Bubble Chamber Relativity    

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In this picture a beam of K^- particles (moving from bottom and upwards) is fired against the liquid contained in a closed chamber (called a "bubble chamber").

The bubble chamber is a detector because the electrons of the atoms in the liquid interact with the charged particles entering the chamber , producing bubbles that make the particle tracks visible (the liquid boils along the tracks!).

The protons in the liquid act as targets.

If you want to know more about bubble chambers, click here