Glossary

An atom consists of a nucleus and electrons moving around it (often called orbital electrons).

An atomic nucleus consists of protons and neutrons. The number of protons in an atomic nucleus determines the chemical element, and the number of neutrons in the nucleus of an element determines the isotope of the observed element.

For example, the nucleus of the element helium has 2 protons (so helium in the periodic table has the ordinal number 2), and its most common isotope also has 2 neutrons, i.e. a total of 4 nucleons (common name for protons and neutrons) and it is denoted as He-4 or helium-4 (4 being the mass number of the nucleus).
Also, the atomic nucleus of an isotope may have an excess of energy compared to the so-called ground state, the lowest energy state of that nucleus. The different energy states of an atomic nucleus of an isotope are called isomers of that isotope.

To describe an atomic nucleus that includes the number of protons, the number of neutrons, and the energy state of the nucleus – the term nuclide is used. For example, the nuclei of different elements are different nuclides, the nuclei of different isotopes of an element are different nuclides, the nuclei of different energy states (i.e. different isomers) of an isotope are different nuclides.

If a nuclide is unstable, which means it has the property of radioactivity (it spontaneously changes into another nuclide or splits into other nuclides), it is called a radionuclide.

The term radioisotope is often used for unstable isotopes that do not have metastable isomers.

Alpha radiation consists of alpha particles emitted from radioactive atomic nuclei during alpha decay. Alpha particles are composed of the atomic nucleus of helium-4. Radioactive nuclei that emit alpha particles are called alpha emitters. The name was introduced before the structure of alpha particles was known.

Alpha radiation is the least penetrating type of radioactive radiation: it does not penetrate into the air farther than a few centimetres, and it can be stopped by plain paper. This means that alpha particles transfer all their energy along their very short trajectory through matter, making them particularly dangerous in living tissue (see equivalent dose). However, they cause real damage only if alpha emitters enter the body (food, drink, breathing) because they are mostly stopped on the skin by the dead surface layer.

Beta radiation consists of beta particles emitted from radioactive atomic nuclei during beta decay. A beta particle is an electron that is formed in the nucleus in the process of beta-minus decay and is emitted from it at high speed. The name was introduced before it was determined that a beta particle was actually an electron. Also, beta-plus decay had not yet been detected, so only the term “beta decay” was used.

In beta-plus decay, in the nucleus a positron is formed instead of an electron. However, upon being emitted from the nucleus, a positron reacts with some of the orbital electrons and both are converted to gamma radiation. This is why radioactive materials do not emit “beta-plus” radiation (which would consist of positrons).

Radioactive nuclei that emit beta radiation are called beta emitters.

Beta radiation is more penetrating than alpha radiation, but not by far. It does not penetrate farther than a few meters in the air, and is stopped by metal foil or thin plastic. Still, it can penetrate the dead surface layer of bare skin and damage the deeper living layers of the skin. It is most harmful if beta emitters are taken into the organism (through food, drink or breathing), but much less than alpha radiation (see equivalent dose). ekvivalentnu dozu).

Gamma radiation consists of electromagnetic waves with the shortest wavelengths, i.e. the highest frequencies and energies of photons. Photons are massless particles that make up electromagnetic waves, so it is colloquially said that photons are particles (quanta) of the energy of electromagnetic waves. The energy of an individual photon is proportional to the frequency of the wave.

Gamma radiation originating from radioactive decay typically has wavelengths about a million times shorter than visible light, so these gamma photons have an energy about a million times greater than photons of visible light. The atomic nucleus emits one gamma photon at each transition from a higher energy state to a lower energy state (if the excess energy is not resolved in any other way; see, for instance, internal conversion).

Gamma radiation is a general term for electromagnetic radiation from nuclear reactions (and therefore from radioactive decay), and electromagnetic radiation generated by annihilation, as well as for electromagnetic radiation of similar and higher frequencies coming to Earth from space (part of cosmic radiation) whose origin is often unknown. The name was introduced at the time of the discovery of radioactivity, when the nature of radiation was not yet known.

The range of gamma radiation is not strictly delimited from the range of X-ray radiation, which generally extends toward lower frequencies. The distinction is traditionally made according to the origin of the radiation: the term X-rays is most often used to denote electromagnetic radiation from processes in which electrons change energy, and gamma radiation for electromagnetic radiation from nuclear processes. For example, gamma radiation of technetium-99m is identical to X-rays of the same frequency generated in a diagnostic X-ray device.

