Adhesive radiation: induced radioactivity, radioactive contamination, decontamination ...
Many people think that radiation is “contagious”: it is believed that if something has been exposed to radiation, it itself becomes its source. These representations have their own rational grain, but the ability of radiation to “transfer” to irradiated things is greatly exaggerated. Many people think, for example, that you can “grab a dose” from the parts of a disassembled X-ray machine, from X-ray images, and even from a radiologist. And how much noise rises when they start talking about gamma radiation of food products for their sterilization! Like, we will have to eat irradiated, which means radioactive food. Absolutely ridiculous rumors are circulating that “microwaves” remain in food heated in the microwave, and that under the influence of bactericidal lamps the air in the room where they burned becomes radioactive.
In this article I will tell you how everything really is.
When radiation gives rise to radiation
In 1934, Frederic and Irene Joliot-Curie, studying the interaction of alpha particles with atoms of various elements, found that some of them - aluminum, boron, magnesium - emit some radiation recorded by the Geiger counter during bombardment with alpha particles, which does not stop immediately after the source of alpha rays is removed, and quickly decreases exponentially. An experiment in the Wilson chamber showed that this radiation is a stream of positrons, a little earlier discovered in cosmic rays. The spouses of Joliot-Curie would not have been Curie if they had not guessed that they had again encountered a phenomenon that the alchemists had been trying to discover for centuries, but never discovered. The alpha particle, which is the nucleus of helium, collided with the nucleus of aluminum, knocking a neutron out of it, and a nucleus of a radioactive isotope of phosphorus was formed. And this conjecture was proved by an extremely subtle and skillful chemical experiment, with the help of which it was possible to isolate and detect an insignificant amount of phosphorus by radioactivity, which could not be seen in any microscope if all its atoms were collected “in a heap”. And this phosphorus also melted before our eyes.
Subsequent experiments discovered that neutrons, especially those slowed by passage through water, paraffin or graphite, have an even greater ability to excite nuclear reactions and activate various substances. With the discovery of nuclear fission reactions producing a huge number of neutrons, this became on the one hand a big problem - not only nuclear fuel, but all the structural elements of the reactors became terribly radioactive. On the other hand, in this way it became possible to obtain the required radionuclides cheaply and in large quantities. Air and soil activated by the neutron flux of a thermonuclear explosion are an additional serious factor in damage, so the "ecological purity" of the hydrogen bomb is nothing more than a myth.
So in which case does irradiation cause nuclear reactions and lead to the appearance of artificial radioactivity?
As I said, neutrons have a special ability for this. It is easy to guess what the reason is: the neutron easily penetrates into the nucleus. He does not need to overcome electrostatic repulsion, like a proton or alpha particle. At the same time, a neutron is the same building material of the nucleus as those protons and neutrons; it is just as capable of entering into a strong interaction. Therefore, the chemical element number zero is the very “philosophical stone” of alchemists. Rather, they could be called "alphysics" if this word had not been used in relation to adepts of ether and torsion fields.
A neutron can cause a nuclear transformation of any energy, up to zero. But other particles must have a sufficiently large energy for this. I already spoke about alpha particles (like protons): they need to overcome the Coulomb repulsion. For light elements, the alpha-particle energy requirement is a few megaelectron-volts — that is, what alpha particles emitted by heavy unstable nuclei possess. And the heavier ones already need tens of MeV - such energy can be obtained only in the accelerator. In addition, with an increase in the mass of the nucleus, it itself is less and less willing to react with the alpha particle: for iron, the addition of nucleons to the nucleus proceeds with an expense rather than with the release of energy. Given the extremely low penetration of alpha particles into the target, it becomes clear
But what about the other particles? Electrons, photons? They do not need to overcome repulsion, but they are reluctant to interact 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 a significant 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 effective.
The spectrum of photons emitted during radioactive decay ends at 2.62 MeV - this is the energy of the thallium-208 quanta, the last member of the radioactive series of thorium-232. And there are very few nuclei whose thresholds for photonuclear reactions are below this value. More precisely, there are two such nuclei: deuterium and beryllium-9
The first reaction proceeds under the influence of gamma radiation above 2.23 MeV, the source of which is thallium-208 (a series of thorium), 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, give rise to radioactive isotopes. But the cross sections of these reactions themselves are small, and therefore noticeable induced radioactivity is possible only at very high radiation intensities. For the implementation of other photonuclear reactions, gamma rays whose energy is measured in tens and hundreds of MeV are already needed. At such energies, not only photons, but in general all particles - electrons and positrons, muons, protons, etc., colliding with nuclei, cause nuclear reactions with sufficiently high efficiency. Beams of such particles obtained at accelerators lead to strong activation of almost 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 the residual radioactivity in two cases:
- from targets exposed to neutrons;
- from targets irradiated in accelerators.
In all other cases, including under the influence of x-ray radiation, beta and gamma radiation (with the exception of the aforementioned beryllium and deuterium), radioactive isotopes of induced radioactivity do not arise. Alpha radiation produces weak and usually short-lived induced radioactivity when exposed to light elements.
