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Introduction

The Thermos Bottle Detector is a development that is based on an earlier design from The University of Mainz, Germany. We modified the design to make it safer for student use. It uses the Cerenkov effect to detect particles. We used a standard thermos bottle with a glass insulation that can be purchased at any supermarket. The detection media is basically tap water, although distilled water would be better. The most expensive part are the photomultiplier tubes. For safety we prefer the PMTs from Electron Tubes. Cerenkov light is analogous to a sonic boom and it happens when ultra relativistic particles travel faster than the speed of light in that media.

Thermos Bottle Detector

Helio Takai, Tara Newman, and Joseph Sundermier

Cherenkov light is a small burst of radiation that is produced when particles, such as cosmic rays, travel through a medium faster than light. The faint light produced can be converted to an electrical signal by using a photo multiplier tube. The electrical signal can be sent to, and analyzed by, a computer to determine the frequency of cosmic rays. The Auger detector in South America is an example of such a detector. We wanted to determine if a miniature model of this type of detector could be made for tabletop use.

We selected ordinary thermos bottles that can be obtained in a supermarket. In order to prevent false triggering of the detectors we painted both the bottle, and the inside of the container, black. We encountered some difficulty in getting the paint to adhere to the plastic.

Next, we fit two photo multiplier tubes (Electron Tubes model P30CW5 http://www.electron-tubes.co.uk/pdf/P30CW5.pdf) into holes cut into the thermos bottle caps. These detectors contained integral Cockroft and Walton power converters. The tubes had a 5.0-volt supply and put out a negative mv signal. The photo multiplier tubes were secured into the cap with black RTV. Black electrical tape was wrapped around the joints to add further structural integrity.

Once constructed, the first order of business was to determine if the detectors saw anything. They were both filled with water and connected to an oscilloscope. After a few minutes, the signal shown on the right was obtained. This signal made us reasonably sure that we were seeing an actual event.

The next order of business was to sandwich the detector between two paddles and look for triple coincidence. It was reasoned that if the paddles saw a signal and the thermos bottle also saw the signal, then we were seeing an actual event. After a while the event on the right was observed. Notice the center trace; it was from the thermos bottle, while the upper and lower traces were from the paddle detectors. We were now reasonably sure that what we were seeing were actual particles traveling through the liquid.

Since an oscilloscope is a specialized tool that many schools don’t posses, we adapted a simple coincidence detector circuit board, called the Berkley board, which is shown on the left. The circuit is constructed from simple components and allowed us to count the number of coincidences every minute. (http://www.lbl.gov/abc/cosmic/documentation/CosmicDetectorManual.pdf)

Our first experiment involved detecting the frequency of counts versus the distance of separation of the thermos bottles. It was reasoned that as the distance between the bottles increase, the angle of observation and therefore the number of counts should decrease. The data table and graph summarize our results. It can be seen that as expected, as the separation increases, the frequency of the counts decreases.

In conclusion, this project was interesting and fun to construct, and has the potential of bringing a rather sophisticated detector into the classroom. With the use of this detector the student can investigate the effects of latitude, east-west, air pressure, humidity, and time of day on cosmic ray flux.

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