In the summer of 1945, World War II ended with the atomic bombing of the Japanese cities of Hiroshima and Nagasaki. However, it was a German scientist who initially discovered nuclear fission. So why didn't the war end with an atomic bomb on London? In other words:why did the Germans fail to make an atomic bomb, but the Allies did?
“We would like to draw your attention to the latest development in nuclear physics that makes it possible to cause an explosion many times larger than that of all known weaponry. The country that takes advantage of this first has an unbeatable lead over the others.”
It is clear language from the German chemist Paul Harteck in a letter to Erich Schumann on April 24, 1939. Schumann, who led the section of the German army that researched new armaments, was well aware of this.
Harteck referred to research by Otto Hahn, a chemist from Berlin, who had accidentally discovered nuclear fission a few months earlier. Hahn bombed uranium with neutrons in his lab in Berlin, which to his surprise resulted in smaller atomic nuclei. He realized that apparently you can break up an atomic nucleus in this way – or rather:split it – in smaller atomic nuclei. Moreover, a large amount of energy was released during this nuclear fission.
A little later it turned out that several neutrons are formed during the fission of one uranium nucleus, so that a chain reaction is possible. The amount of energy that would be released was potentially enormous. At the same time Harteck briefed the German military about this, American physicist George Pegram did the same to the US military. It was the starting point of the race for the atomic bomb.
Heisenberg
In the summer of 1939, the Germans founded the Uranverein (Uranium Club) op. The club consisted of scientists who had to investigate the possibility of an atomic bomb. Members included Kurt Diebner, Paul Harteck, Otto Hahn, Hans Geiger (known for the Geiger counter, an instrument for measuring radioactivity), Carl Friedrich von Weizsäcker and Walther Bothe.
“If there's even the slightest chance that it's possible to build an atomic bomb, we have to work on it,” Bothe said at the first meeting. Even before Germany invaded Poland on September 1, 1939, the investigation into the possible use of nuclear weapons was already taken seriously by the highest military and political administrators.
On September 25, the Uranverein . called also Werner Heisenberg op. Heisenberg was one of the foremost scientists the Third Reich still had in its ranks. He had been awarded the Nobel Prize in Physics in 1932 for his pioneering work in quantum mechanics.
Critical mass
Soon after Heisenberg joined the Uranverein he took on the scientific leadership of the project. The first technical problem was the scarcity of uranium. The Germans had only one uranium mine at their disposal and it was located at Joachimstal in present-day Czech Republic.
Heisenberg was aware that uranium could only be fissioned if it had 92 protons and 143 neutrons in its nucleus, called uranium-235. But in natural uranium ore, uranium-235 is scarce. Most of it is made up of uranium-238 (with three more neutrons in the nucleus), and this isotope doesn't split when bombarded with neutrons.
Heisenberg believed you needed tons of uranium-235 to make a working bomb. This concerned the so-called 'critical mass':the minimum amount of uranium needed to keep a chain reaction going. We now know that the critical mass of uranium is only 52 kilograms; Heisenberg was therefore far wrong with his estimate. In 1940 Otto Frisch and Rudolph Peierls came up with a more realistic estimate:700 grams.
For the Americans, this small amount of uranium required confirmed that an atomic bomb was in principle feasible. Despite his familiarity with the work of Frisch and Peierls, Heisenberg, however, continued to maintain a critical mass of several tons of uranium. This made it unnecessarily difficult for them:where do you get so much uranium from? Later, in a German report from 1942, a critical mass is mentioned between 10 and 100 kilograms, but it is unclear who came up with this and how. It seems that the Germans could not calculate the exact value of the critical mass properly.
Heavy water or graphite?
But whether the exact critical mass was known or not, the challenge was to find a way to separate uranium-235 from natural uranium ore from uranium-238. A difficult task, since the elements have the same chemical properties. The Uranverein had also set himself the goal of building a nuclear reactor. This would enable them to create a working chain reaction, something that no one had done before.
The Uranverein gave priority to building a reactor, because it could also work with natural uranium. Separation of the isotopes uranium-235 and uranium-238 was not necessary in that case. One of the most important mistakes the Germans made was the choice of the so-called moderator for the reactor to be built.
A moderator is the material that inhibits released neutrons from the fission reactions. This is necessary to maintain the chain reaction. Two moderators were obvious:'heavy water' and graphite. Heavy water is a form of water where the two hydrogen atoms in the water molecule have been replaced by two deuterium atoms. These are hydrogen atoms with an (extra) neutron in the nucleus.
