These are identical with the helium nucleus and are emitted with a well-defined speed from unstable nu­clei. They are relatively heavy, being about 7000 times heavier than the orbital electrons with which they collide, and travel relatively slowly compared with 0 particles. The alpha particle carries two positive charges; orbital electrons are therefore attracted to the a particle and pulled away from their parent atom. Thus as a particles traverse a material their energy is quickly dissipated. Because they move in straight lines and lose energy continuously (in effect) in a large number of ionising collisions, a particles of a given energy have a definite range in a given material. Typically a particles travel a few centimetres in air and are stopped by a sheet of paper or the outer layers of the skin.

1.5.2 Beta particles

These are electrons originating in and emitted from unstable nuclei with energies that are found to range from zero to a definite maximum value. Being much lighter than the a particle, the 0 particles generally travel much faster and have less time in which to react with individual atoms as they pass by. Moreover, having only a single negative electric charge, the elec­tric forces are weaker than for the a particle. Thus in comparison to a particles they ionise less, lose their energy less quickly and travel further. Typically 0
particles may travel from zero to several metres in air and about a centimetre in water or living tissue.

It is appropriate at this stage to comment briefly on the emission energies of a and 0 particles. For a given a emitting radioactive isotope the a particles are all ejected with exactly the same energy. It would be expected that 0 emission would be mono-energetic also and not have the range of energies found in practice. This puzzled the early researchers; in 1931 W. Pauli postulated (the postulation was not experimen­tally confirmed until 1956) that 0 emission is accom­panied by a second particle: the neutrino with 0+ and the antineutrino with 0“. The energy of emission is shared between the two particles thus explaining the observed energies of 0 radiation: ranging from a maxi­mum value, when all the available energy is in the 0 particle, to zero, when all the energy is in the neutrino/antineutrino. The neutrino and antineutrino are particles of zero mass and zero charge and thus possessing only energy. They pass through the mass of the_earth without hindrance, perhaps one in 1010 reacting. No further comment will be made here; the reader is referred to the literature for further infor­mation on these ephemeral particles.