Nuclear reaction (NR) - a process in which the nucleus of an atom changes by crushing or combining with the nucleus of another atom. Thus, it must lead to the transformation of at least one nuclide into another. Sometimes, if a nucleus interacts with another nucleus or particle without changing the nature of any nuclide, the process is referred to as nuclear scattering. Perhaps most notable are the fusion reactions of light elements, which affect the energy production of stars and the sun. Natural reactions also occur in the interaction of cosmic rays with matter.
Natural nuclear reactor
The most notable human-controlled reaction is the fission reaction that occurs in nuclear reactors. These are devices for initiating and controlling a nuclear chain reaction. But there are not only artificial reactors. The world's first natural nuclear reactor was discovered in 1972 at Oklo in Gabon by French physicist Francis Perrin.
The conditions under which the natural energy of a nuclear reaction could be generated were predicted in 1956 by Paul Kazuo Kuroda. The only known place inworld consists of 16 sites in which self-sustaining reactions of this type occurred. This is believed to have been approximately 1.7 billion years ago and continued for several hundred thousand years, as evidenced by xenon isotopes (a fission product gas) and varying U-235/U-238 ratios (natural uranium enrichment).
Nuclear fission
The binding energy plot suggests that nuclides with a mass greater than 130 a.m.u. should spontaneously separate from each other to form lighter and more stable nuclides. Experimentally, scientists have found that spontaneous fission reactions of the elements of a nuclear reaction occur only for the heaviest nuclides with a mass number of 230 or more. Even if this is done, it is very slow. The half-life for spontaneous fission of 238 U, for example, is 10-16 years, or about two million times longer than the age of our planet! Fission reactions can be induced by irradiating samples of heavy nuclides with slow thermal neutrons. For example, when 235 U absorbs a thermal neutron, it breaks into two particles of uneven mass and releases an average of 2.5 neutrons.
The absorption of the 238 U neutron induces vibrations in the nucleus, which deform it until it breaks into fragments, just as a drop of liquid can shatter into smaller droplets. More than 370 daughter nuclides with atomic masses between 72 and 161 a.m.u. are formed during fission by a thermal neutron 235U, including two products,shown below.
Isotopes of a nuclear reaction, such as uranium, undergo induced fission. But the only natural isotope 235 U is present in abundance at only 0.72%. The induced fission of this isotope releases an average of 200 MeV per atom, or 80 million kilojoules per gram of 235 U. The attraction of nuclear fission as an energy source can be understood by comparing this value with the 50 kJ/g released when natural gas is burned.
First nuclear reactor
The first artificial nuclear reactor was built by Enrico Fermi and co-workers under the University of Chicago football stadium and put into operation on December 2, 1942. This reactor, which produced several kilowatts of power, consisted of a pile of 385 tons of graphite blocks stacked in layers around a cubic lattice of 40 tons of uranium and uranium oxide. Spontaneous fission of 238 U or 235 U in this reactor produced very few neutrons. But there was enough uranium, so one of these neutrons induced fission of the 235 U nucleus, thereby releasing an average of 2.5 neutrons, which catalyzed the fission of additional 235 U nuclei in a chain reaction (nuclear reactions).
The amount of fissile material required to sustain a chain reaction is called critical mass. The green arrows show the splitting of the uranium nucleus in two fission fragments emitting new neutrons. Some of these neutrons can trigger new fission reactions (black arrows). Some ofneutrons may be lost in other processes (blue arrows). The red arrows show delayed neutrons that arrive later from radioactive fission fragments and can trigger new fission reactions.
Designation of nuclear reactions
Let's look at the basic properties of atoms, including atomic number and atomic mass. The atomic number is the number of protons in the nucleus of an atom, and isotopes have the same atomic number but differ in the number of neutrons. If the initial nuclei are denoted a and b, and the product nuclei are denoted c and d, then the reaction can be represented by the equation you can see below.
Which nuclear reactions cancel out for light particles instead of using full equations? In many situations, the compact form is used to describe such processes: a (b, c) d is equivalent to a + b producing c + d. Light particles are often abbreviated: usually p stands for proton, n for neutron, d for deuteron, α for alpha or helium-4, β for beta or electron, γ for gamma photon, etc.
