This post is the second of three articles first published by Xiaoduo Media in “Front Vision”. Front Vision is a Chinese online science magazine for children. My original English text produced with permission.

War and fascism – everything changes

The research described in the previous chapter involved scientists from across Europe, all sharing information and working towards a common goal. Through scientific papers, they shared their ideas equally, allowing people in many countries to contribute. But In 1933 Adolf Hitler took power in Germany. His fascist ideology, based on lies and hatred, blamed Jewish people and other countries for holding back Germany. Between 1933 and 1939 he and other fascist leaders started to discriminate against Jews and it was clear to all that even more terrible things were coming. Many Jewish scientists left Europe for America, to escape persecution. For example, Otto Frisch and Lise Meitner were refugees in Sweden fleeing the Nazi takeover of Austria when they identified nuclear fission.

Another Jewish refugee was Leo Szilard. In 1933 crossing the street in London he had a vision. If slow neutrons could induce nuclear reactions in elements, what if such a reaction created two neutrons? And each of those neutrons in turn made another reaction? 1-2-4-8-16-32-64-128 .. It grows and grows – a nuclear chain reaction.  In 1939 (now safe in the USA) he heard of the discovery of fission in Uranium and realised the possibility of creating a fission chain reaction. Nuclear fission, theoretical calculations suggested, released unusually large amounts of energy.  A nuclear chain reaction involving fission would suddenly release massive amounts of energy – it would be a bomb of enormous destructive power. 

Szilard wasn’t the only person to realise the potential of nuclear fission chain reactions. In the UK – from September 1939 at war with Germany – James Chadwick (the discoverer of the Neutron and student of Rutherford), led serious investigations into the practicalities of making a nuclear bomb. At the same time, the German government started it’s own investigations, led by Werner Heisenberg. 

In the USA in 1941, researchers created Plutonium from Uranium. They kept their discovery secret. Theoretical studies suggested that Plutonium, like Uranium, was fissile (could undergo nuclear fission) and could be produced in large quantities from Uranium.

How to make an atomic bomb

By 1941, the basic theory of how to make an atomic bomb was known to many physicists. What’s required is to generate a nuclear chain reaction involving nuclear fission. Two materials were known that did this, Plutonium and Uranium. However only one of the isotopes of Uranium was suitable, Uranium-235. Natural Uranium contained a mix of Uranium-235 and Uranium-238 and nobody knew an easy way to separate them out. 

If you somehow managed to get hold of enough of either material, you needed to create a ‘critical mass’ of it. This is where a large enough volume material is tightly packed together such that not too many neutrons escape out, but instead enough hit other fissile atoms and create yet more neutrons and so sustain a chain reaction. 

The concept of critical mass can be understood by thinking of a forest fire. If a tree standing alone in a field is hit by lightning and catches fire it releases heat, but this is lost to the atmosphere and soon the fire stops. This is like fission in a small mass of Uranium, a reaction starts but the neutrons don’t hit more fuel and so fission stops. If the first tree is closely surrounded by dead wood and other trees, the heat it produces heats this wood and causes them to start burning – it starts more chemical reactions – and so the fire spreads growing hotter and bigger. This is like a fission reaction starting within a critical mass of Uranium, most neutrons hit more fuel, creating ever more reactions. Nuclear fuel releases much more energy than wood, so things move much more quickly.  This would release huge quantities of energy and so create an extraordinary explosion. 

Physics is not engineering. Creating enough fissile material and turning it into a working bomb were huge challenges.

Manhattan project

In December 1941, Japan attacked a US naval base called Pearl Harbor and the USA entered the Second World War. Japan and Germany were aligned, so the USA declared war on both Germany and Japan. 

Alerted by a letter from Leo Szilard and Einstein, the US government had already started research into nuclear weapons. Knowing of German expertise in nuclear physics, the goal was to create a bomb before Germany did. Now at war, a new US project was created and given the code name of the Manhattan Project. It had enormous resources available to it. 

The project called on the best scientists in the world. The best physicists in American universities, like Philip Oppenheimer and the young Richard Feynman were called up in secret. Leo Szilard and Enrico Fermi and others had already fled fascism to reach the USA, and the British project team was merged in as the two countries became allies. In time Niels Bohr was involved, having fled his native Denmark in secret to escape Nazi persecution. 

