experimental physics - Explain background pattern in particle tracing image

I'm trying to understand this image of a particle tracing experiment (which can be found all over the net if you google for "bubble chamber"):

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There are two things that I can't figure out:

1. 
   

   The background has some kind of pattern made up of irregularly sized, semi-rectangular patches. Where does that come from?

   
   


2. 
   

   There is a vertical line of 4 circles in the middle of the image, each marked with an X. These seem to be part of the setup, but what is their function? Also, the pattern in the background in these circles is not continuous w.r.t. to the outside pattern, although the traces are. Why?

   
   

In addition, if anyone has the original source or more information about that particular experiment then that would be very welcome, too.

EDIT: It seems that the image stems from a 1980 Fermilab experiment.

Answer

Have you read about the construction and operation of bubble chambers? They are large vessels (with at least one transparent side) and work by rapidly reducing the pressure in the contained fluid so that it is superheated for a time. Doing that requires plumbing; both for filling the chamber with working fluid, and for arranging the pressure drop.

As the working fluid is often cryogenic it was also probably kept in circulation so that it could be periodically re-cooled, calling for more plumbing.

As the working fluid is chosen to be transparent except where bubbles nucleate, you can see through to whatever occupies the far side chamber. (The figure is a negative image so the light background of the image represents a dark background space.)

I would not be surprised if those neat, uniform circles all in a line represent the plumbing in the vessel, and the rest of the structure in the background the panels and welds of the chamber.