Radiation: sources
In a previous post I talked about units of ionizing radiation. Now let's talk about the sources of radiation.
I will not write here about “that you don’t need to touch it” - so much has been written about it, but I’m not Oleg Aizon and I don’t have unique photos of unprecedented radioactive artifacts. I will tell in general - where radiation comes from.
What is radioactive decay? Someone, recalling school knowledge, will answer - this is the phenomenon of the transformation of some elements into others. Someone will give a different, as a rule, equally inaccurate definition. In fact, radioactive decay is any spontaneous change in the state of an atomic nucleus as a system of nucleons, accompanied by the release of energy, the value of which, as a rule, exceeds several kiloelectron-volts. This energy is then carried away by the elementary particles emitted from the nucleus, by electromagnetic radiation quanta, or transferred to the electrons of the atom. In this case, the nucleus itself can change its charge, mass, split into two or more nuclei, or it can remain by itself, only by going into a more stable state.
“External”, easily determined characteristics of an atomic nucleus are its mass Aand charge (or atomic number) Z , measured in the charges and masses of the proton. These are integer values that have a physical meaning of the number of corresponding particles in the composition of the nucleus. The neutron charge is zero, and the mass is almost the same as that of a proton, so calculate the number of neutrons:
. Nuclei with the same charges are called isotopes , with the same masses - isobars , if the same, then, and the other, we are dealing with isomers . Z and A are denoted to the left of the element symbol in the lower and upper indices, respectively.
From what has been said it is obvious that in order for Z to change, the nucleus must leave the charged particle, and for A to change, something heavier than the electron must fly away from the nucleus. So, the following options are possible:
- an electron and an antineutrino or a positron and neutrino (beta decay) fly out - Z changes by one (increases in the case of electron and decreases in the case of positron decay), A - does not change;
- the nucleus, on the contrary, can absorb an electron from the K-level of the atom (K-capture) - Z increases by one (as in beta-plus decay), A does not change, neutrinos are emitted.
- the helium-4 core flies out, the so-called alpha particle (alpha decay) - Z decreases by 2, A decreases by 4;
Beta decay (and electron capture) is the conversion of one of the neutrons into a proton or vice versa, and is a manifestation of a weak interaction that “recharges” one of the nucleon quarks. Together with the electron, an antineutrino is always formed, which takes away part of the energy, while the energy between them is redistributed randomly. Because of this, the energy spectrum of beta radiation is continuous.
And alpha decay occurs simply because any nucleus heavier than iron is energetically more profitable to "lose weight". But while this gain is not more than a few MeV, the energy barrier to removing an alpha particle or any other fragment from the nucleus is too high. And when the energy gain is large enough (but still less than the binding energy), it becomes possible to tunnel an alpha particle outside the nucleus. In addition to an alpha particle, a neutron or proton can fly out of the nucleus in extremely rare cases, or the nucleus is heavier than the alpha particle. And finally, the nucleus can fall into several nuclei, while emitting several neutrons. This is a spontaneous fission that only heavy nuclei are capable of, starting with thorium and uranium.
After the act of decay, an excess of energy may remain in the nucleus and this “warmed up” nucleus must somehow get rid of it. To do this, it emits one or more gamma rays. Sometimes the phenomenon of internal conversion also occurs: energy is not radiated in the form of photons, but is transmitted to electrons that fly out of the atom. Unlike beta rays, conversion electrons have a monoenergetic (linear) spectrum.
In some cases, a core with excess energy can exist for a long time, sometimes even hundreds of years. It does not differ from the same "ordinary" nucleus - neither by charge nor by mass, that is, it is the same chemical element and the same isotope. And here isomers- various. Most often, the lifetime of metastable isomers does not exceed hours, and only a few of them have years. There is only one core for which only the isomeric state is stable: it is tantalum-180. In the ground state, it is beta-active and short-lived (half-life of 8 hours), and its tantalum 180m isomer, it would seem, should either go into the ground state with the emission of gamma rays with an energy of 75 keV, or undergo beta decay, but neither , no one has ever observed: this isomer, in contrast to the short-lived ground state, is stable .
The decay of a nuclear isomer is the only example of radioactive decay, accompanied exclusively by gamma radiation . In all other cases, gamma radiation always exists.exclusively with alpha or beta radiation.
About isotopes and isomers, we said. One more "iso" remains - these are isobars. Nuclei with different nuclear charges and the same mass. Stable isobars usually have charges differing by two units, and between them there is almost always a radioactive isotope. The existence of two stable isobars in neighboring cells of the periodic table is unlikely - this rule is called the Schukarev-Mattauch rule. Only two exceptions are known from it: antimony and tellurium-123 and hafnium-180 and the aforementioned tantalum-180m.
In addition to radioactive substances, some other processes and phenomena, both natural and generated by the human mind, also lead to the appearance of radiation with similar properties.
You probably know about cosmic radiation. Cosmic rays fill the entire Universe, they are protons and heavier nuclei, electrons and gamma rays with exceptionally high energies. The maximum energy recorded by cosmic particles reaches the zept of an electron volt ! it
eV. What is the source of such high-energy particles is impossible to say unequivocally, but particles and gamma-rays with moderate energies - from kilo-giga-electron volts - are generated by stars, including our Sun.
This is the so-called primary cosmic radiation. You can only encounter it if you go into low Earth orbit, or at least go up several tens of kilometers. Despite the high energy, these particles do not reach the surface. Each of these particles, having flown into the atmosphere, causes a whole cascade of nuclear reactions, leading to the formation of many particles - mainly muons - which already reach the Earth. By the way, they fly solely due to the relativistic time dilation: the existence time of a muon - two microseconds - without it would make it possible to fly a muon only half a kilometer with a small one. And another interesting fact related to cosmic muons: they are negatively charged, but the primary cosmic rays are positively charged, since they consist mainly of protons. That is why the Earth has a negative charge, and the ionosphere is positive. Near the surface of the Earth, on average, one muon flies through each square centimeter per minute. About a third of the natural background - about 3.5 μR / h - is due to them. And at the altitude at which passenger planes fly, cosmic rays create a dose rate of several microsievert per hour, which already poses a certain danger to the health of pilots.
