theory of relativity says that mass - is a special form of energy.It follows that it is possible to convert the energy of the mass and energy to the mass.On intraatomic level such reactions occur.In particular, some of the mass of the atomic nucleus may well turn into energy.This occurs through multiple paths.Firstly, the core may break up into a number of smaller nuclei, this reaction is called "collapse".Second, smaller kernels can easily connect to get a large - is the synthesis reaction.In the universe, such reactions are not uncommon.Suffice it to say that the fusion reaction - a source of energy for the stars.But the reaction to the collapse of humanity used in nuclear reactors, as people learned to control these complex processes.But what is a nuclear chain reaction?How to manage it?
What happens in the nucleus of an atom
nuclear chain reaction - a process running in the collision of elementary particles or nuclei with other nuclei.Why the "chain"?This set of sequential single nuclear reactions.This process results in a change in the quantum state and nucleon composition of the starting nuclei appear even new particles - products of the reaction.Nuclear chain reaction, which allows you to explore the physics of the mechanisms of interaction of the nuclei with the nuclei and particles - this is the main method for getting new elements and isotopes.In order to understand the chain reaction, we must first deal with the single.
What is needed for the reaction
In order to implement such a process, as a nuclear chain reaction, it is necessary to bring together the particles (the nucleus and the nucleon, two nuclei) at a distance range of the strong interaction (about one Fermi).If the distances are large, the interaction of charged particles is pure Coulomb.The nuclear reaction in compliance with all laws: conservation of energy, the time, the momentum of the baryon charge.Nuclear chain reaction is indicated by a set of symbols a, b, c, d.The symbol represents the original nucleus, b - the incident particle with - a new outgoing particles, and d is the resulting core.
reaction energy
nuclear chain reaction can take place either with the absorption and release of energy, which is equal to the difference between the masses of the particles after the reaction and before.The absorbed energy determines the minimum kinetic energy of the collision, the so-called threshold of a nuclear reaction in which it can flow freely.This threshold depends on the particles that are involved in the interaction, and their characteristics.At the initial stage, all particles are in a predetermined quantum state.
, the reaction
main source of charged particles that bombard the nucleus is a particle accelerator that provides beams of protons, heavy ions and light nuclei.Slow neutrons obtained through the use of nuclear reactors.To fix the incident charged particles can be used in different types of nuclear reactions - as the synthesis and decay.The probability of them depends on the parameters of particles that collide.This is probably related characteristics such as a section of the reaction - the value of the effective area that characterizes the nucleus as a target for the incoming particles, and which is a measure of the probability of entry of the particle and the nucleus interact.If reactions are participating particle with non-zero spin, the cross section depends on their orientation.Since the spins of the incident particles are oriented not quite chaotic, and more or less orderly manner, all corpuscles are polarized.Quantitative characterization of oriented spins the beam polarization vector is described.
reaction mechanism
What is the nuclear chain reaction?As already mentioned, it is a sequence of simple reactions.Characteristics of the incident particle and its interaction with the nucleus depends on the mass, charge, kinetic energy.The interaction is determined by the degree of freedom of nuclei, which are excited by the collision.Gaining control over all these mechanisms allows for a process such as controlled nuclear chain reaction.
Direct response
If a charged particle that hits the target nucleus, just touches it, the duration of the collision is still necessary to overcome the distance radius of the nucleus.This nuclear reaction is called a straight line.A common characteristic for all reactions of this type is to bring a small number of degrees of freedom.In this process, after the first collision particle has still enough energy to overcome the nuclear attraction.For example, such interactions, as the inelastic neutron scattering, charge exchange, and are straight.The contribution of these processes in response called "total cross section" quite miserable.However, the direct distribution of the products of a nuclear reaction passage to determine the probability of departure from the angle beam direction, quantum numbers, selectivity populated states and determine their structure.
pre-equilibrium emission
If the particle does not leave the field of nuclear cooperation after the first collision, it will be involved in a cascade of successive collisions.This is actually exactly what is called a nuclear chain reaction.As a result of this situation, the kinetic energy of the particle is distributed among the component parts of the kernel.The very same kernel state will gradually become much more complicated.During this process at some nucleon or the whole cluster (group of nucleons) can be concentrated enough energy for the emission of a nucleon from the nucleus.Further relaxation will result in a statistical equilibrium and the formation of the compound nucleus.
