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This article comes from the official account of Wechat: ID:fanpu2019, author: Chen Shaohao

In scientific research, every time scientists discover new natural phenomena or propose new scientific concepts, they will create proper nouns to name them. Once named, complex phenomena and difficult concepts will be remembered or fascinated because the names are intuitive and easy to understand, arousing people's curiosity and exploration. If the name is not good, it will make people misunderstand or discourage people from writing. The purpose of this series of articles is to make a brief study of some important nouns in physics, to study their meanings and to explore the profound physical meanings behind them.

-- author

Written by Chen Shaohao (Massachusetts Institute of Technology)

The hit release of the film Oppenheimer once again drew public attention to the atomic bomb. As the leader of the Manhattan Project, Oppenheimer played an important role in the birth of the first atomic bomb in human history, and was known as the "father of the atomic bomb".

Since Einstein wrote the mass-energy equation in 1905, physicists have known that there is a great deal of energy in tiny nuclei. From the birth of the scientific concept to the practical application, it is necessary to overcome numerous engineering and technical difficulties. Had it not been to compete with Nazi Germany, the United States would not have launched the Manhattan Project to build an atomic bomb with the strength of the whole country. Since its birth, the powerful nuclear weapons have completely changed the course of human history and brought great impact and impact on human society.

In the mass media, many statements about nuclear weapons are vague, and some commonly used terms do not fully accurately describe the real physical meaning behind them. Nowadays, several nuclear powers in the world regard nuclear weapons technology as the top state secret, and the technical details of the manufacture of nuclear weapons cannot be made public. This is one of the reasons why some nuclear weapons terms are ambiguous. Although the technical details of nuclear weapons are state secrets, their basic physical principles are completely public. In academia, there is no secret to the physics associated with nuclear weapons. Based on the principles of physics, this paper will analyze some nouns related to nuclear weapons and study the physical contents behind them.

The atomic bomb has nothing to do with atoms to mention nuclear weapons. Nothing is more familiar to the public than the atomic bomb (Atomic Bomb). Almost everyone in contemporary society has at least heard of the atomic bomb. However, from a scientific point of view, the title atomic bomb itself is very imprecise. In fact, the atomic bomb has nothing to do with the Atom, but has a close relationship with the Nucleus.

The center of the atom is the nucleus, and electrons (Electron) are distributed around the nucleus. The research object of atomic physics (Atomic Physics) is the electrons in the atom, and the function of the nucleus is only to provide an external electric field to the electrons. The study of the internal structure of the nucleus is called nuclear physics (Nuclear Physics). The energy of an atomic bomb comes entirely from the Fission within the nucleus, so the more accurate name of the atomic bomb should be the fission bomb (Fission Bomb), a nuclear bomb (Nuclear Bomb).

Since the atomic bomb is based entirely on nuclear fission, it actually has a scientific name, pure fission nuclear weapons (Pure Fission Weapons). At the end of World War II, the two atomic bombs dropped by the United States on Hiroshima and Nagasaki in Japan were pure fission nuclear weapons.

The nuclear charge used in the atomic bomb is uranium-235 (Umur235) or plutonium-239 (Pu-239). Uranium and plutonium are the names of elements, translated from the English words Uranium and Plutonium respectively. The Chinese names of metal elements usually have gold characters next to them. There are more than 90 kinds of metal elements in the periodic table, all of which have to be named after the Chinese characters next to the gold character, and it is not easy to have some homonyms with English. Speaking of which, it also has something to do with Zhu Yuanzhang, the emperor of the Ming Dynasty. Zhu Yuanzhang laid down a motto that the third word in the name of his descendants must use the word "five elements", that is, "fire, earth, metal, water, wood". In this way, the royal family of the Ming Dynasty, which lasted for more than two hundred years, created many Chinese characters next to the golden characters, many of which had no meaning in themselves, and did not expect to be used in naming metal elements in modern times.