Types of nuclear reactions
Although the number of possible such reactions is huge, they can be sorted by type. Most of these reactions are accompanied by gamma radiation. Here are some examples:
- Elastic scattering. Occurs when no energy is transferred between the target nucleus and the incoming particle.
- Inelastic scattering. Occurs when energy is transferred. The difference in kinetic energies is conserved in the excited nuclide.
- Capture reactions. both charged andneutral particles can be captured by nuclei. This is accompanied by the emission of ɣ-rays. The particles of nuclear reactions in the neutron capture reaction are called radioactive nuclides (induced radioactivity).
- Transmission reactions. The absorption of a particle, accompanied by the emission of one or more particles, is called a transfer reaction.
- Fission reactions. Nuclear fission is a reaction in which the nucleus of an atom is split into smaller pieces (lighter nuclei). The fission process often produces free neutrons and photons (in the form of gamma rays) and releases large amounts of energy.
- Fusion reactions. Occur when two or more atomic nuclei collide at a very high speed and combine to form a new type of atomic nucleus. Deuterium and tritium fusion nuclear particles are of particular interest because of their potential to provide energy in the future.
- Splitting reactions. Occurs when a nucleus is hit by a particle with enough energy and momentum to knock out a few small fragments or break it into many fragments.
- Rearrangement reactions. This is the absorption of a particle, accompanied by the emission of one or more particles:
- 197Au (p, d) 196mAu
- 4He (a, p) 7Li
- 27Al (a, n) 30P
- 54Fe (a, d) 58Co
- 54Fe (a, 2 n) 56Ni
- 54Fe (32S, 28Si) 58Ni
Different rearrangement reactions change the number of neutrons and the number of protons.
Nuclear decay
Nuclear reactions occur when an unstable atom loses energy throughradiation. It is a random process at the level of single atoms, since according to quantum theory it is impossible to predict when an individual atom will decay.
There are many types of radioactive decay:
- Alpha radioactivity. Alpha particles are made up of two protons and two neutrons bound together with a particle identical to a helium nucleus. Due to its very large mass and its charge, it strongly ionizes the material and has a very short range.
- Beta radioactivity. It is high-energy, high-speed positrons or electrons emitted by certain types of radioactive nuclei, such as potassium-40. Beta particles have a greater penetration range than alpha particles, but still much less than gamma rays. Ejected beta particles are a form of ionizing radiation, also known as nuclear chain reaction beta rays. The production of beta particles is called beta decay.
- Gamma radioactivity. Gamma rays are electromagnetic radiation of very high frequency and are therefore high energy photons. They are formed when nuclei decay as they go from a high-energy state to a lower state known as gamma decay. Most nuclear reactions are accompanied by gamma radiation.
- Neutron emission. Neutron emission is a type of radioactive decay of nuclei containing excess neutrons (especially fission products), in which the neutron is simply ejected from the nucleus. This typeradiation plays a key role in the control of nuclear reactors because these neutrons are delayed.
Energy
Q-value of the energy of a nuclear reaction is the amount of energy released or absorbed during the reaction. It is called the energy balance, or Q-value of the reaction. This energy is expressed as the difference between the kinetic energy of the product and the amount of the reactant.
General view of the reaction: x + X ⟶ Y + y + Q……(i) x + X ⟶ Y + y + Q……(i), where x and X are reactants, and y and Y are reaction product, which can determine the energy of a nuclear reaction, Q is the energy balance.
Q-value NR refers to the energy released or absorbed in a reaction. It is also called the NR energy balance, which can be positive or negative depending on the nature.
If the Q-value is positive, the reaction will be exothermic, also called exoergic. She releases energy. If the Q-value is negative, the reaction is endoergic, or endothermic. Such reactions are carried out by absorbing energy.
In nuclear physics, such reactions are defined by the Q-value, as the difference between the sum of the masses of the initial reactants and the final products. It is measured in energy units MeV. Consider a typical reaction in which projectile a and target A yield to two products B and b.
