you could use a set of "light curtains". they are LED sensor / receiver pairs in various lengths that are made for e-stop applications. Usually they e-stop when an operator puts arms or hands thru the curtain (like on a press) but you could use 3-4 lengthwise (to form a box) to do the e-stop when a tube breaks it.
http://www.ab.com/safety/prod_direc...ardshield_safe_light_curtains/type2.html
