What Is a Nuclear Reaction?

Nuclear reaction refers to various changes caused by the interaction between atomic nucleus and atomic nucleus, or atomic nucleus and various particles (such as protons, neutrons, photons or high-energy electrons). During the nuclear reaction, a new atomic nucleus is generated that is different from the incident bomb nucleus and the target nucleus. Therefore, nuclear reactions are the fundamental way to generate various unstable nuclei.

Chinese name: Nuclear reaction
Foreign name: nuclear reaction

Meaning: Refers to particles (such as neutrons, photons, mesons, etc.)
Features: Interaction between nuclei and nuclei

Introduction to Nuclear Reactions

Nuclear reaction refers to the process in which the collision of incident particles (or atomic nuclei) with atomic nuclei (called target nuclei) causes the state of atomic nuclei to change or form new nuclei. The energy, momentum, angular momentum, mass, charge, and parity must be conserved before and after the reaction.

Nuclear fusion Nuclear reaction is a very important natural phenomenon already prevalent in the universe. Except for hydrogen, all existing chemical elements are synthesized by natural nuclear reactions. The nuclear reactions that take place on stars are the source of huge energy emitted by stars.

In addition, cosmic rays cause nuclear reactions on Earth at all times. Most of the carbon 14 in nature is produced by neutrons in the cosmic rays bombarding nitrogen 14. In 1919, the British E. Rutherford bombarded nitrogen with alpha particles of natural radioactive material, and realized the nuclear reaction for the first time artificially. The emergence of accelerators in the early 1930s and the construction of reactors in the early 1940s provided powerful tools for studying nuclear reactions. It has been able to accelerate protons to 5 × 10 ^ 5 mega-electron volts, accelerate uranium nuclei to about 9 × 10 ^ 4 mega-electron volts, and obtain meson beams. The use of high-resolution semiconductor detectors has greatly improved the accuracy of measuring nuclear radiation energy. Developments in nuclear electronics and computer technology have radically improved data access

Nuclear reaction schematic Access and processing capabilities. In the past half-century, thousands of nuclear reactions have been studied, and about 2,000 radionuclides that do not exist in nature have been prepared. More than 300 kinds of basic particles have been found. The properties of nuclide, the law of nuclear transformation, Numerous knowledge of nuclear structure, elementary particles, and the four laws of interaction in nature.

Nuclear reaction definition

1 Nuclear reaction 1

Nuclear particle Let's analyze the definition of "nuclear reaction" in the "original text": "the process by which nuclear nuclei produce new atomic nuclei under the bombardment of other particles is called a nuclear reaction". This definition obviously has certain limitations, and actually describes another A type of nuclear reaction-the artificial transformation of nuclear (including heavy nuclear fission, etc.).

2 Nuclear reaction 2

For radioactive elements like uranium, plutonium and radium, the protons and neutrons in the nucleus can continuously change from high-energy arrangements to low-energy arrangements. This is called a "nuclear reaction" and the excess energy released is called "nuclear nuclear energy ^[1] ".

3 Nuclear reaction 3

When an incident nucleus of a certain energy is used to bombard the nucleus, the change in the nucleus is caused by the interaction between the two. This process is called a nuclear reaction. The first artificial nuclear reaction in history was in 1919, Rutherford used a natural radioactive source ( Po).

4 Nuclear reaction 4

The so-called nuclear reaction refers to the process in which an atomic nucleus is impacted by a particle to release one or several particles. [1] In the process of researching it, experimental workers often use a static laboratory coordinate system to perform actual measurement of data.

5 Nuclear reaction 5

Nuclear reactions and equations The process by which the nucleus changes is called a nuclear reaction. The basis for writing a nuclear reaction equation is that the number of charges before and after the reaction is constant, and the number of masses is also constant.

