Spread the love

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 Curie 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 ₀n¹ 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.

$\displaystyle {}_{0}{{n}^{1}}~\text{ }~\text{ }~\to ~\text{ }~\text{ }~{}_{1}{{H}^{1}}~\text{ }+~\text{ }{}_{-1}{{\beta }^{0}}~\text{ }~+~\text{ }\bar{\nu }$

In 1935, Sir James Chadwick received the Nobel Prize in physics for discovery of neutron.

Properties of Neutron