Why are all elements after lead radioactive
Lexicon> Letter R> Radioactivity
Definition: the phenomenon that certain atomic nuclei transform while emitting high-energy radiation
More specific terms: α-radiation, β-radiation, positron radiation, γ-radiation, neutron radiation
Categories: Basic Concepts, Nuclear Energy, Physical Basics
Author: Dr. Rüdiger Paschotta
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Original creation: 05/17/2011; last change: 03/21/2021
While the matter surrounding us consists largely of atoms with stable atomic nuclei that do not change without external influence, there are also so-called radioactive substanceswhose atomic nuclei are unstable. Such atomic nuclei spontaneously transform into other atomic nuclei, giving off high-energy radiation. One also speaks of ionizing radiation, because this radiation can ionize atoms or molecules, i.e. snatch electrons from them and thus trigger chemical transformations.Different isotopes of a chemical element can be extremely different in terms of radioactivity.
An atomic nucleus is characterized by its Atomic number (the number of protons) and its Mass number (the sum of the numbers of protons and neutrons). The chemical behavior is practically determined solely by the atomic number; each value of the atomic number corresponds to a certain chemical element (e.g. hydrogen, iron or cobalt). For many chemical elements there are versions with different mass numbers, which are known as isotopes. For the stability or the tendency to radioactive decay, not only the atomic number is important, but also the mass number; different isotopes of an element can therefore be extremely different in terms of radioactivity. For example, Cobalt 59 is stable, Cobalt 60 (different notation: 60Co), on the other hand, is highly radioactive. Here 59 and 60 are the mass numbers, while the atomic number of cobalt is always 27.
Radioactive substances occur naturally - for example uranium, thorium and radium in certain ore deposits. In nature, substances such as potassium also consist to a small extent of radioactive isotopes (e.g. potassium 40). Others are produced in certain technical processes, in particular in fission in nuclear reactors that are part of nuclear power plants. In particular, highly radioactive fission products are created here, as well as so-called transuranic elements such as plutonium, some of which are quite long-lived, through “incubation”.
The time of the decay of a single unstable atomic nucleus cannot be foreseen. However, such atomic nuclei decay according to a simple statistical law: As long as the decay has not taken place, the probability that this will happen in the next second is always the same - regardless of how long the atomic nucleus has existed. This leads to the fact that within a certain period of time, the Half-life is called, half of the existing amount of material decays (as long as there are many atomic nuclei). The half-lives of different radioactive substances are extremely different: They can be tiny fractions of a second, but also billions of years or anywhere in between.
Note that the atomic nucleus that is formed when it decays is often itself unstable. So it will later disintegrate itself. This then only decays when a stable atomic nucleus is formed. Because of such decay series, it is often difficult to calculate exactly how the total radioactive radiation decreases over time. This is also due to the fact that decay series can have branches, since some unstable atomic nuclei can decay in different ways.
The article on the term half-life gives quite a few more details.
Types of radioactive decay
Depending on the type of radioactive material, one or more of the following decay mechanisms are possible, which lead to different types of radioactive radiation:Alpha radiation is locally very damaging, but very poorly penetrating.
- During the α-decay (alpha decay) of a nucleus, a helium atomic nucleus (4He) ejected with high energy. The α-radiation (alpha radiation) consists of fast helium nuclei. Since these interact very strongly with other matter, they can deposit a lot of energy locally. B. cause a lot of damage in body cells, but have only a very short range - even in air only a few centimeters.
- During the β-decay (beta decay) a fast electron or positron (anti-electron) is ejected. Such electrons or positrons emit their energy over a longer distance.
- Often immediately after an α or β decay, one or more high-energy photons (γ quanta) are emitted. (Sometimes a so-called metastable atomic nucleus is created, which only emits such photons with a certain delay and then changes to the stable ground state.) This γ-radiation (gamma radiation) is a type of light, but with much higher photon energies. Depending on the size of the photon energy, gamma radiation has a greater or lesser penetration capacity in solid matter. In general, the range is much higher than for α and β radiation. Hard γ radiation can best be shielded by thick layers of lead.
- Intense neutron radiation is mainly generated during nuclear fission. The fast neutrons can transfer a lot of energy, especially to hydrogen atoms, which are known to play an important role in biological materials. They can even penetrate lead very well - but much less water and other hydrogen-containing substances.
