Nuclear energy is a double-edged sword for mankind: the peaceful use of nuclear energy provides a way out for mankind to solve the energy problem; Nuclear weapons, which have killed at least 100,000 people, have always made mankind feel the threat of the end of the world. The emergence of nuclear energy stems from the fission of heavy nuclei and the associated chain reactions. In December 1938, uranium nuclear fission was confirmed, opening the door to the era of nuclear and nuclear weapons, which eventually led to the atomic bomb. In fact, the discovery of heavy nuclear fission went through a tortuous process. What are some of the thrilling stories in this?
Written by | Wang Shanqin
On August 6, 1945, an atomic bomb codenamed "Little Boy" exploded over Hiroshima, Japan. Although its core was only 64 kilograms of enriched uranium, it destroyed most of the city and killed tens of thousands of people in an instant. And in the process, the mass of the entire bomb was lost less than 1 gram.
The development of the powerful atomic bomb required more than 100,000 people of the Manhattan Project, but its basic principle - heavy nuclear fission - was discovered in a crooked way.
Nuclear energy, the atomic bomb and science fiction
In 1903, Ernest Rutherford (1871-1937) of the Cavendish Laboratory at the University of Cambridge and his student Frederick Soddy (1877-1956) calculated that the decay of radioactive material emits at least 20,000 times the chemical energy of a substance of the same mass, and may even reach a million times. In 1904, Sodi predicted that the energy released by the radioactive process could be used or even used as a weapon in the future. He believed that such a weapon could change the fate of mankind and even destroy the world.
Rutherford (left, 1892, age 21) and Thordy (right, 1921, age 44). 丨Image source: Public Copyright
Rutherford and Soddy's predictions predate Einstein's theory of relativity and the mass-energy relation associated with it. Therefore, mankind's understanding of atomic energy predates the birth of the theory of relativity and the mass-energy relationship (1905). Of course, later physicists used Einstein's formula when explaining nuclear energy.
Inspired by Sody's views, United Kingdom science fiction writer Herbert Wells (1866-1946) wrote the science fiction novel The World Set Free in 1913. In this book, the phrase "atomic bombs" appears for the first time. The author imagined that in 1956, when Britain, France, the United States and Germany broke out, a single atomic bomb in the handbag would be enough to destroy half of the city, and these atomic bombs were dropped on all the important cities of the world, and the continuous radioactivity led to continuous fires and caused great damage.
威尔斯(1920年)丨图片来源:George Charles Beresford
The principle of the atomic bomb, which Wells "predicted", was that the radioactive material inside released energy through continuous radioactive decay. However, it has since been proven that it is not possible to obtain nuclear energy on a large scale and make it into a weapon simply through the radioactive decay of matter. Such an "atomic bomb" can only be considered a radioactive contaminant, not a bomb.
The basis that led to the use of atomic energy and the explosion of the atomic bomb was heavy nuclear fission and the chain reaction associated with it. Before heavy nuclear fission was discovered, humans first discovered light nuclear fission and achieved artificial radioactivity.
Light nuclear fission
In 1932, John Cockcroft (1897-1967) and Ernest Walton (1903-1995) of the Cavendish Laboratory accelerated protons with a particle accelerator and bombarded lithium 7 with it, which collided and split into two α particles. This process is called splitting the atom, and it is the fission of the nucleus of an atom for the first time.
Cockrauv (left), Rutherford (center) and Walton (right). 丨Image source: Public Copyright
Cockraugf and Walton also found that the mass loss produced by this process and the energy released fit Einstein's mass-energy equation. They were awarded the Nobel Prize in Physics in 1951.
In 1933, Rutherford gave a lecture. He mentions the work of Kokraoff and Walton to split lithium with protons, but he argues that nuclear energy cannot be used on a large scale: "In these processes, we may get much more energy than protons can provide, but in general, we can't expect to get energy in this way." ”
However, in the same year (1932) that light nuclear fission was realized, the "sharp sword" capable of causing heavy nuclear fission in the future was found, after which it wandered like a ghost in the laboratory for several years, and mankind finally discovered heavy nuclear fission.
