## 2.2.2. Radioactive compounds

Authors: Nathalie Vanhoudt, Nele Horemans

Reviewer: Robin de Kruijff

Learning objectives:

You should be able to:

- describe the process of radioactive decay
- describe the different types of radiation and their interaction with matter
- explain the difference between natural and artificial radionuclides and give examples

Keywords: artificial radionuclides; ionising radiation; naturally occurring radionuclides; radioactive decay

Introduction

Naturally occurring radionuclides are omnipresent in the environment and exposure to radiation is unequivocally related to life on Earth. Every day we are exposed to cosmic radiation, radon exhalation from the soil and radioactive potassium naturally present in our bodies. Moreover, radionuclides and ionising radiation are successfully applied in many domains such as nuclear medicine, research applications, energy production, food preservation and other industrial activities. To be able to positively apply radionuclides or ionising radiation and to evaluate the impact on man and environment in case of a contamination scenario, it is important to understand the process of radioactive decay, the different types of radiation and radionuclides and how ionising radiation interacts with matter.

Radioactive decay

Radioactivity is the phenomenon of spontaneous disintegration or decay of unstable atomic nuclei to form energetically more stable ones (Krane, 1988). Within this process, particles (e.g. protons, neutrons) and/or radiation (photons) can be emitted. This radioactive decay is irreversible and after one or more transformations, a stable, non-radioactive atom is formed.

Radioactive decay is considered a stochastic phenomenon as it is impossible to predict when any given atom will disintegrate. However, the probability per unit time that an unstable nucleus can decay is described by the disintegration or decay constant λ [s-1]. The fact that this probability is constant, forms the basic assumption of the statistical theory of radioactive decay.

Radioactive decay follows an exponential function (Eq. 1, Figure 1) with N0 the number of nuclei at time 0, N(t) the remaining nuclei at time t and λ the decay constant [s-1].

Eq. 1

The decay constant λ is specific for every radionuclide and the half-life t1/2 of a radionuclide can be derived from this constant (Eq. 2).

Eq. 2

The specific half-life of a radionuclide gives the time that is necessary for half of the nuclei to decay. Half-lives can vary between fractions of seconds to many billions of years depending on the radionuclide of interest. For example 238U and 232Th are two primordial radionuclides with half-lives of 4.468 × 109 y and 1.405 × 1010 y, respectively. 137Cs on the other hand is an important radionuclide released during the Chernobyl and Fukushima nuclear power plant accidents and has a half-life of 30.17 y. While other shorter-lived radionuclides released during these accidents (e.g. 131I with a half-life of 8 days) have already decayed, 137Cs has the most substantial long-term impact on terrestrial ecosystems and human health owing to its relatively long half-life and high release rate (Onda et al., 2020).

Figure 1. Illustration of exponential decay and half-life t1/2.

The activity A of a radioactive material is defined by the rate at which decay occurs in the sample and is determined by the amount of radioactive nuclei present at time t and the decay constant λ (Eq. 3). As such, the activity of A sample is a continuously decreasing value following the same exponential curve as presented in Fig. 1.

Eq. 3

The SI-unit to express activity is Becquerel [Bq], equal to one disintegration per second. In a sample with an activity of 100 Bq, it is expected that 100 radioactive disintegrations will occur every second. Also the older non-SI unit Curie (Ci) is still often used to express activity, with 1 Ci being equal to 3.7 × 1010 Bq.

One way by which an unstable nucleus will strive towards a more stable state is by emitting particles and as such creating a new nucleus. During this process, α-particles, protons, neutrons, β--particles and β+-particles can be emitted by the nucleus.

For example, during alpha decay, an α-particle, which is a stable configuration of two protons and two neutrons (4He nucleus), is emitted, resulting in a new nucleus with an atomic number Z that is two units lower (2 protons) and a mass number A that is 4 units lower (2 protons + 2 neutrons) (Figure 2).

Figure 2. Alpha decay.

During beta decay, the nucleus can correct an imbalance between neutrons and protons by transforming one of its nucleons (i.e. converting a neutron into a proton or vice versa). This process can occur in different ways that all involve an extra charged particle (beta particle or electron) to conserve electric charge (Krane, 1988). During β--decay, a neutron is converted into a proton with emission of a highly energetic negatively charged electron (β--particle) and an antineutrino (Figure 3). During β+-decay, conversion of a proton into a neutron is accompanied by emission of a positively charged electron (positron or β+-particle) and a neutrino. In addition, the nucleus can also correct a proton excess by capturing an inner atomic electron to convert the proton into a neutron. This process is called electron capture.

Figure 3. Beta minus decay.

Although fission is usually considered as a process that is artificially induced (e.g. nuclear reactor), some heavy nuclei with an excess of neutrons naturally decay through fission, resulting in two lighter nuclei and a few neutrons. These new nuclei usually also further decay.

A second way by which a nucleus in its excited state will transform into a more stable state, is by emitting energy in the form of highly energetic electromagnetic radiation called photons. During this process, the original nucleus is maintained. Gamma decay is often a secondary process after alpha or beta decay as the nuclei often contain an excess amount of energy after transformation. As the energies of the emitted gamma rays are unique to each radionuclide, gamma ray energy spectra can be used to identify radionuclides in a sample.