Chain Reactions
What is the nuclear chain reaction?This sequence of its constituent parts.That is, multiple successive individual nuclear reactions caused by charged particles appear as reaction products in the previous steps.What is called a nuclear chain reaction?For example, the fission of heavy nuclei, when multiple fission events are initiated from a previous neutron decay.
Features of a nuclear chain reaction
Among all the chemical reactions it received wide distribution chain.Particles with unused connections act as free atoms or radicals.In this process, as the nuclear chain reaction, the mechanism of its occurrence provide neutrons which have the Coulomb barrier and excite the nucleus upon absorption.If there is a need in the medium particle, it causes a chain of subsequent transformations, which will continue to chain scission due to the loss of the carrier particles.
Why lost carrier
There are only two reasons for the loss of carrier particles unbroken chain reactions.The first is the absorption of the particles without secondary emission process.The second - leaving the particles within the scope of the substance that supports the chain process.
Two types of
If in each period a chain reaction comes exclusively single particle of the carrier, you can call this process unbranched.It can not result in the release of energy on a large scale.If there are many carrier particles, it is called branched reaction.What is a nuclear chain reaction with branching?One of the obtained in the previous act of secondary particles continue previously initiated a chain, but the other will create new reactions that will also branch out.With this process will be competing processes lead to breakage.The resulting situation would give rise to specific critical and marginal phenomenon.For example, if the continuity is greater than a purely new chains, the reaction self-support is impossible.Even if excite her artificially introducing into the medium desired number of particles, the process will still fade over time (usually quite quickly).If the number of new chains will exceed the number of breaks, the chain reaction will begin to spread throughout the material.
critical state
critical condition separate state of matter developed a self-sustaining chain reaction, and the area where this reaction is not possible at all.This parameter is characterized by equality between the number of new circuits and the number of possible breaks.As the presence of a free particle carrier, the critical state is the main item in this list, as "the conditions of the nuclear chain reaction."Achievement of this condition can be determined by a number of possible factors.Nuclear fission of heavy elements is excited by only one neutron.As a result of this process, as a nuclear fission chain reaction, there are more neutrons.Consequently, this process may produce branched reaction, wherein the carrier and will act neutrons.In the case where the rate of neutron captures without division or departures (loss rate) will be compensated speed reproduction of the carrier particles, the chain reaction will proceed in a stationary mode.This equation describes the multiplication factor.In the above case, it is equal to unity.In the nuclear power industry with the introduction of negative feedback between the rate of energy release and the multiplication factor is possible to implement control of nuclear reactions.If this ratio is greater than one, then the reaction will grow exponentially.Uncontrollable chain reaction is used in nuclear weapons.
nuclear chain reaction in the energy
reactivity of the reactor is determined by a large number of processes that occur in its core.All these influences are determined by the so-called coefficient of reactivity.Effect of temperature graphite rods, heat transfer fluids or uranium reactivity of the reactor and the intensity of the flow of a process such as nuclear chain reaction, characterized by a temperature coefficient (for the coolant, uranium, on graphite).It is also dependent on the characteristics of the power of barometric indicators of steam parameters.To maintain the nuclear reaction in the reactor necessary converting one element into another.To do this, take into account the conditions of course the nuclear chain reaction - the presence of a substance that is able to divide and allocate themselves from the decay of a number of elementary particles which, as a result, will cause the division of the remaining cores.As such a material often used uranium-238, uranium-235, plutonium-239.During the passage of the nuclear chain reaction, isotopes of these elements will decompose and form two or more other chemical substances.In this process it is emitted so-called "gamma" -rays, an intensive release of energy, formation of two or three neutrons capable of further acts of reaction.There are slow and fast neutrons, because to the core of the atom decayed, these particles should fly at a certain speed.
