am-241 is 1 x 108 Bq
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*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-Life
•After 7 half-lives the activity decreases to about 1% of its original value.
* Half-lives of radioisotopes vary: Uranium-238 4.5 x 109 years Americium-241 4.32 x102 years Polonium-212 0.3 microseconds
•After 10 half-lives the activity reduces
to one thousandth of its original value.
Mathematically the concept of half-lifeCan be defined by :
A = Ao ____
2nThe decay constant and half life are related :
t1/2 = ln(2) = 0.693 λ λ
n= #half-lives
A = Aoe-0.693 t /T1/2
A = Aoe –λT
Combine to form:
t1/2 = 0.693 λ
A researcher has 500 MBq of 32P but cannot use it for 35 days.
What is the activity when she wants to use the material?
A = Ao
2n
After 2 half-lives(n=2)
A = 500 4
= 125 MBq
After 3 half-lives(n=3)
A = 500 8
= 62.5 MBq
For 32P T ½ = 14.3 days
After 1 half-life(n=1)
A = 500 2
= 250 MBq
1.696-35/14.3 x -.693
1/20
e x 500MBqe x 500MBq
days 14.3T d, 35 , MBq 500
A
tA
A = 91.7 MBq
A = Aoe-0.693 t /T1/2
*History
*Radiation : typesalpha, 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)