Pair production- Why do the particles spiral away from each other? So I have read that when the particle-antiparticle pair are produced, they spiral in opposite directions. I have also read that a magnetic field is present causing them to spiral. I understand why they would spiral as they experience a force due the field through $F=Bvq$ but my question is, why is there a magnetic field? What creates it? 
And also, why in this image is one spiral much larger than the other? As if they have the same energy then there velocities will be the same and hence, follow the same path as $r=\frac {mv}{Bq}$.
 A: This shows the scatter of a gamma on an electron in the atoms of the bubble chamber, ejecting it (called a spectator) and at the same time the gamma has enough energy to generate an e+e- pair.

why is there a magnetic field?

So that one can use bqv=mv^2/r and find the momentum of the particles and the charge. Particularly for e+e- the opening angle of the interaction gamma-->e-e+e- is so small that they would not be separable in the bubble chamber photo you show.

What creates it? 

Experimenters so that they can find the momentum. Actually it is a large team of engineers and applied physicists that built the bubble chambers and the magnetic field enclosing them.
The magnets in LHC detectors have huge magnetic fields because the momenta are very large.
They spiral because they are loosing momentum by ionizing the bubble chamber medium, and they are opposite spirals/curvature because they have the opposite charge.The one on the left is an electron because it has the same curvature as the spectator electron.
Here is a cloud chamber photo of cosmic rays  without a magnetic field applied

Low momentum tracks just get deflected and wiggly by scatters. Higher momentum ionize and are straight. Without a magnetic field the two tracks in your image would continue straight and in this case on top of each other and the spectator because the opening angle of what would look like a V is very small for e+e- creation.
