The picture above shows the
apparatus used in this experiment. Briefly:
The two photomultipier
(PM) tubes are facing each other directly
with a radioactive source (a beta
emitter) between them
When the radioactive
source undergoes a beta decay, it will
emit a positive electron (known as a
positron) and a neutrino
The positron quickly
annihilates with an electron producing 2
gamma rays
In order for momentum to
be conserved the two gamma rays must
travel in opposite directions (at same
speed and carrying same energy)
The electronic pulse
counters for PM tube 1 and PM tube 2 will
register counts each time a gamma ray
strikes the respective PM tubes
The coincidence counter is
setup so as to register only those counts
that occur simultaneously in PM 1 and PM
2
Thus each count on the
coincidence counter represents an
annihilation event
Recording the count rate
when the PM tubes are directly facing
each other and then at when they are at
increasing angles to each other we see
that the count rate drops off
dramatically with increasing angle
This shows conclusively the
presence of antimatter!
