Sticky radiation: induced radioactivity, radioactive contamination, deactivation ...

For many people, radiation appears as something “contagious”: it is believed that if something is exposed to radiation, it becomes its source. These ideas have their own rational grain, but the ability of radiation to “switch over” to the irradiated things is very much exaggerated. Many people think, for example, that you can “grab a dose” from the parts of a disassembled X-ray machine, from x-rays, and even from a radiologist. And how much noise rises when they start talking about gamma irradiation of food for their sterilization! Like, we also have to eat irradiated, which means radioactive food. There are altogether ridiculous rumors that microwaves “remain in the food warmed in the microwave”, that under the action of germicidal lamps the air in the room where they burned becomes radioactive.

In this article I will tell you how everything really is.

When radiation generates radiation

In 1934, Frederick and Irene Joliot-Curie, studying the interaction of alpha particles with atoms of different elements, found that some of them — aluminum, boron, magnesium — emit some radiation when bombarded with alpha particles, which is detected by a Geiger counter, which does not immediately stop after the source of alpha rays has been removed, and rapidly decreases exponentially. An experiment in the Wilson chamber showed that this radiation is a stream of positrons, a little earlier discovered in cosmic rays. Spouses Joliot-Curie would not have been Curie, if they had not guessed that they were once again confronted with the phenomenon that the alchemists had been trying to open for centuries, but had never discovered. The alpha particle, which is a helium nucleus, collided with the aluminum nucleus, knocking out a neutron from it, and the nucleus of the radioactive phosphorus isotope was formed. And this conjecture was proved by an extremely subtle and skilful chemical experiment, with the help of which it was possible to isolate and detect by radioactivity an insignificant amount of phosphorus that could not be seen in a single microscope if all its atoms were gathered “in a handful”. And this phosphorus also melted in front of his eyes.
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Subsequent experiments discovered that neutrons, especially those slowed by passing through water, paraffin or graphite, have an even greater ability to initiate nuclear reactions and activate various substances. With the discovery of nuclear fission reactions producing a huge amount of neutrons, on the one hand this became a big problem - not only nuclear fuel, but also all the elements of reactor design became terribly radioactive. On the other hand, in this way it became possible to obtain the required radionuclides cheaply and in large quantities. The air and the ground activated by the neutron flux of a thermonuclear explosion are an additional serious factor of damage, so the “ecological purity” of the hydrogen bomb is no more than a myth.

So in what case does irradiation cause nuclear reactions and lead to the appearance of artificial radioactivity?

As I have already said, neutrons have a special capacity for this. It is not difficult to guess the reason: the neutron easily penetrates the nucleus. It does not need to overcome electrostatic repulsion, like a proton or an alpha particle. At the same time, a neutron is the same building material of a nucleus, as well as those protons and neutrons, just as capable of engaging in strong interaction. Therefore, the chemical element with the number zero is the very “philosopher's stone” of the alchemists. Rather, they could be called "Alfizikov", if this word would not be used in relation to adherents of the ether and torsion fields.

A neutron of any energy, up to zero, can cause a nuclear transformation. But other particles must have a sufficiently large energy. I already said about alpha particles (like protons): they need to overcome the Coulomb repulsion. For light elements, the alpha-particle energy required is several megaelectronvolts — that is, the amount of alpha particles emitted by heavy unstable nuclei. And the heavier ones already need dozens of MeV - such energy can only be obtained in an accelerator. Moreover, with the growth of the mass of the nucleus, it itself less and less willingly reacts with an alpha particle to the reaction: for iron, the addition of nucleons to the nucleus comes at a cost, and not with the release of energy. If we take into account the extremely low penetrating power of alpha particles into the target, it becomes clear that even with a very powerful flux of alpha particles, the intensity of artificial radioactivity is low.

And what about the other particles? Electrons, photons? They do not need to overcome repulsion, but they interact reluctantly with the core. An electron can only enter into electromagnetic and weak interactions and in most cases (with the exception of nuclei that are unstable to electron capture), such a reaction is possible only if the electron transfers considerable energy to the nucleus sufficient to detach the nucleon from the nucleus. The same applies to the photon - only a photon of sufficiently high energy can excite a photonuclear reaction , but an electron much faster than a photon loses energy in a substance, which is why it is less efficient.