The project brought together different cultures.  The many scientists who had fled to the USA from Hungary were called ‘Martians’. This was a joke name as – like aliens from another planet – they spoke an incomprehensible language and were incredibly intelligent.  University scientists didn’t always get along with Army engineers. In his memoirs, General Groves – the leader of the Manhattan project – talks of a meeting where scientists gave him their estimate for how much Plutonium was required to make a bomb. Like an engineer, he assumed an estimate was one with maybe 25% or 50% uncertainty. However to physicists a reasonable estimate was one within a factor of ten.

Those on the project were completely focused on one goal, and solving the many technical challenges that faced them. Knowing that there were two types of bomb, based on Uranium-235 or Plutonium and not knowing which would be best, they decided to build both at the same time.

Little Boy, the Uranium weapon

The Manhattan project managed to secure large supplies of Uranium ore, but this contained a mix of Uranium-238 and the more fissile Uranium-235. These different isotopes are chemically identical, making separating them (a process called Uranium enrichment) a huge job. A huge secret site was built called Oak Ridge to focus on Uranium enrichment.

Various different approaches were considered. While chemically identical, U-238 is slightly heavier (because each atom contains more neutrons) so each method finds ways to use the weight difference to separate them. 

The first method is to spin the Uranium in centrifuges. These spinning containers separate out the lighter U-235. Attempts by the Manhattan project to build machines failed, but this technique is now the preferred method. 

The electromagnetic technique spun Uranium gas through a magnetic field to separate the isotopes. It required huge quantities of copper which was hard to find during wartime. In the end 13 tonnes of silver kept for use as money was used instead. This technique was inefficient, but used known technology and so was used in case the other techniques failed.

The Gaseous diffusion technique relies on the fact that lighter gas diffuses more quickly through a membrane. Huge buildings, four stories high and 0.8km long were built and the technique was very successful. The gas Uranium Hexaflouride was pumped under high pressure into a tube  surrounded by a barrier, a membrane that contained thousands of tiny holes. Molecules made of the slightly lighter Uranium-235 (shown as orange dots) could more easily pass through these holes leaving behind gas contained more Uranium-238 (blue dots). This process separated the gas into a enriched stream and a depleted one. The enriched stream would still contain lots of Uranium-238, but it could be passed through into another set of pipes and through another barrier again and again, becoming each time more and more enriched. The depleted steam could be discarded – it was the Uranium-235 that was needed.

(See https://www.atomicheritage.org/history/isotope-separation-methods for some diagrams of this process)

Finally, the thermal diffusion technique placed hot Uranium gas in 15m tall tubes. The lighter Uranium-235 gathered more at the top – this was another effective way of separating out the isotopes by weight.

The three techniques were used in sequence, to create more and more concentrated Uranium-235. Eventually about 50 kilograms of uranium enriched to 89% uranium-235 was delivered to Los Alamos by July 1945 where it was turned into a bomb.

The design was simple, a little like a gun. A long tube contained two pieces of enriched Uranium. Each piece alone was too small to sustain nuclear fission reactions. Conventional explosives fired one piece down the tube into the other. This joined mass was big enough to become critical  and create a nuclear fission chain reaction and so an explosion. The bomb had the codename “little boy”.

Fat man – Plutonium weapon

Despite being only discovered the year before, Plutonium was seen as another potential fuel for a bomb. Plutonium could be produced from un-enriched Uranium, by generating a controlled nuclear fission reaction. Nuclear fission reactions would produce neutrons, slowed by graphite blocks. The material that slows the neutrons is called the moderator, as it moderates (slows) the speed of the neutrons. Some of these neutrons sustain a steady rate of reactions, others turn Uranium into Plutonium. The rate of reaction was controlled by adding rods that absorbed neutrons to slow or even stop the reactions. The designs ensured there was no ever-increasing chain reaction as was wanted in a bomb, just a steady rate. The reactions produced heat, but the real purpose was creating Plutonium. Importantly, the Uranium fuel could be unprocessed ore – there was no need to enrich the Uranium-235 first.