Today, more than 4000 radionuclides are known and information regarding these radionuclides is compiled in a nuclide chart, which is a two dimensional representation of the nuclear and radioactive properties of all known atoms (Figure 4) (Sóti et al., 2019). In contrast to the periodic table, the nuclide chart arranges nuclides according to their number of neutrons (X-axis) and protons (Y-axis). This chart includes information on half-lives, mass numbers, decade modes, energies of emitted radiation, etc. Different colours are used to represent stable nuclei and specific modes of radioactive decay (e.g. alpha decay, beta decay, electron capture). Sóti et al. (2019) can be consulted for more information regarding the content and use of the nuclide chart and an interactive nuclide chart has been made available by the International Atomic Energy Agency (IAEA).

Figure 4. Illustration of a nuclide chart (designed based on the IAEA interactive nuclide chart) and Krane (1988)).

Naturally occurring radionuclides and artificial radionuclides

Naturally occurring radionuclides such as 238U, 232Th, 226Ra and 40K are omnipresent in the environment and high concentrations can often be found in certain geological materials such as igneous rocks and ores. For example, activity concentrations between 7 and 60 Bq kg-1 of 238U and between 70 and 1500 Bq kg-1 of 40K can be found in the most common rock types (IAEA, 2003). One group of naturally occurring radionuclides are the primordial radionuclides that were created before the formation of planet Earth and have long half-lives of billions of years. While some of these primordial radionuclides exist alone (e.g. 40K), others are the head of nuclear decay chains (e.g. 238U, 232Th and 235U). Through subsequent alpha and beta decay, these radionuclides decay until a stable Pb isotope is formed. Radionuclides such as 238U, 232Th, 226Ra, 210Pb, 210Po, with their own specific chemical and radiological properties, are part of these radioactive decay chains. Similar as for other elements, the chemical form in which these radionuclides occur will determine their behaviour and fate in the environment and finally their possible risk to humans and other biota.

The three radioactive decay chains and the primordial radionuclide 40K contribute most to the external background radiation humans are exposed to. Within the 238U and 232Th radioactive decay chains, two isotopes of the noble gas Rn (222Rn and 220Rn, respectively) are formed. In contrast to the other decay products, this noble gas has the potential to migrate through the pores of rocks towards the soil surface. Through this process, radioactive Rn can be released into the atmosphere resulting in an average activity concentration of 1-10 Bq m-3 in air, although this value is highly dependent on the soil type and composition. Although 222,220Rn itself is inert it can decay to other alpha and beta emitters that can attach to tissues. Especially when inhaled, the decay products of 222,220Rn can cause internal lung irradiation.

In addition, several industries (e.g. metal mining and milling, the phosphate industry, oil and gas industry) are involved in the exploitation of natural resources that contain naturally occurring radionuclides. These activities will result in enhanced concentrations of radionuclides in products, by-products and residues that can lead to elevated (or more bioavailable) radionuclide levels in the environment posing a risk to human and ecosystem health (IAEA, 2003).

Besides the primordial radionuclides and radionuclides that are part of the 238U, 232Th or 235U radioactive decay chains, some radionuclides are continuously formed in the atmosphere through interaction with cosmic radiation. For example, 14C is continuously produced in the atmosphere through interaction of thermal neutrons with nitrogen (14N(n,p)14C).

Artificial radionuclides are those radionuclides that are artificially generated, for example in nuclear power plants, particle accelerators and radionuclide generators. These radionuclides can be generated for different purposes such as energy production, medical applications and research activities.

In the last century, nuclear weapon production and testing, improper waste management, nuclear energy production and related accidents have contributed to the spread of a large array of anthropogenic radionuclides in the environment, including 3H, 14C, 90Sr, 99Tc, 129I, 137Cs, 237Np, 241Am and several U and Pu isotopes (Hu, 2010). Although a wide range of radionuclides were released during the Chernobyl and Fukushima nuclear power plant accidents, most of them had half-lives of hours, days and weeks resulting in a rapid decline of radionuclide activity concentrations (IAEA, 2006, 2020). After the initial release period, 137Cs remained the most important radionuclide causing enhanced long-term exposure risk for humans and biota (IAEA, 2006, 2020). Nonetheless, compared to nuclear weapon production and testing, nuclear accidents contribute only for a small fraction to the environmental contamination (Hu, 2010). Recent maps on the 137Cs atmospheric fallout from global nuclear weapon testing and the Chernobyl accident in European topsoils are presented by Meusburger et al. (2020).

Interaction of ionising radiation with matter

Ionising radiation has the potential to react with atoms and molecules in matter and cause directly or indirectly ionisations, excitations and radicals, which will result in damage to organisms. Although ionising radiation can originate from radioactive decay, it can also be artificially generated (e.g. X-rays) or come from cosmic radiation.