In addition to muons, there are also electrons and neutrons in secondary cosmic rays. The latter play an important role in the formation of the so-called cosmogenic radionuclides.
Secondary cosmic rays have a very high penetrating power. To protect yourself from them, you have to go into deep cellars and mines. Of course, one has to defend oneself not because they are harmful to health - but because they interfere with detecting rare and weak events in nuclear physics experiments, measuring small radionuclide activities, etc. But there is some benefit from them: with their help, it is possible to “shine through” geological structures, large structures (such as the Egyptian pyramids).
By the way, the Earth’s atmosphere is equivalent to about a meter of lead for cosmic rays. Not only one atmosphere protects the Earth and all of us from cosmic rays - besides it there is a magnetic field deflecting charged particles. But one should not underestimate the protective properties of the atmosphere. During geomagnetic inversions, the Earth’s magnetic shield can practically disappear for a certain time, but contrary to the horror stories of alarmists, this will not lead to the cessation of life on Earth, and the level of radiation at the surface will increase only 2-3 times.
Particularly high-energy particles arriving from space cause the formation of a shower of particles, which covers a large area, causing the simultaneous registration of many particles at detectors spaced over considerable distances. These are the so-called wide air showers. Their registration with the help of a variety of spaced detectors makes it possible to determine the energy of the primary particle, and it is in this way that the energies of the highest-energy particles of cosmic rays are determined. In addition, such a particle causes a powerful flash of Cherenkov radiation in the atmosphere.
Earthly sources of short bursts of gamma radiation and high-energy electrons are lightning and other atmospheric discharges.
And the work of human hands are numerous devices that generate streams of high-energy particles and quanta, not necessarily intentionally. Especially for this, there are X-ray tubes and various kinds of accelerators - from small ones that fit almost in the palm of your hand, to the LHC monster, which occupies the territory of several countries. And the sources, as they say in the dry language of official papers, of unused X-ray radiation are any electrovacuum devices. But it is usually able to go outside when the voltage at the anode is tens of kilovolts. So, high-voltage kenotrons, pulsed modulator lamps and traveling-wave microwave lamps, klystrons, etc., become X-ray sources. in radar stations. And also - in the hands of various lovers of home experiments.
You can often hear about the fact that the source of x-ray radiation is the picture tube of a TV or monitor. Maybe, but usually not. The fact is that the glass at the picture tube is quite thick, and the x-ray radiation at an anode voltage of 15-25 kV is too soft to pass through such a glass. Here are kinescopes of projection TVs, which operated at voltages up to 50 kV and had small dimensions and thin walls of the bulb, "X-rayed" even like that. And among the televisions, the ULPTC with their circuit for stabilizing the anode voltage “distinguished themselves”. In this circuit, the GP-5 lamp was used, operating at an anode voltage equal to the voltage at the second anode (i.e. 25 kV), a noticeable anode current passed through it, and the walls of this lamp were thin. As a result, it shone brightly in the X-ray range.
But we will return to radioactivity.
Uranium and thorium became the first radioactive elements known to man. It was on uranium ore that Henri Becquerel discovered a new penetrating radiation, similar to X-ray, it was from her that Maria Skłodowska Curie produced the first grains of radium and polonium.
These elements are a kind of “islands of stability” in the middle of a sea of elements whose life is too short compared to the Earth’s lifetime. They remained from the time when they formed in the bowels of a supernova, during the explosion of which those gases and dust were formed, from which our solar system was then formed. And they are located in the midst of elements whose half-lives are measured in minutes, hours, years, millennia ... So, changing the cell in the periodic table to the one on the right (in beta decay) or one on the left, this element becomes even more unstable and a radioactive element that decays again - And so, until the decay chain finally leads to a stable element - lead or bismuth.
In this regard, in discussions on various forums of radioactive artifacts such as Japanese lenses or uranium glass, as well as the history of depleted uranium in weapons and airplanes, one can often hear a fallacy: they say that uranium and thorium are alpha emitters and in this connection their radioactivity can neglected if they do not enter the body. Yes, uranium-238 and thorium-232 undergo alpha decay, not accompanied by gamma radiation. However, the subsequent members of the uranium-238 series, whose decays quickly follow one after another up to the long-lived uranium-234, are beta-active, and protactinium-234m gives intense gamma radiation.
In addition, in natural uranium, in addition to the 238th isotope, there are always the 235th and 234th isotopes. The specific activity of the first in natural uranium is 21 times lower than
However, it has intense gamma radiation, like uranium-234, whose activity is almost always equal to the activity of uranium-238, since it is in secular equilibrium with it. Therefore, a piece of uranium-238 decently enough “shines” and illuminates the film on which it lies for about an hour. The story with thorium is about the same, with the only difference being that freshly isolated thorium-232 is actually almost pure alpha emitter, and, for example, the thorium glass of Japanese lenses at the time of their manufacture did not pose a special radiation hazard. But as equilibrium is restored in it, within 10-15 years the intensity of beta and gamma radiation of thorium increases significantly, due to the accumulation in it of radium-228 and subsequent members of the series - up to the final “salute” of thallium-208, which gives very hard gamma radiation with an energy of 2.6 MeV. This line is usually the last in the gamma spectra, beyond it there is nothing but cosmic radiation.
The most famous “daughter” of uranium-238 is, of course, radium-226, the same that was discovered by the Curie spouses and with the extraction of which Mayakovsky compared his work:
That is why fresh uranium is not a source of radon-222 (another minus one myth about uranium glass).
