Observing Nuclear Radiation
The basic structure of an atom is a small central nucleus, made up of protons and neutrons, surrounded by electrons.
The atoms of an element always have the same number of protons. However, different isotopes of the element will have different numbers of neutrons.
The nuclei of radiactive substances are unstable. They become stable by radioactive decay. In this process they emit radiation and turn into other elements.
The three types of radation emitts are:
- Alpha particles.
- Beta particles.
- Gamma rays,
Radioactive decay is a random process and is affected by external conditions.
Alpha, Beta and Gamma Radiation
- An Alpha partical is a helium nucleus. It is made up of two protons and two neutrons.
- A beta partical is a high speed electron from the nucleus. It is emitted when a neutron changes to a proton and an electron. The proton remains in the nucleus.
- Gamma radiation is very short wavelength electromagnetic that is emitted from the nucleus.
When a nucleus radiation travels through a material is will collide with the atoms of the material and knock electrons off them creating ions, this is called ionisation.
Alpha particals are relatively large, so they have lots of collisions with atoms - they are strongly ionising, because of these colisions, the alpha particles do not penetrate far into a material. They can be stopped by a thin sheet of paper, human skin or a few centimetres of air. Alpha particals have a positive charge and are deflected by electric and magnetic fields.
Beta Paricles and Gamma Radiation
Beta particles are much smaller and faster than alpha particles so they are less ionising and penetrate further. They are blocked by a few meters of air or a thin sheet of aluminium. Beta paricles have a negative charge and are deflected by electric and magnetic fields in the opposite sense to alpha particles.
Gamma rays are electromagnetic waves so they will travel a long way through a material before colliding with an atom. They are weakly ionising and very penetrating. Several centimeters of lead or several metres of concrete are needed to absorb most of the radiation. Gamma rays are not deflected by electric or magnectic fields.
We can measure the radioactivity of a sample of a radioactive material by measuring the count rate from it. The radioactivity of a sample decreases over time. How quickly the count raet falls nearly to zero depends of the material. Some take a few mintues, others take millions of years.
We use the idea of a half life to measure how quickly the radioactivity decreases. It is the time taken for the count rate from the original substance to fall half its inital value.
Or we can define it as they time taken for the number of unstable nuclei in a sample to halve.
A half life is the same for any sample of a particular material.
Radioactivity At Work
- Alpha sources are used in smoke alarms. The alpha particles are not dangerous because they are very poor at penetrating. The source needs a half life of several years.
- Beta sources are used for thickness control in the manufacture of things like paper. Alpha particals would be stopped by a thin sheet of paper and all the gamma rays would pass through it. The source needs a half life of many years, so that decreases in the count rate are due to changes in the thickness of the paper.
- Gamma and beta sources are used as tracers in medicine. The source is injected or swallowed by the patient. The source needs a half life of a few hours so that the patient is not exposed to unneccessary radioactivty.
- Gamma sources are also used to sterilise medical equipment and prevert food spoilage.
Radioactivity at Work
If nuclear radation enters living cells, it causes ioisation which damages cells and may cause cancers. If a source of radiation is outdie the body alpha particles will be stopped by clothing or skin. Gamma and beta radiation are more dangerous because they may pass through skin and damage cells.
If the sourse of radiation is inside the body, eg it is inhaled, alpha radiation is most likely to harm you as it is most ionising.