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Detector Information

Muon Detectors
Contents of Set-ups

More Information

Data Acquisition
Data Analysis

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How the muon detectors work

As cosmic rays hit the upper atmosphere, they interact with nuclei in the atmosphere. Muons, which are one of the decay products of that interaction, travel through the atmosphere to the surface of the earth at close to the speed of light.

The muon detector paddles have several components: first, a scintillating plastic that emits light when exposed to high-energy particles (muons, photons, electrons, etc.); next, a plastic light guide to direct the light from the scintillator to the photomultiplier tube (PMT); then a PMT where a photon initiates a reaction that produces about 100,000 electrons, thereby creating an amplified electrical signal; and finally, a base which supplies the PMT with power. This apparatus is covered with two layers of thick paper, white on the inside to reflect light back into the paddle and black on the outside to prevent any visible light photons from entering the system.

When a high-energy particle contacts the scintillator, a signal is generated that will travel from the base, through an interface board, to the computer that displays your results. When doing experiments, you will always use at least two paddles, because a single paddle will sometimes give off signals for things that aren’t muons. You are looking for a coincidence, which occurs when there is a signal from two or more paddles within a few hundred nanoseconds. This is such a short time because the particles are traveling at high speeds. A coincidence almost always represents a muon that has traveled a path intersecting both paddles.

Detailed explanation of how the detectors work

What your detector set-up includes

Each set-up loaned to a participating teacher includes the following:

Quantity Description
(1) Laptop PC running LINUX OS
  • Detector Paddles
    • plastic scintillator
    • light guide
    • P.M.T.
(1) Board and Base Power Supply

Data Acquisition Board

  • Schematic diagram
  • Layout
  • Photo


  • (1) PC serial cable
  • (3) Green signal cables
  • (3) Daisy chain voltage cables
(3) Detector support brackets


How to assemble your detector set-up

1) Plug in the power supplies to the computer and the data acquisition board

2) Connect one end of the serial cable to the port on the computer and the other to the data acquisition board

3) Now turn on the computer (serial cable must be connected before booting!)

4) Connect green signal cables from each paddle to the front of the DAQ

5) Connect a power cable from the DAQ to a paddle, then use two more cables to "daisy chain" the power from the first paddle to the second and from the second paddles to the third.

6) Verify that the bases of the paddles are set at the correct voltage and then turn them on!

Detailed detector assembly instructions (with pictures!)

Advanced Detector Operation Instructions

Instructions for optimizing the operating voltage The paddles have been individually tested before being packaged in a set-up and loaned to teachers. They have each been labeled with the voltage at which they best perform. Should the singles rates differ significantly from the singles rate on the label, check for light leaks and verify that the voltage dial has not been unlocked. Then, you may want to follow this step-by-step procedure for determining the best voltage to set each photomultiplier tube. You may also call the PARTICLE Fellow.

Instructions for checking the counting efficiency The paddles sent out to teachers run at >95% efficiency. This is a step-by-step procedure for checking the efficiency of a single "test" paddle with three "good" paddles. The efficiency test is a comparison of the count rate of muons passing through all four paddles and the count rate of muons passing through all three "good" paddles but not the test paddle. A diagram of the proper configuration of the paddles is included.

Instructions for the electronics (pdf) This document explains how to connect power, connect the serial cable, connect the signal cables from the counters, use the SimpleDaq program, and adjust thresholds. It also includes a note about grounding.


Department of Physics and Astronomy, University of Rochester, PO Box 270171, Rochester, NY 14627-0170, (585)-275-5306