Radioisotopes as Sources of Ionizing Radiation

(Transcript of the lesson commentary.)

From the structure of the atom through radioactivity to ionizing radiation

When passing through matter, ionizing radiation releases electrons from atoms and can thus disrupt molecules, damage organic tissue or lead to structural changes in matter. We are constantly exposed to the effects of ionizing radiation as it is a natural part of our environment. Different types of ionizing radiation come both from radioactive elements contained in the Earth’s crust and from space in the form of cosmic rays and we also receive a certain dose of radiation from artificially created sources. Each type of ionizing radiation has its own specific properties and for each type there is a way to effectively shield it or, on the contrary, use its properties.

The essence and effects of ionizing radiation could not have been explained in the past without the discovery of radioactivity and experiments leading to an understanding of the composition of the atom. A number of scientists took care of this, starting with Wilhelm Conrad Röntgen with his mysterious gas-discharge radiation, Henri Becquerel studying the radiation of unexposed uranium ore and Maria Skłodowska-Curie, who named the ability of matter to emit radiation by radioactivity.

Ernest Rutherford contributed greatly to the expansion of knowledge about the mysterious invisible radiation but also about the structure of the atom. It was him who divided radiation into three groups according to the depth of their penetration and proved that alpha radiation is actually a stream of helium atom nuclei, while beta radiation is composed only of electrons. A few years later, based on the famous experiment with the bombardment of gold foil with alpha particles, he made another important discovery explaining the nature of the composition of the atom. He came to the conclusion that almost all the mass of an atom is concentrated in a very small volume of the positively charged nucleus and the much lighter electrons rotating around it.

The correctness of Rutherford’s planetary model of the atom was later explained using quantum physics by Niels Bohr, according to which electrons in an atom rotate around a small nucleus in certain orbits and remain there without losing their energy. Only going into a higher or lower orbit would require the absorption or emission of a certain amount of energy. The discoveries of subatomic particles of the nucleus — the proton, discovered in 1917 by Rutherford, and the neutron, discovered by James Chadwick in 1932 — contributed to a complete understanding of the composition of the atom.

An atomic nucleus consists of protons and neutrons, the number of protons being the atomic number, which uniquely identifies the element. Atoms with the same number of protons but different numbers of neutrons are referred to as isotopes of that element. Isotopes have the same chemical properties but differ in their physical properties. Some of the isotopes are unstable and when releasing a particle from the nucleus, it shifts the ratio of protons to neutrons in them to a more stable state. This process is called radioactive decay or transmutation. When alpha or beta radiation is emitted, the atomic number changes and thus the element is transmuted into another element. In gamma radiation, only a photon is emitted and the nucleus just releases excess energy.

Characteristics of individual types of radiation

We can call radiation not only electromagnetic waves, but they can also be a stream of particles, for example positively charged helium nuclei or negatively charged electrons. The penetration of the radiation depends on the distance it travels before the moving particle is stopped by the successive ionization of the atoms or the photon is absorbed by the atom. Ionizing radiation can be divided into alpha, beta and gamma rays, X-rays, cosmic rays and neutrons. Cosmic rays are primarily made up of high-energy protons, which, after colliding with the upper atmosphere, initiate a shower of secondary radiation.

Due to the weight of the alpha radiation particles, it has the smallest depth of penetration into matter. It is actually a stream of positively charged helium nuclei, which can be reliably shielded by an ordinary sheet of paper or the top layer of human skin. In alpha decay, the element’s atomic weight is reduced by four — the decaying nucleus loses two protons and two neutrons, forming a helium nucleus. Typical sources of alpha radiation are uranium, thorium and radium.

Beta radiation, on the other hand, is a stream of electrons or positrons that arise in the nucleus of an atom during the decay of a neutron or proton. The depth of penetration is greater than that of alpha particles but beta radiation can still be shielded by a few centimetres of water or a few millimetres of aluminium. In beta decay, the atomic number increases by 1. A typical source of beta radiation is caesium.

The third type is gamma radiation. It is a stream of photons with a wavelength of less than 100 pm. It has a very high penetration and ionizes indirectly through particles that are created after the interaction of gamma rays with matter. Gamma radiation is produced together with alpha or beta radiation and can be shielded by a thick layer of lead.

X-rays are also indirectly ionizing short-wave radiation with a wavelength between 10 nm and 100 pm. It has a great penetration depth and can be shielded with a thick layer of concrete or lead. X-rays are not produced by the decay of an atom’s nucleus but rather when electrons from higher energy levels fill a vacancy in a lower orbit.

