Neutron, the third “elementary” particle was object of a long hunt. Just from Mendeleyev table one could deduce that atomic change by a unit but sometimes by more or even by a fraction. The mass of one mole of nitrogen molecules is 28 g, of oxygen 32 g while that of chlorine 71 g. With the advent of mass spectroscopy by Aston in 1920 and first measurements on neon it was clear that the same nuclei can bring the same electric charge but different masses. A heavy particle, like proton, but electrically neutral was missing.
Some experiments tried to produce neutrons from protons collapsing with electrons, but failed. Two teams, a German one (Walther Bothe and Herbert Becker) and a French one (Irene Joliot-Curie and her husband, Frederic) were close to the Nobel prize on the neutron. Bothe and Becker in 1930 noticed that beryllium emits highly penetrating radiation while irradiated with alpha particles from polonium. Mrs and Mr Joliot observed emission of high energy protons, if on the path of this mysterious radiation a paraffin block was placed. But both teams explained the effects by the well known gamma rays.
Original Chadwick's instruments for studies of neutrons [Deutsche Museum, Munchen]
Only James Chadwick, a collaborator of Hans Geiger and a civilian prisoner in Germany during the 1stWW did the experiments carefully: he put beryllium and polonium into a vacuum chamber and observed reactions of the mysterious radiation with different atoms. He also determined the neutron mass: 1.0067 time the mass of the proton. Soon he got the Nobel prize. In 1940, with Jozef Rotblat, his Polish student, flew to the USA, to work on the Manhattan project.
In 1930, the German physicists Walther Bothe and Herbert Becker noticed something odd. When they shot alpha rays at beryllium (atomic number 4) the beryllium emitted a neutral radiation that could penetrate 200 millimeters of lead. In contrast, it takes less than one millimeter of lead to stop a proton. Bothe and Becker assumed the neutral radiation was high-energy gamma rays.
Marie Curie's daughter, Irene Joliot-Curie, and Irene's husband, Frederic, put a block of paraffin wax in front of the beryllium rays. They observed high-speed protons coming from the paraffin. They knew that gamma rays could eject electrons from metals. They thought the same thing was happening to the protons in the paraffin.
Chadwick said the radiation could not be gamma rays. To eject protons at such a high velocity, the rays must have an energy of 50 million electron volts. An electron volt is a tiny amount of energy, only enough to keep a 75-watt light bulb burning for a tenth of a trillionth of a second. The alpha particles colliding with beryllium nuclei could produce only 14 million electron volts.
The law of conservation of energy states that energy can neither be created nor destroyed. It certainly looked as if energy was being created along with the neutral radiation.
Chadwick had another explanation for the beryllium rays. He thought they were neutrons. He set up an experiment to test his hypothesis.
Chadwick put a piece of beryllium in a vacuum chamber with some polonium. The polonium emitted alpha rays, which struck the beryllium. When struck, the beryllium emitted the mysterious neutral rays.
In the path of the rays, Chadwick put a target. When the rays hit the target, they knocked atoms out of it. The atoms, which became electrically charged in the collision, flew into a detector.
Chadwick's detector was a chamber filled with gas. When a charged particle passed through the chamber, it ionized the gas molecules. The ions drifted toward an electrode. Chadwick measured the current flowing through the electrode. Knowing the current, he could count the atoms and estimate their speed.
Chadwick used targets of different elements, measuring the energy needed to eject the atoms of each. Gamma rays could not explain the speed of the atoms. The only good explanation for his result was a neutral particle.
To prove that the particle was indeed the neutron, Chadwick measured its mass. He could not weigh it directly. Instead he measured everything else in the collision and used that information to calculate the mass.
In above demonstration, you saw that if you shot a copper penny into a target, one of the target pennies would fly out. If you knew what your target was made of, but did not know what sort of projectile was hitting it, you could calculate the mass of the projectile from the speed of the target particle.
For his mass measurement, Chadwick bombarded boron with alpha particles. Like beryllium, boron emitted neutral rays. Chadwick placed a hydrogen target in the path of the rays. When the rays struck the target, protons flew out. Chadwick measured the velocity of the protons.
Using the laws of conservation of momentum and energy, Chadwick calculated the mass of the neutral particle. It was 1.0067 times the mass of the proton. The neutral radiation was indeed the long-sought neutron.
Neutrons due to their penetrating properties can be used to see interior of different objects. Neutrons are stronger scattered on lighter atoms than on heaviers. Area strongly scattering neutrons are darker. You can see how water is travellin through the flower, pollution inside vacuum pump, construction of weapons and bullets. There is also shown the difference between wet and dry sponge.