As for the suffixes of uranium-235 and plutonium-239, the numbers "235,239" are the Mass Number of the element. The nucleus consists of neutrons and protons, and the sum of the number of protons and neutrons is the mass number. There are generally many different isotopes (Isotope) of the same element. Isotopes have the same number of protons and different numbers of neutrons, so they also have different mass numbers. For example, uranium has 92 protons, and the mass numbers of the two common isotopes are 235 and 238, respectively, so they are called uranium-235 and uranium-238.

Both uranium-235 and plutonium-239 are fission materials, which occur under the impact of neutrons. Fission means that the heavier nucleus splits into two lighter nuclei, releasing energy due to mass loss. Einstein's mass-energy equation E=mc2 means that the energy of a microscopic particle is equal to its rest mass times the square of the speed of light. The value of the speed of light is so large that a small mass loss can produce a lot of energy. This is the energy source of the atomic bomb.

The process of heavy nuclear fission also produces neutrons. The newly generated neutrons hit the nearby uranium nuclei and cause them to fission, resulting in an exponential increase in the number of fission nuclei, which is called nuclear chain reaction (Nuclear Chain Reaction). The number of fission nuclei can be increased a hundredfold in just one microsecond. The chain reaction can release the energy needed for the explosion in a short time. It takes only a few kilograms of fissile material to fission, and the energy released is enough to destroy a city.

Figure 1: chain reaction in an atomic bomb. The nucleus of uranium-235 fission under the impact of neutrons, producing two smaller nuclei and neutrons. The newly generated neutrons hit other uranium-235 nuclei, causing more fission. | Source: the key for www.nuclear-power.com to detonate an atomic bomb is to squeeze the fissile material together to reach the critical mass (Critical Mass) needed to produce a chain reaction. The existing atomic bomb can be detonated in two ways. The first is the gun style. In this way, like a bullet hitting a target, the thrust of conventional explosive explosion is used to shoot part of the fissile material into another fissionable material at high speed, so that the fissile material is closely bound together to reach the critical mass. The second is implosion. In this scheme, conventional explosives are on the outer layer and fissile material on the inside. When a conventional explosive explodes, it produces inward pressure and squeezes the fissionable material so that the fissionable material reaches the critical mass.

The heat released by nuclear fission expands the fissile material, resulting in a greater distance between uranium atoms. Because the cross section of the nuclear reaction decreases rapidly with the increase of distance, the probability of the newly generated neutrons hitting the uranium atom is greatly reduced, which leads to the cessation of the chain reaction. One of the keys to the design of an atomic bomb is to keep fissile material from being blown up for as long as possible so that it can be fully utilized. The nuclear reaction cross section (Nuclear cross section) mentioned above is a physical quantity used to measure the probability of reaction and has the dimension of area.

Hydrogen bombs have little to do with hydrogen. Hydrogen bombs (Hydrogen Bomb) are more powerful nuclear weapons than atomic bombs. Because of the great lethality of the hydrogen bomb, Oppenheimer, the father of the atomic bomb, boycotted the US government to build a hydrogen bomb out of humanitarian considerations, but his security permit was revoked, which meant the end of his political career. The movie Oppenheimer spends a lot of time on the detailed process of the security clearance hearing. The opposition of famous scientists such as Oppenheimer and Einstein did not stop the development of the hydrogen bomb. Years after the atomic bomb was built, the hydrogen bomb was also developed in 1951.

The design of the hydrogen bomb makes clever use of the energy generated by nuclear fusion (Fusion). Contrary to the process of nuclear fission, the process of nuclear fusion is the fusion of two lighter nuclei to form heavier nuclei. The nuclear fusion process used by the hydrogen bomb is as follows: the deuterium nucleus fuses with the tritium nucleus to produce a helium nucleus and a neutron.