Thorium also has its own radium in its row - two hundred and twenty-eighth. Since the equilibrium in the thorium series is quickly established, radium-228, and with it radon-220, is not long in coming.
Radon is an inert gas. In this regard, it would seem that it should not have a high degree of radiotoxicity, since it is practically not absorbed and does not accumulate. They thought so for a long time, and even when they knew a lot about the dangers of radiation, radon baths were the most popular method of treatment.
But the fact is that radon (that is uranium 222, that of thorium 220), standing in the middle of the radioactive row, quickly turns into one of the radioactive isotopes of lead (214 for radon and 212 for thoron), which settles in the lungs and remains there forever. Rather, until it decays. And already he (and the subsequent members of the series — in the uranium series, for example, polonium-210) effectively and efficiently irradiates the lungs. It is radon and its decay products that make the main contribution to the annual radiation dose.
By the way, these radioactive decay products of radon constantly fall on our heads. And if you measure the background radiation in the street in heavy rain, it turns out that it has grown - sometimes even 2-3 times. This is not the “Chernobyl rain” and the consequences of Fukushima, it’s just the decay products of radon from a kilometer-long atmosphere gathered on the surface of the earth.
Then these lead and bismuth-214 will turn into a relatively long-lived (22 years) lead-210, which can be used to determine how much time has passed since the moment when the sediment layer at the bottom of the sea or another reservoir was blocked by new layers.
And they are also readily absorbed by lichens, for example, reindeer moss, which the deer then feed on. The concentration of daughter products of radon decay in lichens is many times higher than their initial content in rainwater and soil. The lead-210 content in the reindeer moss reaches 500 Bq / kg, which leads to a high content of this nuclide (and therefore polonium-210) in the reindeer meat - and in the bones of the representatives of the peoples of the far north, which this meat (as well as fish, in which is also high in lead-210) are fed. The result is a 35 times higher annual dose than a resident, for example, Moscow.
In addition to uranium and thorium with “daughters”, sources of natural radioactivity are a number of elements that have, in addition to stable, radioactive natural isotopes. Among them there are isotopes that were formedduring the reign of Peas before the birth of the solar system. Their half-lives, like that of uranium and thorium, exceed the lifetime of the solar system, and even the universe. Others have relatively short half-lives that do not allow them to survive from ancient times. They could not have formed during the decay of other radioactive isotopes, which means that somewhere there must be a different source of their appearance. These are cosmic rays.
High-speed protons, crashing into the nuclei of atoms, both themselves cause nuclear reactions, and lead to the birth of neutrons and high-energy gamma rays, which cause new nuclear reactions. As a result, each of the cosmic protons flying into the atmosphere leads to the formation of not only a bunch of muons and electrons, but also to the formation of many unstable nuclei - cosmogenic radionuclides. Due to the fact that they are formed constantly, they are always present in the atmosphere, despite the relatively short (from seconds to thousands of years) lifetime. Perhaps the most important of cosmogenic radionuclides is carbon-14, formed under the action of cosmic rays from nitrogen. Other examples are beryllium-7, which, together with the decay products of radon, is easily detected in rainwater by the characteristic gamma radiation, tritium.
Some cosmogenic radionuclides did not form in the Earth’s atmosphere under the influence of cosmic rays, but they arrived with these cosmic rays. These are chlorine-36 and beryllium-10.
Cosmogenic radionuclides are important tracers for studying various natural processes of substance transfer, radioactive “clocks” for dating (everyone knows about the radiocarbon method), but their role in creating the natural radiation background is small - no one can compete with potassium in this - 40. Their (mainly carbon-14) activity in the human body is only slightly less than potassium-40 activity, however, the decay energy of the latter is one and a half MeVa, and that of carbon-14 is 156 keV. Accordingly, the dose from it is an order of magnitude lower - only about 15 μSv / year.
The peculiarity of potassium is that it is the most important vital element for almost any life form. And at the same time, potassium is inseparable from radioactive potassium-40, which causes its very noticeable radioactivity. The activity of a gram of natural potassium is 31 Bq / g, and the potassium activity in the human body is approximately 60 Bq / kg. This activity creates an annual dose of 170 μSv / year - somewhere a little less than one tenth of the total radiation dose.
Bananas, as you know, are rich in potassium, and hence its radioactive isotope. Potassium, in fact, a lot of things are rich - dried apricots, dates, nuts, and in general bananas are not a leader among them, but still there is a lot of potassium in it. An average banana contains about half a gram of potassium, which corresponds to 15-16 becquerels of potassium. This activity, as well as the magnitude of the contribution to the radiation dose caused by the consumption of one banana (estimated as 0.1 μSv) during the Crash Island accident, was jokingly nicknamed the “banana equivalent."
In fact, the "banana equivalent" in dose terms is almost zero. The fact is that the concentration of potassium in the body is a pretty constant thing. The body perceives any serious deviation in the concentration of potassium in the tissues very painfully and carefully maintains this concentration within narrow limits. If a lot of potassium enters the body, a lot of potassium is excreted by the kidneys. Not enough potassium - the kidneys will save potassium with all their might. But its content in the body will remain unchanged. So the eaten banana will not change the amount of potassium in the body, which means that it will not create an additional dose of radiation.
There is still rubidium-87. It also behaves in the body like potassium, but because of its rarity, its contribution to the dose is small - something in the region of 6 μSv / year.
