Building a new Cosmic Ray Detector

For the past five years I have been fortunate to have a QuarkNet cosmic ray muon detector to use for teaching and for research with students. It’s a fun piece of real particle physics hardware, and lets me and my students go hands-on with the same muon-tracking tech we see at Fermilab, CERN, and other facilities.


QuarkNet Series 6000 CRMD. Photo: QuarkNet/Fermilab


One of the limiting factors preventing more teachers and students from using these detectors is their high cost. Without QuarkNet making these available for us to borrow, these detectors would cost us several thousand dollars each.

In the past few years I’ve seen projects to detect cosmic rays in a variety of ways, notably CRAYFIS and DECO that use the silicon imaging chip in your phone’s camera when the lens is covered or in a very dark environment. This method uses techniques similar to the silicon detectors like the pixel and strip trackers in the CMS detector at CERN. One disadvantage is the tiny area and volume of that phone sensor chip, which probably intercepts less than one muon per minute.

A more promising route for classroom use is the MIT detector, called the Cosmic Watch. This project uses inexpensive hardware to bring higher level, scintillator based cosmic ray muon detectors to teachers and students for a target price of $100, about 98% less than the absolute cost of the more capable QuarkNet detector.


Cosmic Watch CRMD. Photo:

I’m in the process of acquiring the components to build a Cosmic Watch, starting with the silicon photomultiplier from SensL in Ireland. They were kind enough to offer the preferred detector chip in small quantities to students and teachers for a discount┬áprice of $60 with available $15 shipping (no word on how long they will run this discount).


Mounted SenseL SiPM. Photo: SenseL

The MIT Cosmic Watch project website and Git repository include all of the information you need to build the detector, including source files for having the PCBs produced, dimensions for the scintillator, parts lists, and instructions for assembly and use. [2/20 Note: The newest version, which includes microSD capability and coincidence capability with two units and other revisions, can be found at the v2 Git repository here.)

Future posts here will discuss the similarities and differences between the QuarkNet and Cosmic Watch detectors, including sensitivity, data collection process, and coincidence signal detection.

Right now I’m simply going to build one for myself to experience the build and learn more about it. I’m thinking maybe local teachers would be interested in a build workshop to put together, test, and take home their own Cosmic Watch. Watch here for updates — I anticipate building this over spring break (March 17-25).