Enrico Fermi, piled up Uranium oxide and graphite blocks in a squash court in a sports ground at Chicago University and so in 1942 created the first (man-made) self-sustaining nuclear reaction in Chicago Pile-1, the ancestor of all nuclear reactors. Soon a small secret town called Hanford was built to house a number of bigger nuclear reactors all dedicated to producing Plutonium. Once Plutonium had been produced, it could be chemically separated from the Uranium material.

The design of the weapons was done at a site called Los Alamos. This required intense work from many of the world’s best physicists. To know if the bomb would work, they had to understand how it would work. To start with they calculated the energy released by a single fission reaction. This required knowledge of the exact mass of the material before and after, to calculate the missing mass converted to energy according to the Einstein equation.

That was the easy part. To create a sustained chain reaction, a large amount of fissile material needs to be suddenly pressed together to make a critical mass. A problem with the Plutonium was that it contained a mix of different isotopes, meaning the fission reactions acted in a different way. The material produced within the reactors contained more plutonium-240 (a very reactive isotope) than expected. This would start fission reactions much sooner than the plutonium-239. If a gun design was used, like in Little Boy, then the plutonium-240 would fission early and push the Plutonium apart before most of the material – the plutonium-239 – could fission.  This would cause the bomb to fail to explode properly.

So a different and more complicated bomb design was required, one that involved taking a sphere of Plutonium and surrounding it with conventional explosives. The physicists had to write equations to model the behaviour of the Plutonium pressed together by explosives, then track the fission reactions as a critical mass was formed. Imagine trying to understand in detail what happens in the heart of a bomb, and then add on the complexity of understanding the fission reactions. Many equations and long difficult calculations were required and because Plutonium behaves different from Uranium, all the calculations were different from those required for the Uranium bomb design. The concept was simple but the design was complicated. The explosives were made with a complicated three-dimensional shapes to focus the shock wave through them and down onto the ball of Plutonium. The yellow detonator, started different conventional explosions with the outer layer of explosives. The curved shape at the based helped join the different shock waves into a single spherical wave that was then increased by two further layers of conventional explosives. This then compressed three different layers, of Aluminium, plastic and then Uranium, each with a different purpose.

Inside all this, the red Plutonium contained a polonium-beryllium initiator, a device that when compressed provided a good source of neutrons to initiate the chain reaction. The size and round shape of the bomb compared to ‘little boy’ led to it being called ‘fat man’. 

They ran tests of the explosives (without any Plutonium) and used X-rays to see what happened inside, but still any big mistake in these calculations and the bomb wouldn’t work.

German efforts

Scientists who before the war had cooperated now found themselves on opposite sides of a vicious war. Non-Jewish German scientists like Werner Heisenberg might not be supporters of Hitler’s fascism, but most chose to stay and support their country as it fought a war. 

Heisenberg was asked to lead efforts in Germany finding uses for nuclear fission. He understood as well as anyone the possibility of making a bomb, but focused mostly on the potential for energy generation, rather than weapons. Some people speculate that he did this as he didn’t want to give Hitler such a powerful weapon, but no-one knows for sure.

The Manhattan Project was started precisely to get a bomb before Germany, but nobody knew that Heisenberg would fail to deliver. To slow down any German efforts, the UK army, working with locals, attacked a site in Norway that was a source of heavy water, useful for nuclear research. In 1944 Heisenberg gave a talk in Switzerland, a neutral country. A US agent was sent with instructions to kill him if the talk suggested that Germany was close to having a bomb, but of course there was no need.

Testing and use in war

By 1945 the Manhattan project had achieved so many things. It had produced enough Uranium-235 for a single bomb and larger quantities of Plutonium. We’ve seen how complicated the calculations required for bomb design were – what was needed now was a test, to see if nuclear weapons actually worked. 

A version of the fat man weapon was built and a site deep in the desert chosen. At 05:29 on the 16th July the device was exploded at the Trinity Test site. A huge flash of light lit up the desert, with colour changing from yellow, red to purple. A ball of fire slowly rose leaving behind a column of smoke, forming a characteristic mushroom shape. After about 40 seconds the blast wave reached the observers. Enrico Fermi dropped a series of pieces of paper and saw how far they were blown by the wind. He had already made a rough calculation and from the distance the paper was blown he estimated the size of the explosion to be the same as that created by exploding 10,000 tonnes of TNT. 

