Discovery of neutron

Neutron was discovered by Chadwick in the year 1932.

When beryllium nucleus was bombarded with alpha particles of energy 5 MeV emitted from polonium, carbon was formed along with highly penetrating radiation known as beryllium radiation.

James Chadwick had subjected Nitrogen and Argon to the Beryllium radiation. He concluded that the new particles have no charge and have mass roughly equal to the mass of proton.

He named this neutral particles as neutrons.

He represented this nuclear reaction as follows

_{4}^{9}\textrm{Be} + _{2}^{4}\textrm{He} \xrightarrow{\hspace*{1cm}} \begin{bmatrix} _{6}^{13}\textrm{C} \end{bmatrix} \xrightarrow{\hspace*{1cm}}_{6}^{12}\textrm{C} + _{0}^{1}\textrm{n} \;+ Q

Procedure :

  •  A Schematic diagram of Chadwick’s apparatus is shown in figure. The \alpha – particles were emitted from the source ‘S’ (Polonium) and were allowed to bombard the Beryllium layer.
    Discovery of Neutron
  • The particles coming out from the Beryllium were allowed to be incident on paraffin block. It emits high energy protons which were then allowed to pass through an ionization chamber. ‘I’ through a window ‘W’.
  • Hence the ionization current due to protons was amplified and measured with help of an electrometer. Thus the neutron was discovered by Chadwick.

Properties of neutron :

  • It is an uncharged particle and hence the electric and magnetic fields have no effect on it.
  • Being neutral particles, it posses very high penetrating power and has very low ionization energy.
  • It is stable inside the nucleus and unstable outside the nucleus. Its half-life period is about 1000 sec.
  • The speed of the neutrons can be slowed down by passing them through heavy water, paraffin wax, graphite which are used as moderators in a Nuclear reactor.
  • The mass of Neutron is nearly equal to that of a proton (1.0748 \times 10^{-27} \;kg)