Radiation is not always scary: everything you wanted to know about it
After the accident at the Fukushima nuclear power plant, another wave of panic radiophobia swept the world.
Iodine disappeared from the sale in the Far East, and the manufacturers and sellers of dosimeters not only sold all the instruments available in the warehouses, but also collected pre-orders for six months or a year in advance.
But is radiation terrible? If you flinch every time with this word, the article is written for you.
What is radiation? So called various types of ionizing radiation, that is, the one that is able to detach electrons from atoms of matter. The three main types of ionizing radiation are usually designated in Greek letters alpha, beta and gamma. Alpha radiation is a stream of helium-4 nuclei (almost all helium from balloons was once alpha radiation), beta is a stream of fast electrons (less often positrons), and gamma is a stream of high-energy photons. Another type of radiation is the neutron flux. Ionizing radiation (with the exception of X-rays) is the result of nuclear reactions; therefore, neither mobile phones nor microwave ovens are its sources.
Of all the arts, cinema is the most important for us, and of the types of radiation, gamma radiation. It has a very high penetrating power, and theoretically no barrier can protect against it completely. We are constantly exposed to gamma irradiation, it comes to us through the thickness of the atmosphere from space, breaks through the layer of soil and the walls of houses. The reverse side of such permeability 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 the substance, knocking electrons out of it due to the photoelectric effect, hard - scatters on electrons, while the photon is not absorbed and retains a significant part of its energy, so the probability of destruction of molecules in such process is much less.
Beta radiation in its effects close to gamma radiation - it also knocks electrons out of atoms. But with external irradiation, it is completely absorbed by the skin and the tissues closest to the skin, without reaching the internal organs. Nevertheless, this leads to the fact that the stream of fast electrons transfers considerable energy to the irradiated tissues, which can lead to radiation burns or provoke, for example, a cataract.
Alpha radiation carries considerable energy and large momentum, which allows it to knock electrons out of atoms and even atoms themselves out of molecules. Therefore, the “destruction” caused by it is much more - 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, the penetrating power of alpha particles is extremely small: they are absorbed by the uppermost layer of the skin. But when ingested, alpha-active isotopes are extremely dangerous: remember the infamous tea with alpha-active polonium-210, which Alexander Litvinenko was poisoned.
But the first place in the hazard rating is undoubtedly occupied by fast neutrons. The neutron does not have an electric charge and therefore interacts not with electrons, but with the nuclei — only with a “direct hit”. A stream 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 deviates to the side, almost without losing energy. And when colliding with a hydrogen nucleus (proton), the neutron transmits to it about half of its energy, knocking out a proton from its place. It is this fast proton (or, to a lesser extent, the nucleus of another light element) that causes ionization in the substance, 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 that cause great damage. In addition, neutrons are the very radiation that causes induced radioactivity in the irradiated substances, that is, converts stable isotopes into radioactive. This is an extremely unpleasant effect: for example, after alpha, beta and gamma active dust being in the outbreak of a radiation accident, vehicles can be washed off, but it is impossible to get rid of neutron activation — the body itself radiates (by the way, this the damaging effect of the neutron bomb that activated tank armor).
In nature, neutron radiation is very small. In fact, the risk of being exposed to it exists only when a nuclear bombardment or a serious accident at a nuclear power plant occurs with the melting and release into the environment of a large part of the reactor core (and then only in the first seconds).
Radiation can be detected and measured using various sensors. The simplest of them are ionization chambers, proportional counters and gas-discharge Geiger-Muller counters. They are a thin-walled metal tube with a 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 principal difference between the sensors is only in the magnitude of the applied voltage: at low voltages, we have an ionization chamber, at large - 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 transmitted to the gas. The Geiger-Muller counter only counts particles, but it is very easy to receive and process readings from it: the power of each pulse is sufficient to directly bring 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 level of radiation. For its alignment, 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 make removable. In addition, “end counters” are used to increase sensitivity to beta and alpha radiation: this is a disc with a bottom as one electrode and a second helical wire electrode. The lid of the end meters is made of very thin (10 × 20 μm) mica plates, through which soft beta radiation and even alpha particles easily pass.
Semiconductors and scintillators
Instead of an ionization chamber, you can use a semiconductor sensor. The simplest example is the usual diode to which a blocking voltage is applied: when an ionizing particle enters the pn junction, it creates additional charge carriers that lead to the appearance of a current pulse. To increase the sensitivity, so-called pin diodes are used, where between the layers of p- and n-semiconductors there is a relatively thick layer of undoped semiconductor. Such sensors are compact and allow you to measure the energy of particles 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 gas discharge.
Another principle is counting and measuring the brightness of flares that occur in some substances during the absorption of particles of ionizing radiation. These flashes cannot be seen with the naked eye, but special highly sensitive devices — photomultiplier tubes — are capable of that. They even allow you to measure the change in brightness over time, which characterizes the energy loss of each individual particle. Sensors on this principle are called scintillator.