Neutrons
In 1920, Rutherford proposed that the nucleus of an atom is composed of positively charged protons and neutrally charged particles. The latter was later named "neutron".
In 1931, Germany physicist Walther Bothe (1891-1957) and his student Herbert Becker (date of birth and death unknown) discovered that when α particles released by polonium decay bombard beryllium, boron or lithium, they produce a highly penetrating radiation that is not affected by electric field forces. They think it's gamma rays.
At the beginning of 1932, Pierre · Curie (1859-1906) and Marie Curie (1867-1934) ·daughter Elena · Joliot-Curie (1897-1956) and her ·son-in-law Jean Joliot-Curie (1900-1958) and her son-in-law Jean Joliot-Curie (1900-1958) (hereafter referred to as "Joliot") This radiation has also been found in experiments. Elena and Jorio also discovered that the energy of this neutral radiation is high: when they bombard paraffin or any other hydrogen-containing compound, they knock out protons. They still think it's gamma rays.
伊莲娜与约里奥 (1935)丨图片来源:Ph. Coll. Archives Larbor
James Chadwick (1891-1974) of the Cavendish Laboratory was incredulous when he saw the paper published by Elena and Yorio. Gamma rays, or less capable X-rays, can deflect electrons when they bombard them (the "Compton effect"), but how can protons, which are thousands of times more massive than electrons, be knocked out of the nucleus by gamma rays? Chadwick, a Rutherford student, had long known about Rutherford's assumptions about the existence of neutrons, so it was natural to guess that these neutral radiations were most likely neutrons. In order to confirm this guess as soon as possible, he immediately threw himself into intense experiments.
In February 1932, Chadwick proved that the highly energetic neutral radiation was not gamma rays, but a group of uncharged particles with a mass almost identical to that of a proton, and these properties corresponded to the properties of the hypothetical neutron, which was neutron.
Chadwick丨Image source: Nobel Prize official website (www.nobelprize.org)
Chadwick soon realized that bombarding the nucleus with uncharged neutrons would be more efficient than positively charged α particles and protons, because it was not affected by the electric field forces of negatively charged extranuclear electrons and positively charged nuclei. In addition, it is relatively easy to obtain neutrons: let the α particles released by the decay of radioactive elements such as radium and polonium bombard beryllium-9, making it carbon-12 and releasing 1 neutron.
The discovery and confirmation of neutrons played a key role in the development of nuclear physics. Hans Bethe (1906-2005), a leading figure in the field of nuclear physics, argued that the era before 1932 was the prehistoric era of nuclear physics; In 1932, the era of nuclear physics began, because neutrons were discovered in that year.
For discovering and confirming neutrons, Chadwick was awarded the Nobel Prize in Physics in 1935. Bote, Becker, Elena and Jorio all missed out on the Nobel Prize in Physics.
Artificially radioactive
In January 1934, Elena and Jorio discovered that certain α particles bombarded (irradiated) aluminum foil, and even after the α particle source was removed, the aluminum foil was still radioactive. After determining that there were no problems with the Geiger counter, they guessed: in this process, α particles combine with the aluminum nucleus to form radioactive phosphorus-30 and release a neutron, which then decays into silicon-30.
Through chemical experiments, Elena and Jorio prove that phosphorus is indeed present in the product. This means that the stable aluminum nucleus is artificially converted into a radioactive isotope of phosphorus. At this point, they discovered artificial radioactivity.
The discovery of artificial radioactivity was a great leap forward in the field of nuclear physics. It makes it possible for radioactive elements to no longer be limited to those heavy elements, but may extend to the entire periodic table, and humans can artificially produce radioactive isotopes of various elements.
When Elena presented her artificially obtained radioactive material to Marie Curie, the great physicist and chemist was so pleased with this important achievement of her daughter and son-in-law that she excitedly put her finger into a test tube containing artificially created radioactive phosphorus (which was so weak that it would have no consequences) to feel the precious results of the experiment. Elena recalls that it was the last time her mother was so excited. In July 1934, Marie Curie died of illness.