The spectrum of photons emitted during radioactive decay ends at 2.62 MeV - this is the energy of thallium-208 quanta, the last member of the radioactive thorium-232 series. And there are very few nuclei whose thresholds for photonuclear reactions are below this value. More specifically, there are two such nuclei: deuterium and beryllium-9

$$

$$\ gamma + {} ^ {2} \ textrm {H} \ rightarrow p + n$$

$$

$$\ gamma + {} ^ {9} _ {4} \ textrm {Be} \ rightarrow n + ^ {8} _ {4} \ textrm {Be}$$

The first reaction proceeds under the action of gamma radiation above 2.23 MeV, the source of which is thallium-208 (thorium series), the second is enough 1.76 MeV - radiation of bismuth-214 (a series of uranium-radium).

These reactions yield neutrons, which, in turn, interacting with other nuclei, produce radioactive isotopes. But the cross sections of these reactions themselves are small, and therefore noticeable induced radioactivity is possible only with very high radiation intensity. For the implementation of other photonuclear reactions, gamma quanta are already needed, whose energy is measured in tens and hundreds of MeV. At such energies, not only photons, but in general all particles - electrons and positrons, muons, protons, etc., colliding with nuclei, cause nuclear reactions with a sufficiently high efficiency. The beams of such particles, obtained at accelerators, lead to the strong activation of practically any initially non-radioactive targets.

So, indeed, in some cases, when exposed to radioactive radiation on a substance, radioactive isotopes are formed. But usually a serious radiation hazard is represented by residual radioactivity in two cases:

• from targets exposed to neutrons;
• from targets irradiated in accelerators.

In all other cases, including under the action of X-rays, beta and gamma radiation (with the exception of the aforementioned beryllium and deuterium), radioactive isotopes of induced radioactivity do not occur. Alpha radiation produces weak and usually short-lived induced radioactivity when exposed to light elements.

Neither X-ray irradiation, nor exposure to other radiation - ultraviolet, microwave, etc., causes artificial radioactivity. Foodstuffs and drugs sterilized by radioactive radiation, seeds irradiated to increase germination and new varieties, stones irradiated to give them color (if it is not irradiated in the neutron channels of a nuclear reactor) do not become radioactive. X-ray parts, protective clothing of the radiologist and he himself are not radioactive!

To illustrate this, I had a little experience. Hiring an alpha source of americium-241 with an activity of 1 MBq in a neighboring laboratory (this is about 100 times the activity of the source contained in the HIS-07 smoke detector, which is not difficult to buy even on Aliexpress - ATTENTION! Illegal trafficking of radioactive substances - Article 220 Of the Criminal Code! ), I put an aluminum plate under it. As a result, as in the experience of Joliot-Curie (which used a much more powerful source), I had to get phosphorus-30, which decays into silicon-30 and a positron with a half-life of 2.5 minutes (and also a neutron, which is also anyone can activate). However, after half an hour of exposure (to establish an equilibrium between the birth and decay of phosphorus-30), I could not detect any noticeable radioactivity from the aluminum plate. I tried to use a Geiger counter with a mica window (they detected positrons in the same way as electrons), as well as a scintillation detector (which effectively registers them in the 511 keV line corresponding to the annihilation process). The reason for the failure of the experiment was that nuclear reactions under the action of alpha particles rarely occur, and even though in my experience aluminum was exposed to at least half a billion alpha particles, during this time only a few thousand radioactive atoms were formed, most of which during the exposure just broke up. Perhaps I would have been able to detect positrons in Wilson’s chamber due to the almost zero natural background of positrons, but I haven’t finished it yet (when I do it will be a good topic for the article).

Invisible radioactive dirt

In most cases, with the exception of those described above, contamination by radioactive isotopes on the surface of things and objects is taken for induced radioactivity. The fact is that with a half-life period of months, years, and decades, the amount of substance emitting frightening levels of radiation is truly negligible. Remember milligrams of radium, which gives 8.4 R / h at a distance of a centimeter? He has a half-life of 1600 years. And if the half-life is 1.6 years, and the energy of gamma-quanta is the same as that of radium? Then this milligram will “shine” at the same distance already 8400 R / h.

When dealing with radioactive isotopes, in most practical cases their number is negligible. These are the so-called indicator quantities , judged by their radioactivity. And in such cases, the phenomenon of adsorption — precipitation and “sticking” of a substance onto the interface - surface arises in its entire growth.