Directly ionising radiation consists of charged particles such as alpha or beta particles with sufficient kinetic energy to cause ionisations. When colliding with electrons, these particles can transfer part of their energy resulting in ionisations. Alpha particles have usually a high energy, typically around 5 MeV. Due to their relatively high mass, high kinetic energy and their charge, they have a high ionising potential. When interacting with matter, an alpha particle follows, due to its high mass, a relatively straight and short path along which ionisations and excitations occur (Figure 5). During each interaction, a small amount of the particle’s energy is transferred until it is finally stopped. This will result in a lot of damage in a small area, hence its high ionising potential. As their penetration depth is low, the alpha particle can be stopped by a few centimetres of air or a sheet of paper (Figure 5). This means that alpha particles cannot penetrate the skin resulting in low hazard in case of external irradiation. On the other hand, when present inside the body (in case of internal contamination), much more damage can be induced due to its high ionising capacity and the lack of a shielding barrier. Their property to deposit all their energy in a very small area makes alpha emitters perfectly suited for the local treatment of tumour cells. A targeting biomolecule to which an alpha emitter (or a radionuclide that decays into an alpha emitter) is chemically bound can be injected intravenously to spread through the body and accumulate in specific body tissues or cells where it locally irradiates tumour metastases.

Beta particles are high speed electrons or positrons emitted during radioactive decay. Due to their low mass, usually high kinetic energy and their charge, they have a lower ionising potential compared to alpha radiation but a higher penetration depth. In contrast to alpha particles, beta particles do not follow a linear path when interacting with matter. When colliding with other electrons, beta particles can change direction, resulting in a very irregular interaction pattern (Figure 5). In air, beta particles have a penetration potential from several decimetres up to a few meters while this is reduced to centimetres when interacting with solids or liquids. Care has to be taken when selecting the best shielding material as beta particles can also generate Bremsstrahlung, which is electromagnetic radiation produced when the beta particle is deflected in the electric field of an atomic nucleus (Figure 5). Materials with low atomic number such as Plexiglas or aluminium are preferred to minimize the additional risk of Bremsstrahlung production (Figure 5). As beta particles can penetrate the human tissue up to a few millimetres, it forms both an external and internal risk.

In the case of indirect ionising radiation such as gamma radiation, charged particles are first created through energy transfer from the radiation field to matter which will then cause ionisations. Not all types of electromagnetic radiation are considered ionising radiation. Only radiation with a short wavelength will have sufficient energy to induce ionisations such as gamma radiation, X-rays and high energy UV radiation. However, the interaction with matter is fundamentally different between charged and uncharged particles such as gamma radiation. While charged particles interact with many particles at the same time, resulting in a lot of ionisations, uncharged particles will mainly not interact with particles along their pathway through matter but there is a likelihood that they will interact. When they interact, ionisations are induced indirectly as energy is first transferred to release charged particles (such as electrons) that will in turn cause ionisations (Figure 5). As such, due to its lack of charge and minimal mass, gamma radiation has a high penetration potential, forming an important internal and external risk. Lead is a commonly used shielding material for gamma radiation (Figure 5). Nonetheless, the high penetration potential and the difference in interaction with tissues of different density, forms the basis to use X-rays in internal imaging techniques for medical and industrial purposes.

Figure 5. Illustration of the interaction of alpha, beta and gamma radiation with matter.

References

Hu, Q.-H., Weng, J.-Q., Wang, J.-S. (2010). Sources of anthropogenic radionuclides in the environment: a review. Journal of Environmental Radioactivity 101, 426-437. https://doi.org/10.1016/j.jenvrad.2008.08.004.

IAEA (2003). TRS 419 Extent of environmental contamination by naturally occurring radioactive material (NORM) and technological options for mitigation. International Atomic Energy Agency. Vienna, Austria.

IAEA (2006). Environmental consequences of the Chernobyl accident and their remediation: Twenty years of experience. International Atomic Energy Agency. Vienna, Austria.

IAEA (2020). TECDOC 1927 Environmental transfer of radionuclides in Japan following the accident at the Fukushima Daiichi Nuclear Power Plant. International Atomic Energy Agency. Vienna, Austria.

Krane, K. (1988) Introductory Nuclear Physics. John Wiley & Sons, Inc.

Meusburger, K., Evrard, O., Alewell, C., Borrelli, P., Cinelli, G., Ketterer, M., Mabit, L., Panagos, P., van Oost, K., Ballabio, C. (2020). Plutonium aided reconstruction of caesium atmospheric fallout in European topsoils. Scientific Reports 10:11858. https://doi.org/10.1038/s41598-020-68736-2.

Onda, Y., Taniguchi, K., Yoshimura, K., Kato, H., Takahashi, J., Wakiyama, Y., Coppin, F., Smith, H. (2020). Radionuclides from the Fukushima Daiichi Nuclear Power Plant in terrestrial systems. Nature Reviews Earth & Environment 1, 644-660. https://doi.org/10.1038/s43017-020-0099-x.

Sóti, Z., Magill, J., Dreher, R. (2019). Karlsruhe Nuclide Chart – New 10th edition 2018. EPJ Nuclear Sciences and Technologies 5, 6. https://doi.org/10.1051/epjn/2019004.