(ORDO NEWS) — After the accident at the Fukushima nuclear power plant, the world was swept by another wave of panic radiophobia. In the Far East, iodine disappeared from sale, and manufacturers and sellers of dosimeters not only sold out all the devices in their warehouses, but also collected pre-orders for six months or a year in advance. But is radiation so terrible? If every time you flinch at the word, the article is written for you.
What is radiation? This is the name for various types of ionizing radiation, that is, one that is capable of taking electrons from atoms of a substance. The three main types of ionizing radiation are usually denoted by the Greek letters alpha, beta and gamma. Alpha radiation is a flux of helium-4 nuclei (almost all helium from balloons was once alpha radiation), beta is a flux of fast electrons (less often positrons), and gamma is a flux of high-energy photons. Another type of radiation is neutron flux. Ionizing radiation (with the exception of X-rays) is the result of nuclear reactions, so neither mobile phones nor microwaves are sources of it.
Cinema is the most important of all types of art for us, and gamma radiation is the most important type of radiation. It has a very high penetrating power, and theoretically no obstacle is able to completely protect against it. We are constantly exposed to gamma radiation, it comes to us through the atmosphere from space, breaks through the soil layer and the walls of houses. The downside of such pervasiveness is a relatively weak destructive effect: of a large number of photons, only a small part will transfer its energy to the body. Soft (low-energy) gamma radiation (and X-rays) mainly interacts with matter, knocking out electrons from it due to the photoelectric effect, hard – scattered by electrons, while the photon is not absorbed and retains a noticeable part of its energy.
Beta radiation is close to gamma radiation in its effect – it also knocks electrons out of atoms. But with external irradiation, it is completely absorbed by the skin and tissues closest to the skin, without reaching the internal organs. Nevertheless, this leads to the fact that the flow of fast electrons transfers significant energy to the irradiated tissues, which can lead to radiation burns or provoke, for example, cataracts.
Alpha radiation carries significant energy and momentum, which allows it to knock electrons from atoms and even atoms themselves from molecules. Therefore, the “destruction” caused by it is much greater – it is believed that by transferring 1 J of energy to the body, alpha radiation will cause the same damage as 20 J in the case of gamma or beta radiation. Fortunately, alpha particles are extremely small in penetration: they are absorbed by the topmost layer of the skin. But when ingested, alpha-active isotopes are extremely dangerous: remember the infamous tea with alpha-active polonium-210, which was poisoned by Alexander Litvinenko.
But the first place in the hazard rating is undoubtedly occupied by fast neutrons. The neutron has no electric charge and therefore interacts not with electrons, but with nuclei – only with a “direct hit”. A flux of fast neutrons can pass through a layer of matter on average from 2 to 10 cm without interacting with it. Moreover, in the case of heavy elements, colliding with the nucleus, the neutron only deflects to the side, almost without losing energy. And when it collides with a hydrogen nucleus (proton), a neutron transfers about half of its energy to it, knocking the proton out of its place. It is this fast proton (or, to a lesser extent, the nucleus of another light element) that causes ionization in matter, acting like alpha radiation. As a result, neutron radiation, like gamma quanta, easily penetrates into the body, but is almost completely absorbed there, creating fast protons, causing great destruction. In addition, neutrons are the very radiation that causes induced radioactivity in the irradiated substances, that is, converts stable isotopes into radioactive ones. This is an extremely unpleasant effect: for example, after being in the focus of a radiation accident, alpha-, beta- and gamma-active dust can be washed off from vehicles, but it is impossible to get rid of neutron activation – the body itself radiates (by the way, this was the basis for the striking effect of a neutron bomb that activated the armor of tanks).
Dose and power
When measuring and assessing radiation, so many different concepts and units are used that it is no wonder for an ordinary person to get confused. The exposure dose is proportional to the amount of ions generated by gamma and X-rays per unit mass of air. It is customary to measure it in X-rays (R). The absorbed dose shows the amount of radiation energy absorbed by a unit mass of a substance. Previously, it was measured in rads (rad), and now – in grays (Gr). The equivalent dose additionally takes into account the difference in the destructive power of different types of radiation. Previously, it was measured in “biological equivalents of rad” – rem (rem), and now – in sievert (Sv).