Gamma radiation is much more penetrating than alpha or beta radiation, especially if it consists of high-energy photons. Also, it does not stop completely in a material, but only weakens gradually. For instance, a layer of lead of about 0.7 cm is required to halve the gamma radiation intensity of caesium-137, and of about 5 cm to reduce the radiation intensity below 1%.

During alpha decay, the radioactive nucleus emits a particle traditionally called the alpha particle, i.e. the atomic nucleus of helium-4 (consisting of 2 protons and 2 neutrons). This is why the new nucleus, which was formed by such a change in the previous radioactive nucleus, has 2 protons and 2 neutrons less.

In most types of radioactive nuclei that decay by alpha decay, the newly formed nucleus has an excess of energy that is resolved by emitting one or more gamma photons. This is why gamma radiation often appears simultaneously with alpha radiation (see metastable isomers).

Depending on the type of the previous nucleus (ancestor), the newly formed nuclei (descendants) can themselves be radioactive and decay in the same or a different type of decay.

There are two types of beta decay, beta-minus and beta-plus, and originally the name “beta decay” referred only to the beta-minus decay, which was first discovered.

In beta-minus decay, the radioactive nucleus emits one electron, which was named beta particle when it was not known that it was an electron and that there was another type of beta decay. There are no electrons in the atomic nucleus, but when it decays, one of its neutrons is converted into a proton and an electron (preserving the total charge). The electron is emitted from the nucleus, and the proton remains in it, so the nucleus does not change its mass number, but its charge (ordinal number) increases by one, i.e. the nucleus passes into the next element of the periodic table.

It was later discovered that in beta-minus decay, a neutrino (more precisely: electronic antineutrino) is formed, a neutral particle of insignificant rest mass (if any) that easily passes through matter without any effect, so it is not significant for describing radioactive radiation.

In most types of radioactive nuclei that decay by beta-minus decay, the newly formed nucleus has an excess of energy that is resolved by emitting one or more gamma photons. This is why gamma radiation often appears simultaneously with beta radiation (see metastable isomers).

In beta-plus decay, the radioactive nucleus emits a single positron, which is the antiparticle of the electron (positron is the traditional name for the antielectron). An antielectron has the same mass (and some other properties) as an electron, but has the opposite charge. The key property of particle-antiparticle pairs is that they are converted into gamma radiation (which is called annihilation) when they come into contact with each other. This is why the emitted positron, together with some of the neighbouring orbital electrons with which it will come into contact after being emitted from the nucleus, is converted into two gamma photons, which is why the beta-plus radioactive material emits gamma radiation.

Since there are no positrons in the atomic nucleus, during beta-plus decay one of its protons is converted into a neutron and a positron (preservation of the total charge). The positron is emitted from the nucleus, and the neutron remains in it, so the nucleus does not change its mass number, but its charge (ordinal number) decreases by one, i.e. the nucleus passes into the previous element of the periodic table.

It was later discovered that in beta-minus decay, a neutrino (more precisely: electronic neutrino) is formed, a neutral particle of insignificant rest mass (if any) that easily passes through matter without any effect, so it is not significant for describing radioactive radiation.

In most types of radioactive nuclei that decay by beta-plus decay, the newly formed nucleus has an excess of energy that is resolved by emitting one or more gamma photons. This emission usually occurs virtually simultaneously with the emission of positrons or gamma radiation caused by its annihilation (see metastable isomers).

Depending on the type of the beta-plus radioactive nucleus, a certain percentage of beta-plus decay takes place as the so-called electron capture. In this case, instead of emitting a positron, the nucleus absorbs, i.e. “captures” the nearest orbital electron.

Depending on the type of the previous nucleus (ancestor), the nuclei formed through beta decay (descendants) can themselves be radioactive and decay in the same or a different type of decay.

Gamma decay is the process by which an atomic nucleus transitions from a higher energy state to a lower energy state by emitting a gamma photon. The nucleus remains the same isotope, but is no longer the same isomer.

During alpha decay, and particularly beta decay, newly formed nuclei often have excess energy (formed as isomers in the excited state). Most often, this excess is resolved by gamma emission, which is virtually simultaneous with alpha or beta emission (within millionths of a billionth of a second). Some types of newly formed nuclei, however, retain this excess energy for a long time, and only later pass into a lower state: an atomic nucleus in such an excited state is called a metastable isomer.

In these metastable isomers, gamma emission is clearly separated in time from the alpha or beta emissions that precede it, which is why the name “gamma decay” makes sense because the transition to a lower energy state takes place in accordance with the radioactive decay law.

