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
Neutrons are the constituent particles of all
nuclei, except hydrogen.
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.
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.
0n1 → 1H1
neutrons are neutral,
they can easily
penetrate any nucleus.
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.