The effective dose also takes into account the different sensitivity of different organs to radiation: for example, irradiating the hand is much less dangerous than the back or chest. Previously it was measured in the same rem, now – in sieverts. Converting some units of measurement to others is not always correct, but on average it is generally accepted that an exposure dose of 1 R of gamma radiation will bring the same harm to the body as an equivalent dose of 1/114 Sv. The conversion from glad to gray and rem to sievert is very simple: 1 Gr = 100 rad, 1 Sv = 100 rem. To convert the absorbed dose into an equivalent dose, the so-called. A “radiation quality factor” of 1 for gamma and beta radiation, 20 for alpha radiation and 10 for fast neutrons. For example, 1 Gy of fast neutrons = 10 Sv1000 rem. The natural equivalent dose rate (DER) of external exposure is usually 0.06 – 0.10 μSv / h, but in some places it may be less than 0.02 μSv / h or more than 0.30 μSv / h. The level of more than 1.2 μSv / h in Russia is officially considered dangerous, although in the aircraft cabin during the flight the DER can be many times higher than this value. And the ISS crew is exposed to radiation with a power of about 40 μSv / h.
In nature, neutron radiation is very insignificant. In fact, the risk of being exposed to it exists only in the event of a nuclear bombardment or a serious accident at a nuclear power plant with the melting and release of most of the reactor core into the environment (and even then only in the first seconds).
Gas discharge meters
Radiation can be detected and measured using a variety of sensors. The simplest of these are ionization chambers, proportional counters, and Geiger-Muller gas-discharge counters. They represent a thin-walled metal tube with gas (or air), along the axis of which a wire is stretched – an electrode. A voltage is applied between the housing and the wire and the flowing current is measured. The fundamental difference between the sensors is only in the magnitude of the applied voltage: at low voltages we have an ionization chamber, at high voltages – a gas-discharge counter, somewhere in the middle – a proportional counter.
Ionization chambers and proportional counters allow you to determine the energy that each particle has transferred to the gas. The Geiger-Müller counter only counts particles, but the readings from it are very easy to obtain and process: the power of each pulse is sufficient to directly output it to a small speaker! An important problem of gas-discharge counters is the dependence of the counting rate on the radiation energy at the same radiation level. For its equalization, special filters are used that absorb part of the soft gamma and all beta radiation. To measure the flux density of beta and alpha particles, such filters are made removable. In addition, to increase the sensitivity to beta and alpha radiation, “end counters” are used: this is a disc with a bottom as one electrode and a second spiral wire electrode.
Semiconductors and scintillators
A semiconductor sensor can be used instead of an ionization chamber. The simplest example is a conventional diode, to which a blocking voltage is applied: when an ionizing particle hits the pn junction, it creates additional charge carriers, which lead to the appearance of a current pulse. To increase the sensitivity, so-called pin diodes are used, where there is a relatively thick layer of undoped semiconductor between the p- and n-semiconductor layers. These sensors are compact and can measure particle energy with high accuracy. But the volume of the sensitive area is small, and therefore the sensitivity is limited. In addition, they are much more expensive than gas discharge ones.
Another principle is counting and measuring the brightness of flares that occur in some substances when particles of ionizing radiation are absorbed. These flashes cannot be seen with the naked eye, but special highly sensitive devices – photomultiplier tubes – are capable of this. They even measure the change in brightness over time, which characterizes the energy loss of each individual particle. Sensors based on this principle are called scintillator sensors.
For shielding against gamma radiation, heavy elements such as lead are most effective. The higher the number of an element in the periodic table, the stronger the photo effect is manifested in it. The degree of protection also depends on the energy of the radiation particles. Even lead attenuates radiation from cesium-137 (662 keV) only by half for every 5 mm of its thickness. In the case of cobalt-60 (1173 and 1333 keV), two-fold attenuation will require more than a centimeter of lead. Only for soft gamma radiation, such as radiation from cobalt-57 (122 keV), a sufficiently thin layer of lead will be serious protection: 1 mm will weaken it ten times. So, anti-radiation suits from films and computer games in reality only protect against soft gamma radiation.