From the moment radioactivity was discovered until 1934, scientists dealt only with those radioactive elements that exist in nature. In 1934, Frederic and Irene Joliot-Curie, studying the formation of free neutrons under the influence of a stream of alpha particles, found that after the cessation of irradiation, the aluminum target continues to emit some particles (which later turned out to be positrons), the flux of which quickly decayed. So the first artificial synthesis of a radioactive isotope was carried out:
The formation of radioactive phosphorus was proved chemically: when aluminum that became radioactive was dissolved in hydrochloric acid, all the activity went into the liberated gas in the form of phosphorous hydrogen. Then the Joliot-Curie spouses also showed the formation of other artificial radioactive isotopes: by irradiation of boron with alpha particles, radioactive nitrogen was obtained, and by irradiation of magnesium - aluminum. The alchemists' dream of turning some elements into others came true. More productive was the use of recently created charged particle accelerators, with the help of which it was possible to synthesize not only many radioactive isotopes of known elements, but also those elements that did not exist in nature. The first of these was Emilio Segre's technetium, discovered in 1937, the name of which has since indicated its artificial origin. Then there were France, astatine,
Finally, it was discovered, perhaps the most powerful source of new artificial isotopes: nuclear fission.
As I said above, for heavy nuclei, the whole existence of a whole nucleus is less energetically advantageous than its destruction. Nevertheless, the nucleus remains intact, since there is a significant energy barrier between the states of the “whole nucleus” and “individual fragments”. The probability of spontaneous overcoming of such a barrier even for the heaviest nuclei - uranium, thorium, transuranium elements - is insignificant. It is much larger if the detachable fragment is an alpha particle, which determines the alpha activity of such nuclei. But there remains a very small chance that the core will fall apart into several approximately identical “pieces” that will immediately fly apart under the influence of electrostatic repulsion. But the probability of nuclear fission increases sharply if the core is "heated up", excited by any particle from the outside. The easiest way to do this is with a neutron: he does not need to overcome the Coulomb barrier. The excited core is deformed and then broken. It is important that fission usually produces not only “fragments”, but also free neutrons, which also turn out to be able to cause fission in other nuclei. This process is the basis of all nuclear energy of our time, and it produces a huge number of the most diverse radioactive isotopes: nuclear "fragments" can be almost any, and we can detect and isolate them or not, it is determined only by their life time. A powerful neutron flux generated during an intense nuclear reaction (especially in a nuclear explosion) is capable of producing very heavy transuranic elements. Einsteinium and fermium became such "offspring of a nuclear explosion." And lighter plutonium, americium,
The reprocessing of irradiated nuclear fuel and the irradiation by neutrons of various elements in reactors has become an effective and cheap source of almost any radioactive isotopes, allowing them to be obtained in any quantities - from small control sources for calibrating pocket dosimeters that come with them and do not pose a serious danger, to those in the beam from which even bacteria die almost instantly, and the air glows like a light bulb.
The radioactive isotope as a radiation source has one property, which is both an advantage and a disadvantage. It "works" on its own, not depending on anything. It is impossible to “turn off” a radioactive source - only hide it behind a thick layer of lead.
But the fission reaction can (and should) be controlled. A prerequisite for a self-sustaining fission reaction is that the number of neutrons that are produced during fission events is sufficient to replenish both those neutrons that are spent on fission itself and those that left the active zone without causing fission: were absorbed or captured, or simply flew beyond it. This is a critical condition. More neutrons are formed than necessary — the reaction accelerates, increasing its intensity exponentially, like an avalanche. Not enough neutrons - the reaction is dying away.
Nuclear reactors are usually considered primarily as sources of neutrons. Around such a research reactor (or several), a whole scientific center is usually built in which a variety of studies and experiments are carried out, which require an intense neutron flux. These are studies of the crystal structure using neutron diffraction, various methods of chemical analysis based on the conversion of stable elements into radioactive isotopes (neutron activation analysis), the study of the effect of radiation on matter, including biomolecules and living organisms in general, and much more.
One of the options for such a reactor is a pulsed nuclear reactor. This is almost an atomic bomb in the view of some popularizers of nuclear physics: "if we take two pieces of uranium and put them together, we will get a funnel half a mile in diameter." This is exactly what happens in a pulsed reactor: a critical mass is formed for an instant when one piece of uranium quickly flies past another. The neutron burst, which is formed in this case, can be thousands of times more intense than the neutron flux of a conventional energy or research reactor.
A nuclear reactor is a good source of neutrons, but stationary, expensive, bulky and dangerous. In an ordinary laboratory or in the field, either California-252, which generates neutrons through spontaneous fission, or sources based on the reactions of alpha particles with beryllium, boron or aluminum, are used to produce a neutron flux. However, such sources are of low intensity and inevitably produce gamma radiation together with neutrons. Such sources have an alternative in the form of the so-called neutron tube.
In fact, this is also a reactor, only a thermonuclear: A nuclear fusion reaction is carried out in a neutron tube. True, much more energy is spent on its implementation than is released, but it gives a neutron flux. And most importantly, a switched off neutron tube is practically safe (with the exception of some activation of the elements of its structure, and some amount of tritium inside the tube) and in this sense is similar to an X-ray tube. Nuclear fusion occurs on a target from tritium under the influence of deuterium nuclei - deuterons, accelerated by a gas discharge in deuterium.
Ionizing radiation is not a new phenomenon. Contrary to popular beliefs (I already wrote about some myths on this topic in previous articles), the proportion of anthropogenic radiation sources in the radiation dose of the vast majority of people is very small. However, it is anthropogenic sources that pose the greatest danger of acuteradiation damage. Natural terrestrial radiation almost never threatens life directly - the only exception is work on the development of some of the richest uranium deposits. But artificial sources have already managed to kill a lot of people. These are physicists who worked with uranium and plutonium and fell under the outbreaks of SCR, and the victims of the bombing of Hiroshima and Nagasaki, and the victims of Chernobyl and other lesser-known radiation accidents. There were cases when people were killed by a lost or stolen radiation source, or when people unknowingly found themselves in a zone of intense radiation and gained lethal doses in seconds.
I will tell about this - or rather, about radiation safety, in the next article.
Radiation: Everyday life of the radiochemical laboratory
Radiation: units of measurement
Radiation: risks, safety, protection
I will not write here about “that you don’t need to touch it” - so much has been written about it, but I’m not Oleg Aizon and I don’t have unique photos of unprecedented radioactive artifacts. I will tell in general - where radiation comes from.