Yelena conducted her research under the direction of her mother, Mary. Elena began to study radioisotopes with her mother at an early age and discovered natural radioactivity. She received her doctorate in 1925. 丨Image source: Public Copyright
In 1935, Elena and Yorio were awarded the Nobel Prize in Chemistry for their discovery of artificial radioactivity.
The controversy over "transuranic elements".
After the news of the discovery of artificial radioactivity reached Italy, Enrico Fermi (1901-1954), at the suggestion of team member Gian Wick (1909-1992), shifted the focus of research from theory to experiment, and immediately prepared experimental equipment with the team to bombard (irradiate) targets of various elements with neutrons to make more radioactive isotopes.
In the beginning, Fermi's team's experiments were always unsuccessful. Later he put paraffin in front of the target. The protons in paraffin turn fast neutrons into slow neutrons, giving them more time to interact with the nucleus, greatly improving the efficiency of the experiment. Fermi's team bombarded almost all the elements known at the time, obtaining 22 radioactive isotopes.
When the Fermi team bombarded the elements thorium 90 and uranium 92, they found that the properties of the elements produced were very different from those of thorium and uranium. Fermi et al. consider them to be elements 93 and 94, i.e., transuranic elements. Fermi's results were well received by his peers, and many teams followed suit.
However, the German chemist and physicist Ida Noddack (1896-1978) strongly questioned Fermi's conclusions. Noddack and her husband (Walter Noddack, 1893-1960), a prominent expert in the field of rare earths, discovered the element rhenium 75 in 1925 with their collaborators.
Photograph of Nodak, circa 1940. 丨Image source: Public Copyright
In his paper "On Element 93", Nodax pointed out that Fermi analyzed the reaction products by excluding only lead and elements heavier than lead, but not elements lighter than lead, so it could not be proved that the product was an element heavier than uranium. Only by excluding all light elements can the product be proved to be transuranic.
Nodak argues that Fermi may not have made new, heavier elements, but rather existing, lighter elements that were produced by uranium splitting, saying: "It is conceivable that the nucleus would split into several large fragments that were isotopes of known elements and would not be adjacent elements [element 93] of the irradiated element [element 92]." ”
Nodaq actually predicted the possibility of heavy nuclear fission. If this explanation is true, then what Fermi actually discovered was heavy nuclear fission. However, Nodaq did not have uranium and therefore could not do this experiment, and she did not give a theoretical proof. In addition, she was only an "unpaid collaborator" at the time, with a low status in academia. What's more, the scientific community at the time generally did not believe that a small neutron could break a heavy nucleus and make it split. As a result, Nodak's paper was widely ridiculed by his peers at the time.
In 1935 or 1936, Nodak and her husband asked the famous German chemist Otto Hahn (1879-1968) to mention Nodak's criticism of Fermi's work in lecture notes or writings. Despite their previous attention, Hahn categorically rejected them. Because Hahn thinks Nodak's views are ridiculous, quoting her will only make himself a joke in the academic world.
Before the storm
It was understandable that Hahn ignored Nodak's request, as he was also exploring the subject at the time.
In 1934, Lise Meitner (1878-1968) invited Hahn to follow up on Fermi's research. The two have worked together for a long time, but the two have not worked together for more than ten years.
Hahn was initially reluctant to repeat Fermi's experiment. However·, Aristid von Grosse told Hahn that Fermi may have discovered an isotope of protactinium 91, not transuranic element. Hahn became immediately intrigued by the subject and agreed to work with Meitner to verify whether the product was a lower quality protactinium or a higher quality transuranic element.
Hahn with Meitner in the laboratory, 1912. 丨Image source: Public Copyright
In 1935, Hahn recruited an excellent assistant, Fritz Strassmann (1902-1980). In this way, the trio began the experiment in full swing.
