Module 3: Interaction of Radiation with Matter

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MODULE 3: INTERACTION OF RADIATION WITH MATTER

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During the radioactive decay process, a particle and/or a photon are emitted from the parent atom. The particles emitted carry energy proportional with their mass and speed. The photons carry energy proportional to their frequency. The photons and the particles interact with the surrounding matter. This interaction depends on their type (alpha, beta, gamma, neutrons, etc.), mass, electrical charge, energy, and on the composition of the surrounding materials

3.1 Alpha Radiation

Since the alpha particle is basically a He nucleus (2 protons & 2 neutrons), it is the largest and most massive type of radiation (except for fission fragments). Additionally, the interaction of alpha particles with matter is very strong due to the alpha particle's electrical charge of 2 units. Alpha trajectories can be deviated by both electric and magnetic fields. The major energy loss mechanism for alpha particles is electronic excitation and ionization. The specific ionization of an alpha particle is very high, in the order of thousands of ion pairs per centimetre of air.

Because of the strong interaction of alpha particles with matter, they have a short range; a sheet of paper, the surface layer of dead skin (epidermis), or a few centimetres of air can easily stop them. Consequently, there is no concern for external irradiation of people. However, when gamma radiation is emitted together with alpha particles, precautions against external irradiation caused by gamma rays should be taken into account.

When inhalation or ingestion of an alpha emitting radioactive material occurs, internal irradiation becomes a major concern. The alpha particles interact strongly with the surrounding internal tissues (live tissue). All of their energy is absorbed inside the body, potentially causing damage to the cells. Therefore, special precautions are taken when handling open, volatile sources of alpha emitting radionuclides.

3.2 Beta Radiation

Positrons, positively charge beta particles, interact with electrons from surrounding matter through the process called annihilation of radiation, producing 2 gamma rays (see chapter 2.5).

Electrons are negative beta particles with relatively light mass. Their interaction with matter can be characterised as average. There are 2 main mechanisms of interaction that are important from the point of view of radiation protection.

3.2.1 Excitation and Ionization

The interaction between the electric field of a beta particle and the orbital electrons of the absorbing medium leads to inelastic collisions that generate electronic excitation and ionization. Because of the continuous spectrum of beta particles, the specific ionization (the number of ion pairs created per cm of air) decreases from approximately 200 ion pairs with increase of beta energy, reaching a minimum of approximately 60 ion pairs at beta energy of 1 MeV.

3.2.2 Bremsstrahlung

The second important mechanism of reducing energy of beta particles is "bremsstrahlung". When a high-speed charged particle passes through a medium, it occasionally undergoes a substantial nuclear scattering, which results in the emission of continuous electromagnetic energy called bremsstrahlung or "braking radiation". This energy is in the range of X-rays (lower electromagnetic energy than gamma rays) and becomes more energetic if the stopping material is made of heavy materials such as metals of high Z. The use of light materials reduces "bremsstrahlung". This is why light materials such as Plexiglass are used to absorb beta radiation.

When gamma emission follows beta disintegration, protection against gamma rays is also required. In this situation, we need to stop the beta particles first with light materials and then gamma and bremsstrahlung radiation with heavy material (lead or other metal).

Because of the continuous energy distribution, absorption of beta particles in material is also continuous. The range, however, has a maximum value for different materials, and is related to the maximum energy of the beta particles. For example, tritium (H-3) has a low maximum beta energy (0.018 MeV) and a maximum range in air of 6 mm. On the other hand, P-32 has a higher maximum beta energy (1.71 MeV) and a maximum range in air of 7.9 m.

3.3 Gamma Radiation

Gamma rays are photons (quanta of light) and have no electric charge and no rest mass. Therefore, the interaction of gamma rays with matter is weak. There are 3 mechanisms that are important from the point of view of radiation protection.

3.3.1 Photoelectric Effect

An electron is emitted from an atom (ionization process) with energy equal to the energy of the gamma ray and the gamma ray dissapears. The electron then moves through matter and loses its energy as described for beta interactions. This is the predominant effect at low gamma energies.

3.3.2 Compton Scattering

The gamma ray interacts with an electron, causing an increase in the electron's energy. A new gamma ray with a smaller energy is then emitted. The electron interacts as explained earlier. The new gamma ray can escape from the matter or can be absorbed through the photoelectric effect. The Compton effect is the predominant effect at intermediate gamma energies.

3.3.3 Pair Production

High-energy gamma rays are absorbed and two particles are created (an electron and a positron) and share the energy of the gamma ray. The electron interacts with matter, as explained above for beta interaction. The positron loses its energy through ionization or excitation. If it is stationary, the positron interacts with an electron creating two gamma rays with energies of 511 keV each (annihilation radiation). These two gamma rays can escape or interact with matter through the Compton scattering or Photoelectric effect.

The absorption of gamma rays obeys an exponential law. There is no definite range of absorption for gamma rays in matter. Protection against gamma rays (as well as against X-rays) is best obtained with heavy materials (lead or other metals), as well as with large quantities of concrete or other materials. For example, the earth's atmosphere protects us against high-energy gamma rays and other high-energy radiation coming from outer space.

3.4 Neutrons

Neutrons, which have rest mass but are electrically neutral, undergo weak interactions with matter. Their mechanism of interaction is through collisions. Having a mass similar to that of the protons, their greatest interactions occur with atoms of Hydrogen (like billiard balls colliding with each other). After a number of collisions, the neutron's energy decreases and is finally totally absorbed. Due to the high content of water in human tissue, neutrons are considered very hazardous. Protection against neutrons can be obtained with materials containing H or other light nuclei (like water, wax, or concrete).

The interaction of neutrons with boron nuclei is the main mechanism used for neutron detection: 

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As a result of this nuclear reaction, alpha particles and gamma rays are emitted with energies of 480 keV and could be detected by the instrument. Therefore, detection of neutrons is an indirect process.