The Germans eventually opted for heavy water, thanks in part to Walter Bothe. He had concluded after experiments that graphite was unsuitable as a moderator. But his conclusion was wrong:he had tested graphite with too many impurities. A graphite moderator had to be as pure as possible. The choice for heavy water caused the Germans all kinds of extra problems. Heavy water could only be obtained – and in small quantities – from a factory in Norway. As a result, the German project was severely delayed.
The Americans, on the other hand, did experiment with pure graphite. Graphite was much more readily available than heavy water. The result:by December 1942, the Americans in Chicago had completed the world's first working reactor, with a pure graphite moderator. In the end, the Germans failed to build a working reactor. They achieved their best attempt in March 1945 – although it had long been clear that Germany would perish – in a cave below the town of Haigerloch. The number of neutrons released was the highest so far, but not yet sufficient to keep the chain reaction going.
Plutonium
The fact that the Germans did not come to a critical reactor may have prevented them from experimenting with plutonium. Plutonium is the element that can be formed by decay reactions of uranium-238. U-238 can capture a neutron and turn into uranium-239. Subsequently, a neutron can change twice into a proton, via beta decay, after which plutonium-239 is formed. This element is as fissile as uranium-235, noted the American physicist Louis Turner, and the idea soon reached Germany via Von Weizsäcker. The advantage of plutonium is that it is easier to separate from uranium. So you can get started with natural uranium, without having to separate uranium-235 from it.
Heisenberg probably also saw the benefits of plutonium. Knowing the difficulties the Germans had in building a reactor, however, he believed that a plutonium bomb would not be a realistic possibility until the distant future—when the war was long overdue. He therefore decided not to discuss the plutonium alternative publicly during a conference at the Ministry in the summer of 1942.
Heisenberg was convinced that science needed time, something Hitler and his right-hand man Albert Speer would never understand as non-scientists. Heisenberg did not want to be put under unpleasant pressure to come up with results. However, the Americans set to work with plutonium very energetically, as witnessed by the second atomic bomb on Nagasaki that worked on plutonium.
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'Total war'
It was not just purely scientific problems that the German researchers had to contend with. In the summer of 1941, what Hitler had been looking forward to for years began:the attack on the Soviet Union. The advance initially went well, but in the winter the German offensive ran aground hopelessly in the harsh Russian winter. German soldiers died en masse from cold, hunger and exhaustion. But Hitler wanted to carry on. He had to and would defeat his nemesis Stalin.
Germany was preparing for a 'total war':the entire society – including civilians – was deployed for warfare.
The fact that even Carl Friedrich von Weizsäcker and Walther Bothe were drafted into military service indicated that the German leaders did not expect much from the Uranverein. Fortunately, thanks to his good connections, Heisenberg was able to prevent the two from being forced to leave his project.
However, since it appeared that the Uranverein would not help Nazi Germany with a nuclear weapon in the foreseeable future, the Heereswaffenamt decided – the German Arms Division – to hand over the entire project to the Nazi Ministry of Education in early 1942.
Moreover, the financing was in danger of drying up. Albert Speer, however, was able to be convinced by Heisenberg to continue to support the research. All in all, the German researchers were therefore not given ample space to work on the atomic bomb, which slowed their progress considerably.
Misplaced sense of superiority
The German approach was in stark contrast to the way the Americans got to work on their Manhattan project. The top-secret project in the Nevada desert, which eventually led to the US atomic bomb, employed tens of thousands of people and the government invested billions of dollars in it. Heisenberg probably never realized that the path to a practically applicable nuclear weapon would require so much money and the commitment of so many people.
The Americans knew that the Nazis were trying to initiate nuclear fission. For example, the intelligence service knew in 1940 that the Germans had increased surveillance of the Norsk Hydro factory, near the town of Vermork in occupied Norway. Norsk Hydro was the only factory in the world producing heavy water at the time. The Allies made several attempts to sabotage the factory, all of which failed.
The Germans, on the other hand, were not aware of the American project, partly due to a lack of intelligence. In fact, they had the – misguided – sense of superiority that they were miles ahead and could not be overtaken by anyone. Illustrative of this is that when German scientists learned of the US bomb on Hiroshima after the war, they thought it was a hoax. wash.
Due to the combination of Heisenberg's lack of knowledge of critical mass and the isolation of uranium-235, the lack of raw materials and the low priority that the Uranium Project was given after the outbreak of 'total war', it is not surprising that Heisenberg believed that a working atomic bomb would not be a realistic option until well after the end of the war. He never had to make the moral decision as to whether he should help Hitler with a weapon of mass destruction.
- Bohr, Heisenberg and the bomb
- Oldest nuclear weapons material recovered
- Supercomputer simulates nuclear explosion
- How an atomic bomb works (atomicarchive.com)