The bomb contained 6.2kg of Plutonium of which 1kg was transformed by the fission reactions. Of that mass, about a gram was converted directly into energy. Enough energy to turn the desert sand beneath into glass.

The initial reaction of the scientists watching the test was joy at their success, but slowly they became solemn as they realised how terrifying a weapon they’d created. The blast wave spread outwards and was heard over a hundred kilometres away. A fake cover story was invented to explain it as the new bomb was still secret. The political implications spread around the world.

Nazi Germany had been defeated a few months earlier, but the war against Japan continued. The Japanese armed forces had been mostly defeated outside of Japan but refused to surrender. Faced with the prospect of having to invade Japan the US Army estimated that this would mean half a million US soldiers and 5 million Japanese people would be killed. The American president turned to his new atomic weapons as an alternative. On 6 August 1945 the Little Boy Uranium-235 weapon was dropped on the Japanese city of Hiroshima. Three days later a Plutonium fat man weapon was exploded over Nagasaki and six days after that, Japan surrendered and the war was over. Many of the scientists who worked on the Manhattan project argued the bombs should not have been used and were appalled that 10,000s of civilians had been killed by what they had built. The alternative argument is that they ended the war early and so saved more lives that way. Probably people will never agree which interpretation is correct.

Aftermath and impact on science and beyond

Nuclear physics moved in only a few years from experiments on tables in laboratories to a huge engineering project. Theoretical physicists would produce equations that within days were being used to design new machines and industrial processes.

Some of those who worked on the Manhattan project moved back into basic research. Having demonstrated it’s practical value, physicists had little difficulty finding the funding for building the bigger and bigger machines required to explore deeper inside the atom. The search began to find out what smaller particles neutrons and protons were built up from.

Some others stayed in weapons research. The process of fusing small atoms together – nuclear fusion – produces even more energy than fission and within 10 years a fusion weapon – the hydrogen bomb – had been made and tested. Others focused on more peaceful uses of nuclear fission. This included fields like medicine but many thought of the heat produced by the Manhattan reactors and saw it as a source of cheap energy.

This post is the first of three articles first published by Xiaoduo Media in “Front Vision”. Front Vision is a Chinese online science magazine for children. My original English text produced with permission.

Around 1.7 billion years ago, a thick layer of rock rich in the element Uranium began to heat up. The tiny blobs of matter in the centre of the Uranium atoms split into two new atoms, releasing energy and radiation. Special conditions caused more and more atoms to split and produce heat until the water in the rock layer boiled away, making things calm again. This rock layer is now in Gabon, Africa and is the only known example of a natural nuclear reactor. 

In the 19th Century, physicists still had no idea such things were possible. Geologists studying thick layers of sediment argued the world must be hundreds of millions old, as these layers formed so slowly on the modern earth. Previously people in Europe had regarded the Christian Bible as evidence that the earth was only thousands of years old. Physicist Lord Kelvin calculated that even if the earth started out extremely hot, that it would be become cold after only a few million years. He argued that the existence of volcanoes today suggested the earth could not be very old and so the geologists were wrong. His arguments were based on the fact that nobody knew of a mechanism that could heat the earth from within. Scientists had discovered many elements (Uranium was identified in 1789) and had a good understanding of chemistry, but believed atoms to be fundamental unbreakable units.

Discovery of radioactivity

The first hint that there was something more to be discovered came in 1896 when French scientist Henri Becquerel was experimenting with some Uranium compounds. He was investigating the effects of exposing them to sunlight, but the discovery came when he left them in a closed drawer with some photographic plates. When he took them out, he discovered the plates were fogged. 

This discovery led to many more experiments of the strange properties of Uranium. The first step was to introduce a magnet, affecting the space between the Uranium and the photographic plates.  This showed that whatever was coming out from the Uranium, it came in three different types. The first type was pushed away by the magnet, the second pulled towards it and the third not affected by the magnet at all. By 1897, New Zealand scientist Ernest Rutherford had named these types Alpha, Beta & Gamma after the first 3 letters of the Greek alphabet. He showed that they had different properties. Alpha particles were positively charged and did not travel far – a few centimetres of air could stop them. Beta particles are negatively charged and could be stopped by a millimetre of Aluminium. Gamma particles were unaffected by a magnet and could pass through solid materials – it took a few centimetres of lead to stop them.

