Bubble chamber picture demonstrating existence of antineutron
The dark lines
in this picture are produced by charged particles as they force their way
through the bubble chamber liquid. At the arrowed point, an antiproton
beam particle 
that
has entered from the top of the picture and annihilated with a proton 
to
produce a neutron 
and
an antineutron 
.The
continues
(see dotted line) and itself annihilates with a proton producing
a 'star’ of
5 tracks with a total energy consistent with the rest mass of an and
a proton being
converted into energy.
For a full description of this event,
click here.