Strassmann. 丨Image source: https://www.uni-hannover.de/en/universitaet/freunde-foerderer/alumni/geschichten/fritz-strassmann
From 1934 to early 1938, the trio discovered more than 10 previously unknown isotopes. They believed that they were all isotopes of transuranic elements and "confirmed" elements 93 to 96, confirmed uranium-239 in the product and measured its half-life of 23 minutes. However, they still can't get access to the true Element 93 and heavier elements. Hahn and Strassmann improved the chemistry of the experiment, while Meitner designed the new experiment.
During this period, Yelena and the physicist Pavle Savić (1909-1994), who came from Yugoslavia, were also following up on Fermi's experiments. They found that after uranium was bombarded by neutrons, there was an element with a half-life of 3.5 hours, which could be an isotope of element 90 thorium.
Savage (before 1969)丨Image source: Public Copyright
Hahn et al. consider this conclusion absurd, as it means that a slow neutron bombards a uranium nucleus, but can knock out a α particle. In addition, the paper does not fully affirm the contributions of Hahn and others, which Hahn is unhappy about. The Hahn trio did not find this thorium isotope with a half-life of 3.5 h in subsequent experiments.
In January 1938, Hahn wrote to Elena and Savage, pointing out that their research was wrong and asking them to retract it. The two did not reply, but continued their experiments. They found that lanthanum 57 could be used as a carrier to extract this element. Thus, in the second paper, Elena and Savage announced that the newly discovered isotope was not an isotope of thorium, but possibly an isotope of actinium of element 89. Strassmann urged Hahn to read the paper, but Hahn refused in protest.
In May 1938, Hahn met Jolio at an international conference in Rome and told him privately: "I have not publicly criticized your wife because she is a woman." But she was wrong. When Jorio returned to France, he conveyed this opinion of Hahn to his wife Elena.
Elena and Savage decided to continue experimenting. In May, they published their third paper on uranium. This time, they determined that the product of the neutron bombardment of uranium, the new isotope, resembles lanthanum 57. The duo did not believe that uranium 92 would lose so many protons and neutrons after being bombarded and become lanthanum. Therefore, they believe that this is a new, extremely difficult to explain transuranic element.
In July 1938, Meitner fled Germany and moved to Sweden. From 1933 onwards, she was under constant threat, persecuted by Hitler's racial policies because of her Jewish origins. But at that time she was still an Austrian, and her situation was not so dangerous. On March 12, 1938, when Germany annexed Austria, Meitner lost her Austria citizenship and became a German, Germany's racial laws began to take effect on her, and her research funding was soon stopped and she was in extreme danger. In order to avoid more terrible persecution, Meitner began to prepare for his escape from that time, and finally escaped in July.
Since then, Hahn has collaborated with her through correspondence.
In September 1938, Elena and Savage published their recent results again in the Proceedings of the France Academy of Sciences (Comptes Rendus). According to Strassmann's recollections, after reading the paper, he determined that Yelena and others not only did not make any mistakes, but also gave a correct research path. Excited, he ran upstairs and said to Hahn, "You must read this paper." ”
Hahn smoked his cigar and replied arrogantly, "I'm not interested in what our close friend has written lately. Strassmann was not discouraged, and he insisted on recounting the best parts of the paper of Elena and others in front of Hahn. Hahn was stunned. He put the cigar he hadn't had time to finish on the table, and immediately went with Strassmann to repeat the experiment of Yelena and others.
There is another version of the story (possibly provided by Hahn): Hahn saw Yelena and Savage's new article, strongly questioned the conclusions, and gave it to Strassmann to read, and the two began to repeat the experiment.
In either case, Hahn and Strassmann began experimenting with separating the elements in the autumn of 1938.
Heavy nuclear fission
Hahn and Strassmann used lanthanum as a carrier to isolate elements such as actinide that might be produced; They also use barium as a carrier to separate elements such as radium that may be produced. They quickly identified 16 isotopes, three of which were previously unknown. They guessed it was an isotope of radium.
On November 10, Hahn visited Copenhagen at the invitation of Niels Bohr (1885-1962). He discussed these results with Bohr, Meitner, and Frisch (1904-1979).
