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Radioactivity
Some atomic nuclei are unstable.
A substance containing unstable nuclei is said to be radioactive.
Unstable nuclei become stable by emitting various types of radiation.
This process is called nuclear decay.
 
When a nucleus decays, the resulting nucleus is called a daughter nucleus.
Some indication as to the degree of instability of a nucleus is given by its neutron/proton ratio.
 
For example:
has a neutron to proton ratio of 8/6 = 1.33 and is unstable with a half-life of 5730 years
has a neutron to proton ratio of 7/6 = 1.17 and is stable
has a neutron to proton ratio of 6/6 = 1 and is stable
has a neutron to proton ratio of 5/6 =  0.83 and is unstable with a half-life of about 20 minutes
 
Three types of radiation will be considered here: α, β and γ radiations.
 
Information about the radiations can be obtained by passing them through a magnetic field, as shown below.
 
 
The green crosses represent a magnetic field acting into the plane of the diagram.
See here and here for a reminder about magnetic fields.
These observations suggest that
1. alpha radiation consists of positively charged particles having a relatively low charge to mass ratio, of about 4.8107Ckg-1
2. beta radiation consists of negatively particles having a relatively high charge to mass ratio, of about (-)1.761011Ckg-1
3. gamma radiation carries no charge
 
α Radiation
Looking at the charge to mass ratio, we can say that an a particle is the same as a helium nucleus.
Therefore we can say that when an a particle is emitted the atomic number of the nucleus decreases by 2 and the mass number decreases by 4.
α radiation is not very penetrating.
α particles can be absorbed by a few 10's of cm of air at normal atmospheric pressure or a couple of sheets of paper.
 
β Radiation
Again, from the charge to mass ratio (and measurements of their kinetic energy), we can say that β radiation consist of fast moving electrons.
However, these particles are emitted by nuclei made of protons and neutrons!
To explain this, we suggest that a neutron in the nucleus decays into a proton and an electron.
The electron is then emitted.
This can be represented by the following nuclear equation
 
What was not shown in the diagram above is the fact that b radiations come in two types.
Some radioactive substances emit particles with the same magnitude of charge to mass ratio but having a positive charge.
 
To distinguish between these two types, we write β for the negative particles and β+ for the positive type.
 
β+ Radiation
β+ radiation consists of particles having the same mass as electrons but having positive charge.
These particles are the "anti-particles" of electrons and are called positrons.
To explain this type of radiation we suggest that a proton decays into a neutron and a positron, as described by this equation
β radiation is more penetrating than α.
β particles can be stopped by a few sheets of a metal such as aluminium.
 
γ Radiation
The fact that this radiation is not deflected by a magnetic field shows that is is uncharged.
γ radiation has been shown to be short wavelength electro-magnetic radiation.
 
γ radiation is more penetrating than α and β and we need thick sheets of a dense metal such as lead to absorb it.
 
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