Alpha particles 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.
Characteristics of Alpha Particles
Key characteristics of alpha particles are summarized in few following points:
- Alpha particles are energetic nuclei of helium and they are relatively heavy and carry a double positive charge.
- Typical alpha particle have kinetic energy about 5 MeV. This is due to the nature of alpha decay.
- Pure alpha decay is very rare, alpha decay is frequently accompanied by gamma radiation.
- Alpha particles interact with matter primarily through coulomb forces (ionization and excitation of matter) between their positive charge and the negative charge of the electrons from atomic orbitals.
- Alpha particles heavily ionize matter and they quickly lose their kinetic energy. Therefore alpha particles have very short ranges. On the other hand they deposit all their energies along their short paths.
- For example, the ranges of a 5 MeV alpha particle (most have such initial energy) are approximately only 0,002 cm in aluminium alloy or approximately 3.5 cm in air.
- The stopping power is well described by the Bethe formula.
- The Bragg curve is typical for alpha particles and for other heavy charged particles and describes energy loss of ionizing radiation during travel through matter.