YUKAWA THEORY
In order to explain the origin of
strong nuclear forces which are believed to exist in between nucleons, Yukawa
in 1935 proposed a theory of nuclear forces known’s as the Meson theory of
nuclear forces. He predicted the existence of particle having rest mass
intermediate the rest masses of an electron and proton and named as Meson.
According to Yukawa’s theory the
force between two nucleons is due to the continuous rapid exchange of mesons
between them as the electric force between the two charged particles is due to
the exchange of photons. The most important particle of the nuclear force is
that it has finite range. It decreases extremely rapidly when interacting
particles are more than one Fermi apart. Experiments show that in practice
there is a critical length beyond which this interaction does not exist. In
this respect a nuclear force differs fundamentally from a force with infinite
range such as the electric force. We must therefore expect that the dimension
of length. Such a potential was first proposed by Yukawa which has the form
Yukawa potential
Where g is positive constant and r i
Yukawa explained on the theoretical
ground that for a finite nuclear range 10^-13 cm
The rest mass of meson should be 200
times the rest mass of an electron. The first particle of intermediate mass to
be detected was u meson with rest mass 200 times the rest mass of electron. The
u meson was found to have weak interactions with nuclei and did not
Agree with Yukawa theory that mesons
have strong nuclear interactions with nuclei.
This problem was solved with the
discovery of pi-mesons (pions) in 1947. The pions were found to have strong
interactions with nuclei and they are believed to be Yukawa’s particles. The
nuclear force is caused by the continuous rapid exchange of pions between the
nucleons. The pions exist in three states pi-positive, pi-negative, pi-neutral.
The rest mass of charged pion is 273me and that of neutral pion is 264me, where
me is rest mass of electron. The exchange reactions involved in nuclear forces
are
Which shows that pi-positive and
pi-negative are exchange particles of p-n
And n-p forces while pi-neutral is
exchange particle for p-p and n-n forces.
