What is ionizing radiation?

Ionizing radiation

Ionizing radiation is electromagnetic waves and particles that can, directly or indirectly, cause the ionization of the atoms and molecules of the matter they pass through. When passing through matter, ionizing radiation manages to remove, by virtue of its energy, electrons from atoms (or molecules), thus creating a pair of charged particles.

A fundamental distinction between the different types of ionizing radiation is that between radiation of a corpuscular nature, made up of subatomic or nuclear particles with a certain mass and, often, an electric charge (protons, α particles, deuterons, etc.), and radiation of a wave nature (photons), with zero mass and no electric charge.

Ionizing radiation is further divided into directly ionizing and indirectly ionizing.

Heavy charged particles (electrons, β particles, α particles, etc.) are directly ionizing; photons (X-rays and g-rays) and neutral particles (neutrons) are indirectly ionizing.

Directly ionizing radiation ionizes matter due to the Coulomb interaction between the moving particle and the electrons of the medium. When passing through matter, directly ionizing radiation releases its energy mostly through elastic collisions with atomic electrons. In turn, the electrons freed by the collision acquire energy which they subsequently lose by colliding with other atomic electrons. This mechanism produces ionization at a distance from the primary beam. The loss of energy of the heavy charged particles is linearly proportional to the density of the matter crossed and the very mechanism by which it occurs, characterized by numerous collisions, each one taking away a very small part of the energy available. This means that the length of the path in the matter is almost the same for every particle of given energy.

Indirectly ionizing radiation, on the other hand, acts by releasing, in the elementary interaction with the atoms or nuclei of the medium, charged particles capable of subsequently giving rise to ionization phenomena. Photons of energy between 10 keV and 10 MeV (X-rays and g-rays) undergo three main types of interaction with matter: photoelectric effect, Compton effect, pair creation.

• Photoelectric effect: The photon is completely absorbed by an atomic electron, which gains enough energy to escape the atomic bond.

• Compton effect: the photon hits an electron in the outer orbits of the atom. This results in a diffusion of the incident photon (with lower energy than the original one) and the expulsion of the hit electron.

• Pair production: while the previous two processes can be understood in almost ballistic terms, this last type of process is complex since it involves the transformation of the photon into an electron-positron pair.

In all three cases, as a result of the interaction process there is at least one charged particle.

The penetration of indirectly ionizing radiation into matter is much greater than that of charged particles. Due to their high penetration power, to effectively attenuate X and g radiation, heavy materials with a high Z atomic number, i.e. with high electronic density, must be used, such as: lead, tungsten, barite concrete, etc.

Neutral particles (neutrons) lose their energy through collisions with atomic nuclei. The nuclei can be weakly accelerated or crushed. The fragments (being made up of protons and neutrons) lose energy in a similar way to the particles described previously. To attenuate neutron beams, the best materials are those with a high content of protons and light nuclei, such as: water, paraffin, concrete, etc.

Sources of ionizing radiation

There is a natural, background radiation to which the general population is continuously exposed. On average, more than three quarters of exposure to ionizing radiation is due to sources of natural origin, mainly cosmic rays, deriving from the sun, and radon (radioactive gas produced by the decay of uranium and thorium atoms present in rocks, and therefore also present in building materials).

The remaining amount of radiation is instead determined by human sources, in the medical, industrial and commercial fields. Medical procedures are responsible for approximately 96% of the entire exposure of the general population to ionizing radiation from human sources.

Quantities and units of measurement

To quantify the effect of energy transfer within the human body and to evaluate its effects, specific quantities have been defined:

• absorbed dose (D)

• equivalent dose (H)

• effective dose (E)

Absorbed dose (D): is defined as the ratio between the average energy released by ionizing radiation to matter in a certain volume element and the mass of matter contained in that volume element. It can cause biological effects in the affected tissues, the extent of which varies depending on the energy released, the organ affected, the age of the person, etc. The SI unit of measurement for absorbed dose is the Gray (Gy). By definition, 1 Gy corresponds to the absorption of 1 J of radiant energy per kg of matter (1 J/kg).

It is not sufficient to use the absorbed dose as the sole term to measure exposure in radiation protection and to estimate the associated risk, because the effects of radiation depend not only on the absorbed dose but also on the type of radiation, on the spatial and temporal distribution of the energy absorbed within the human body and by the radio-sensitivity of the exposed tissues or organs: equal doses imparted by different types of radiation produce different biological damage.

Knowledge of the absorbed dose, therefore, is insufficient to predict both the severity and probability of biological effects.

Since different types of radiation produce different effects on tissues, the concept of equivalent dose (H) was introduced.

Equivalent dose (H): it is the product of the absorbed dose by the radiation weight factor. The equivalent dose evaluates the different danger of radiation incident on a given tissue taking into account the so-called radiation weighting factors, wR. The reference radiation (photons) is by definition assigned a wR equal to 1. The product of the average absorbed dose in an organ or tissue, DT, by the wR factor, is called the "equivalent dose" in the tissue or organ T , HT (HT = wR * DT). Unit of measurement: Sievert (Sv). 1 Sv = 1Gy = 1 J/1kg.

To take into account the different radiosensitivity of the different organs and tissues of the human body, the effective dose quantity, E, was introduced.

Effective dose (E): it is a radioprotection quantity that takes into account the different radio-sensitivity of different tissues. It is calculated by adding the average value of the equivalent dose of each organ and tissue, each multiplied by a weighting factor, wT, which takes into account the different radiosensitivity of the irradiated organs and tissues. Unit of measurement: Sievert (Sv).

Biological effects of ionizing radiation

The physical damage caused to humans by ionizing radiation can be divided into three main categories:

• deterministic somatic damage,

• stochastic somatic damage,

• stochastic genetic damage.

The damage that occurs in the irradiated individual is called somatic, while the damage that occurs in his offspring is called genetic.

Deterministic somatic damage is divided into early, from hours to weeks, and late. They have the following characteristics:

• threshold dose, below which they do not manifest themselves (more precisely, at the threshold dose, 1% of the exposed population will manifest tissue reactions). Split doses and chronic exposures at low dose rates are less harmful than acute doses (higher threshold dose),

• the severity of the damage increases as the dose increases,

• the latency period is usually short, days or weeks. It is rarely longer (e.g. years for cataracts),

• direct cause-effect relationship,

• they are the result of exposure to high doses. Indicatively, the sensitivity threshold of the human organism "as a whole" is, for an acute irradiation, of the order of 0.25 Gy, a value around which the first slight and transitory haematological alterations begin to appear.

Stochastic somatic damages are those for which only the probability of occurrence, and not the severity of the damage, is related to the irradiation dose. Stochastic somatic damage is essentially leukemia and solid tumors. They are completely indistinguishable from tumors induced by other carcinogens, but a statistical correlation with exposure to ionizing radiation has been demonstrated and there is experimental radiobiological evidence. They can manifest up to several decades after exposure. It should be noted that only the probability of the pathology occurring is related to the exposure dose, not the severity of the pathology ("all or nothing" law).

Stochastic genetic damage is that which occurs in the offspring of parents exposed to ionizing radiation, following the damage caused to the cells of the germ line. They can manifest themselves both as malformations of the fetus and as hereditary pathologies, which, if dominant, manifest themselves already in the first generation, if recessive, they can appear after several generations or never.