French-Polish scientist Marie Curie invented the term ‘radioactivity’ to describe these mysterious effects. Working with her husband Pierre she started to study the chemistry of the mineral called ‘pitchblende’ from which Uranium was extracted. 

She had noticed that pitchblende was actually more radioactive than purified Uranium. She inferred that pitchblende must therefore contain another unknown element even more radioactive than Uranium. Pierre and Marie dissolved large amounts of pitchblende in acid and using standard chemical techniques, by 1898 had extracted a black powder made up of a new element, 330 times more radioactive than Uranium. They called it Polonium, after the latin for Poland, her native country.

Being thorough, they also studied the liquid left after they had extracted the Uranium and Polonium from the Pitchblende. It was still radioactive! They realised the liquid contained another unknown element, but in really small quantities. They had to dissolve a lot of pitchblende to get the Polonium, but it was too expensive to get the amount they needed. 

They contacted a factory who extracted Uranium from pitchblende. The factory was piling up what was left once the Uranium was extracted, worthless to them but exactly what the Curies needed. In simple conditions – no more than a shed – they processed many 20kg batches of this waste, working on a much bigger scale. The work to extract the new element was complex and hard, but by 1902 they had enough to prove it was another new element, called Radium.

For her work on radioactivity, Marie Curie became the first person who has won two Nobel Prizes in different subjects, one in Physics, the other in Chemistry. Her work to isolate these new highly radioactive elements was vital for future research. Many of the future experiments we’ll describe used small samples of Radium to provide a strong source of radiation.

Exploring radioactivity

Radioactivity could not be explained by existing models of the atom, so new ones were needed. The old ideas of the atom as an unbreakable unit had to be discarded, as radioactivity involved breaking atoms into pieces. Physicists started to try and understand what things are within the atom. 

In 1897 English physicist J.J. Thomson was experimenting with putting high voltage across two metal plates in a vacuum tube. He saw a stream of particles passing between the plates and was able to show they were negatively charged and extremely light. He called these particles electrons – and showed they must have come from inside atoms.

Normal atoms are electrically neutral, so he speculated that the inside of an atom was a mixture of negatively charged electrons and unknown positively charged parts. He saw them as all mixed together, like pieces of fruit within a cake. This was called the ‘plum-pudding’ model of the atom. In 1899 Becquerel speculated correctly that the beta particles given off by radioactivity might be electrons travelling at high speed.

By 1901 the Curies, using material that had extracted themselves, performed important studies of the heat energy given off by radioactive elements. They showed that the heating never stopped, producing large amounts of heat over time. This was not explainable by existing physics. The ‘laws of thermodynamics’ state that energy can never be created or destroyed, just transferred. Heating a room by burning coal turns chemical energy into heat energy, but eventually the coal is used up and it stops. Radioactive heating was different and could not be explained at the time.

In 1903 Ernest Rutherford (a student of J.J. Thomson) revisited the arguments made by Lord Kelvin against a very ancient earth and showed that the geologists were correct. Slow radioactive heating from elements like Uranium over billions of years is what keeps the earth hot inside

What is radioactivity and what are its effects?

Most elements are very stable, but radioactive ones are not. Completely randomly a ‘parent’ atom breaks apart, turning into a different lighter ‘daughter’ element. The process of radioactive decay is messy, producing energy and three different types of particles which are fired out at speed. These are the three types of radiation, alpha, beta and gamma.

Radioactivity doesn’t break the laws of thermodynamics as there is a conversion of some matter into energy. Eventually all of a radioactive element decays and it turns into some other element. An element’s ‘half-life’ is the time it takes for half of it to decay. This can vary from seconds – for some very radioactive elements – to billions of years for Uranium and Thorium.

The radiation emitted travels until it hits something and stops, transferring its energy into what it hits. This can cause damage, particularly to organic molecules like those that you and I are made of. High levels of radiation damage tissues in a variety of ways and cause ‘radiation sickness’ that can be extremely dangerous. Even low levels of radiation can damage DNA and make cancers more likely. Beta and gamma particles are the lightest and cause the least damage, but travel the furthest. If radioactive elements touch or even enter the body – through the lungs or stomach – it’s the most dangerous as the body directly receives Alpha radiation which causes the most damage.