To protect against gamma radiation, heavy elements such as lead are most effective. The greater the number of the element in the periodic table, the stronger the photoelectric effect appears in it. The degree of protection depends on the energy of the radiation particles. Even lead weakens radiation from cesium-137 (662 keV) only twice for every 5 mm of its thickness. In the case of cobalt-60 (1173 and 1333 keV), more than a centimeter of lead will be required for two-fold attenuation. Only for soft gamma radiation, such as cobalt-57 radiation (122 keV), a sufficiently thin layer of lead will be a serious defense: 1 mm will weaken it ten times. So, radiation suits from movies and computer games in reality only protect against soft gamma radiation.
Beta radiation is completely absorbed by the protection of a certain thickness. For example, the beta radiation of 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 just 0.6 mm of aluminum. But lead to protect against beta radiation should not be used: too fast inhibition of beta electrons leads to the formation of X-rays. In order to completely absorb strontium-90 radiation, less than 1.5 mm of lead is needed, but another centimeter is needed to absorb the X-ray radiation that is formed!
The easiest way to protect against external alpha irradiation is to do this: a sheet of paper is enough. However, most of the alpha particles do not pass in the air and five centimeters, so protection may be required unless in the case of direct contact with a radioactive source. Where it is more important to protect against alpha-active isotopes entering the body, for which a respirator mask is used, and ideally a sealed suit with an isolated respiratory system.
Finally, hydrogen-rich substances best protect against fast neutrons. For example, hydrocarbons, the best option is polyethylene. When 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, convert 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 should be at least 10 cm. So it turns out to be not much lighter than lead protection against gamma radiation.
More than three quarters of the human body consists 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 appearance 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 that contain a sulfhydryl group - SH (adenosine triphosphatase, succinic oxidase, hexokinase, carboxylase, cholinesterase) are inactivated. At the same time, the processes of biosynthesis and energy metabolism are disrupted, proteolytic enzymes are released from the destroyed organelles into the cytoplasm, self-digestion begins. In the risk group, in the first place, the germ cells, the precursors of the blood cells, the cells of the gastrointestinal tract and the lymphocytes are found, but neurons and muscle cells are quite resistant to ionizing radiation.
Drugs that can protect against the effects of radiation, began to be actively developed in the middle of the XX century. Only some aminothiols, such as cystamine, cysteamine, aminoethylisothiuronium, turned out to be more or less effective and suitable for mass use. In fact, they are donors - SH groups, putting them at risk instead of “relatives”.
Radiation around us
In order to collide with face-to-face radiation, accidents are not at all necessary. Radioactive substances are widely used in everyday life. Potassium has a natural radioactivity - a very important element for all living things. Because of the small impurity of the K-40 isotope in natural potassium, fonit is dietary salt and potash fertilizers. Some older lenses used glass mixed with thorium oxide. The same element is added to some modern electrodes for argon welding. Until the middle of the twentieth century, radium-based instruments were actively used (radium was replaced by less dangerous tritium in our time). Some smoke detectors use an alpha emitter based on americium-241 or highly enriched plutonium-239 (yes, the same one from which nuclear bombs are made). But do not worry - the harm to health from all these sources is much less harm from anxiety about this.
Dose and power
When measuring and assessing radiation, many concepts and units are used.
- - Exposure dose is proportional to the number of ions that create gamma and X-rays per unit mass of air. It is usually measured in X-rays (P).
- - Absorbed dose - the amount of radiation energy absorbed by a unit mass of a substance. Previously, it was measured in rads (rad), and now it is measured in grays (Gy).
- - Equivalent dose additionally takes into account the difference in the destructive ability of different types of radiation. Previously, it was measured in “biological equivalents of rad” - rem (rem), now in sievert (Sv).
- - The effective dose takes into account the different sensitivity of organs to radiation: thus, it is less dangerous to irradiate the arm than the back or chest. Previously, it was measured in the same rem, now - in Sievert.
Conversion of some units of measurement to others is not always correct, but it is considered that an exposure dose of 1 G of gamma radiation will cause the same harm to the body as the equivalent dose of 1/114 Sv. The translation of glad to gray and the banks in sievert is very simple: 1 Gr = 100 glad, 1 Sv = 100 bar. To convert the absorbed dose to the equivalent, a radiation quality factor of 1 for gamma and beta radiation, 20 for alpha radiation, and 10 for fast neutrons is used. For example, 1 Gy of fast neutrons = 10Sv = 1000 rem.
- - The natural equivalent dose rate (DER) of external exposure is usually 0.06 - 0.10 μSv / h, but in some places it can be less than 0.02 μSv / h or more than 0.30 μSv / h. The level of more than 1.2 mSv / h in Russia is officially considered dangerous, although in the cabin of an aircraft during a flight, the MED may be many times higher than this value. And the crew of the ISS is exposed to radiation with a power of about 40 µSv / h.