玻尔（1922）丨图片来源：AB Lagrelius & Westphal
Frisch was the son of Meitner sister Auguste Meitner Frisch (1877-1951·· Auguste Meitner Frisch (1877-1951). He was an excellent theoretical physicist who had worked in Germany, and when Hitler began to pursue a policy of racial persecution in 1933, he immediately left Germany and went to United Kingdom to work with Patrick Blackett (1897-1974) on cloud chamber technology and artificial radioactivity. Because of his outstanding talents, he was recruited by Bohr to Copenhagen to do research with Bohr (for a period of 5 years).
Frisch's ID photo during the Manhattan Project丨Image source: Los Alamos Laboratory
There was no breakthrough in this discussion. After returning to Berlin, Hahn continued his experiments. After many days of experiments, measurements, and analysis, Hahn and Strassmann made a breakthrough on December 16 and 17, 1938, when they confirmed that the three unknown isotopes could be separated from all other elements, but not from barium's carrier, meaning that they were most likely barium, not radium.
Barium is element 56 and is 40% lighter than uranium. At the time, it was thought that it was impossible for uranium to be converted into barium by losing more than 100 nucleons, because it was impossible for neutrons to have so much energy to strip away so many nucleons. Hahn and Strassmann faced the same predicament as Elena and Savage.
On December 19, Hahn wrote to Meitner to inform her of his latest findings. "We are getting closer and closer to a terrible conclusion that our radium isotopes behave not like radium, but like barium," the letter said...... Maybe you can come up with some fantastic explanations. We ourselves realize that it (uranium) cannot be split into barium. Now we want to test the actinium isotope derived from 'radium', which behaves not like actinides, but like lanthanum. In fact, by this time Hahn was already inclined to think that uranium was split by neutrons.
Through beta decay, radium decays into actinium and barium decays into lanthanum. As long as the product is actinium or lanthanum, it can be determined whether the parent element is radium or barium. Hahn and Strassmann immediately began to conduct this experiment.
On December 20, Hahn called the editor of the journal Die Naturwissenschaften (Natural Sciences) to inform him of his findings and to ask them to expedite the publication of his paper. The editor promised to postpone an issue of a paper scheduled for publication to make room for Hahn's paper, on the condition that Hahn's paper must be submitted on the 23rd. Hahn arranged for a typist to type out the paper on the 22nd.
On December 21, Hahn and Strassmann determined the results of the experiment: the decay product of an unknown element was lanthanum, not actinium. So, that mysterious isotope is indeed an isotope of barium, not radium.
This means that Elena and Savage identified an isotope similar to lanthanum at the time, which is actually an isotope of lanthanum, which is a product of barium decay; It's just that they didn't know it at the time and kept thinking of it as some kind of puzzling transuranic element.
On the same day (21st), Meitner received a letter written by Hahn on the 19th, and she was also shocked by the result. "At the moment, it is difficult for me to assume such a complete rupture, but in nuclear physics we experience so much amazement that we cannot categorically say: 'It's impossible. Then she told Hahn that she was going to Khonelf for a week-long holiday starting on the 23rd. If there is a new letter, please send it there.
Meitner in 1906 (at the age of 28)丨Image source: Public Copyright
Although he has not yet received a reply from Meitner, Hahn, who had wavered two days ago, has now strengthened his conviction that after the neutron bombardment of uranium, one of the products of the uranium nucleus is barium, which has since decayed into lanthanum. In order to prevent Elena and Savage from coming to the same conclusion and preemptively publishing their results, Hahn can't wait to announce their results immediately.
On the 21st, before receiving a reply from Meitner, he wrote to Meitner again to say that they had confirmed that the product was barium and not radium, and Hahn mentioned that although he thought the results were physically absurd, they could not continue to be kept secret. The paper will be submitted tomorrow or the day after tomorrow. A copy will be sent to her.
On December 22, the paper was submitted to the editorial office. The paper is not signed by Meitner. That evening, Hahn sent a copy of the paper to Meitner, who at this time did not know that Meitner was about to go on vacation. This important paper was published on January 6, 1939.
Why do uranium nuclei split?
