How do we know the direction from which cosmic rays arrive at the Museum?

As we saw in the previous question, the tubes are mounted in coincidence, i.e. only if a pair of tubes registers a signal within a very short time interval (1), that signal is recorded with its time stamp and we attribute it to a particle that has passed through the detector in the direction joining the two tubes:

_{Fig B3-1 cross-section of two tubes}

_{Cross-section of two tubes crossed by a particle. Image: Wearbeard}

However, the tubes are not points - two points define a line - but are finite in size, so particles can come from a different set of directions, larger the closer together the tubes are, and leave a coincidence signal in both. Graphically:

_{Fig. B3-2 Front and side views of two detectors showing schematically the directions from which a particle can come and pass through the two Geiger tubes.}

This severely limits the directionality of our detector, but if we were to measure directions more accurately by separating the tubes much further apart, we would have far fewer coincidences, which is not desirable in a museum; we want no more than a few seconds to pass without an event.