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Chapter: 11th 12th std standard Class Physics sciense Higher secondary school College Notes

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Neutron - Discovery and Properties of neutrons

In 1930, two German Physicists Bothe and Becker found that when beryllium was bombarded with α -particles, a highly penetrating radiation was emitted. This radiation was capable of traversing through a thick layer of lead and was unaffected by electric and magnetic fields. At that time, the radiation was thought to be γ−rays.

Neutron - Discovery

 

In 1930, two German Physicists Bothe and Becker found that when beryllium was bombarded with α -particles, a highly penetrating radiation was emitted. This radiation was capable of traversing through a thick layer of lead and was unaffected by electric and magnetic fields. At that time, the radiation was thought to be γ−rays.

 

In 1932, Irene Curie and F. Joliot found that those radiations were able to knock out protons from paraffin and similar substances that are rich in hydrogen. Chadwick in the same year discovered that the emitted radiation consists of particles of mass nearly equal to proton and no charge. He called them as neutrons. The above reaction can be written as

 

4Be9 + 2He4 6C12 + 0n1

 

where on1  represents neutron.

 

Properties of neutrons

 

1.     Neutrons are the constituent particles of all nuclei, except hydrogen.

 

2.     Neutrons are neutral particles with no charge and mass slightly greater than that of protons. Hence, they are not deflected by electric and magnetic fields.

 

3.     Neutrons are stable inside the nucleus. But outside the nucleus they are unstable. The free neutron decays with an emission of proton, electron and antineutrino, with half life of 13 minutes.

 

                                                               i.      0n1 1H1 + 1e0 + ν

4.     As  neutrons  are  neutral,  they  can  easily  penetrate  any nucleus.

 

5.     Neutrons are classified according to their kinetic energy as slow neutrons and (b) fast neutrons. Both are capable of penetrating a nucleus causing artificial transmutation of the nucleus.

 

Neutrons with energies from 0 to 1000 eV are called slow neutrons. The neutrons with an average energy of about 0.025 eV in thermal equilibrium are called thermal neutrons.

 

Neutrons with energies in the range between 0.5 MeV and 10 MeV are called fast neutrons. In nuclear reactors, fast neutrons are converted into slow neutrons using moderators.

 

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