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Distribution of Cosmic Rays

We can measure the direction of the muons that reach the Earth's surface. We need two counters. If we require both counters to fire at the same time, the path of the muon must have gone through both detectors. Therefore we know approximately what direction the muons came from. Measure the rate of muons as a function of the angle of the two detectors with respect to the vertical.

Muon Angular Distribution

It is much harder to tell the difference between upward going and downward traveling muons. It turns out that almost all muons are coming from the upper atmosphere and very few come from, or through, the Earth. But to measure that ourselves, we must be able to define a coincidence over a short enough time so that hits from the top and then the bottom counter are in coincidence but if the hits appear on the bottom and then on the top, we exclude them. The muons move at near the speed of light (3x108 m/s), so if the counters are 30 cm apart, the time between the two hit is Dt = distance/speed = 0.3m/(3x108 m/s) = 1x10-9s. The muon pulses can not be located in time better than a few ns (ns = 10-9s). So this is hard! We could not accomplish this with the first version of the DAQ card (DAQ1). But a second version DAQ2 was designed at Fermi and University of Washingtion with better time capability, that has become the standard QuarkNet readout card throughout the world.