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Fermilab | Batavia, IL | USA

View Blog | Read Bio

Particle detectors don’t like the light

 Editor’s note:
Bob’s most excellent particle detector adventure, part 4.

 Bob Peterson continues to travel with his QuarkNet particle detector around the edge of Africa recording remnants of cosmic rays. This offers a chance to study how cosmic ray recordings differ on land and sea and at different latitudes. The data will be accessible to high school students and teachers in several countries who use similar detectors to learn about particle physics.  

Read his previous posts here:
*The voyage begins

*Turning the detector on

*Other science on the sea

 27 April 2011
R/V Polarstern
Latitude: 11-15.6 S
Longitude: 2-13.2 W
Ship velocity 10.9 knots
Ship course 322.9 ° T

Progressively, over the last three days, the data from the cosmic ray muon detector has become more problematic.

How a QuarkNet detector reads a cosmic ray remenant. Credit: Fermilab/QuarkNet

Here’s how our system works. Plastic scintillator is covered with aluminum foil and then with black paper and tape to make it “light-tight.” Any light leaking in will incorrectly be recorded as a particle interaction and make the data unreliable. A photo multiplier tube, or PMT, is attached to the wrapped scintillator; this assembly is called a counter. Up to four of these can be connected to the data acquisition card. The data acquisition system, or DAQ, sends data to the computer via the USB port. When a cosmic ray muon passes through the scintillator it causes a few photons to be emitted in the scintillator material. These are picked up by the photo multiplier tube, converted to an electrical pulse and amplified. Each photo multiplier tube sends its signal to the data acquisition system.

It is not working like that today.

One of the counter channels has become variable; first rising in counts and then falling. We look for what we call coincidences, two signals, one from each photo multiplier tube, received within a short time. These are reported to the computer; all other signals are vetoed as likely background noise from the photo multiplier tubes

I have been seeing far fewer coincidences than the data acquisition system should be recording. This has affected the overall data flow, causing it sometimes to fall to zero.

Looking back over the several days, I seemed to notice a day/night dependency. So, I decided to disassemble the counter “stack” and see if there was a possible light leak in the fourth counter; of course, that was the bottom counter. Sure enough, the bottom counter had been poorly assembled and much of the wrapping tape had let go and opened several “seams”. There were not major gaps, but they were large enough to let light leak into the photo multiplier tubes. I’m glad I found this out. I inspected all the counters and found a few other suspect areas.

QuarkNet cosmic ray muon detector tool kit. Credit: Fermilab

With the gaps addressed, the detector is back on the air and the data looks healthy.

Meanwhile, the Polarstern progresses north and west. The weather is now warm and sticky with humidity. We approach the Inter Tropical Convergence Zone where the global circulation cells collide causing an upwelling of air. Showers can be expected in a day or two.

Also, expected is the equator. When we cross over, the “pollywogs”, or people who have never crossed the equator, will be “baptized” by Neptune. I’ve been warned to wear old clothes and be prepared to throw them away. I suppose I really can’t be considered a sailor ’til I submit.

Wonder if I should get an earring? What do you think?

— Bob Peterson

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