Gamma decay is one of two forms of isomeric transition. In addition to gamma decay, the metastable isomer can get rid of excess energy by passing it to an orbital electron. Such an isomeric transition is called internal conversion. An orbital electron emitted from an atom in this way is not considered beta radiation (nor is it accompanied by the emission of neutrinos).

For example, the use of metastable technetium-99m is widespread in medical diagnostic imaging. The isomeric transition of technetium-99m to the basic isomer of technetium-99 takes place during a half-life of 6 hours, in 88% of cases as gamma decay, and in 12% of cases as internal conversion. vremenom poluraspada od 6 sati, i to u 88% slučajeva kao gama raspad, a u 12% slučajeva kao unutarnja konverzija.

Fission is the splitting of a heavy atomic nucleus into two equal parts, resulting in the emission of several neutrons called fission neutrons (typically, two to three neutrons) and gamma radiation. The newly formed atomic nuclei, which are called fission products, move at high speeds after fission (they have high energy). One fission releases about one hundred million times more energy than one chemical reaction during coal combustion.

Fission as a way of radioactive decay of some types of naturally unstable atomic nuclei is also called spontaneous fission. This type of spontaneous change of the nucleus, which truly deserves the name “decay”, was observed only a few decades after the discovery of radioactivity. Spontaneous fission takes place in accordance with the radioactive decay law, as do other forms of spontaneous decay. For example, uranium-238 is dominated by alpha decay with a half-life of about 4.5 billion years, with the simultaneous occurrence of spontaneous fission, with a half-life that is about two million times longer (per one gram of uranium-238, one fission occurs every 2.5 minutes, and 750,000 alpha decays every minute).

In some types of atomic nuclei, instantaneous fission can be induced after the nucleus is hit by a neutron; such fission is called induced fission. If the newly formed fission neutrons cause further induced fissions in the same way, and their fission neutrons do the same, and so on, a chain reaction called chain fission occurs.

During the explosion of a nuclear bomb, chain fission involves a huge number of atomic nuclei in a very short time. The chain fission that occurs in a nuclear reactor is regulated mainly by acting on fission neutrons. Such fission is called controlled fission.

The term radioactivity in the qualitative sense means the appearance or property of spontaneous change (decay) of unstable atomic nuclei. Natural materials on Earth contain fewer or more such unstable nuclei that decay faster or slower. The same term is used for the quantitative description of a phenomenon: the radioactivity (in short, activity, if it is clear from the context what kind of activity it is) of a body or quantity of a substance (a sample) is equal to the number of radioactive decays in this sample per unit of time. The unit of measure of this activity, becquerel (Bq), denotes one decay per second.

For instance, the natural radioactivity of the average human is over 4000 Bq, which means that over 4000 radioactive decays occur naturally in the human body every second. The radioactivity of all the oceans on Earth is estimated at tens of thousands of billions of Bq, and the entire Earth’s crust at about a thousand times more.

The old unit of activity, curie (Ci), which amounts to 37 billion Bq, was defined as the activity of one gram of radium-226.

The activity of a sample of N radionuclides whose decay constant is λ amounts to A = λN.

When describing the radioactivity of a relatively homogeneous material, the activity per unit mass is usually indicated (e.g. Bq/kg) or activity per unit of volume (e.g. Bq/m³). The terms specific activity or activity concentration are used for both units.

The course of radioactive decay over time is described by the radioactive decay law: the number of unstable nuclei in a sample, which have not yet decayed, decreases exponentially over time.

The same statement can, in other words, be formulated as follows: after a certain time interval, for which the term half-life is used, the number of non-decomposed nuclei is halved from the beginning of the interval, and after another interval the number of the remaining undecomposed nuclei is halved again, and so on.

This means that – of an initially determined number of undecomposed nuclei – after one half-life, one half remains, a quarter remains after two half-lives, one-eighth remains after three half-lives, etc. The half-life is determined (with measurements) for a particular type of radionuclide, but also for a particular type of decay if it occurs in multiple ways.

Below, additional explanations and mathematical versions of this law are presented.

Radioactive or spontaneous decay of unstable atomic nuclei and other unstable particles is a random process, which means its time of occurrence is not predictable for a single particle. It can only be statistically predicted how many of a multitude of identical particles will decay and when. If the moment of decay were predetermined for specific identical particles, they would not be identical because their lifetimes would differ. Instead, only the expected or mean lifetime can be determined for an individual particle (calculated by dividing the half-life of the particle type by ln2).

