Three types of emission

Nature of radioactive emissions, ionising effects and penetrating powers.

There are three types of radioactive emissions - alpha radiation, beta radiation and gamma radiation.

These radiations not only travel through air. They also go through solid. How far a radiation can go through a substance is related to its penetrating power. The further a substance can travel through a substance, the greater is its penetrating power. Alpha radiation is the least penetrating and gamma radiation is the most penetrating.

In air, alpha radiation can travel a few centimetres. Beta radiation can travel a few tens of centimetres. Gamma radiation can travel many metres.

In solid, alpha radiation can be stopped by paper. Beta radiation goes through paper, but is stopped by thin aluminium sheet. Gamma radiation can go through many metres of lead. For the same thickness, we would expect it to be a lot more difficult to go through lead than to go through aluminium, because lead is a lot denser than aluminium.

The penetrating power of a radiation is related to its ionising effect. Ionising effect is the ability of a radiation to create ions. When beta radiation travels through air, it knocks out electrons from air molecules. The air molecule is said to be ionised. In doing so, the beta radiation loses energy. After travelling for some distance, it creates more and more ions in the air. Eventually, it loses all its energy. The distance over which this happens is the distance that the radiation penetrates into the air. If a radiation produces more ions in the air over the same distance, it is said to have a larger ionising effect. Then it would lose energy more quickly and stop after a shorter distance.

The larger the ionising effect of a radiation, the smaller the penetrating power. The converse is also true. Since alpha radiation has the smallest penetrating power, it must have lost the most energy over a short distance. So it has the largest ionising effect. Since gamma radiation has the largest penetrating power, it must have the smallest ionising effect.

Deflection of radioactive emissions in electric fields and magnetic fields

What are alpha, beta and gamma radiations, really?

Actually, alpha radiation is made up of particles with mass. Each particle in alpha radiation is in fact the same as the nucleus of a Helium atom, which is made up of 2 protons and 2 neutrons. So it has a positive charge. An alpha particle has an energy of 3 - 5 MeV, and travels at about 5% the speed of light.

Beta radiation is also made up of particles that have mass. The particles are actually electrons. So each particle has a negative charge. Beta particles have a wide range of energy, from a few keV to a few tens of MeV. Those at the higher end of the range travels very close to the speed of light.

Gamma radiation on the other hand, are made up of particles that are massless (has no mass). It is like light or X ray. We can think of it as electromagnetic radiation with higher frequency than X ray. We can also think of it as being made up of photons, which are particles with no mass but that travel at the speed of light. Gamma radiation has no electrical charge.

The different electrical charges make the three radiations behave differently in electric and magnetic fields. Remember that alpha particles are positive, beta particles are negative and gamma radiation is neutral.

This means that when they go through an electric field, alpha particles will experience a force in the direction of the field and bend in that direction, and beta particles will experience a force in the opposite direction and bend in the opposite direction. Gamma radiation will go straight. Being neutral, gamma radiation is not affected by the field.

When they travel through magnetic field, alpha particles will experience a force with a direction given by Fleming's left hand rule. They will bend in the direction of this force. Beta particles being negative will experience a force in the opposite direction and bend opposite to the alpha particles. Gamma radiation will again go straight since it is neutral and not does experience any force.

Meaning of radioactive decay

How do these radiations come out from the nuclei of atoms?

Lets look at some examples. A uranium-238 atom can emit both alpha and gamma radiations from its nucleus. The nucleus of uranium-238 is made up of 238 neutrons and protons. Of these, 92 are protons. This information is written like this: ²³⁸₉₂U. There are 238 - 92 = 146 neutrons. When there are more neutrons than protons, a nucleus tends to be unstable. This means that it could break up into small fragments. Uranium-238 does this be ejecting an alpha particle and a photon of gamma radiation:

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He + ⁰₀γ

Since an alpha particle is really the same as the nucleus of a Helium atom, it is reprsented by ⁴₂He, where 2 means there are two protons and 4 means there are altogether 4 protons and neutrons. A gamma radiation photon is represented by ⁰₀γ. γ is a Greek letter that is pronounced "gamma". The 0's mean no protons or neutrons. After ejecting these, the uranium nucleus would have 2 protons and 2 neutrons fewer. So the new nucleus, or daughter nucleus, has 92 - 2 = 90 protons. The element whose atoms have 90 protons each is called Thorium. This daughter nucleus is represented by ²³⁴₉₀Th, where 234 is total the remaining number of protons and neutrons. The original uranium has become smaller by losing some protons and nucleus. We say that uranium has undergone radioactive decay. The decay products are a Thorium-234 nucleus, an alpha particle and a gamma ray photon.

Another type of radioactive decay happens in the emission of beta radiation. An example is when carbon-14 decays. The most common type of carbon atoms is carbon-12. A nucleus of a carbon-12 atom has 6 protons and 6 neutrons. On the other hand, a nucleus of a carbon-14 atom has 6 protons and 8 neutrons. This is represented by the symbol ¹⁴₆C. It is unstable. The way it tries to become more stable is by changing one of its neutrons into a proton and an electron. This would appear to be a rather unusual reaction compared to simply ejecting a few particles. After the neutron has changed, the electron is ejected as a beta particle and the new proton stays. So the nucleus now has one neutron fewer but one proton more. That is, it now has 7 protons and 7 neutrons. A atom with 7 protons is a nitrogen atom. In this case it is represented by ¹⁴₇N. The complete process can be written as an equation like this:

¹⁴₆C → ¹⁴₇N + e

where e, which stands for an electron, is the ejected beta particle.

References

Nuclear radiation
Radioactivity