Types of Radioactivity
 
 

 

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Radioactivity: what is it?

All substance are made of atoms.
These have electrons (e) around the outside,
and a nucleus in the middle.

The nucleus consists of
protons
(p) and neutrons (n),
and is extremely small.
(Atoms are almost entirely made of empty space!)

In some types of atom, the nucleus is unstable, and will decay into a more stable atom. This radioactive decay is completely spontaneous.

It's not the same as what happens in a nuclear power station (where neutrons whizz around and hit uranium nuclei, causing them to split).

The nucleus is in the middle of the atom
This form of Lithium is NOT radioactive - it's just an example of a simple atom.
Most radioactive substances have many more particles in their nucleus.

You can heat the substance up, subject it to high pressure or strong magnetic fields - in fact, do pretty much whatever you like to it - and you won't affect the rate of decay in the slightest.

That's what we've believed for some time - but recent research makes us wonder if this is true after all. There is some evidence of slight seasonal variations in the rates of radioactive decay of some isotopes, which may be due to temperature differences. Or may be nothing at all. Or may be due to something else we don't know about yet. For now, for GCSE exams, let's go along with the idea that the rate of radioactive decay does not depend on external factors.

When an unstable nucleus decays, there are three ways that it can do so.
It may give out:-

  • an alpha particle (we use the symbol )
  • a beta particle (symbol )
  • a gamma ray (symbol )

Many radioactive substances emit particles and particles as well as rays.

In fact, you won't find a pure source: anything that gives off rays will also give off and/or too.

 

That's the basic information. Click here for more details:


Alpha particles

Alpha particles are made of 2 protons and 2 neutrons.

This means that they have a charge of +2, and a mass of 4
(the mass is measured in "atomic mass units", where each proton & neutron=1)
We can write them as , or, because they're the same as a helium nucleus, .

Alpha particles are relatively slow and heavy.

They have a low penetrating power - you can stop them with just a sheet of paper.

Because they have a large charge, alpha particles ionise other atoms strongly.



More about particles:
More Alpha

Beta particles

Beta particles have a charge of minus 1, and a mass of about 1/2000th of a proton. This means that beta particles are the same as an electron.
We can write them as or, because they're the same as an electron, .

They are fast, and light.

Beta particles have a medium penetrating power - they are stopped by a sheet of aluminium or plastics such as perspex.

Beta particles ionise atoms that they pass, but not as strongly as alpha particles do.



More about particles

Gamma rays

Gamma rays are waves, not particles.
This means that they have no mass and no charge. So we sometimes write .

Gamma rays have a high penetrating power - it takes a thick sheet of metal such as lead, or concrete to reduce them significantly.

Gamma rays do not directly ionise other atoms, although they may cause atoms to emit other particles which will then cause ionisation.

We don't find pure gamma sources - gamma rays are emitted alongside alpha or beta particles. Strictly speaking, gamma emission isn't 'radioactive decay' because it doesn't change the state of the nucleus, it just carries away some energy.



More about rays:
 

Summary
  • Alpha particles are easy to stop, gamma rays are hard to stop.

   Penetrating power

  • Particles that ionise other atoms strongly have a low penetrating power, because they lose energy each time they ionise an atom.

  • Radioactive decay is not affected by external conditions.

  • You need to know the information in this table:-
Type of Radiation Alpha particle Beta particle Gamma ray
Symbol or or or
(can look different,
depends on the font)
Mass (atomic mass units) 4 1/2000 0
Charge +2 -1 0
Speed slow fast very fast (speed of light)
Ionising ability high medium 0
Penetrating power low medium high
Stopped by: paper aluminium lead



We talk about "radioactive isotopes" - but what's an isotope?

For a start, just because something is called an isotope doesn't necessarily mean it's radioactive.
You can think of different isotopes of an atom being different "versions" of that atom.

Consider a carbon atom.
It has 6 protons and 6 neutrons - we call it "carbon-12" because it has an atomic mass of 12 (6 plus 6).
If we add a neutron, it's still a carbon atom, but it's a different isotope of carbon.
One useful isotope of carbon is "carbon-14", which has 6 protons and 8 neutrons. This is the atom we look for when we're carbon dating an object.

So isotopes of an atom have the same number of protons, but a different number of neutrons.

Video clip: Radioactive isotopes            
Showing the types of radioactivity and their penetrating power

Let's see how much you've learned:


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