Radioactive decay as a phenomenon
What is radioactive decay? Someone, recalling school knowledge, will answer - this is the phenomenon of the transformation of some elements into others. Someone will give a different, as a rule, equally inaccurate definition. In fact, radioactive decay is any spontaneous change in the state of an atomic nucleus as a system of nucleons, accompanied by the release of energy, the value of which, as a rule, exceeds several kiloelectron-volts. This energy is then carried away by the elementary particles emitted from the nucleus, by electromagnetic radiation quanta, or transferred to the electrons of the atom. In this case, the nucleus itself can change its charge, mass, split into two or more nuclei, or it can remain by itself, only by going into a more stable state.
“External”, easily determined characteristics of an atomic nucleus are its mass Aand charge (or atomic number) Z , measured in the charges and masses of the proton. These are integer values that have a physical meaning of the number of corresponding particles in the composition of the nucleus. The neutron charge is zero, and the mass is almost the same as that of a proton, so calculate the number of neutrons:
. Nuclei with the same charges are called isotopes , with the same masses - isobars , if the same, then, and the other, we are dealing with isomers . Z and A are denoted to the left of the element symbol in the lower and upper indices, respectively. From what has been said it is obvious that in order for Z to change, the nucleus must leave the charged particle, and for A to change, something heavier than the electron must fly away from the nucleus. So, the following options are possible:
- an electron and an antineutrino or a positron and neutrino (beta decay) fly out - Z changes by one (increases in the case of electron and decreases in the case of positron decay), A - does not change;
- the nucleus, on the contrary, can absorb an electron from the K-level of the atom (K-capture) - Z increases by one (as in beta-plus decay), A does not change, neutrinos are emitted.
- the helium-4 core flies out, the so-called alpha particle (alpha decay) - Z decreases by 2, A decreases by 4;
Beta decay (and electron capture) is the conversion of one of the neutrons into a proton or vice versa, and is a manifestation of a weak interaction that “recharges” one of the nucleon quarks. Together with the electron, an antineutrino is always formed, which takes away part of the energy, while the energy between them is redistributed randomly. Because of this, the energy spectrum of beta radiation is continuous.
And alpha decay occurs simply because any nucleus heavier than iron is energetically more profitable to "lose weight". But while this gain is not more than a few MeV, the energy barrier to removing an alpha particle or any other fragment from the nucleus is too high. And when the energy gain is large enough (but still less than the binding energy), it becomes possible to tunnel an alpha particle outside the nucleus. In addition to an alpha particle, a neutron or proton can fly out of the nucleus in extremely rare cases, or the nucleus is heavier than the alpha particle. And finally, the nucleus can fall into several nuclei, while emitting several neutrons. This is a spontaneous fission that only heavy nuclei are capable of, starting with thorium and uranium.
After the act of decay, an excess of energy may remain in the nucleus and this “warmed up” nucleus must somehow get rid of it. To do this, it emits one or more gamma rays. Sometimes the phenomenon of internal conversion also occurs: energy is not radiated in the form of photons, but is transmitted to electrons that fly out of the atom. Unlike beta rays, conversion electrons have a monoenergetic (linear) spectrum.
In some cases, a core with excess energy can exist for a long time, sometimes even hundreds of years. It does not differ from the same "ordinary" nucleus - neither by charge nor by mass, that is, it is the same chemical element and the same isotope. And here isomers- various. Most often, the lifetime of metastable isomers does not exceed hours, and only a few of them have years. There is only one core for which only the isomeric state is stable: it is tantalum-180. In the ground state, it is beta-active and short-lived (half-life of 8 hours), and its tantalum 180m isomer, it would seem, should either go into the ground state with the emission of gamma rays with an energy of 75 keV, or undergo beta decay, but neither , no one has ever observed: this isomer, in contrast to the short-lived ground state, is stable .
The decay of a nuclear isomer is the only example of radioactive decay, accompanied exclusively by gamma radiation . In all other cases, gamma radiation always exists.exclusively with alpha or beta radiation.
About isotopes and isomers, we said. One more "iso" remains - these are isobars. Nuclei with different nuclear charges and the same mass. Stable isobars usually have charges differing by two units, and between them there is almost always a radioactive isotope. The existence of two stable isobars in neighboring cells of the periodic table is unlikely - this rule is called the Schukarev-Mattauch rule. Only two exceptions are known from it: antimony and tellurium-123 and hafnium-180 and the aforementioned tantalum-180m.
Cosmic rays and other non-radioactive radiation sources
In addition to radioactive substances, some other processes and phenomena, both natural and generated by the human mind, also lead to the appearance of radiation with similar properties.
You probably know about cosmic radiation. Cosmic rays fill the entire Universe, they are protons and heavier nuclei, electrons and gamma rays with exceptionally high energies. The maximum energy recorded by cosmic particles reaches the zept of an electron volt ! it
eV. What is the source of such high-energy particles is impossible to say unequivocally, but particles and gamma-rays with moderate energies - from kilo-giga-electron volts - are generated by stars, including our Sun.This is the so-called primary cosmic radiation. You can only encounter it if you go into low Earth orbit, or at least go up several tens of kilometers. Despite the high energy, these particles do not reach the surface. Each of these particles, having flown into the atmosphere, causes a whole cascade of nuclear reactions, leading to the formation of many particles - mainly muons - which already reach the Earth. By the way, they fly solely due to the relativistic time dilation: the existence time of a muon - two microseconds - without it would make it possible to fly a muon only half a kilometer with a small one. And another interesting fact related to cosmic muons: they are negatively charged, but the primary cosmic rays are positively charged, since they consist mainly of protons. That is why the Earth has a negative charge, and the ionosphere is positive. Near the surface of the Earth, on average, one muon flies through each square centimeter per minute. About a third of the natural background - about 3.5 μR / h - is due to them. And at the altitude at which passenger planes fly, cosmic rays create a dose rate of several microsievert per hour, which already poses a certain danger to the health of pilots.
