The State Meteorological Agency (AEMET) has warned us: during the next few days a significant amount of suspended dust from the Sahara Desert will significantly affect the Canary Islands. And also to the Iberian Peninsulaalthough to a lesser extent. Some Spanish media are reporting that Saharan dust has the capacity to deposit radioactive elements on Spanish soil. And yes, it is true.

In July 2023, a team of French researchers published in Earth System Science Data a very interesting scientific article in which he confirmed that, indeed, the dust from the Sahara that sometimes reaches Europe contains particles of radioactive elements, such as cesium or beryllium, which end up depositing on the ground. Furthermore, this analysis revealed a large variability in the properties and amount of dust. And the larger and heavier particles are deposited closer to their origin than the smaller and lighter ones.

These French scientists verified that some of the radioactive elements they identified, such as cesium, beryllium or lead, probably came from the nuclear tests carried out by France in the Sahara in the 1960s. However, they also found traces of plutonium linked to the nuclear tests carried out by the United States and the Soviet Union at that same time. There is still much to do to understand precisely what impact these radioactive particles have on our ecosystem, but These scientists suggest that can alter the terrestrial radioactive balance, cloud formation or solar energy production. They could also impact people’s health.

What is radioactivity

Radioactivity is the naturally occurring process that explains how an unstable atomic nucleus loses energy in the attempt to reach a more stable state. And to achieve this it emits radiation. One or several even smaller elementary particles with a negative electrical charge, which we call electrons, orbit around the nucleus. The nucleus, in turn, is made up of one or more protons, which are particles with a positive electrical charge. The simplest atom that we can find in nature is protium (hydrogen-1), an isotope of hydrogen that has a single proton in its nucleus and a single electron orbiting around it.

The problem is that matter is not only composed of protium, but also of many other more complex and heavier chemical elements, and which, therefore, have more protons in their nucleus and more electrons orbiting around them. How is it possible that there is more than one proton in the nucleus If all of them have a positive electric charge? It is reasonable to think that they could not be very close together because having the same elemental electrical charge would repel each other. And yes, this idea is coherent. Those responsible for solving this dilemma are neutrons, the particles that coexist with protons in the atomic nucleus.

Unlike protons, neutrons have a neutral overall electrical charge, so they do not “feel” either the electromagnetic repulsion or attraction to which protons and electrons are exposed. The function of neutrons is none other than to stabilize the nucleus, allowing several protons that would otherwise repel each other to coexist in it. And they manage to do so thanks to the action of one of the four fundamental forces of nature: the strong nuclear interaction.

The other three forces are the electromagnetic interaction, gravity, and the weak nuclear interaction. Physicists usually place the Higgs field at this same level, which is another fundamental interaction that explains how particles get their massbut to facilitate its understanding, the texts usually include the four that I mentioned a little above as fundamental forces because they are in some way with which we are all familiar.

The nucleons, which are the protons and neutrons of the atomic nucleus, manage to stay together and overcome the natural repulsion that the protons face because the presence of neutrons allows the strong nuclear force to act as a glue capable of imposing itself to the electromagnetic force. The strong nuclear interaction has a very limited range, but at short distances its intensity is enormous. The important thing about all this is that neutrons, as I told you a few lines above, act by stabilizing the atomic nucleus, so that as an atom has more protons it will also need more neutrons in its nucleus so that the strong attractive force manage to overcome the repulsive electromagnetic force.

Interestingly, the balance between the number of protons and neutrons is very delicate. An atom is stable if its nucleus has a precise number of nucleons and the distribution of these between protons and neutrons allows the strong nuclear interaction to act as “glue.” For this reason in nature we can only find a finite amount of chemical elements: those included in the periodic table with which we are all more or less familiar. Any other combination of protons and neutrons would not allow this fine balance to be maintained, giving rise to an unstable atom.

What differentiates a stable atom from an unstable one is that in the nucleus of the latter the strong nuclear interaction and the electromagnetic force are not in balance, so the atom needs to modify its structure to reach a state of lower energy that allows it adopt a more stable configuration. A stable atom is “comfortable” with its current structure and does not need to do anything, but an unstable one needs to give up some of its energy to reach the lower energy state we just talked about.

In that case, how does the atom manage to get rid of part of its energy? The answer is surprising: resorting to a quantum mechanism known as the “tunnel effect” that allows you to do something that a priori seems impossible, and which is nothing other than overcome an energy barrier. This quantum effect is complex and very unintuitive, but fortunately, we do not need to delve into it to clearly understand how radioactivity works. What is important is that we know that an unstable atom has four different mechanisms at its disposal that can help it modify its structure to adopt a stable configuration: alpha, beta, inverse beta and gamma radiation.

The first of these mechanisms, alpha radiation, allows the atom to get rid of a part of its nucleus by emitting an alpha particle, which is made up of two protons and two neutrons. The next mechanism is beta radiation, which requires a neutron from the atomic nucleus to transform into a proton, and during this process it also emits an electron and an antineutrino. Reverse beta radiation works just the opposite of beta radiation: a proton transforms into a neutron and this process emits an antielectron and a neutrino, which are the antiparticles of the electron and antineutrino emitted by beta radiation.

And finally, gamma radiation, which is the most energetic and most penetrating of all, requires the emission of a high-energy photon, commonly known as a gamma ray, so the atomic nucleus maintains its original structure. Some of these high-energy photons are capable of passing through very thick concrete walls and lead sheets, making this the most dangerous form of radiation of all.

As we have just seen, radioactivity allows unstable atoms to shed part of their energy in order to reach a less energetic and more stable state, but what really happens with that energy? The principle of conservation of energy says that it cannot be destroyed, so it is necessarily taken away by the particles emitted by the unstable atom as a result of any of the four forms of radiation we just talked about. This energy causes the emitted particles to be ejected like tiny bullets that have the ability to interact with the matter they find in their path.

Image | 光曦刘

More information | AEMET | Earth System Science Data

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