Discovery of Neutron
Discovery of Neutron :-
Rutherford had discovered the nucleus in 1911, and had observed the proton in 1919. However, it seemed there must be something in the nucleus in addition to protons as there exist some atoms of the same element, which exhibit the same chemical properties, but differ in mass(called Isotopes).
For example, hydrogen was found to have three types of nuclei with mass ratio 1:2:3 (called hydrogen, deuterium and tritium, i.e., 1H1,1H2 and 1H3).
Therefore, the nuclei of deuterium(1H2) and tritium(1H3) must contain, in addition to a proton, some neutral matter(because atom as whole is neutral).
In 1928, a German physicist, Walter Bothe, and his student, Herbert Becker, bombarded beryllium(Be) with alpha particles emitted from polonium and found that some type of rays are emitted which have high penetrating power but low ionizing power and are electrically neutral (not deflected by electric and magnetic fields), which they interpreted to be high-energy gamma photons.
These rays were found to have energies about 7Mev(estimated the value of energy by the absorption of these rays by lead sheets of different thickness). But this value of energy is more than the energy of gamma-rays. So they cannot be γ-rays.
In 1932, Irene Joliot-Curie(one of Madame Curie’s daughters) and her husband, Frederic Joliot-Curie, decided to further investigate Bothe’s penetrating radiation.
They found that these rays emitted from beryllium do not have ionizing power (no ionization in the ionization chamber held in front of rays emitted from beryllium) but when paraffin wax is held between the rays and ionization chamber, they showed high ionizing power.
This discovery was amazing because γ-ray photons have no mass.
Madam Curies interpreted the results as the action of γ-ray photons on the hydrogen atoms in paraffin. She thought that this is because of Compton Effect, in which photons impinging on a metal surface eject electrons.
But there was a problem, as proton is 1,836 times heavier than electron and γ-ray photons do not have enough energy to knock out protons from paraffin. Also the energy of protons ejected from paraffin wax was found to be 4.5 MeV so if we consider rays emitted from beryllium to be γ-ray photons, there energy must be 55 MeV to eject protons from wax.
This problem was solved by James Chadwick in 1932. He repeated the experiments at the Cavendish Laboratory in Cambridge, England. He proved that the beryllium emissions are not γ-rays but are neutral particles with a mass approximately equal to that of the proton. He named these particles as neutron. Neutron is represented by 0 n1 because its atomic number is zero (no charge) and atomic mass is 1u.
Chadwick represented that bombardment of α-particles on beryllium by the following equation…
4Be9 + 2He4 → 6C12 + 0n1
Neutron is stable inside the nucleus but it is unstable outside the nucleus and decays into a proton, an electron and an antineutrino (another elementary particle) and has a mean life of about 1000s.
In 1935, Sir James Chadwick received the Nobel Prize in physics for discovery of neutron.
Properties of Neutron
(Discovery of Neutron)
1. These are neutral particles because they are not deflected by electric and magnetic fields.
2. As they are changeless, their ionizing power is zero.
3. Mass of neutron is 1.675 x 10-27 kg(slightly heavier than proton). It is stable in the nucleus.
4. Neutron is not stable outside the nucleus. It converts into proton by emitting β-particle(fast moving electron) and another particle antineutrino.
5. It has a mean life of about 1000s(about17minutes).
Uses of Neutron
(Discovery of Neutron)
- Neutrons are used in Artificial radioactivity.
- Thermal neutrons are used to study crystal structure.
- Thermal neutrons are used in nuclear fission.
- Neutrons are used in biology and Medical Science. Neutrons are used in the treatment of cancer.
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