Radioactive elements are today tightly controlled, but that wasn’t always the case.

Radioactivity: discovery of uses and dangers

When they were first discovered, people used radioactive elements in many ways. Adding Uranium to glass gives it a beautiful green colour that was popular. Before it was properly understood, people claimed radioactivity was healthy and they sold radioactive waters as cures to various ailments. Radium is luminescent, meaning it glows in the dark, so following its discovery people found uses for it. Starting from 1917 was used as paint on watches in the USA. Slowly it was realised that the workers handling the radium were suffering a whole range of serious health issues. It took decades for the mostly female workers – called ‘the radium girls’ – to get compensation from their employer for the harm done to them.

The first known victim of radioactivity was Marie Curie. She and Pierre suffered from radiation sickness while processing pitchblende. They felt unwell and had burns on their hands from handling the highly radioactive material. Marie Curie herself died in 1934 from exposure to radiation. Her working papers – even her body – became highly radioactive and are now stored in a lead box for safety.

Understanding atoms

Scientists continued to study radioactivity. Small samples of radioactive elements were used as sources of radioactivity meaning hugely important experiments could be done on a table-top. In 1909 Ernest Rutherford placed a radioactive source of alpha particles in a glass tube from which air had been removed. Over time the vacuum filled with a gas formed from the alpha particles. He was able to show that this gas was the element Helium.

It was clear that radioactivity included pieces of atoms. Beta particles were negatively charged electrons and alpha particles the positively charged part of Helium atoms. In 1909 Ernest Rutherford designed an experiment to fire alpha particles at a thin piece of gold foil, using a detector to see what happened to them. Placing the detector behind the gold foil showed that most alpha particles passed straight through, as expected. Placing the detector in front of the foil brought a surprise – a few alpha particles ‘bounced off’ the gold foil. This experiment showed the structure of atoms. They are mostly empty space, which is why most alpha particles pass straight through. The ones that bounce back must have hit the main mass of an atom and this  part of the atom must be small, explaining why only a few hit it. This small mass in the middle is called the nucleus of the atom and things related to the nucleus are called nuclear.

By 1913 Rutherford and Niels Bohr, a Danish theoretical physicist had devised the  Rutherford–Bohr model of the atom. This model was a combination of experimental data and a part of theoretical physics called quantum mechanics. 

This combination of theory and practice continued, as scientists searched for the different parts of the atom. In 1920 the Proton was named. This is a fundamental particle within the atom that has a positive charge and a large mass. In 1932 the Neutron was named, a particle within the atom with neutral charge and large mass. 

What’s inside an atom? 

Atoms are a little like the solar system. At the centre is a massive object, the positively charged nucleus of the atom. Circling this massive object – like planets orbiting the sun – are the much smaller negatively charged electrons. Electrons are more familiar to us. Atoms sharing electrons are what form the chemical bonds that join atoms into compounds. Electricity is the flow of electrons between atoms. 

The nucleus is a mixture of neutrons and protons strongly joined together. The number of protons in an atom is the atomic number, as shown on a periodic table. It determines which element it is. To be electrically neutral, the number of protons is matched by the same number of orbiting electrons, which determines its chemical properties. 

A hydrogen atom is a single proton orbited by a single electron. But to be stable, larger nuclei need to include Neutrons, joined tightly with the Protons. The nucleus of Helium typically contains two protons and two neutrons. Atoms can contain different numbers of neutrons and still be the same element, they are known as different isotopes. Adding or removing Neutrons doesn’t affect the electrons so it’s the same element with the same chemistry. For example Helium is typically Helium-4 (2 protons and 2 neutrons make 4), but Helium-3 can also be found. It has 2 protons and only 1 neutron. 

Radioactivity is created when a nucleus spontaneously breaks up into smaller pieces. In natural radioactive decay, large fragments become new atomic nuclei (with fewer protons and neutrons) and smaller parts make up the Alpha, beta and gamma types of radiation. As part of the breakdown of the atom, some mass is converted into energy, which is why the particles all move extremely fast.

