Multiple Simultaneous Cosmic Ray Muons

Purpose

The primary incoming cosmic rays (mostly protons) interact with air molecules high up in the atmosphere, about 10 km up. These interactions result in an extensive air shower consisting of an extremely large number of particles, mostly pions. Pion are bounds states of a quark and an anti-quark. They are the lowest mass particle that contains quarks, about ten times less massive than a proton or neutron, which is why they are the most plentifully produced. The charged pions live for only about 10 ns decaying to a muon and a neutrino. The muon is exactly like an electron except that its mass is about 100 time larger. Like the electron, it is an elementary particle. Since it doesn't contain quarks, it doesn't interact strongly but only by the electromagnetic interaction. As a result, most of the muons reach the ground before interacting. These are what are detected by your detector paddles.

One primary cosmic ray will lead to a large number of muons reaching the ground. They will be spread out over an area approximately 1 km in diameter. These muons will hit the ground more or less simultaneously. Because of the difference in heights at which they are produced and their different path lengths to reach the ground, the arrive times will be spread out over a few micro seconds. By arranging, the detector paddles horizontally, we should find that two, or more, of them will be in coincidence some small fraction of the time. By determining this fraction, you will get a measure of the typical density of muons resulting from an extensive air shower.

Procedure

Make two stack of detector paddles and measure the rate at which cosmic ray muons hit the two stack 'simultaneously.".

1. Make two vertical stacks of two detectors each. Try to make sure the areas overlap as much as possible.
2. Run the "multifold" experiment.
3. Set the following configuration.

Delay Time:	1.0 μs
Gate Width:	2.0 μs
Channel Enable:	1, 2, 3, 4
Coincidence Level:	4

4. Count for ten minutes or longer and note the number of times all four detectors have hits.
5. Determine the ratio of two muon hits to one muon hit. What fraction of time when you detect one muon do you detect two?
6. Repeat the measurement outdoors. Does the rate of two muon hits change? Why?
7. Try varying the horizontal spacing of the two stacks. Plot the rate of two muon hits as a function of the spacing between the stacks

Questions

• How large is the background?
• That is, given that the rate of cosmic ray muons hitting the ground is about 10 per second, what is the probability that two cosmic ray from independent showers will hit the detectors in the 1 micro second window?
• How large is this rate compared to your observed rate of two simultaneous muons?