Radiochemists all the time have to fight with adsorption. Because of it, it is possible to completely lose a radioactive isotope during operations with it simply due to the fact that he has completely settled on the walls of the test tube or cup. It is necessary to select the composition of the “background” solution, but some of the isotope is still lost, and alas, often unknown. It is necessary to do parallel experience in absolutely the same conditions (up to test tubes from one box) or add an exit tag to the solution — another radioactive isotope of the same chemical element . And you can sit in a galosh in another way: the isotope, the solution of which was previously contained in the glass, settled on the wall and, despite subsequent washing and rinsing with acid, then distilled water, got into the next sample. The glass at the same time seemed absolutely, immaculately clean.

Any thing can seem just as immaculately clean, but nevertheless, radiating dirt is present on its surface (as well as inside pores communicating with it, cracks, etc.). And not only a thing: in the zone of radiation damage, skin and hair of affected people, animal hair can become radioactive. And not in all cases, this activity is easily removed. In most cases, the decontamination of highly contaminated objects is difficult, and in many cases it becomes unsuccessful.

Unlike induced radioactivity, which is usually firmly fixed on its carrier, contamination with radionuclides is located on its surface and therefore easily passes to other objects on people's hands and then enters their body, exposing it to internal radiation.

Decontamination - methods and means

The simplest method of decontamination is regular washing with soap or other surfactants. This is a method that is suitable for almost everything - you can wash the asphalt, the walls of the house, the living person, and the rare painting or violin with soap. In the latter case, this is done carefully, wiping the surface with a wrung fabric swab moistened in a soapy solution and immediately wiping with the same tampon with clean water, and then removing the remaining water with filter paper. Thus, the radiation of a violin lying in the hottest days of the Chernobyl disaster next to the open window of the Kiev house and “shining” close to 1 mR / h “conditionally” close, was reduced to an acceptable level and thus saved the instrument. There are specialized decontamination agents that contain, in addition to surfactants, complexing agents (such as EDTA), ion exchange resins, zeolites and other sorbents. Complexing agents promote the transfer of radionuclides that form cations into solution, and ion-exchange components and sorbents, on the contrary, remove them from solution, translating into a coherent form, but not on a deactivated surface. So, it is well known (and actively used in our laboratory) a Novosibirsk decontamination tool “Protection”, which operates according to this principle.

But such a means is often not enough: radionuclides are firmly bound to the surface, are deep in pores and microcracks. In such cases, it is necessary to use much more rigid methods - to treat the surface with acids, dissolving the surface layer of metal and a rust peel on it, and promoting desorption of radioactive contamination. Also used are strong oxidizing agents that destroy organic pollutants on the surface, on which radioactive dust also adheres. At a nuclear power plant for decontamination of equipment, a two-bath decontamination method is often used when the parts are first treated with an alkaline solution of potassium permanganate and then with acid.
For metal surfaces, an effective method of decontamination is the electrochemical method. The goal is about the same - to remove the surface layer of metal, layers of corrosion, impregnated with radionuclides. But the amount of liquid radioactive waste is sharply reduced, since it is possible to use the minimum amount of electrolyte. This is the so-called semi - dry electrolytic bath - a cloth or felt impregnated with electrolyte is placed on the surface to be deactivated and a second electrode is placed on top of it). The decontaminated part or surface is the anode, and as the cathode, usually a lead sheet is used that is easily deformable to tightly fit the surface being deactivated.

Sandblasting was also used to deactivate hard-to-remove radioactive contaminants, such as, for example, from helicopters flying over the emergency Chernobyl reactor. However, it generates a huge amount of radioactive dust, severely damages the surface being decontaminated, and generally has low efficiency.

If suddenly, God forbid, you get into the zone of radioactive contamination and you need to urgently deactivate something, then I recommend dishwashing detergent (“Fae”, etc.) or any laundry detergent with oxalic acid. You can also use such household cleaners for plumbing as Cif, they already have acid.
From induced radiation, deactivation usually does not help. After all, its source is located deep in the radiating object - neutrons have a very high penetrating power. But not always the impossibility of deactivation means that the radiation source is connected with it.

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Induced radiation is a real phenomenon, but it is so overgrown with myths that it itself has become a kind of myth. In reality, the formation of induced radioactivity must be taken into account in a number of cases, but in the usual handling of radioactive substances and other sources of ionizing radiation, there is no need to be afraid of induced radiation. But pollution with radionuclides is not only more real, but also more dangerous.

On - "Duga". Photo by Mike Deere .