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19 July, 2002

I brought a portable cosmic ray detector to River Falls ! And we (maybe) broke it - while making it better ! Has that ever happened to you ?

The detector is a simple "lightweight" design by Howard Matis of Lawrence Berkeley Labs. To register a count, it relies on coincidence : a simultaneous "hit" in each of 2 individual detectors. Since this type of detector is noisy, firing occasionally by itself, 2 detectors are used in coincidence to help eliminate random noise.

The detector uses a piece of plastic doped with a fluorescent molecule. High energy parcticles stream though the plastic at about 10 Hz. These are usually muons, a kind of high-energy electron created in the Earth's atmosphere by protons from the Sun. As the muons pass through the plastic, they disturb the fluorescent molecules and cause them to radiate a faint violet-blue light. A PMT (photomultiplier tube) senses the light and converts it to an electronic signal.

This system has been used for years in both astrophysics and parcticle accelerators like the ones at FermiLab and SLAC (the Stanford Linear Accelerator). It's very similar to the detection system we'll use in our WALTA (Washington Area Large-scale Time-coincidence Array), where we're planning to install detectors at schools throughout the greater Seattle region, to detect the large footprints of ultra-high energy parcticles.

Jim Madsen of the University of Wisconsin at River Falls suggested I take it to the South Pole ! So I brought it along to Wisconsin when I attended the Astronomy in the Ice course, and we worked on making a robust shipping crate and reinforcing the PMT-scintillator junction, which, even though held together with epoxy, has proven very vulnerable to breakage.

Other plans for the detector include : a) a hot-air balloon flight to reproduce the data that led to the discovery of cosmic rays by Victor Hess. b) piggybacking on a NASA high altitude balloon experiment, to collect data from above most of the atmosphere. c) sending the detector to Sweden with TEAs Jason Petula and Mats Petterson. d) sharing the detector among local WALTA teachers.

Lots of sources contributed to the construction of the detector. QuarkNet & FermiLab donated the PMTs, the most expensive part. Howard Matis donated the circuit board and the analog IC chips. A NASA mini-grant provided $ 400 toward construction. I paid for the rest ! Students did almost all the original soldering and housing construction. Hans-Gerd Berns of the University of Washington did the troubleshooting...

Howard Matis' website


My web page on the detector :

Current Status : something went wrong with the board while Jim & I were working with the shipping box & PMT splints. It's coming back to me, and I'll be a) fixing it, b) taking it to Vernier software in Portland to get help on data collection, c) demonstrating it in our next WALTA workshop for teachers in early August, d) shipping it to Jason to ake on his trip to Sweden.

Wow !

The detector as it arrived at River Falls. Jim Madsen & I went to a lot of trouble to make it stronger, so it might have a chance of arriving at the South Pole ready to collect data.

Jim Madsen fits the highly reinforced scintillator/PMT inside the top box of the detector.

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