- The fission products can also be viewed as radioactive radiation with very high energy, but only a short range in matter.
Various other types of radioactive decay also exist, but will not be discussed in detail here.
Often atomic nuclei are formed during radioactive decay, which in turn are unstable. This results in whole Chains of decay. For example, the uranium-radium series begins with uranium 238, which is finally converted into a stable lead isotope with many intermediate steps.
The article on radiation discusses various aspects of radiation in more detail.
Generation of heat
The energy released during radioactive decay is initially contained in the emitted radiation. It is converted into heat when this radiation is absorbed in a material. However, noticeable warming only occurs when a material consists to a large extent of a radioactive substance with a very short half-life, so that very many decays take place per second.Some artificially produced radioactive materials generate a lot of heat.
Such materials are not found in nature because they would have long since disintegrated. Natural radioactive emitters such as uranium have so long half-lives that they do not noticeably heat up. However, the radioactive waste that occurs in the form of spent fuel elements from nuclear power plants generates a lot of heat for some time, which makes it even more difficult to handle. If the resulting decay heat is not dissipated effectively enough, the resulting enormous heat can damage containers, for example, so that hazardous material can escape. If the fuel elements are still in a large nuclear reactor, if the cooling is inadequate, the so-called decay heat alone (even after the nuclear chain reaction has ended completely) can lead to a so-called core meltdown within a few hours or days if the reactor is not adequately cooled. This has happened several times in the history of nuclear energy use in the context of major nuclear accidents.
There are also some relatively exotic uses for the heat generation of radioactive substances. See the article on radionuclide batteries, which also briefly discusses some related technical approaches.
In the interior of the earth, too, the production of heat through radioactive decay plays an essential role. Although the power density of this heat source is extremely low, it takes place in a very large volume. So it can still contribute significantly to the fact that the temperature of the earth increases inward.
Biological hazard of radioactivity
When radioactive radiation is absorbed in body cells, concentrated energy is transferred to individual molecules of the cell, which can destroy these molecules or convert them into other molecules. This disrupts the function of the cells. Damage to the genetic material in the cell nucleus is particularly delicate; These can lead to the death of the affected cells, but also to degeneration, which can become the cause of a cancerous tumor if the body's repair and defense mechanisms cannot prevent this.
Radioactive substances outside the body can essentially only damage the body through γ radiation, since α radiation is already absorbed in the outermost layers of the skin, which are made up of dead cells anyway, and β radiation cannot penetrate deeply either. If, on the other hand, α or β emitters get into the body (incorporated are) - for example by inhalation into the lungs or by swallowing - these are even particularly harmful, as they can then irradiate body cells directly.
The biological hazard posed by radioactive substances depends not only on the type and strength of the radiation emitted, but also to a large extent on the chemical properties. These determine whether the respective substance prefers to get into certain parts of the body and how long it stays there. Some examples:The health risks depend very much on which radioactive isotopes are involved and whether they are in the body or something outside.
- The radioactive noble gas radon constantly escapes from the ground - especially from soils containing uranium. It can get into houses, accumulate there, especially if there is insufficient ventilation, and it can be inhaled by the residents. Although it hardly gets into the body, it irradiates the cells of the lungs and can cause lung cancer there. A large proportion of non-smoking lung cancer cases are thought to be due to radon.
- Substances such as radium and plutonium are stored by the body particularly in bones and can therefore be prone to bone cancer and blood cancer (leukemiadue to the formation of blood in the bones).
- If the smallest plutonium particles are inhaled, they can be deposited in the lungs and cause lung cancer there. Even the smallest amounts below a microgram are sufficient for this.
- Radioactive iodine, which is produced in the fuel rods of nuclear reactors and can be released in the event of nuclear accidents, is highly volatile and can therefore be inhaled or ingested with food. It is mainly concentrated in the thyroid gland and can therefore easily cause thyroid cancer.
The health effects of high doses of radiation are relatively well researched. On the other hand, it is still not certain what effects low radiation doses have. This is difficult to determine because such effects can occur after a very long time, but with a low probability. When a large number of people are examined in an epidemiological study, it is usually not known exactly what radiation and what other harmful effects they were previously exposed to. Any damage is then difficult to assign to a specific cause.
