- Some elements which radiate energy of itself without any excitation from outside is called radioactive elements , e.g. Uranium, Radium, Thorium and Polonium.

- This phenomenon of matter radiating energy of itself is called the natural radioactivity .

Experiments show that radioactive elements emit three types of radiation.

- Alpha (α) particle

- Beta (β) particle

- Gamma (γ) ray

Characteristics of the three types of radiation

three types of radiation

Radioactive decay

A nucleus, which has too many or too few neutrons gains extra energy, and becomes unstable. It tends to emit radiation such as α-particles, β-particles and γ-rays until a stable atom is reached. This emission ofα-particles or β-particles is called Radioactive decay .

Activity and Half-life

Half-life of a sample of radioactive element is defined as the time taken for half of the unstable nuclei to decay.


For example, the half-life of radium is 1600years. If there are 40g of radium initially, half of radium(20g) is decayed in first 1600 years. Next 1600 years, half of 20g radium (10g) is decayed. After next 1600years, half of 10g radium (5g) is decayed.


Activity and Half-life Example 1


There is 1kg of Strontium (Sr) that has the half-life of 30years.

(a) How many grams of Strontium are remained after 60 years?

(b) If 125g of strontium are remained, how long does it take?


Activity and Half-life example 2

(a) 125g of strontium are remained 60years later.

(b) 90years later.

Dangers of radiation

Radiation (α-particle, β-particle, γ-ray) can cause the following to human being.

- Damage to living cells

- Genetic changes in living cells.

- Cancer

Safety precautions

- Avoid unneccessary exposure.

- Avoid direct contact with the source.

- Never point the source to any people.

- Radioactive sources must be stored in places out of reach of the public.

- Sources must be stored inside lead box to aboid leakage of radiations.

- Thick lead shields should be installed.

- Radiation symbols must be displayed at all places where radioacitive sources are used.

- Radiation workers have to wear special protective clothing and gloves.

- Use film badges to measure the amount of radiation the radiation workers receive during a certain period.