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Detection of radioactivity

Radioactivity is the emission of sub-atomic particles from the nuclei of atoms.

An atom is very small, usually about 1/10,000,000,000 m in diameter.  It is made up of an even smaller nucleus at the center, with electrons going around the nucleus.  The nucleus is about 100,000 times smaller than the atom itself.  Electrons moving around the nucleus and define the size of the atom.  Sub-atomic particles are particles that are much smaller than an atom.  Examples are electrons, protons, neutrons and the nucleus itself.  Radioactivity is about such particles coming out from nuclei (plural of nucleus).

Why would such particles come out from the nucleus?  The short answer is that the nucleus can have a lower potential energy when it emits the particles.  Atoms of elements that can do this are said to be radioactive.  Actually, most of the familiar elements like oxygen, carbon, hydrogen and so on are not radioactive.   Examples of radioactive elements are uranium and plutonium.

Detection of alpha-particles, beta-particles and gamma-rays

Three types of radiation are usually emitted by radioactive nuclei - alpha, beta and gamma.  An alpha particle consists of 2 protons and 2 neutrons.  It is identical to the nucleus of a helium atom.  A beta particle is just an electron.  Gamma ray is not made up of particles with mass.  It is like X ray, but much more penetrating.  

The three types of radiation are invisible to the eye.  So how can we detect them?

There are two common ways.  

One way is to use a photographic film wrapped in a thin plastic so that light cannot go in.  What does this mean?  To understand this, we must know how a photographic film works.  The older type of camera (not the digital type) uses photographic film.  It is important that the film is not exposed to light.  That means light must not fall not it at first.  When we buy it from a shop, it is usually inside a small plastic tube.  After putting it inside the camera, the camera pulls out the film automatically.  The camera will allow light to fall on it for a very short time when we take a picture.  When this happens, the light causes a chemical reaction to take place on parts of the film where light has fallen.  These parts are said to be exposed.  After that, the film must be carefully kept in the dark.  We must not just open the camera and allow light to fall on the film.  When we finish using a roll of film, the camera will automatically rewind the film back into the plastic tube.  Then it is safe to take out the tube and bring it to a photo shop.  The photo shop people will put this film into a dark room or a machine and treat the film with chemicals.  This step is called developing.  This will both give colour to the exposed part (where light falls when the camera takes the picture) and change the chemicals on the film so that they will no longer be affected by light.  Depending on the type of film, the parts exposed to light will usually get darker after developing. 

This is how a photographic film in thin plastic can be used to detect radiation.  The idea is that although light cannot go through the plastic, radiation can.  If radiation goes through and falls on the film, the effect is the same as light falling on it - the film is exposed.  Then when the film is developed, it will become darker, as if light has fallen on it.  But since we know that light could not through the plastic, it must be radiation.  On the other hand, if the film does not get darker after developing, then it means there is no radiation around.  In this way, the photographic film in thin plastic can be used to detect radiation. 

Another way is to use a Geiger-Muller counter.  This is a device that makes a clicking sound when radiation falls on it.  The more radiation, the more it clicks.  

The photographic film in thin plastic and the Geiger-Muller counter both respond the same way to alpha, beta and gamma radiation.  How can we tell which radiation it is?

The way is to make use of the following properties:

- alpha radiation cannot go through paper

- beta radiation can go through paper but cannot go through thin aluminium sheet

 - gamma radiation can go through paper and thin aluminium sheet but cannot go through a few centimetres of lead

Suppose that we have a piece of radioactive substance and a Geiger-Muller counter.  When the counter is near the substance it makes a clicking sound.  If we want to know which type of radiation the counter is detecting, one way is to get ready a piece of paper and a thin aluminium sheet.  Put these between counter and radioactive substance one at a time.  Start with paper.  If clicking stops, then it is alpha radiation.  If not, try the aluminium sheet.  If it stops clicking, it is beta radiation.  If not, then it must be gamma radiation.

Random emission of radioactivity in direction and time

Suppose you have a small piece of radioactive substance.  It probably looks like a small lump of metal or plastic, nothing special.  If you bring a Geiger-Muller counter near it, the counter starts clicking.  Each click means that a particle of radiation is detected once.  This particle comes from the nucleus of one of the atoms in the substance.  If you listen carefully, you may be able to convince yourself that the clicking is completely random.  There is no repeating pattern to the sound at all.  This is simply because the radiation is emitted randomly.  After the nucleus of one atom has emitted a particle, we will never know when the nucleus of the another atom will emit the next particle. 

If we move the counter around the substance, keeping it at roughly the same distance, we would find no obvious change in the rate of clicking.  This means that the directions of the particles are also completely random.