Nuclear reaction principle

In nuclear reactions, particles used to bombard nuclei are called incident particles or bombarded particles, bombarded nuclei are called target nuclei, particles emitted by nuclear reactions are called outgoing particles, and nuclei generated by the reaction are called remaining nuclei or product nuclei. The nuclear reaction where the incident particle a bombs the target core A, emits the emitted particle b, and generates the remaining core B can be expressed by the following equation:

A + a B + b or abbreviated as: A (a, b) B

If a and b are the same kind of particles, they are scattered, and are divided into elastic scattering and inelastic scattering according to whether the remaining core is in the ground state or the excited state. A (a, a) A and A (a, a ) A * It means that a given incident particle and target nuclear energy often have more than one kind of nuclear reaction. Each nuclear reaction is called a reaction channel. The reaction path is composed of an incident path and an exit path. The incident particles and the target nucleus form the incident track, and the outgoing particles and the remaining nucleus form the exit track. There can be several exit channels for the same incident channel, and there can also be several incident channels for the same exit channel.

The probability of a nuclear reaction occurring is characterized by the nuclear reaction cross section. A nuclear reaction can only occur if the conservation conditions such as mass, charge, energy, momentum, angular momentum, and parity are met. The corresponding reaction channel is called open, or open channel for short, otherwise it is closed. The nuclear reaction process is always accompanied by the absorption or release of energy. The former is called an energy absorption reaction, and the opposite is called an exothermic reaction. The reaction energy is usually expressed by Q, which is equal to the difference between the kinetic energy of the system after the reaction and before the reaction. It can be indicated in the nuclear reaction equation, for example:

³ H + ² H ⁴ He + n + QQ = 17.6MeV

For energy-absorbing reactions, a nuclear reaction can only be initiated when the kinetic energy of the incident particles is above the threshold energy (Eth).

Nuclear reactions are qualitative changes by their nature, but they are different from general chemical reactions. A chemical reaction is simply a rearrangement of atoms or ions, leaving the nucleus unchanged. Therefore, in a chemical reaction, one atom cannot become another atom. A nuclear reaction is the transfer of particles between the nucleus of an atom, causing an atom to be converted into another atom, and the atom undergoes a qualitative change. The energy effect of nuclear reactions is much greater than that of chemical reactions. Nuclear reaction energy is often measured in mega-electron volts, while chemical reaction energy is usually only a few electron volts. For example, nuclear reactions can not be achieved by general chemical methods, but use many experimental techniques and theories of modern physics. First, high-energy nuclear "cannonballs", such as helium, hydrogen, and deuterium nuclei, must be generated artificially. These "cannonballs" are used to violently impact other nuclear nuclei, causing nuclear reactions. All kinds of accelerators are used to artificially generate charged high-energy particles and use them as nuclear "cannonballs" for nuclear reactions. When people discovered neutrons in 1932, they not only had a correct understanding of the structure of the nucleus, but also found that neutrons are a new type of nuclear "cannonball". Since neutrons are not charged, there is no electrical repulsive force between them and the nucleus. Therefore, it is much better to use it to generate nuclear reactions than other high-energy particles that are charged. Some factories have nuclear reactors.

Nuclear reactions are generally divided into four categories: decay, particle bombardment, fission, and fusion. The former are spontaneous nuclear transitions, while the latter three are artificial nuclear reactions (ie, non-spontaneous nuclear reactions performed artificially).

It is worth mentioning that the decay, fission, and fusion of nuclear reactions are preliminary studies of high school physics for atomic physics. The gradual civilianization of nuclear reactions has become one of the basic knowledge learned by students.

Classification of nuclear reactions

According to different incident particles, nuclear reactions can be divided into three types: Neutron nuclear reactions, such as neutron elastic scattering (n, n), inelastic scattering (n, n ), neutron radiation capture (n, ) , Nuclear reactions (n, p), (n, ) that emit charged particles, such as neutron fission reaction (n, f), nuclear reactions that emit two particles (n, 2n), (n, pn), etc .; Nuclear reactions of charged particles, such as proton-induced nuclear reactions (p, ), (p, n), (p, p), (p, p ), (p, ), (p, 2n), etc., caused by deuteron Nuclear reactions (d, n), (d, p), (d, ), etc., nuclear reactions caused by particles (, n), (, 2n), (, p), etc., nuclear reactions caused by heavy ions (12C, 4n), (22Ne, 6n), etc .; photonuclear reactions, that is, nuclear reactions caused by photons, such as (, n), (, p), (, ), (, f) and so on.

