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)