It is conceivable that radiation below a certain threshold value has no effect, or possibly even positive effects. However, it is at least as well conceivable and, based on simple models, plausible that z. B. the probability of developing cancer depends linearly on the radiation dose; then reducing the dose would only reduce the probability, but not bring it to zero. Because there is some evidence in favor of this linear dose-effect relationship (which is also based on e.g. the International Commission on Radiological Protection), the precautionary principle requires that radioactive radiation exposure be reduced as far as possible - even below the level that can be proven to cause damage.
Units for the quantification of radioactivity
The number of radioactive decays of an amount of substance per second is given in Becquerel (Bq). The shorter the half-life of a radioactive substance, the more Becquerel per kilogram of the substance you get.
Of course, it is not the number of decays that is biologically relevant, but rather the energy deposited in the body's cells by radiation. This is denoted by the unit Sievert; This is about the radiation energy deposited per kilogram of body tissue, weighted according to the type (biological effectiveness) of the radiation. If a certain amount of a substance, referred to in Becquerel, gets into or near the body, the radiation dose measured in Sievert (or millisievert, microsievert) is higher, the more the radiation is absorbed in the body.
The load z. B. in the vicinity of a damaged nuclear power plant is often given in millisieverts per hour (mSv / h); this indicates the radiation dose that would be received from outside radiation by staying at the respective location for one hour. If one were to absorb radioactive substances into the body, the radiation dose could be considerably higher.
Generation of radioactivity when using nuclear energy
Today's use of nuclear energy is based on the fission of heavy atomic nuclei. The fissile materials like uranium and plutonium are inherently radioactive; However, the radioactivity of the uranium is particularly weak because the half-life is very long, so the decay takes place very slowly. If, however, the nuclear fission takes place (triggered by neutron radiation from the nuclear fission itself), fission products arise, which for the most part are comparatively short-lived. This means that their radiation is initially very strong, but depending on the substance, it also subsides within seconds to centuries. While fresh uranium fuel rods are only slightly radioactive and therefore hardly dangerous, extremely strong radioactivity is generated during reactor operation - so strong that the fuel rods still give off a lot of heat even after the nuclear fission has been stopped (see above). These Decay heat In the event of insufficient cooling, it can even easily lead to the melting of the fuel rods, i.e. to a Meltdownwhich must be considered particularly dangerous (see also the article on reactor safety). Note that the radioactive inventory of a typical nuclear reactor is far larger than that which e.g. B. released the Hiroshima bomb. It is therefore imperative to prevent even a small part of this material from being released.
Various materials can be converted into radioactive substances, especially by means of neutron irradiation, even without nuclear fission. For example, cobalt 59 can be converted into radioactive cobalt 60 during neutron irradiation in a reactor, which is used, among other things, for radiation therapy against cancer, for the sterilization of food and for material examinations. In nuclear reactors there is also undesired activation of materials, e.g. B. by certain elements in the steel, so that the steel becomes radioactive not only through contamination, but also through this activation. However, this radioactivity is much weaker than that of the fission products.
Elimination of radioactivity
The radioactivity of substances cannot be influenced by common physical and chemical processes; you can only collect the radioactive substances and keep them as safe as possible, for example in stable containers that are deposited in a deep repository.In principle, the lifespan of radioactive waste can be greatly reduced by targeted irradiation; however, a practicable procedure has not yet been developed.
However, there are special physical processes with which atomic nuclei can be converted so that radioactivity is directly affected. This assumes that the atomic nuclei of the respective substance are irradiated. As a rule, the radioactivity is increased first. However, if short-lived substances are produced in this way, the radioactivity can be reduced in the long term. This is the principle behind the idea of Transmutation as a method of breaking down certain radioactive waste, namely the transuranic elements in the radioactive waste from nuclear reactors. These transuranic elements (e.g.Plutonium) have relatively long lifetimes (half-lives), but can be split by intensive neutron irradiation. The resulting fission products are currently much more radioactive, but largely decay within a few centuries. In this way, the remaining waste could become so little radioactive again within “only” a few centuries (instead of within hundreds of thousands of years) that it would only pose a minor problem. Unfortunately, this procedure is impractical, as explained in the article on transmutation.
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See also: radiation, nuclear fission, half-life, fuel rod, nuclear fuel, nuclear energy, reactor safety, radioactive waste, transmutation
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