Figure 2: deuterium and tritium nuclei are fused to produce a helium nucleus and a neutron and release energy. Wiki source: deuterium and tritium are both hydrogen isotopes. Like hydrogen, both deuterium and tritium have a proton in the nucleus and an electron outside the nucleus. Because the chemical properties of elements are completely determined by extranuclear electrons, the chemical properties of hydrogen, deuterium and tritium are exactly the same, and their elements are all gases. The difference is the number of neutrons and the number of masses. The mass numbers of hydrogen, deuterium and tritium nuclei are 1, 2 and 3, respectively. Deuterium is Deuterium in English and means "second" in Greek. Tritium is Tritium in English and means "third" in Greek. Deuterium (pronouncing Dao) and tritium (pronouncing Chuan) are homophonic with English. The chemical properties of the simple substance are reflected next to the gas character, and the following two strokes and three strokes represent their mass numbers respectively. These two words can be said to be a model of the combination of transliteration and free translation.

There is a mass deficit in the process of nuclear fusion. According to the mass-energy equation, the defective mass is converted into energy. A huge number of nuclear fusion is going on in the interior of the sun all the time, and the fusion energy radiates outward in the form of electromagnetic waves. Part of the electromagnetic wave (including visible light) propagates to the earth and becomes the source of almost all energy on the earth.

Fusion material per unit mass produces more energy than nuclear fission. It is worth noting that this is not why a hydrogen bomb is more powerful than an atomic bomb. Nuclear weapons that make full use of fusion energy are called pure fusion weapons (Pure Fusion Weapons), which exist only in theory and have not been made in reality. The current hydrogen bomb does make use of nuclear fusion, but the main function of nuclear fusion is to produce new neutrons, which in turn causes more fission of fissile material and produces more fission energy. The main energy of the hydrogen bomb still comes from nuclear fission, and the energy produced by nuclear fusion of hydrogen isotopes is basically negligible. From this point of view, the hydrogen bomb has little to do with hydrogen isotopes, so the name hydrogen bomb is not rigorous. Since nuclear fusion must require very high temperatures, hydrogen bombs are more professionally called "thermonuclear bombs" (Thermonuclear Bomb).

There are two main designs for hydrogen bombs. The first is fusion enhanced fission nuclear weapons (Boosted Fission Weapons). This is an improvement of the implosion atomic bomb. When an atomic bomb explodes, a lot of fissile material is not used because the chain reaction stops due to the explosion of fissile material. Deuterium and tritium are re-injected into the center of the fissile material uranium, and the high temperature produced by the explosion can cause the fusion reaction of deuterium and tritium. The neutrons produced by fusion hit the outer fission material, triggering a new chain reaction of nuclear fission. This makes more full use of fissile material and ends up releasing more than twice as much energy. Because this design uses both the isotopes of fission material uranium and fusion material hydrogen, it is also known as "hydrogen-uranium bomb" (Uranium Hydride Bomb).

The second scheme is proposed by Teller and Ulam, so it is also known as the Teller-Uram scheme. Teller is considered to be the father of the hydrogen bomb. In the movie Oppenheimer, Teller appeared as a villain and testified against Oppenheimer at the hearing.

The hydrogen bomb designed in this scheme, also known as a two-stage thermonuclear weapon (Staged Thermonuclear Weapons), consists of two fusion-enhanced fission bombs mentioned above, one called a primary bomb and the other a secondary bomb. In the first stage, conventional explosions detonate the primary nuclear bomb. In the second stage, the high-energy X-rays produced when the primary nuclear bomb explodes detonate the secondary nuclear bomb. This process, called radiation implosion, is more efficient than the first stage and releases much more energy than a primary nuclear bomb. This scheme combines fusion-enhanced fission and fission-enhanced fusion technology, so it can release energy more fully.

Chen Shaohao, bachelor of physics from Tsinghua University, PhD in atomic and molecular physics from Tsinghua University, was a postdoctoral fellow at the University of Colorado Boulder in the United States, successively worked at Louisiana State University and Boston University, and now works at the Massachusetts Institute of Technology, engaged in high-performance computing.

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