On the morning of December 23, Meitner left Stockholm for Kong Elf as planned. Later, her nephew, Frisch, came to visit her. At this point, Meitner didn't know that Hahn had submitted her paper yesterday and that it wasn't signed to her. Since both the copy of the paper and the letter sent by Hahn on the 21st were sent to Stockholm, it was impossible for her to see the contents of the letter until she returned to Stockholm.
In Konelf, Meitner handed over the letter sent by Hahn on the 19th to Frisch. After reading it, Frisch did not believe that barium would be produced after the uranium nucleus was bombarded, and he ran out to ski. However, Meitner pursued Frisch relentlessly, and said as he chased it. Frisch was persuaded and decided to consider the possibility of the uranium nucleus being split.
They think of the droplet model proposed by George Gamow (1904-1968) in 1935 and perfected by Fritz Kalckar (1910-1938) and Bohr in 1937. This model assumes that the nucleus resembles a droplet. But Kalka and Bohr believed that heavy nuclear droplets were difficult to break. Frisch used to get along with Kalka (see image below; Kalka died in 1938 at the age of 27).
From left to right: Milton Plesset (1908-1991), Bohr, Kalka, Edward Teller (1908-2003) and Frisch. From January to August 1934, Teller was a visiting scholar in Copenhagen with Bohr, so this photograph should have been taken during this period. 丨Image source: AIP Emilio Segrè Visual Archives, Wheeler Collection
Within the framework of the droplet model, Frisch carried out the calculations in cooperation with Meitner. They found that the charge of the uranium nucleus was large enough to almost completely overcome the surface tension constraints, and was therefore on the verge of rupture, like an unstable water droplet. The knock of the neutron causes the uranium nucleus to become ellipsoidal, then its "waist" to tain, and then to break off from the "waist" and split into two small "droplets".
Droplet model of heavy nuclear fission. 丨Image source: Hullernuc
They also calculated that such a split would release 200 MeV (1 MeV = 1.6×10-13 joules) of energy. Where does this energy come from? Meitner remembered that she had heard Einstein's lecture on the theory of relativity, and that she was struck by the mass-energy relation in it.
Meitner calculates this mass difference by empirical formula for calculating the mass of the nucleus (1.67 × 10-27 kg), multiplying this value by the square of the speed of light (9×1016) gives a value (3.0×10-11 joules) that is almost equal to the energy produced after fission (3.2×10-11 joules). Since the quality difference itself is an estimate, the small difference between 3.0 and 3.2 is negligible. This result means that the uranium nucleus may indeed have split.
Borrowing terminology from the field of biology, Frisch used the word "fission" for the first time to name the uranium nuclear fission process. Upon his return to Copenhagen, Denmark, Frisch told Bohr about his findings. Bohr immediately understood, slapped his forehead with the palm of his hand and said: "Why are we so idiots!" ”（“What idiots we have been !” ）
Frisch then used cloud chambers (one of his research areas during his time in the United Kingdom was cloud chamber technology) to trace the trajectories of reaction products and directly prove in an intuitive, physical way that neutrons did indeed fission after they collided with uranium nuclei.
Therefore, the hypothesis put forward by the Noldak 4 years ago is correct: after the uranium was bombarded by neutrons, fission occurred. It was only then that it was discovered that the element obtained by Fermi's team was not transuranic, and that they had actually discovered heavy nuclear fission for the first time, but missed out on the accolade. Elena and Savage also missed out on the accolade.
On February 11, 1939, Meitner and Frisch's paper on theoretical explanations was published in Nature. On February 18, Free's paper on uranium fission using a cloud chamber was also published in Nature.
However, before the publication of these two papers, the relevant news was transmitted by Bohr to the United States. When Bohr arrived in Washington in January, he told Gamow the news. Gamov called Teller and said, "Bohr just came in and he's gone crazy." He said that a neutron could split uranium. Teller immediately thought of the inexplicable observations of the Fermi team and immediately understood that it was fission.
On January 26, 1939, Bohr and Fermi co-chaired the Fifth Washington Conference on Theoretical Physics in Washington, D.C., and the news of uranium fission caused a sensation in the venue. Physicists at Colombia University quickly replicated the results in the lab and determined that uranium fissioned by slow neutrons was predominantly uranium-235.