According to the energy of incident particles, nuclear reactions can be roughly divided into three categories: low-energy nuclear reactions, the energy of incident particles is less than 108 electron volts, and for lighter heavy ions, the average energy of each nucleus is less than 107 electron volts (such as 108 electron volts) Carbon 12 cores), also belongs to the category of low-energy nuclear reactions, the number of outgoing particles of low-energy nuclear reactions is at most 3 to 4; medium-energy nuclear reactions, the energy of incident particles is between 108 and 1010 electron volts; high-energy nuclear reactions, incident particles The energy is greater than 1010 electron volts.

Controlled nuclear fusion

Because fusion materials are deuterium and tritium ions, they all have a positive charge, so if you want to occur fusion, you must first overcome the electromagnetic potential. In order to overcome the electromagnetic potential, the deuterium and tritium ions must have a high energy and be sufficiently dense. Fusion has a term called Lawson's criterion, which means that if the output energy is greater than the input energy, the product of the three: temperature, density, and energy constraint time must be greater than a fixed value. In order to achieve the Lawson criterion, a good energy confinement method is essential. Obviously, it is impossible to confine plasma with a temperature of about 100 million degrees Celsius by using a furnace bottle. The following are the main constraints:

Magnetic confinement We have learned in high school that electrons and ions will spin in a magnetic field without freely escaping the magnetic lines of force. Therefore, magnetic fields are a good way to constrain high-temperature plasma. There are many types of magnetically constrained devices, such as tokamak, spherical rings, star spinning devices, and so on.

2. Inertia constraints . The core idea of inertial confinement is to use the inertia of these ions to raise the temperature density to a level that can be fused when they are too late. The hydrogen bomb is actually a kind of inertial restraint, but it is not a controlled fusion. This is because in order to achieve restraint, it relies on the atomic bomb to detonate on the outside and squeeze it inward. Inertial confinement fusion devices have been struggling to seek to replace the atomic bomb and squeeze inward in other gentler ways. The mainstream solution is to rely on high-power lasers.

3. Gravitational constraints . This is the way the sun and all the stars are constrained, just don't think about it on earth.

4. Other constraints . There are many strange people who have proposed some strange fusion restraint methods, but they have not made any substantial progress. . .

Nuclear reaction characteristics

1. Chain reaction

Some nuclear reactions have a chain reaction phenomenon, such as the nuclear reaction of U-235 and neutrons: As long as one neutron bombards U-235, three neutrons will be emitted, and three neutrons will bomb U-235. Nine neutrons are generated, and in this continuous manner, within a few microseconds, the reaction proceeds very violently and emits huge energy. A reaction with this characteristic is called a chain reaction. The explosion of the atomic bomb can be so violent because of a chain reaction.

2. Accompanying nuclear radiation

In the nuclear reaction between U-235 and neutrons, if the reaction is not sealed, the neutrons produced will be emitted to the surrounding environment at the speed of light, forming radiation. Even small particles moving at the speed of light can produce radiation. The radiation cannot be seen or touched. But it can be measured by the instrument. A small amount of radiation has no effect on the human body, and humans also use radiation for their benefit. For example, hospitals use X-rays to perform chest radiographs for patients. Radiotherapy is a more common method for treating cancer. The principle is to use radiation to kill cancer cells. However, when the amount of radiation is large, it will cause harm to the human body. For example, X-rays can be used to detect diseases, but if pregnant women are X-rayed, they may cause baby deformities or genetic mutations. Similarly, patients receiving radiation therapy may experience side effects such as hair loss, nausea, and fatigue. Larger doses of radiation can also cause genetic mutations in adults and induce diseases such as leukemia (blood cancer) and skin cancer. A large amount of radiation will burn or even burn all living matter.

3.Efficient

4.Clean and pollution-free

Nuclear energy is a clean, non-polluting new energy source: Take France as an example: from 1980 to 1986, the proportion of French nuclear power in the total power generation ranged from 24% to 70%. During this period, France's total power generation increased by 40%, and emissions Sulfur was reduced by 9% and dust was reduced by 36%. The air quality has improved significantly.