Nuclear Power Plant Isar, Germany

Spent nuclear fuel pool

Turbine of a nuclear plant



We define radioactivity as the spontaneous emission of particles (alpha particles, beta particles, neutrons) or radiations (range, K capture), or both at the same time, coming from the disintegration of certain nuclides that form them, due to an arrangement in its internal structure.

Radioactive decay occurs in unstable atomic nuclei, that is, those that do not have enough binding energy to keep the nucleus together due to an excess of protons or neutrons.

Radioactivity can be natural or artificial. In natural radioactivity, the substance already has it in the natural state. In artificial radioactivity, radioactivity has been induced by irradiation.

Types of radiation

Among the light elements, the most frequent radiations are:

  • Beta radiations b -, which are electrons from the nucleus
  • Beta radiation b+ , which are positrons from the nucleus
  • Gamma rays (g), which are high-energy electromagnetic waves
  • Electronic capture (K disintegrations)

Radiations a are characteristic of heavy elements.

Each type of radioactive emission has different penetration power in the matter and different ionisation power (ability to remove electrons from the atoms or molecules with which it collides). They can cause serious damage to living beings.

Alpha particle

Alpha particles (α) or alpha rays are a form of radiation with high ionizing, corpuscular energy and with a low penetration capacity due to the high cross section. They consist of two protons and two neutrons joined by a strong force. The alpha particles belong to the elion family. The beta decay is mediated by a weak force, while the alpha decay is mediated by a strong force.

Alpha particles are typically emitted by heavy element radioactive nuclides, eg, isotopes of uranium, thorium plutonium, radio, etc., in a process called alpha decay. Sometimes, this decomposition leaves the nuclei in an excited state and, consequently, the excess energy can be eliminated with the emission of gamma rays.

Alpha rays, due to their electrical charge, interact strongly with matter and, therefore, are easily absorbed by materials and can travel only a few centimeters in the air. They can be absorbed by the outermost layers of human skin and, therefore, are not life threatening unless the source is inhaled or ingested. In this case, the damages would be, on the other hand, greater than those caused by any other ionizing radiation. If the dose were high enough, all the typical symptoms of radiation poisoning would appear.

Beta particle

Beta radiation is a form of ionizing radiation emitted by certain types of radioactive nuclei.

Beta radiation takes the form of beta particles (β), which are high-energy particles, ejected from an atomic nucleus in a process known as beta decay. There are two forms of beta decay, β - and β +, which respectively emit an electron or a positron.

In β-decay, a neutron becomes a proton, an electron and an electron antineutrino.

In β + decay (observable in proton-rich nuclei), a proton interacts with an electron antineutrino to obtain a neutron and a positron (the direct disintegration of the proton in the positron has not yet been observed).

The interaction of beta particles with matter generally has a ten-fold greater range of action and an ionizing power equal to one tenth as compared to the interaction of alpha particles. They are completely blocked with a few millimeters of aluminum.

Gamma rays

Gamma rays are electromagnetic radiation produced by radioactivity. They stabilize the nucleus without changing its proton content. Normally, radiation usually accompanies another type of emission. They penetrate deeper than a or beta radiation, but they are less ionizing.

Gamma rays can cause serious damage to the nucleus of cells, so they are used to sterilize medical equipment and food.


A radionuclide is the set of radioactive nuclei of the same species. All the radioactive nuclei that form a radionuclide have a well-defined radioactivity, common to all of them, that identifies them; in the same way that a type of chemical reaction identifies the elements that participate.

Radioactividad - partículas emitidas

Quantitatively, radioactivity is a statistical phenomenon. For this reason, to evaluate it, we must observe the behavior of a set of nuclei of the same species. By the law of large numbers, a radioactive constant λ is defined as the probability of disintegration of a nucleus per unit of time.

With this definition, the number N of radioactive nuclei of the same species that are found in a substance at time t is given by N = No · e-λt, where it is not the number of radioactive nuclei that existed before the time elapsed. time t. Actually, hardly a radioactive substance is formed by a single radionuclide, although each of its components to disintegrate is transformed into a different nucleus which, in turn, can also be radioactive.