Shortly before that, Bohr had sworn to Frisch that he would keep it a secret; Then, he felt sorry for Frisch because the news had spread too quickly.
When the news reached Berkeley, Calif., on the West Coast, Luis Alvarez (1911-1988), who was in a barbershop, was shocked because he and his students had been bombarding uranium with neutrons in search of transuranic elements, but had never expected fission. He told the barber to stop the haircut and head straight to the radiation lab.
Alvarez relayed the news to J. Oppenheimer. Robert Oppenheimer, 1904-1967), Oppenheimer did not believe it, and theoretically argued that uranium nuclei could not be fissioned. But the experiment soon showed the energy released after the neutron bombarded uranium. Within 15 minutes, Oppenheimer believed that the uranium nucleus had fissioned.
Chain reaction with the atomic bomb
In February 1939, Hahn and Strassmann published a second paper on the subject, predicting that uranium fission could release neutrons at the same time. Jorio's team quickly proved that uranium fission would release more than two neutrons, and published a paper on the subject in March 1939.
Obviously, the released neutrons will also bombard other uranium nuclei, and the process will continue rapidly in a snowballing fashion, forming a chain reaction that releases enormous amounts of energy.
Nuclear fission of uranium-235 nuclei丨Image source: MikeRun
Previously, the Hungary nuclear physicist Leo Szilard (1898-1964) had speculated in 1933 about a similar path to the chain reaction, which could be used to make atomic bombs. In 1934, he deduced the equation for the chain reaction and proposed the concept of "critical mass" (when the mass of fissioned material exceeds the critical mass, the chain reaction can sustain itself and produce a nuclear explosion).
1915年时的西拉德丨图片来源：Szilard, Leo (February 1979). "His version of the facts". Bulletin of the Atomic Scientists.
However, at that time, no one (including Silard) expected the fission of the heavy nucleus, and Silard did not know what kind of elements could produce a chain reaction when bombarded. He wanted to use neutrons to bombard each of the 92 elements known at the time one by one to find out. However, he was unable to apply for funding to conduct such an experiment. Szilard filed a patent application for a chain nuclear reaction. In order to prevent this discovery from being used by Germany and other countries to create nuclear weapons, he gave the patent to the Admiralty of United Kingdom and asked the Admiralty to keep it secret.
By 1939, it was known that uranium would fission under slow neutron bombardment and could trigger a chain reaction. Since then, the United States, the Soviet Union, Germany, United Kingdom, and Japan have each begun to explore the possibility of building atomic bombs, and to varying degrees put them into practice before the end of World War II.
Oppenheimer, convinced of uranium nuclear fission, only took a few minutes to discuss the chain reaction and the possibility of building an atomic bomb. A week later, sketches of the atomic bomb appeared on the blackboard in his office.
Frisch returned to United Kingdom from Denmark and, together with Rudolf Peierls (1907-1995), calculated that the critical mass of pure uranium-235 in a chain reaction was about 1 pound (about 0.45 kg) or 2 pounds. In 1940, Frisch and Peiers wrote the "Frisch-Peierls memorandum" and called bombs using uranium chain reactions "super-bombs", and they designed the world's first atomic bomb crash model.
However, most people who knew nuclear physics at the time did not believe that any country could have built an atomic bomb at that time. Natural uranium has three isotopes: uranium-234, uranium-235 and uranium-238. Uranium-238 accounts for 99.28%, and it will fission under the bombardment of fast neutrons, but the neutron energy released during fission is lower than the energy of the incident neutron, and it cannot make other uranium-238 nuclei fission, so it cannot start the chain reaction. Uranium-235 can undergo a chain reaction, but only 0.714% of natural uranium.
Most of the uranium must be separated from uranium-238 and the concentration of uranium-235 increased to more than 80% (preferably 90%) in order to become weapons-grade uranium. This requires a high level of industrial capacity, and it may not be possible to achieve it with the power of the whole country. Plutonium-239, which was later manufactured, could also be used to make atomic bombs, but mass production of plutonium-239 would require the nation's efforts.
