YUKAWA THEORY (FULL EXPLAIN)

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

s the separation between two nucleons.

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.