Nuclear reaction refers toNucleusAnd atomic nuclei, or atomic nuclei and various particles (such asproton，neutron，photonOr high-energy electrons).In the process of nuclear reaction, new atomic nuclei will be generated which are different from the incident bomb nucleus and target nucleus.Therefore, nuclear reaction is the fundamental way to generate various unstable nuclei.
A nuclear reaction is an incident particle (orNucleus）And atomic nucleus (calledTarget nucleus）Collision results in atomic nucleusstateThe process of changing or forming a new nucleus.Pre and post reactionenergy、momentum、angular momentum、quality、chargeAndParityMust be conserved.Nuclear reaction isuniverseExtremely importantnatural phenomena。The existing chemical elements, except hydrogen, are all synthesized by natural nuclear reactionfixed starThe nuclear reaction on thefixed starA source that radiates enormous energy.
Schematic diagram of nuclear reaction
In addition,cosmic raysIt causes nuclear reaction on the earth all the time.natureC 14 is mostly neutron bombardment in cosmic raysnitrogen14.British in 1919E. RutherfordNatural radioactiveAlpha particleBombardment of nitrogen is realized manually for the first timenuclear reaction。The emergence of accelerators in the early 1930s and the early 1940sreactorThe completion of the project provides a powerful tool for studying nuclear reactions.Has been able toprotonAccelerate to 5 × 10 ^ 5 trillionElectron volt, willuraniumThe atomic nucleus accelerates to about 9 × 10 ^ 4 mev and can obtainMeson beam。high resolutionSemiconductor detectorThe use ofnuclear radiationAccuracy of energy.Nuclear electronicsandcomputerThe development of technology has fundamentally improved the ability to acquire and process data.In the past half century, thousands of nuclear reactions have been studied to produce radioactivity that does not exist in naturenuclideAbout 2000 species, more than 300 species foundElementary particle, obtained information aboutnuclideNaturenuclear transmutationLaws, nuclear structure, elementary particles, and the laws of four kinds of interactions in nature, as well as a large amount of knowledge of their interrelationships.
When incident with certain energynucleonWhen bombarding the atomic nucleus, the interaction between the two causes the change of the atomic nucleus. This process is called nuclear reaction. The first artificial nuclear reaction in history was in 1919rutherfordUse naturalRadioactive Source(Polonium Po).
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The so-called nuclear reaction refers to the process of an atomic nucleus being hit by a particle and releasing one or several particles [1]. In the process of its research, experimenters often use staticLaboratory coordinate system, for actual measurement of data.
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The process of nuclear reaction and nuclear transformation of equation is called nuclear reactionNuclear reaction equationThe basis is before and after reactionchargeNumber unchanged,qualityThe number remains the same.
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nuclear reaction
In nuclear reactions, the particles used to bombard atomic nuclei are called incident particles or bombarding particles, the bombarded atomic nuclei are called target nuclei, the particles emitted by nuclear reactions are called outgoing particles, and the atomic nuclei generated by reactions are called residual nuclei or product nuclei.The nuclear reaction in which the incident particle a bombards the target nucleus A, emits the outgoing particle b and generates the residual nucleus B can be expressed by the following equation:
A+a → B+b or A (a, b) B
If a and b are the same kind of particles, it is scattering, and the residual nucleus is located atground statestillexcited stateAnd divided intoElastic scattering and inelastic scattering, represented by A (a, a) A and A (a, a ′) A *, there is often more than one nuclear reaction between a given incident particle and target nuclear energy.Each nuclear reaction is called aReaction channel。Reaction channelIt is composed of incident channel and outgoing channel.The incident particles and the target core form the incident channel, and the outgoing particles and the residual core form the exit channel.The same incident channel can have several outgoing channels, and the same outgoing channel can also have several incoming channels.
The probability of a certain nuclear reactionNuclear reaction cross sectionTo characterize.Only when the quality quantitycharge、energy、momentum, angular momentum, parity, etcReaction channelCalled open, or simplyMake a detourAnd vice versaCloister。The nuclear reaction process is always accompanied by the absorption or release of energy. The former is calledEnergy absorption reaction, the opposite is calledExothermic reaction。The reaction energy is usually expressed as Q, which is equal to the difference between the kinetic energy of the system after reaction and before reaction, and can be indicated in the nuclear reaction equation, for example:
Nuclear reaction is essentially a qualitative change, but it is similar to the generalchemical reactionDifferent.The chemical reaction is justatomorIonsOfPermutation and combinationThe nucleus remains unchanged.Therefore, in chemical reactions, aatomIt cannot become another atom.Nuclear reaction isatomThe transfer of particles between nuclei leads to the transformation of one atom into another, and the atom undergoes a qualitative change.The energy effect of nuclear reactions is much greater than that of chemical reactions.Nuclear reaction energyIt is usually measured in megaelectron volts, while the chemical reaction energy is generally only a few electron volts.For example, nuclear reaction can not be realized by general chemical methods, but by using many experimental techniques and theories of modern physics.First, we need to generate high-energy nuclei manually“shell”, such asHelium nuclei 、hydrogen atomNuclearDeuterium atomNuclear, etc., use these "shells" to hit others violentlyNucleusSo as to cause nuclear reaction.All kinds of accelerators are designed to artificially generate charged high-energy particles to be used as nuclear "shells" for nuclear reactions.When neutrons were discovered in 1932, people not only had a correct understanding of the structure of atomic nuclei, but also found that neutrons were a new type of nuclear "shell".Since the neutron is not charged and there is no electric repulsion force between it and the atomic nucleus, the effect of using it to produce nuclear reaction is much better than using other charged high-energy particles.Some factories havenuclear reactor。
Nuclear reactions are generally divided into four categories:decay、Particle bombardment、fissionandfusion。The former is spontaneousnuclear transmutationThe last three are artificial nuclear reactions (that is, non spontaneous nuclear reactions conducted by artificial methods).