Beta radiation is completely absorbed by the shielding of a certain thickness. For example, beta radiation from cesium-137 with a maximum energy of 514 keV (and an average of 174 keV) is completely absorbed by a layer of water 2 mm thick, or only 0.6 mm of aluminum. But lead should not be used to protect against beta radiation: too fast deceleration of beta electrons leads to the formation of X-rays. To completely absorb the radiation of strontium-90, you need less than 1.5 mm of lead, but to absorb the resulting X-ray radiation, you need another centimeter!
There is a well-established myth about the “protective” effect of alcohol, but it has no scientific basis. Even if red wine contains natural antioxidants that could theoretically act as radioprotectors, their theoretical benefits are outweighed by the practical harms of ethanol, which damages cells and is a neurotoxic poison. The extremely tenacious popular recommendation to drink iodine so as not to “get infected with radiation” is justified only for a 30-kilometer zone around a newly exploded nuclear power plant. In this case, potassium iodide is used to prevent radioactive iodine-131 from entering the thyroid gland (half-life is 8 days). The tactic of lesser evil is used: let it be better that the thyroid gland be “clogged” with ordinary, not radioactive iodine. And the prospect of getting a thyroid dysfunction pales before cancer or even death. But outside the zone of infection, swallowing tablets, drinking an alcoholic solution of iodine or smearing it on the front of the neck does not make any sense – it has no preventive value, but it is easy to earn iodine poisoning and turn yourself into a lifelong patient of an endocrinologist.
The easiest way to protect yourself from external alpha-irradiation is a sheet of paper. However, most of the alpha particles do not pass even five centimeters in the air, so protection may be required unless in case of direct contact with a radioactive source. It is much more important to protect against the ingress of alpha-active isotopes into the body, for which a respirator mask is used, and ideally a sealed suit with an isolated breathing system.
Finally, hydrogen-rich substances are the best shielding against fast neutrons. For example, hydrocarbons, the best option is polyethylene. Experiencing collisions with hydrogen atoms, the neutron quickly loses energy, slows down and soon becomes unable to cause ionization. However, such neutrons can still activate, that is, transform into radioactive, many stable isotopes. Therefore, boron is often added to the neutron protection, which very strongly absorbs such slow (they are called thermal) neutrons. Alas, the thickness of polyethylene for reliable protection must be at least 10 cm. So it turns out to be not much lighter than lead protection against gamma radiation.
The human body consists of more than three-quarters of water, so the main effect of ionizing radiation is radiolysis (decomposition of water). The resulting free radicals cause an avalanche cascade of pathological reactions with the emergence of secondary “fragments”. In addition, radiation damages chemical bonds in nucleic acid molecules, causing disintegration and depolymerization of DNA and RNA. The most important enzymes containing a sulfhydryl group – SH (adenosine triphosphatase, succinate oxidase, hexokinase, carboxylase, cholinesterase) are inactivated. In this case, the processes of biosynthesis and energy metabolism are disrupted, proteolytic enzymes are released from the destroyed organelles into the cytoplasm, and self-digestion begins. The risk group is primarily sex cells, the precursors of blood cells,
Drugs capable of protecting against the effects of radiation began to be actively developed in the middle of the 20th century. Only some aminothiols, such as cystamine, cysteamine, aminoethyl isothiouronium, turned out to be more or less effective and suitable for mass use. In fact, they are donors – SH groups, substituting them under attack instead of “relatives”.
Radiation around us
Accidents are not necessary to face radiation “face to face”. Radioactive substances are widely used in everyday life. Potassium is naturally radioactive – a very important element for all living things. Due to the small admixture of the isotope K-40 in natural potassium, dietary salt and potassium fertilizers “phonite”. Some older lenses used thorium oxide glass. The same element is added to some modern argon welding electrodes. Until the middle of the twentieth century, radium-based devices with illumination were actively used (in our time, radium has been replaced by the less dangerous tritium). Some smoke detectors use an alpha emitter based on americium-241 or highly enriched plutonium-239 (yes, the same one that makes nuclear bombs).
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