In addition to muons, there are also electrons and neutrons in secondary cosmic rays. The latter play an important role in the formation of the so-called cosmogenic radionuclides.
Secondary cosmic rays have a very high penetrating power. To protect yourself from them, you have to go into deep cellars and mines. Of course, one has to defend oneself not because they are harmful to health - but because they interfere with detecting rare and weak events in nuclear physics experiments, measuring small radionuclide activities, etc. But there is some benefit from them: with their help, it is possible to “shine through” geological structures, large structures (such as the Egyptian pyramids).
By the way, the Earth’s atmosphere is equivalent to about a meter of lead for cosmic rays. Not only one atmosphere protects the Earth and all of us from cosmic rays - besides it there is a magnetic field deflecting charged particles. But one should not underestimate the protective properties of the atmosphere. During geomagnetic inversions, the Earth’s magnetic shield can practically disappear for a certain time, but contrary to the horror stories of alarmists, this will not lead to the cessation of life on Earth, and the level of radiation at the surface will increase only 2-3 times.
Particularly high-energy particles arriving from space cause the formation of a shower of particles, which covers a large area, causing the simultaneous registration of many particles at detectors spaced over considerable distances. These are the so-called wide air showers. Their registration with the help of a variety of spaced detectors makes it possible to determine the energy of the primary particle, and it is in this way that the energies of the highest-energy particles of cosmic rays are determined. In addition, such a particle causes a powerful flash of Cherenkov radiation in the atmosphere.
Earthly sources of short bursts of gamma radiation and high-energy electrons are lightning and other atmospheric discharges.
And the work of human hands are numerous devices that generate streams of high-energy particles and quanta, not necessarily intentionally. Especially for this, there are X-ray tubes and various kinds of accelerators - from small ones that fit almost in the palm of your hand, to the LHC monster, which occupies the territory of several countries. And the sources, as they say in the dry language of official papers, of unused X-ray radiation are any electrovacuum devices. But it is usually able to go outside when the voltage at the anode is tens of kilovolts. So, high-voltage kenotrons, pulsed modulator lamps and traveling-wave microwave lamps, klystrons, etc., become X-ray sources. in radar stations. And also - in the hands of various lovers of home experiments.
You can often hear about the fact that the source of x-ray radiation is the picture tube of a TV or monitor. Maybe, but usually not. The fact is that the glass at the picture tube is quite thick, and the x-ray radiation at an anode voltage of 15-25 kV is too soft to pass through such a glass. Here are kinescopes of projection TVs, which operated at voltages up to 50 kV and had small dimensions and thin walls of the bulb, "X-rayed" even like that. And among the televisions, the ULPTC with their circuit for stabilizing the anode voltage “distinguished themselves”. In this circuit, the GP-5 lamp was used, operating at an anode voltage equal to the voltage at the second anode (i.e. 25 kV), a noticeable anode current passed through it, and the walls of this lamp were thin. As a result, it shone brightly in the X-ray range.
But we will return to radioactivity.
Uranus and thorium and their daughters
Uranium and thorium became the first radioactive elements known to man. It was on uranium ore that Henri Becquerel discovered a new penetrating radiation, similar to X-ray, it was from her that Maria Skłodowska Curie produced the first grains of radium and polonium.
These elements are a kind of “islands of stability” in the middle of a sea of elements whose life is too short compared to the Earth’s lifetime. They remained from the time when they formed in the bowels of a supernova, during the explosion of which those gases and dust were formed, from which our solar system was then formed. And they are located in the midst of elements whose half-lives are measured in minutes, hours, years, millennia ... So, changing the cell in the periodic table to the one on the right (in beta decay) or one on the left, this element becomes even more unstable and a radioactive element that decays again - And so, until the decay chain finally leads to a stable element - lead or bismuth.
In this regard, in discussions on various forums of radioactive artifacts such as Japanese lenses or uranium glass, as well as the history of depleted uranium in weapons and airplanes, one can often hear a fallacy: they say that uranium and thorium are alpha emitters and in this connection their radioactivity can neglected if they do not enter the body. Yes, uranium-238 and thorium-232 undergo alpha decay, not accompanied by gamma radiation. However, the subsequent members of the uranium-238 series, whose decays quickly follow one after another up to the long-lived uranium-234, are beta-active, and protactinium-234m gives intense gamma radiation.
In addition, in natural uranium, in addition to the 238th isotope, there are always the 235th and 234th isotopes. The specific activity of the first in natural uranium is 21 times lower than
However, it has intense gamma radiation, like uranium-234, whose activity is almost always equal to the activity of uranium-238, since it is in secular equilibrium with it. Therefore, a piece of uranium-238 decently enough “shines” and illuminates the film on which it lies for about an hour. The story with thorium is about the same, with the only difference being that freshly isolated thorium-232 is actually almost pure alpha emitter, and, for example, the thorium glass of Japanese lenses at the time of their manufacture did not pose a special radiation hazard. But as equilibrium is restored in it, within 10-15 years the intensity of beta and gamma radiation of thorium increases significantly, due to the accumulation in it of radium-228 and subsequent members of the series - up to the final “salute” of thallium-208, which gives very hard gamma radiation with an energy of 2.6 MeV. This line is usually the last in the gamma spectra, beyond it there is nothing but cosmic radiation.The most famous “daughter” of uranium-238 is, of course, radium-226, the same that was discovered by the Curie spouses and with the extraction of which Mayakovsky compared his work:
Harassing a single word for the sake of aBut there is almost no radium in fresh uranium. Before him another 245 thousand years to wait for the decay of uranium-234 and then 75 thousand years - thorium-230 with the beautiful name "ion". But in uranium ore, radium is in equilibrium with uranium and its activity is equal to it, uranium, activity. Therefore, uranium ore is much more radioactive than uranium itself.
thousand tons of verbal ore ...
