are energetic nuclei of helium
. The production of alpha particles is termed alpha decay. Alpha particles consist of two protons and two neutrons
bound together into a particle identical to a helium nucleus. Alpha particles are relatively large and carry a double positive charge. They are not very penetrating
and a piece of paper can stop them. They travel only a few centimeters but deposit all their energies along their short paths. In nuclear reactors
they are produced for example in the fuel
(alpha decay of heavy nuclei). Alpha particles are commonly emitted by all of the heavy radioactive
nuclei occuring in the nature (uranium, thorium or radium), as well as the transuranic elements (neptunium, plutonium or americium). Especially energetic alpha particles
(except artificially accelerated helium nuclei) are produced in a nuclear process, which is known as a ternary fission
. In this process, the nucleus of uranium is splitted into three charged particles
(fission fragments) instead of the normal two. The smallest of the fission fragments most probably (90% probability) being an extra energetic alpha particle.
Interaction of Alpha Particles with Matter
Since the electromagnetic interaction extends over some distance, it is not necessary for an alpha particles to make a direct collision with an atom. They can transfer energy simply by passing close by. Alpha particles interact with matter primarily through coulomb forces between their positive charge and the negative charge of the electrons from atomic orbitals. In general, the alpha particles (like other charged particles) transfer energy mostly by:
- Excitation. The charged particle can transfer energy to the atom, raising electrons to a higher energy levels.
- Ionization. Ionization can occur, when the charged particle have enough energy to remove an electron. This results in a creation of ion pairs in surrounding matter.
Creation of pairs requires energy, which is lost from the kinetic energy of the alpha particle causing it to decelerate. The positive ions and free electrons created by the passage of the alpha particle will then reunite, releasing energy in the form of heat (e.g. vibrational energy or rotational energy of atoms). There are considerable differences in the ways of energy loss and scattering between the passage of light charged particles such as positrons and electrons and heavy charged particles such as fission fragments, alpha particles, muons. Most of these differences are based on the different dynamics of the collision process. In general, when a heavy particle collides with a much lighter particle (electrons in the atomic orbitals), the laws of energy and momentum conservation predict that only a small fraction of the massive particle’s energy can be transferred to the less massive particle. The actual amount of transferred energy depends on how closely the charged particles passes through the atom and it depends also on restrictions from quantisation of energy levels.
See also: Interaction of Heavy Charged Particles with Matter