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Pair Production
Making Matter and Antimatter
A gamma photon with enough energy can spontaneously produce a
particle and its antiparticle.
The minimum necessary energy that a photon must carry to
produce an electron/positron pair is equal to the combined mass of
the pair.
0.51MeV
Any extra energy appears as the kinetic energy of the particles
involved.
0.51MeV
The minimum energy of the gamma photon is
2 x 0.51MeV= 1.02MeV
Conserving momentum
In any process momentum has to be conserved.
Momentum = mass x velocity
Here the momentum of the
loaded cannon is zero
The mass of the
cannon ball x its
velocity
The mass of the cannon
x its velocity
These quantities will add to zero
Linear momentum
Between snooker balls the momentum is always conserved
Total mass x
velocity before the
collision
=
Total mass x velocity after
the collision
This happens when nuclei emit particles
We have seen that a gamma photon with the right energy can
produce a pair ( a particle and its antiparticle). If this was to happen
as shown below momentum could not be conserved.
Pair production galways involves a target nucleus to conserve
momentum.
The target nucleus plays no other role
N
+ve charge
-ve charge
S
Detection chamber showing
a perpendicular magnetic
field.
Charged particles follow a curved
path depending on the charge
The curvature of the path depends on
the mass and velocity of the particle
Often electron tracks look
like this
This has the same curvature as the
particle track above, but this particle
has greater mass