This can be expressed like this: a + A → B + B, or even in a more compact notation - A (a, b) B. Types of energies in a nuclear reaction and the meaning of this reactiondetermined by the formula:
Q=[m a + m A - (m b + m B)] c 2, which coincides with the excess kinetic energy of the final products:
Q=T final - T initial
For reactions in which there is an increase in the kinetic energy of the products, Q is positive. Positive Q reactions are called exothermic (or exogenous).
There is a net release of energy, since the kinetic energy of the final state is greater than in the initial state. For reactions in which a decrease in the kinetic energy of the products is observed, Q is negative.
Half-life
The half-life of a radioactive substance is a characteristic constant. It measures the time required for a given amount of matter to be reduced by half through decay and therefore radiation.
Archaeologists and geologists use the half-life to date on organic objects in a process known as carbon dating. During beta decay, carbon 14 is converted to nitrogen 14. At the time of death, organisms stop producing carbon 14. Because the half-life is constant, the ratio of carbon 14 to nitrogen 14 provides a measure of the age of the sample.
In the medical field, the energy sources for nuclear reactions are radioactive isotopes of Cob alt 60, which has been used for radiation therapy to shrink tumors that will later be removed surgically, or to kill cancer cells in inoperabletumors. When it decays into stable nickel, it emits two relatively high energies - gamma rays. Today it is being replaced by electron beam radiotherapy systems.
Isotope half-life from some samples:
- oxygen 16 - infinite;
- uranium 238 - 4,460,000,000 years;
- uranium 235 - 713,000,000 years;
- carbon 14 - 5,730 years;
- cob alt 60 - 5, 27 years old;
- silver 94 - 0.42 seconds.
Radiocarbon dating
At a very steady rate, unstable carbon 14 gradually decays into carbon 12. The ratio of these carbon isotopes reveals the age of some of Earth's oldest inhabitants.
Radiocarbon dating is a method that provides objective estimates of the age of carbon-based materials. Age can be estimated by measuring the amount of carbon 14 present in a sample and comparing it to an international standard reference.
The impact of radiocarbon dating on the modern world has made it one of the most significant discoveries of the 20th century. Plants and animals assimilate carbon 14 from carbon dioxide throughout their lives. When they die, they stop exchanging carbon with the biosphere, and their carbon 14 content begins to decline at a rate determined by the law of radioactive decay.
Radiocarbon dating is essentially a method for measuring residual radioactivity. Knowing how much carbon 14 is left in the sample, you can find outthe age of the organism when it died. It should be noted that the results of radiocarbon dating show when the organism was alive.
Basic methods for measuring radiocarbon
There are three main methods used to measure carbon 14 in any given sampler proportional calculation, liquid scintillation counter and accelerator mass spectrometry.
Proportional gas counting is a common radiometric dating technique that takes into account the beta particles emitted by a given sample. Beta particles are decay products of radiocarbon. In this method, the carbon sample is first converted to carbon dioxide gas before being measured in gas proportional meters.
Scintillation fluid counting is another method of radiocarbon dating that was popular in the 1960s. In this method, the sample is in liquid form and a scintillator is added. This scintillator creates a flash of light when it interacts with a beta particle. The sample tube is passed between two photomultipliers and when both devices register a flash of light, a count is made.
The Benefits of Nuclear Science
The laws of nuclear reactions are used in a wide range of branches of science and technology, such as medicine, energy, geology, space and environmental protection. Nuclear medicine and radiology are medical practices that involve the use of radiation or radioactivity for diagnosis, treatment, and prevention.diseases. While radiology has been in use for almost a century, the term "nuclear medicine" began to be used about 50 years ago.
Nuclear power has been in use for decades and is one of the fastest growing energy options for countries seeking energy security and low emission energy saving solutions.
Archaeologists use a wide range of nuclear methods to determine the age of objects. Artifacts such as the Shroud of Turin, the Dead Sea Scrolls and the Crown of Charlemagne can be dated and authenticated using nuclear methods.
Nuclear techniques are used in agricultural communities to fight disease. Radioactive sources are widely used in the mining industry. For example, they are used in non-destructive testing of blockages in pipelines and welds, in measuring the density of punched material.
Nuclear science plays a valuable role in helping us understand the history of our environment.
