Re: how is spontaneous creation of particles from gamma-rays done?

Dear James,

A massless gamma ray photon traveling through vacuum will never decay
into other real particles.  The reason is that a massless initial state
such as a gamma ray photon can not simultaneously conserve both energy
and momentum in a decay to two or more particles.  Photons are stable.

But gamma ray photons do "pair convert".  This must take place near the
nucleus of some atom.  The gamma ray and the nucleus can interact (because
the nucleus is charged) and the nucleus can recoil carrying just enough
kinetic energy and momentum to allow the conservation laws to hold in
a reaction such as

    gamma ray + nucleus  -->  particle + antiparticle + nucleus

If the recoil conditions are not just right, then no new particles are
created and the gamma ray simply Compton scatters off the nucleus.

Here is an example of a well-known particle decay involving gamma
ray conversion which has been documented numerous times both in old
bubble chamber photographs and in modern computerized detectors:
A neutral pion is unstable and most of the time will decay
into two photons.  These photons can each interact with the nucleus of
an atom of the detector material and out of the collision can emerge
an electron and a positron (an antielectron).  The neutral pion and the
neutral gamma ray photons do not leave tracks in the detector, while
the charged electrons and positrons do leave tracks.  If both gamma
ray photons pair convert, then the tracks look like two V's pointing
back to the location of the original neutral pion.


               \                             /
                \                           /  +
               - \                         /  e
              e   \                       /
                   \                     /
                    \                   /
                     \                 /
             +        \               /          -
            e          \             /          e
      ------------------             --------------------


                                                                           
Of course, gamma ray photons can convert, in the field of a nucleus,
into other particle-antiparticle pairs besides electrons and positrons
if the energy of the incident gamma ray is high enough.  But electrons
and positrons are most common because of their relatively small mass.


--Randall J. Scalise    http://www.phys.psu.edu/~scalise/