LOJ Feb 2004

Radioactivity 2

Ionizing Power and Penetrating Power

LOJ Feb 2004

Recap...

• Alpha radiation consists of helium nuclei, particles are made up of two protons and two neutrons.

• Beta radiation consists of high-energy electrons emitted from the nuclei of atoms. (For each electron emitted, a neutron in the nucleus becomes a proton.)

• Gamma radiation is very short wavelength electromagnetic radiation.

LOJ Feb 2004

Ionizing Power and Penetrating Power

• These are linked. • The penetrating power of nuclear

radiation depends upon the ionizing power of the radiation.

• The radiation continues to penetrate matter until it has lost all of its energy.

LOJ Feb 2004

Ionizing Power and Penetrating Power

• The further it can penetrate into the substance the more spread out the ionization it causes will be,

• So….. the more localized the ionization the less penetrating power it will possess.

LOJ Feb 2004

Alpha Particles

• are the least penetrating as they are

• the most densely ionizing. • They are completely absorbed

by 10 cm of air, 0.01 mm lead or a sheet of paper.

LOJ Feb 2004

Alpha Particles

• This means that if a given number of alphas are fired at a target they will all cause ionization near the surface of the material, resulting in the effects of the radiation being concentrated in a small volume.

• The double charge and considerable mass of the alpha in comparison with the other nuclear radiation forms explains why the impact on matter is so great.

LOJ Feb 2004

Beta Particles

• A beta particle can penetrate quite deeply into matter before its energy has been used up.

• Its penetrating power is therefore moderate (absorbed by 1m air, 0.1 mm lead or 3mm aluminium sheet).

LOJ Feb 2004

Beta Particles

• Betas have only about 1/8000 of the mass of an alpha particle and only half of the charge.

• Therefore their interaction with matter as they passes through is far less severe.

• So the effects of interaction (ionization) are much more spread out.

LOJ Feb 2004

Gamma Rays

• have an ionizing power so low that they penetrate very deeply into matter before most of the energy has been used up.

• Their penetrating power is therefore very high (about 99.9% is absorbed by 1 km of air or 10 cm lead).

LOJ Feb 2004

Gamma Rays

• Gamma rays are pure energy - no charge and no mass - therefore their interaction with matter is much less than the other two forms of nuclear radiation.

LOJ Feb 2004

Absorption of radiation

alpha () radiation - is easily absorbed by a few centimetres of air or a thin sheet of paper;

beta ( radiation - easily passes through air or paper but is mostly absorbed by a few millimetres of metal;

gamma () radiation - is very penetrating and requires many centimetres of lead or metres of concrete to absorb most of it.

LOJ Feb 2004

Penetration Power