The initial radionuclide is called father, and the derivative, son. This situation can continue along multiple filiations and the set of all is called family or radioactive series. In this case, the relationship that gives the number of radioactive nuclei present is more complex because, in addition to taking into account the number of each of them at the initial moment, we must consider that, by disintegrating some, others are formed.

The problem is simplified when you want to achieve radioactive equilibrium (also known as secular equilibrium in natural radioactive series), which is when a long enough time has passed since the filiation process has begun, because then the rhythm of the disintegrations is imposed by the radionuclide that has the smallest radioactive constant.

Natural radioactive nuclei

In nature, there are about 300 different nuclides, of which 25 are radioactive with a sufficiently long period for it to exist even today; another 35 have a much shorter period and are continuously created and disintegrated in the radioactive series.

Artificial radioactive nuclei

More than 1000 artificial radionuclides have been created and identified. The radioactive series are named after the parent nuclide of longer periods. There are four. Three of these radioactive series are natural: that of thorium, that of uranium and that of actinium, which end in their own stable isotopes of lead.

These isotopes have respectively mass numbers 208, 206 and 207. Regarding the neptunium series, as the radionuclides that compose it have a short period compared to the duration of the geological ages, it is not found in nature and has been obtained artificially The last nuclide in this series is the isotope 209 of bismuth.

Origin of radioactivity

Radioactivity was discovered in 1896 by Antoine-Henri Becquerel, who, when making studies on the phosphorescence of the substances, observed that a uranium mineral was able to veil some photographic plates that were stored at his side.

The effects of radioactivity on human health

There are two main health effects caused by radiation, which act in the short and long term and also at shorter and larger distances.

Radiation causes health problems by killing cells in the body, and the amount and type of damage caused depends on the dose of radiation received and the time during which the dose is extended.

In the event of a nuclear accident emergency workers can receive a maximum of 100 millisieverts (mSv) for an action to save assets. If the emergency action is to save lives, a radiation exposure of a maximum of 250 mSv is allowed.

If a person receives between 250 millisieverts (mSv) and 1 sievert (Sv) in a single day, the radioactive exposure provokes symptoms of radiation poisoning. These symptoms of radiation poisoning can be nausea, damage to the lymph nodes and damage to the bone marrow.

If the radioactive dose is increased to 3 Sv, these same effects are more serious with a probability of contracting infections due to a reduced number of white blood cells in the body; With treatment, survival is likely but not guaranteed.

Larger doses, in addition to the symptoms mentioned above, will cause hemorrhage, sterility and skin detachment; an untreated dose of more than 3.5 Sv will be fatal, and death is expected even with treatment for doses of more than 6 Sv.

The level of radiation decreases with the square of the distance from its source, so that someone who is twice as far from an external source will receive a quarter of the radiation.

In general, receiving a higher dose in less time causes more acute damage, since higher doses kill more cells, while the body may have had time to repair some damage with more time between doses.

However, radioactive material that spreads to a wider area can cause long-term health effects through prolonged exposure, especially if they enter the food chain or are inhaled or ingested directly.

Carrying radioactive materials into the body also presents the greatest danger to atoms that undergo alpha decomposition, since alpha particles are not very penetrating and are easily absorbed by a few centimeters of air. It was polonium-210 alpha emitter that was used to assassinate Alexander Litvinenko in 2006.

The radioactive isotopes of iodine, which suffer a beta decay, can accumulate in the thyroid gland and cause thyroid cancer. Attempts to prevent this involve the distribution of pills that include non-radioactive iodine 127 and that flood the thyroid, preventing the absorption of radioactive iodine.

For single doses, such as those in medical examinations, the risk of developing cancer later is estimated at around 1 in 20,000 per mSv received.

It is estimated that the absorption of a cumulative dose of 1 Sv over a longer period of time can cause cancer in 5% of people.

However, there is disagreement about whether very small doses, comparable to the level of background radiation, actually contribute to health effects.

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Last review: December 15, 2018