It is worth mentioning that the decay, fission and fusion of nuclear reaction areHigh School PhysicsA preliminary study of atomic physics.The gradual popularization of nuclear reaction has become one of the basic knowledge for students to learn.
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According to the different incident particles, nuclear reactions can be divided into three categories: ①Neutron nuclear reaction, such as neutronelastic scattering (n,n)、Inelastic scattering(n, n '), neutron radiation capture (n, γ), nuclear reaction emitting charged particles (n, p), (n, α), etc., such as neutronsFission reaction(n, f), nuclear reaction emitting two particles (n, 2n), (n, pn), etc.; ②Nuclear reactions with charged particles, such as nuclear reactions (p, γ), (p, n), (p, p), (p, p '), (p, α), (p, 2n) caused by protons, nuclear reactions (d, n), (d, p), (d, α) caused by deuterons, nuclear reactions (α, n), (α, 2n), (α, p) caused by α particles,Heavy ionNuclear reaction (12C, 4n), (22Ne, 6n), etc.; ③Photonuclear reaction, i.ephotonNuclear reaction caused, such as (γ, n), (γ, p), (γ, α), (γ, f), etc.
According to the energy of incident particles, nuclear reactions can be roughly divided into three categories: ① low-energy nuclear reactions. The energy of incident particles is less than 108 electron volts. For lighter heavy ions, the average energy of each nucleon is less than 107 electron volts (such as the 108 electron volt carbon 12 nucleus), which also belongs to the category of low-energy nuclear reactions. The number of emitted particles of low-energy nuclear reactions is up to 3-4; ②Intermediate energy nuclear reaction, the incident particle energy is between 108~1010 electron volts; ③For high-energy nuclear reactions, the incident particle energy is greater than 1010 electron volts.
Controlled nuclear fusion
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Because the fusion material is the ion of deuterium and tritium with positive charge, the electromagnetic potential must be overcome before fusion occurs.In order to overcome the electromagnetic potential, deuterium tritium ions must have high energy and sufficient density.In fusion, there is a term called Lawson criterion, which means that if the output energy is greater than the input energy, the product of temperature, density and energy constraint time must be greater than a fixed value.In order to reach Lawson's criterion, a good energy constraint mode is essential, obviously usingstoveIt is impossible for the bottle to restrain the plasma with a temperature of about 100 million degrees Celsius.The following are the main constraint modes:
1. Magnetic restraint。In high school, we learned that electrons and ions will spin under a magnetic field, but will not escape from the magnetic field line of force freely. Therefore, magnetic field is a good way to restrain high-temperature plasma.There are many kinds of magnetic confinement devices, such as Tokamak, spherical ring, star spinner, etc.
2. Inertial constraint。The core idea of inertial confinement is to use the inertia of these ions to raise the temperature density to a fusion level before they can disperse.Hydrogen bomb is actually a kind of inertial constraint, but it is not controlled fusion.This is because in order to achieve the constraint, the atomic bomb is detonated outside and then squeezed inward.inertial confinementFusion deviceWhat I have been trying to seek is to replace the atomic bomb and squeeze inward in other gentler ways.The mainstream solution is to rely on high-power lasers.
3. Gravitational constraint。This is the constraint mode of the sun and all the stars, so don't think about it on the earth.
4. Other constraints。Many strange people have proposed some strange fusion constraints, but they have not made any substantive progress...
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1. Chain reaction
nuclear reaction
Some nuclear reactions have the phenomenon of chain reaction, such as the nuclear reaction of U-235 and neutrons: as long as one neutron bombards U-235, three neutrons will be released, and when three neutrons bombard U-235 again, nine neutrons will be generated. If this continues, in a few microseconds, the reaction will be very intense and emit huge energy. The reaction with this feature,We call it a chain reaction.Atomic bombThe reason why the explosion was so violent was that a chain reaction occurred.
In the nuclear reaction between U-235 and neutrons, if the reaction is not sealed, the generated neutrons will shoot to the surrounding environment at the speed of light, forming radiation.Small particles moving at the speed of light can generate radiation.Radiation cannot be seen or touched.But you caninstrumentMeasured.A small amount of radiation has no effect on the human body, and human beings also use radiation to benefit mankind. For example, hospitals use X-ray to do chest radiography for patients. Radiotherapy is a commonly used method to treat cancer. Its principle is to use radiation to kill cancer cells.But more radiation will do harm to human body.For example, X-ray can be used to check diseases, but if pregnant women take X-ray, it may lead to infant malformation orGene variation。Similarly, patients receiving radiotherapy will have hair loss, nausea, fatigue and other side effects.A higher dose of radiation will also cause gene mutation in adults, inducingleukemia(Blood cancer)Skin cancerAnd other diseases.A large amount of radiation can burn or even kill all living materials.
3. Efficient
4. Clean and pollution-free
Nuclear energy is a clean and pollution-free new energy:FranceFor example, from 1980 to 1986, Francenuclear powerThe proportion in the total power generation is from 24% to 70%. During this period, the total power generation in France has increased by 40%, while the sulfur emission has decreased by 9%, and the dust has decreased by 36%.Air qualitySignificantly improved.