The last type of indirect ionizing radiation is a stream of elementary subatomic particles without electric charge — neutrons. Neutrons are created mainly during fission in nuclear reactors or by combining sources of alpha radiation and light elements. They spread over long distances but can be shielded by several meters of water or other hydrogen-rich substances.

Natural sources of ionizing radiation

Almost everything around us can be a source of ionizing radiation. Radioactive elements, and therefore potential sources of radiation, are found in soil, air, water, food, and even in the human body. Other radiation is constantly falling on us from space. All these sources are called “natural” and the dose of radiation we receive from these sources in a year is called “natural background”. About 4/5 of the total annual dose that a person receives is from natural sources.

The dose of natural background depends on the specific location and altitude where we live. The bedrock in different locations contains different amounts of radioactive elements, and if, for example, there is a larger amount of uranium or thorium in the rocks, this also increases the amount of radon in the surrounding air. At higher altitudes, a higher dose of cosmic rays are added to the ground radiation.

The source of cosmic rays is mainly our Sun, a smaller part comes from the surrounding space. However, only a small part of this radiation falls on the Earth's surface, the majority is captured by the Earth’s magnetic field. In the upper layers of the atmosphere, radiation particles create secondary radiation showers of various types when they collide with air molecules. The contribution of cosmic rays to the annual dose is approximately 0.25—0.4 mSv and its fluctuations depend mainly on the activity of the Sun.

Soil and bedrock contain certain amounts of radioactive elements with higher concentrations mainly found in volcanic rocks. Subsequently, building elements made of these materials are also radioactive, so even the walls of buildings can be a source of ionizing radiation. The annual dose depends on the composition of the soil and averages about 0.4 mSv. However, there are places in the world where the natural background is many times higher than the average and reaches over 100 mSv.

Radioactive elements, as they grow, absorb plants from the soil and they continue to enter the food chain so everything we eat and drink is radioactive to some extent. The amount of radioactive substances in food depends on the composition of the soil in which it was grown and the way the plant or animal absorbs the radioactive elements. Mushrooms, for example, are much more absorbent in this regard than most other plants. In addition to substances from the soil, plants also absorb the radioactive carbon isotope 14C from carbon dioxide in the air. Food adds 0.2—0.6 mSv to the annual dose.

Radioactive sources of radiation are also contained in the air. It is mainly radioactive radon gas with a relatively short half-life that can accumulate in unventilated residential buildings located above uranium and thorium-bearing rocks. Its danger lies in long-term inhalation, in which radon emits alpha radiation when it decays in the lungs. The annual dose per inhabitant is approximately 0.2—3.0 mSv.

The human body itself is also a natural source of radiation. It contains the very important elements carbon and potassium and both of these elements have their radioactive isotopes. Every second, several thousand radioactive changes occur in our body. The annual dose we receive from our own body is approximately 0.4 mSv and depends primarily on our body weight.

Sources of radiation created and used by man

In addition to natural radioactive sources, the number of artificial sources of ionizing radiation also increases with the development of scientific knowledge and medicine. And medicine is the field where these man-made resources are used the most, primarily in diagnosis and medical therapy. Artificial resources have also found their place in the food industry, science and many other fields. From these artificial sources we receive an average annual dose of 0.3 to 0.6 mSv. The specific value depends mainly on the frequency and types of medical procedures that we have to undergo.

If we wanted to imagine artificial radioactive sources more concretely, we have to certainly start with medicine. Modern medicine cannot be imagined without diagnostic devices that use X-rays both in the classic form of images and in the sophisticated form of computed tomography. Radioisotopes are also used in liquid form, when they are injected into the bloodstream or given with certain substances, so that the area where the substances with radioisotopes are deposited can then be seen in scintigraphy. High and precisely focused doses of radiation are used in medicine to treat and remove tumours.

Practice has shown that strong sources of gamma radiation destroy harmful microorganisms and therefore doses of this radiation are used, for example, to sterilize surgical instruments, to protect historical monuments against fungi and mould, or to extend the shelf life of food products. In industry, gamma radiation sources are used as precision gauges to measure the thickness or density of materials, to examine their internal structure, or to assess the quality of welds and to detect invisible defects.

In technology, some radioisotopes are used as markers to detect leaks in closed systems. Trace amounts of radioisotopes can also be found in home smoke detectors, when glass is dyed with uranium or during the production of superphosphate fertilizers.

Spent nuclear fuel from nuclear power plants is also a strong artificial source of ionizing radiation, but due to its strict isolation from the environment, its contribution to human exposure is practically negligible. Conversely, burning coal, which always contains small amounts of uranium, thorium or radium, concentrates radioactive isotopes, so that a coal-fired power plant contributes about three times more to the radiation burden of local residents than a nuclear power plant.