That is why fresh uranium is not a source of radon-222 (another minus one myth about uranium glass).
Thorium also has its own radium in its row - two hundred and twenty-eighth. Since the equilibrium in the thorium series is quickly established, radium-228, and with it radon-220, is not long in coming.
A few words about radon
Radon is an inert gas. In this regard, it would seem that it should not have a high degree of radiotoxicity, since it is practically not absorbed and does not accumulate. They thought so for a long time, and even when they knew a lot about the dangers of radiation, radon baths were the most popular method of treatment.
But the fact is that radon (that is uranium 222, that of thorium 220), standing in the middle of the radioactive row, quickly turns into one of the radioactive isotopes of lead (214 for radon and 212 for thoron), which settles in the lungs and remains there forever. Rather, until it decays. And already he (and the subsequent members of the series — in the uranium series, for example, polonium-210) effectively and efficiently irradiates the lungs. It is radon and its decay products that make the main contribution to the annual radiation dose.
By the way, these radioactive decay products of radon constantly fall on our heads. And if you measure the background radiation in the street in heavy rain, it turns out that it has grown - sometimes even 2-3 times. This is not the “Chernobyl rain” and the consequences of Fukushima, it’s just the decay products of radon from a kilometer-long atmosphere gathered on the surface of the earth.
Then these lead and bismuth-214 will turn into a relatively long-lived (22 years) lead-210, which can be used to determine how much time has passed since the moment when the sediment layer at the bottom of the sea or another reservoir was blocked by new layers.
And they are also readily absorbed by lichens, for example, reindeer moss, which the deer then feed on. The concentration of daughter products of radon decay in lichens is many times higher than their initial content in rainwater and soil. The lead-210 content in the reindeer moss reaches 500 Bq / kg, which leads to a high content of this nuclide (and therefore polonium-210) in the reindeer meat - and in the bones of the representatives of the peoples of the far north, which this meat (as well as fish, in which is also high in lead-210) are fed. The result is a 35 times higher annual dose than a resident, for example, Moscow.
About potassium, bananas and other oranges
In addition to uranium and thorium with “daughters”, sources of natural radioactivity are a number of elements that have, in addition to stable, radioactive natural isotopes. Among them there are isotopes that were formed
High-speed protons, crashing into the nuclei of atoms, both themselves cause nuclear reactions, and lead to the birth of neutrons and high-energy gamma rays, which cause new nuclear reactions. As a result, each of the cosmic protons flying into the atmosphere leads to the formation of not only a bunch of muons and electrons, but also to the formation of many unstable nuclei - cosmogenic radionuclides. Due to the fact that they are formed constantly, they are always present in the atmosphere, despite the relatively short (from seconds to thousands of years) lifetime. Perhaps the most important of cosmogenic radionuclides is carbon-14, formed under the action of cosmic rays from nitrogen. Other examples are beryllium-7, which, together with the decay products of radon, is easily detected in rainwater by the characteristic gamma radiation, tritium.
Some cosmogenic radionuclides did not form in the Earth’s atmosphere under the influence of cosmic rays, but they arrived with these cosmic rays. These are chlorine-36 and beryllium-10.
Cosmogenic radionuclides are important tracers for studying various natural processes of substance transfer, radioactive “clocks” for dating (everyone knows about the radiocarbon method), but their role in creating the natural radiation background is small - no one can compete with potassium in this - 40. Their (mainly carbon-14) activity in the human body is only slightly less than potassium-40 activity, however, the decay energy of the latter is one and a half MeVa, and that of carbon-14 is 156 keV. Accordingly, the dose from it is an order of magnitude lower - only about 15 μSv / year.
The peculiarity of potassium is that it is the most important vital element for almost any life form. And at the same time, potassium is inseparable from radioactive potassium-40, which causes its very noticeable radioactivity. The activity of a gram of natural potassium is 31 Bq / g, and the potassium activity in the human body is approximately 60 Bq / kg. This activity creates an annual dose of 170 μSv / year - somewhere a little less than one tenth of the total radiation dose.
Bananas, as you know, are rich in potassium, and hence its radioactive isotope. Potassium, in fact, a lot of things are rich - dried apricots, dates, nuts, and in general bananas are not a leader among them, but still there is a lot of potassium in it. An average banana contains about half a gram of potassium, which corresponds to 15-16 becquerels of potassium. This activity, as well as the magnitude of the contribution to the radiation dose caused by the consumption of one banana (estimated as 0.1 μSv) during the Crash Island accident, was jokingly nicknamed the “banana equivalent."
In fact, the "banana equivalent" in dose terms is almost zero. The fact is that the concentration of potassium in the body is a pretty constant thing. The body perceives any serious deviation in the concentration of potassium in the tissues very painfully and carefully maintains this concentration within narrow limits. If a lot of potassium enters the body, a lot of potassium is excreted by the kidneys. Not enough potassium - the kidneys will save potassium with all their might. But its content in the body will remain unchanged. So the eaten banana will not change the amount of potassium in the body, which means that it will not create an additional dose of radiation.
There is still rubidium-87. It also behaves in the body like potassium, but because of its rarity, its contribution to the dose is small - something in the region of 6 μSv / year.
