We have four key types of radiation, namely: alpha, beta, gamma and neutron. The four types differ in mass, energy and how deep they penetrate objects.
Alpha Particle
The alpha particle (α) is a helium nucleus produced from the radioactive decay of heavy metals and some nuclear reactions. Alpha decay often occurs among nuclei that have a favorable neutron/proton ratio, but contain too many nucleons for stability. The alpha particle is a massive particle consisting of an assembly of two protons and two neutrons and a resultant charge of +2.
The alpha particles are the least penetrating radiation. The major energy loss for alpha particles is due to electrical excitation and ionization. As an alpha particle passes through air or soft issue, it loses, on average, 35 eV per ion pair created. Due to its highly charged state, large mass and low velocity, the specific ionization of an alpha particle is very high.
Beta Particle
The beta particle (β) is an ordinary electron or positron ejected from the nucleus of a beta-unstable radioactive atom. The beta has a single negative or positive electrical charge and a very small mass.
The interaction of a beta particle and an orbital electron leads to electrical excitation and ionization of the orbital electron. These interactions cause the beta particle to lose energy in overcoming the electrical forces of the orbital electron. The electrical forces act over long distances; hence, the two particles do not have to come into direct contact for ionization to occur.
The amount of energy lost by the beta particle depends upon both its distance of approach to the electron and its kinetic energy. Beta particles and orbital electrons have the same mass; as a result, they are easily deflected by collision. Because of this, the beta particle follows a convoluted path as it passes through absorbing material. The specific ionization of a beta particle is low due to its small mass, small charge, and relatively high speed of travel.
Gamma Ray
Gamma ray (γ) is a photon of electromagnetic radiation with a very short wavelength and high energy. It is emitted from an unstable atomic nucleus and has high penetrating power. Since gamma radiation has a very high penetrating power, a small fraction of the original stream will pass through several feet of concrete or several meters of air. The specific ionization of a gamma is low compared to that of an alpha particle, but is higher than that of a beta particle.
Related: Gamma Spectroscopy Measurement Technique
Neutron
Neutrons have no electrical charge and have nearly the same mass as a proton. A neutron (n) is hundreds of times larger than an electron, but one quarter the size of an alpha particle. The source of neutrons is primarily nuclear reactions, such as fission, but they are also produced from the decay of radioactive elements. Because of its size and lack of charge, the neutron is somewhat difficult to stop, and has a relatively high penetrative power.
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Neutrons may collide with nuclei causing one of the following reactions:
- Inelastic scattering
- Elastic scattering
- Radioactive capture
- Fission
Inelastic scattering causes some of the neutron’s kinetic energy to be transferred to the target nucleus in the form of kinetic energy and some internal energy. This transfer of energy slows the neutron, but leaves the nucleus in an excited state. The excitation energy is emitted as a gamma ray photon. The interaction between the neutron and the nucleus is best described by the compound nucleus mode; the neutron is captured, and then re-emitted from the nucleus along with a gamma ray photon. This re-emission is considered the threshold phenomenon. The neutron threshold energy varies from infinity for hydrogen (inelastic scatter cannot occur) to about 6 MeV for oxygen, to less than 1 MeV for uranium.
Elastic scattering is the most likely interaction between fast neutrons and low atomic mass number absorbers. The interaction is sometimes termed to as the “billiard ball effect”. The neutron shares its kinetic energy with the target nucleus without exciting the nucleus.
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Radioactive capture(n, γ) takes place when a neutron is absorbed to produce an excited nucleus. The excited nucleus regains stability by emitting a gamma ray.
The fission process for uranium (U235 or U238) is a nuclear reaction whereby a neutron is absorbed by the uranium nucleus to form the intermediate (compound) uranium nucleus (U236 or U239); the compound nucleus fissions into two nuclei (fission fragments) with the simultaneous emission of one to several neutrons. The fission fragments produced have a combined kinetic energy of about 168 MeV for U235 and 200 MeV for U238, which is dissipated, causing ionization. The fission reaction can occur with either fast or thermal neutrons.
Fast neutrons rapidly degrade in energy by elastic collisions when they interact with low atomic number materials. As neutrons reach thermal energy or near thermal energies, the likelihood of capture increases. In reactor facilities, the thermalized neutron continues to scatter elastically with the moderator (slowing-down medium) until it is absorbed by fuel or non-fuel material or until it leaks from the core. The secondary ionization caused by the capture of neutrons is important in the detection of neutrons. For example, neutrons will interact with B-10 to produce Li-7 and He-4. The lithium and alpha particles share the energy and produce ‘secondary ionizations’ that are easily detectable.
Also read: Radio Frequency (RF) and Microwave Spectral Analysis