Thus, when Bohr arrived in United States, he declared that the atomic bomb could not be built unless United States became a huge factory.
In 1942, Fermi built the first nuclear reactor in human history at the University of Chicago, laying the foundation for the peaceful use of nuclear energy and the subsequent mass production of plutonium-239.
Since then, the Manhattan Project has rapidly advanced the process of building an atomic bomb. After witnessing the progress of the United States in building the atomic bomb, Bohr did not take back his words, he lamented: United States has indeed become a huge factory.
On July 16, 1945, United States successfully detonated the world's first atomic bomb, which exploded with a force equivalent to 20,000 tons of TNT. Less than a month later, two atomic bombs bombed Hiroshima and Nagasaki.
In 1945, Hahn was awarded the Nobel Prize in Chemistry in 1944 for "for his discovery of the fission of heavy nuclei." It's not fair to Meitner and Strassmann to share the award. At the time, Hahn was still in an Allied internment camp and did not receive his award until 1946.
Although he won the Nobel Prize for the discovery of nuclear fission, Hahn had regrets. After the neutron bombardment of uranium, some of the uranium did fission, but some of the uranium did convert to elements 93 and 94, which Hahn did not detect at the time. The Nobel Prize for transuranic elements later fell into someone else's hands. In the midst of fierce competition and almost rapid change, Hahn and others could not imagine that another part of the uranium nucleus had indeed become transuranic while confirming the fission phenomenon of uranium nuclei. In the field of scientific research, there is often a surprise followed by a surprise.
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[1] Richard Rhodes, The Making of the Atomic Bomb, Simon & Schuster, 1986 （中译本：《原子弹出世记》，李汇川 等 译，李汇川 校；世界知识出版社，1990年；《横空出世》，江向东，廖湘彧 译；方在庆 译校，2023；此书还有上海科技教育出版社译本，本文未参考此译本。 ）
[2] Robert Jungk, Heller als tausend Sonnen. Das Schicksal der Atomforscher (Stuttgart, 1956) (英译本Brighter than a Thousand Suns: A Personal History of the Atomic Scientists,中译本《比一千个太阳还亮——原子科学家的故事》,原子能出版社,1991年)
[3] Winifred Conkling Radioactive!: How Irène Curie and Lise Meitner Revolutionized Science and Changed the World （中译本《她们开启了核时代：不该被遗忘的伊蕾娜·居里与莉泽·迈特纳》，王尔山 译 上海科技教育出版社，2017年）
[4] Noddack, Ida (1934). On Element 93. 47(37): 653-655.
[5] Joliot-Curie, Irène; Savić, Pavle (1938). "On the Nature of a Radioactive Element with 3.5-Hour Half-Life Produced in the Neutron Irradiation of Uranium". Comptes Rendus. 208 (906): 1643.
[6] Hahn, O.; Strassmann, F. (1939). "On the Detection and Behavior of the Alkaline Earth Metals Produced by the Irradiation of Uranium by Neutrons". Natural Sciences (in German). 27 (1): 11–15. Received 22 December 1938.
[7] Hahn, O.; Strassmann, F. (February 1939). "Detection of the formation of active barium isotopes from uranium and thorium by neutron irradiation; Evidence of further active fragments during uranium fission". Natural sciences. 27 (6): 89–95.
[8] Meitner, Lise, & Frisch, O. R. (1939). Disintegration of Uranium by Neutrons: a New Type of Nuclear Reaction. Nature. 143 (3615): 239–240.
[9] Frisch, O. R. (1939). Physical Evidence for the Division of Heavy Nuclei under Neutron Bombardment. Nature. 143 (3616): 276.
[10] Otto R. Frisch, "The Discovery of Fission – How It All Began", Physics Today, V20, N11, pp. 43-48 (1967).
[11] Bethe, H. A.; Winter, George (January 1980). "Obituary: Otto Robert Frisch". Physics Today. 33 (1): 99–100
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