Neither x-ray irradiation nor the effects of other radiation - ultraviolet, microwave, etc., cause the appearance of artificial radioactivity. Food and medicine sterilized by radiation do not become radioactive, seeds irradiated to increase germination and new varieties, stones irradiated to give them a color (if this is not radiation in the neutron channels of a nuclear reactor). The details of x-ray units, the protective clothing of the radiologist, and he himself are not radioactive!To illustrate this, I spent a little experience. By renting an alpha-source of americium-241 with an activity of 1 MBq in a nearby laboratory (this is about 100 times the activity of the source contained in the HIS-07 smoke detector, which is easy to buy even on Aliexpress - ATTENTION! Illegal circulation of radioactive substances - Article 220 Criminal Code of the Russian Federation!), I put an aluminum plate under it. As a result, as in the Joliot-Curie experiment (which used a much more powerful source), I had to get phosphorus-30 decaying to silicon-30 and a positron with a half-life of 2.5 minutes (and also a neutron, which is also what Something can activate). However, after half an hour of exposure (to establish an equilibrium between the production and decay of phosphorus-30), I could not detect any noticeable radioactivity from the aluminum plate. For this, I tried to use a Geiger counter with a mica window (positrons are detected by it in the same way as electrons), as well as a scintillation detector (which effectively records 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 influence of alpha particles are rare and even though that in my experience, aluminum was exposed to at least half a billion alpha particles, during this time only several thousand radioactive atoms were formed, most of which simply decayed during irradiation. Perhaps I would have been able to detect positrons in Wilson’s chamber thanks to the almost zero natural background of positrons, but I haven’t completed it yet (when I do, this will be a good topic for the article).
Invisible Radioactive Mud
In most cases, with the exception of the above, pollution induced by radioactive isotopes on the surface of things and objects is taken as induced radioactivity. The fact is that with a half-life of months, years, and tens of years, the amount of substance that emits frightening levels of radiation is truly insignificant. Remember the milligram of radium, which gives 8.4 R / h at a distance of a centimeter? It has a half-life of 1,600 years. And if the half-life is 1.6 years, and the energy of gamma rays 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 to the interface — rises to its full height .
Radiochemists have to fight adsorption all the time. Because of it, you can completely lose the radioactive isotope during operations with it simply because all of it is donkey on the walls of the test tube or glass. It is necessary to select the composition of the “background” solution, but part of the isotope is still lost, and alas, often unknown. One has to do a parallel experiment under absolutely the same conditions (up to test tubes from one box) or add an exit mark to the solution - another radioactive isotope of the same chemical element. And you can sit in the galoshes in another way: the isotope, the solution of which was previously contained in a glass, settled on the wall and, despite subsequent washing and rinsing with acid first, then with distilled water, fell into the next sample. At the same time, the glass seemed absolutely, impeccably clean.
Any thing may seem equally immaculately clean, but nonetheless, radiating dirt on its surface (as well as inside pores, crevices, etc.) that communicate with it. And not only a thing: in the area of radiation damage, the 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 objects heavily contaminated with radionuclides is difficult, and in many cases it becomes unsuccessful.
Unlike induced radioactivity, which is usually firmly fixed on its carrier, contamination with radionuclides is on its surface and therefore easily passes to other objects, to people's hands and then gets into their body, exposing it to internal radiation.
Decontamination - methods and tools
The easiest way to decontaminate is to wash with soap and other surfactants. This is a method that is suitable for almost everything - you can wash asphalt, the walls of a house, a living person, and a rare painting or violin with soap. In the latter case, this is done carefully, wiping the surface with a squeezed cloth swab dipped in soapy water and immediately rubbing it with the same swab of clean water, and then removing the remaining water with filter paper. Thus, the radiation of the violin lying on the hottest days of the Chernobyl disaster near the open window of the Kiev house and "shining" about 1 mR / h "conditionally" closely, was able to be reduced to a completely acceptable one, and thereby save the instrument. There are specialized decontamination products that contain, in addition to surfactants, complexing agents (such as EDTA), ion exchange resins, zeolites and other sorbents. Complexing agents facilitate the transfer of cation-forming radionuclides into solution, while ion-exchange components and sorbents, on the contrary, remove them from the solution, converting them into a bound form, but not on a deactivated surface. So, it is well known (and actively used in our laboratory) the Novosibirsk means for decontamination "Protection", which works on this principle.
But such a tool is often not enough: radionuclides are firmly bound to the surface, located deep in pores and microcracks. In such cases, it is necessary to use much more stringent methods - to treat surfaces with acids that dissolve the surface layer of metal and the crust of rust on it, and contribute to the desorption of radioactive contaminants. They also use strong oxidizing agents, which destroy organic pollution on the surface, which also adheres to radioactive dust. At a nuclear power plant, a two-way decontamination method is often used to decontaminate equipment, when parts are first treated with an alkaline solution of potassium permanganate and then with acid.
For metal surfaces, the electrochemical method is an effective method of decontamination. The goal is roughly the same - to remove the surface layer of metal, corrosion layers, impregnated with radionuclides. But the amount of liquid radioactive waste is sharply reduced, since a minimal amount of electrolyte can be used. This is the so-called semi - dry electrolytic bath - a fabric or felt impregnated with electrolyte is applied to the decontaminated surface and a second electrode is placed on top of it). The decontaminated part or surface is the anode, and usually a lead sheet is used as the cathode, easily deformable to fit the deactivated surface tightly.
To decontaminate hard-to-remove radioactive contaminants, such as, for example, from helicopters flying over the emergency Chernobyl reactor, sandblasting was also used. However, it generates a huge amount of radioactive dust, severely damages the decontaminated surface, and generally has low efficiency.
If suddenly, God forbid, you find yourself in a zone of radioactive contamination and you need to urgently deactivate something, then I recommend a dishwashing detergent (Fairy, etc.) or any washing powder with the addition of oxalic acid. You can also use household plumbing cleaners such as Cif, which already have acid in them.
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 far from always the impossibility of decontamination means that the radiation source is associated 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 normal handling of radioactive substances and other sources of ionizing radiation, one does not need to be afraid of induced radiation. But contamination with radionuclides is not only more real, but also more dangerous.
On KDPV - ZGRLS "Duga". Photo by Mike Deere .