Human handiwork
From the moment radioactivity was discovered until 1934, scientists dealt only with those radioactive elements that exist in nature. In 1934, Frederic and Irene Joliot-Curie, studying the formation of free neutrons under the influence of a stream of alpha particles, found that after the cessation of irradiation, the aluminum target continues to emit some particles (which later turned out to be positrons), the flux of which quickly decayed. So the first artificial synthesis of a radioactive isotope was carried out:
The formation of radioactive phosphorus was proved chemically: when aluminum that became radioactive was dissolved in hydrochloric acid, all the activity went into the liberated gas in the form of phosphorous hydrogen. Then the Joliot-Curie spouses also showed the formation of other artificial radioactive isotopes: by irradiation of boron with alpha particles, radioactive nitrogen was obtained, and by irradiation of magnesium - aluminum. The alchemists' dream of turning some elements into others came true. More productive was the use of recently created charged particle accelerators, with the help of which it was possible to synthesize not only many radioactive isotopes of known elements, but also those elements that did not exist in nature. The first of these was Emilio Segre's technetium, discovered in 1937, the name of which has since indicated its artificial origin. Then there were France, astatine,
Finally, it was discovered, perhaps the most powerful source of new artificial isotopes: nuclear fission.
As I said above, for heavy nuclei, the whole existence of a whole nucleus is less energetically advantageous than its destruction. Nevertheless, the nucleus remains intact, since there is a significant energy barrier between the states of the “whole nucleus” and “individual fragments”. The probability of spontaneous overcoming of such a barrier even for the heaviest nuclei - uranium, thorium, transuranium elements - is insignificant. It is much larger if the detachable fragment is an alpha particle, which determines the alpha activity of such nuclei. But there remains a very small chance that the core will fall apart into several approximately identical “pieces” that will immediately fly apart under the influence of electrostatic repulsion. But the probability of nuclear fission increases sharply if the core is "heated up", excited by any particle from the outside. The easiest way to do this is with a neutron: he does not need to overcome the Coulomb barrier. The excited core is deformed and then broken. It is important that fission usually produces not only “fragments”, but also free neutrons, which also turn out to be able to cause fission in other nuclei. This process is the basis of all nuclear energy of our time, and it produces a huge number of the most diverse radioactive isotopes: nuclear "fragments" can be almost any, and we can detect and isolate them or not, it is determined only by their life time. A powerful neutron flux generated during an intense nuclear reaction (especially in a nuclear explosion) is capable of producing very heavy transuranic elements. Einsteinium and fermium became such "offspring of a nuclear explosion." And lighter plutonium, americium,
The reprocessing of irradiated nuclear fuel and the irradiation by neutrons of various elements in reactors has become an effective and cheap source of almost any radioactive isotopes, allowing them to be obtained in any quantities - from small control sources for calibrating pocket dosimeters that come with them and do not pose a serious danger, to those in the beam from which even bacteria die almost instantly, and the air glows like a light bulb.
And then, draining the gas and starting the reactor ...
The radioactive isotope as a radiation source has one property, which is both an advantage and a disadvantage. It "works" on its own, not depending on anything. It is impossible to “turn off” a radioactive source - only hide it behind a thick layer of lead.
But the fission reaction can (and should) be controlled. A prerequisite for a self-sustaining fission reaction is that the number of neutrons that are produced during fission events is sufficient to replenish both those neutrons that are spent on fission itself and those that left the active zone without causing fission: were absorbed or captured, or simply flew beyond it. This is a critical condition. More neutrons are formed than necessary — the reaction accelerates, increasing its intensity exponentially, like an avalanche. Not enough neutrons - the reaction is dying away.
Nuclear reactors are usually considered primarily as sources of neutrons. Around such a research reactor (or several), a whole scientific center is usually built in which a variety of studies and experiments are carried out, which require an intense neutron flux. These are studies of the crystal structure using neutron diffraction, various methods of chemical analysis based on the conversion of stable elements into radioactive isotopes (neutron activation analysis), the study of the effect of radiation on matter, including biomolecules and living organisms in general, and much more.
One of the options for such a reactor is a pulsed nuclear reactor. This is almost an atomic bomb in the view of some popularizers of nuclear physics: "if we take two pieces of uranium and put them together, we will get a funnel half a mile in diameter." This is exactly what happens in a pulsed reactor: a critical mass is formed for an instant when one piece of uranium quickly flies past another. The neutron burst, which is formed in this case, can be thousands of times more intense than the neutron flux of a conventional energy or research reactor.
A nuclear reactor is a good source of neutrons, but stationary, expensive, bulky and dangerous. In an ordinary laboratory or in the field, either California-252, which generates neutrons through spontaneous fission, or sources based on the reactions of alpha particles with beryllium, boron or aluminum, are used to produce a neutron flux. However, such sources are of low intensity and inevitably produce gamma radiation together with neutrons. Such sources have an alternative in the form of the so-called neutron tube.
In fact, this is also a reactor, only a thermonuclear: A nuclear fusion reaction is carried out in a neutron tube. True, much more energy is spent on its implementation than is released, but it gives a neutron flux. And most importantly, a switched off neutron tube is practically safe (with the exception of some activation of the elements of its structure, and some amount of tritium inside the tube) and in this sense is similar to an X-ray tube. Nuclear fusion occurs on a target from tritium under the influence of deuterium nuclei - deuterons, accelerated by a gas discharge in deuterium.
Afterword
Ionizing radiation is not a new phenomenon. Contrary to popular beliefs (I already wrote about some myths on this topic in previous articles), the proportion of anthropogenic radiation sources in the radiation dose of the vast majority of people is very small. However, it is anthropogenic sources that pose the greatest danger of acuteradiation damage. Natural terrestrial radiation almost never threatens life directly - the only exception is work on the development of some of the richest uranium deposits. But artificial sources have already managed to kill a lot of people. These are physicists who worked with uranium and plutonium and fell under the outbreaks of SCR, and the victims of the bombing of Hiroshima and Nagasaki, and the victims of Chernobyl and other lesser-known radiation accidents. There were cases when people were killed by a lost or stolen radiation source, or when people unknowingly found themselves in a zone of intense radiation and gained lethal doses in seconds.
I will tell about this - or rather, about radiation safety, in the next article.
All articles in the series
Radiation: Everyday life of the radiochemical laboratory
Radiation: units of measurement
Radiation: risks, safety, protection