page 3-2
Ionising radiations

Types of ionising radiation
Alpha, beta and gamma
Radioactive atoms give out ionising radiations. However, it turns out that there are three distinct types of radiation, each with very different properties. We call these radiations alpha (a), beta (b) and gamma (g). The reason that they have very different properties is because the radiation is made up of something different each time.
What are the radiations made of?
Alpha and beta radiations are streams of particles, whereas gamma radiation is part of the electromagnetic spectrum. Let’s look at them in more detail.
Interactive graphic of alpha radiation
Picture 3. Alpha particles.
Ionising alpha
Alpha particles are the same as the nuclei of helium. This means that they have a (relatively) large mass. The atomic number of helium is 2 so an alpha particle carries a double positive charge. Alpha particles don’t travel very fast – because they have such a large mass. This means that they tend to collide with plenty of other atoms. Therefore, they cause a lot of ionisation by pulling electrons off the atoms. All these collisions mean that they lose energy quickly, so they have a short range in air and they are easily stopped by anything solid – even a piece of paper will stop alpha radiation.
Interactive graphic of beta radiation
Picture 4. A stream of beta particles
Fast beta
Beta radiation is a stream of fast moving electrons. These particles have very little mass (about 7000 times lighter than an alpha particle) and travel close to the speed of light (300,000 km/s). They tend to pass through the air and solid matter without many collisions with other atoms. So beta radiation is only weakly ionising. However, it means that it has a long range in air and will pass through paper, and thin sheets of aluminium and steel. However, it is stopped by lead or thick pieces of other metals.
Interactive graphic of gamma radiation
Picture 5. Gamma radiation.
Penetrating gamma
Gamma radiation is at the high frequency end of the electromagnetic spectrum. It has a very short wavelength (much less than the radius of an atom) and will pass through atoms with very little chance of being deflected or absorbed. It has an extremely long range in air and will get through thin samples of most materials without any noticeable decrease in intensity. However, its intensity is reduced by lead or very thick pieces of other metals. The thicker the sample, the greater the reduction in intensity.

Question 3-2.

a) Look at the table below. There are three blanks. Fill in each blank with the correct word (make sure you spell them correctly).

Radiation What it is Charge
alpha positive
beta particle
wave none
b) Look at the sentences below. Fill in the blank with the correct word (make sure you spell them correctly).
The most penetrating radiation is radiation.
The most ionising radiation is radiation.
Summary                   Close
  • There are three main types of ionising radiation:
    alpha, beta and gamma
  • Alpha and beta are particles
  • Alpha particles are positive helium nuclei
  • Beta particles are fast moving electrons
  • Gamma radiation is made of high frequency electromagnetic waves
  • Gamma radiation is the most penetrating