As an example, the natural radioactive decay of Uranium-238 creates alpha radiation and conversion of the atom in thorium-234. It’s also possible for Uranium atoms to split in two, to create two new atoms both much lighter than Uranium. Instead of a small piece coming out of the nucleus (like for natural radioactivity), the nucleus splits into two large pieces and some smaller ones. This process is called nuclear fission.

Discovery of nuclear fission

Scientists continued to find new sources of radioactivity and devise new experiments. Around 1934, Enrico Fermi, an Italian scientist, performed a series of experiments with a mixture of radon and beryllium that was a source of neutrons. He discovered that by firing neutrons at 22 different elements he could make them radioactive. The neutrons were added to the nucleus, causing it to become unstable and so break apart.

Running the experiments, they noticed that the same experimental apparatus transformed elements more quickly when placed on a wooden table than if sat on a stone surface. It seemed the material beneath was having some effect on the neutrons passing between the source and the target – perhaps some were passing through the table and bouncing back up to the target.. Speculating that the speed of the neutrons was affected by the material they passed through, they put paraffin wax directly between the source of neutrons and the target. The effect was dramatic –  the neutrons were slowed down by the paraffin and became much more effective at changing the target material. To change an atomic nucleus, a neutron needs to come close and   become joined to it. If it is travelling fast, it is less likely to do so. Think of meteorites speeding close to planets – if they are travelling fast then they speed right past even if they get close. If  humans send a probe to a planet, they usually slow it down as it gets close to ensure it is ‘captured’ by the planet by going into orbit or landing on the surface. Slowing neutrons has a similar effect, they are more likely  to be captured by an atomic nucleus than if they are moving  faster. 

In 1938 German scientists firing neutrons at Uranium found some odd results. They had bombarded Uranium with neutrons and observed the element Barium produced as a result. This was odd as Barium had 40% less mass than Uranium. Normal radioactive decay never causes such a dramatic drop in mass from parent to daughter element. Lise Meitner working with her nephew Otto Frisch, interpreted this as an example of a nucleus splitting into two, something she called nuclear fission. The neutrons hadn’t just caused the Uranium nucleus to become unstable, but had made it split into two large pieces: Barium and the gas Krypton. This was something new. 

Mass into energy

The mass of a nucleus is always less than the total mass of the individual neutrons and protons that it formed from. For example, an alpha particle is two protons and two neutrons joined together, but it’s mass is 0.8% less than the mass of those four particles measured as individual protons and neutrons. The difference is because of something called nuclear binding energy. This energy is required to keep the nucleus together, as the protons want to move apart due to their electric charge. Things are strange in the nucleus, it’s a place where energy and mass are interchangeable, as shown by the Einstein equation:  

E = mc²

Where E = energy, m = mass and c is the speed of light. 

The speed of light is very fast – meaning only a tiny amount of mass converts to a huge amount of energy. If a nucleus changes, like in radioactive decay or fission, there will be a change in the required nuclear binding energy and so in the mass of the particles before and after. If the total mass afterwards is smaller, then the difference will be converted into energy. 

The amount of energy released varies a lot. Think of the amount of heat released by burning wood or coal in a fire, a typical chemical reaction. Using SI units, the amount of energy released per amount of material changed is around 400 kJ/mol. That’s enough to heat a room on a cold day.

For normal radioactive decay, heat is released, as Marie Curie first described. The total energy released is about 10,000,000 kJ/mol. This is a huge amount more, but remember this is the amount released after all the radioactive element has decayed. Coal releases its chemical energy quickly but this energy release from radioactive decay may be spread out over billions of years. This is one of the (many) reasons we don’t heat our rooms by simply putting radioactive elements in it.

For a fission reaction compared to radioactivity, about a hundred times more energy is released, measured in billions of kJ/mol. Unlike radioactivity, we can control the speed of fission reactions. The rest of this magazine will describe the many ways people have sought to harness the incredible potential of this source of energy.

From theory to practice

Natural radioactivity is slow and can’t be controlled. The Uranium deposit in Gabon created heat because neutrons slowed by water created nuclear fission reactions, but this was an unusual occurrence. However the experiments in the 1930s showed that it was possible to artificially cause nuclear reactions. Of course people speculated how this could be a  new source of energy, now reactions could be controlled. Very few imagined how quickly and dramatically war would turn these theoretical discussions into real and practical matters of life and death.