Demonstration of the Existence of the Antineutron

This picture, taken in the Berkeley 30-inch propane bubble chamber in 1958, demonstrates the existence of the antineutron – the antiparticle of the neutron .

At the point marked by the arrow, an incoming antiproton beam particle undergoes a 'charge exchange' reaction:

The kinetic energy of the interacting is estimated to be about 50 MeV.

The antineutron formed in the process travels an actual distance of 9.5 cm before annihilating in a characteristic 'annihilation star' which is consistent with the process

The visible energy released in the star is about 1500 MeV, which is approaching the amount expected when the rest mass of an antineutron and a proton is 'converted' into energy. The remaining energy may be assumed to have been carried off by netral pions ().

For full details click here.