An alpha particle is made up of two protons and two neutrons - equivalent to the nucleus of a helium atom.
Whenever an alpha particle makes contact with electrons two are captured and the particle becomes a helium atom.
Mass:
Alpha particles have a mass of 6.64 x 10{U-27} kg which is equivalent to 4 times the mass of a hydrogen nucleus.
Charge:
They have an electric charge of 3.2 x 10{U-19} coulombs or +2e, which is twice the charge of a proton.
Range:
Alpha particles have a range of only a few centimetres in air, the range usually being less than 5 cm. They are easily stopped by paper or the skin.
Behaviour:
Alpha particles can be deflected by both magnetic and electric fields. In the case of electric fields they will be attracted by the negative and repelled by the positive. In magnetic fields they deflect at right angles to the field according to Fleming's Left-hand Rule.
(2) Detection:
Alpha particles can be detected in a variety of ways, although it must be remembered that their penetrating power is very low so detectors which require the particles to pass through a 'casing' have limited value.
Scintillations: if alpha particles strike a surface coated with zinc sulphide then fluorescence (glowing) occurs at the point at which the particle hits. This method was used by Ernest Rutherford during the early part of the 20th century.
Cloud chambers: as the alpha particles move through the air they remove electrons from the atoms of the air. Ions (electrically-charged particles) are therefore formed. If these ions are produced in an atmosphere rich in water vapour then droplets of water appear where the ions are produced. The path of the alpha particles therefore becomes visible.
Geiger-Muller tube and ratemeter: a suitable G-M tube with a very thin entry window can be used to detect alpha particles. By connecting the tube to a suitable ratemeter the number of particles entering can be monitored.
(3) General:
Alpha particles travel at about one tenth of the speed of light, which in particle terms is relatively slow.
Alpha emitters occur as the heavier, unstable elements on the periodic table, beginning with bismuth, number 83. They can also be emitted by isotopes of the rare earth elements, from neodymium (number 60) to lutetium (number 71).
(4) Discovery:
The discovery of the alpha particle is closely linked to the study of radioactivity and the search for the structure of the atom. In 1896 H. Becquerel discovered the radioactivity of uranium, which led to extensive research into radioactivity and the exact nature of radioactive emissions.
The alpha particle was discovered and named by E. Rutherford in 1899 as a result of experiments aimed at understanding the structure of the atom.
(5) Biological effects:
Alpha particles are easily stopped by the skin, all the energy being absorbed and penetration being a fraction of a millimetre. However, because of the relatively large mass and low speed, intense ionisation of body tissue occurs along the track taken by the particle. This implies that severe damage can occur to cells exposed to alpha particles and is of particular concern if the alpha particles are being emitted by a source taken into the body. The damage to human tissue by radiations depends upon the intensity; alpha particles give intense radiation effects, but in a small area.
(6) Applications:
Alpha particles have few applications, largely because of their limited range and their inability to penetrate even very thin materials.
They are used in nuclear research facilities around the world because of their ability to remove electrons from other particles, thus producing ions.
Because of their electric charge they can be accelerated to very high speeds and used to bombard the nuclei of other atoms. It is by bombardment of this nature that particle research can continue.
An alpha-emitting sample provides the detecting device inside smoke detectors. The detector has a source of particles and a detector. If smoke particles enter the smoke detector then alpha particles strike them and are observed. By monitoring this change in the presence of alpha radiation the alarm is automatically triggered.