For identical particles or nuclei, and for a certain type of their decay, it can be determined (based on measurements) what the probability is that an individual particle decays in one unit of time (e.g. per second). This probability is called the decay constant and is denoted by the symbol λ. This means the following: the number of particles that will decay in a very short time interval (exactly: in an infinitesimal interval) is equal to the product of the decay constant, that time interval and the number of particles at the beginning of the interval (in mathematical symbols: dN = - λNdt).

The above statement is one of the forms of the radioactive decay law. From this equation, two other mathematical equations are derived that are more commonly used in calculations:

or

In these formulas, N is the number of particles or nuclei that have not yet decayed at time t, while N0 is the number of particles at time t=0. The half-life T is related to the decay constant by the λ relation.

Radioaktivna Radioactive and other ionising radiation (such as X-rays, cosmic radiation, etc.) generates ion pairs in substances (by various mechanisms, e.g., directly ejecting electrons from atoms or molecules). In living tissues, this can cause disturbances in the functioning of cells or their important components (such as genetic material). In addition to immediate tissue damage, and even the death of the irradiated organism in case of high doses, the biological effects can be long-term, such as malignant processes and disorders in the offspring caused by damage to reproductive cells.

The radiation dose system (absorbed, equivalent and effective) is designed to determine the health consequences risk based on the absorbed energy.

The absorbed dose is equal to the ratio of the absorbed radiation energy in a tissue and the mass of that tissue. The unit gray (Gy) is equal to the absorbed dose of one joule per kilogram. (If differences in irradiation at different points in the tissue are described, the ratios of energy absorbed to mass should be calculated for very small volumes of tissue, from point to point.)

However, the biological effects of irradiation depend not only on the absorbed dose, but also on the type of radiation and the type of irradiated tissue (or organ).

The equivalent dose calculates the differences in the harmfulness of certain types of radiation. It is calculated by multiplying the average absorbed dose in a tissue or organ by the number called the radiation weighting factor. For instance, for beta and gamma radiation, the factor is the smallest and amounts to 1, for alpha radiation it is the largest and amounts to 20, and for neutrons it depends on their speed. The unit for the equivalent dose is called the sievert (Sv), to emphasize the difference compared to the absorbed dose (although it has the same structure, joules per kilogram).

The effective dose also takes into account the different sensitivity of individual tissues to radiation. It is calculated exclusively for the entire body (organism) by multiplying the equivalent doses for all tissues with their respective tissue weighting factors, and then adding them up. For example, bone marrow has a weighting factor of 0.12 and skin of 0.01. The effective dose unit is the same as for the equivalent dose (Sv). težinskim faktorom tkiva i potom zbroje. Primjerice, koštana srž ima težinski faktor 0,12, a koža 0,01. Jedinica za efektivnu dozu ista je kao za ekvivalentnu (Sv).

The effective dose approximately describes the overall risk that irradiation poses to human health. For example, a single irradiation of 5 Sv causes the death of about 50% of irradiated people within 30 days. A dose of 1 Sv can cause temporary symptoms of radiation sickness, and later develops into cancer in 5% of cases.

Low doses, as considered in the field of RW management, are expressed in parts-per-thousand of Sv, i.e. in millisieverts (mSv), and the risk of such doses is generally reduced to a low probability of developing cancer. For doses smaller than ten mSv per year, this risk is not really proven, but – for the purpose of regulating radiation protection – a linear extrapolation of risks from higher dose areas is generally accepted (ICRP and IAEA recommendations, EU and national regulations).

Nuclear fuel is the term for radioactive material in which controlled chain fission takes place in a nuclear reactor. This fission process will change the composition and concentrations of radionuclides in the fuel over time to a point that it will no longer be usable to continue the process in the designed way. Then the fuel is called spent nuclear fuel, abbreviated to SNF (or used nuclear fuel or irradiated nuclear fuel), and is removed from the reactor and replaced with new fuel. Due to the accumulation of fission products in the fuel, the radioactivity of spent fuel at the time of removal from the reactor is typically about a billion times higher than at the beginning of its use.

At the Krško Nuclear Power Plant, the nuclear fuel is enriched uranium in the form of uranium dioxide (which contains about 5% uranium-235 as opposed to the 0.71% contained in natural uranium; the rest is uranium-238). The fuel cycle lasts 18 months, after which about half of the fuel in the reactor is replaced, which means that the service life of the fuel is about 3 years.

An agreement is a mutual understanding between two parties. It often leads to the signing of a contract.

A convention is an agreement usually between two states, which is less formal than a treaty.

A treaty is a formal agreement between two or more states for some undertaking of public benefit.