IMAGE BY JOHNNY JOHNSON HIGH-VOLTAGE SOURCE and simple circuitry detect the passing cosmic-ray particles.

Now you're ready to assemble the chamber. The unit has to be airtight, so run continuous beads of silicone aquarium cement where the layers join and put a heavy weight on top or clamp things while the adhesive sets. Make sure also that the channels that hold the lead wires are especially well sealed. The speaker provides an audible output, but for more exacting work I count my cosmic rays on a digital pedometer that I bought for $15 from Radio Shack (catalogue no. 63-618). When this gadget is jostled enough to swing its tiny magnetic pendulum to one side, the magnet pulls together two fine metal strips inside an encapsulated switch, completing a circuit. By bypassing the switch with one of your own, the pedometer can be made to count almost anything that is not producing events too often. The limit seems to be about five times a second, which is just fine for counting cosmic rays. To convert the pedometer, remove the battery cover and nip away at the extreme right side of its plastic case using a small pair of pliers. Then cut off the exposed switch and solder on leads from the detector circuit.

You'll need a variable high-voltage supply to operate the apparatus. Before you power things up for the first time, be absolutely certain that no high-voltage wires are exposed and be extremely careful to avoid any possibility of a dangerous shock. When you're sure that everything is safe, apply 600 volts to start and slowly raise the potential until you just begin to register counts. This setting is your chamber's threshold voltage. The count rate will rise with the applied potential until essentially all the ionizing particles that enter the chamber are detected. At that point (about 1,200 volts for my detector), the count rate levels off. This ¿plateau¿ should extend for several hundred volts. As you raise the voltage even higher, secondary effects generate spurious counts, and so the rate rises again. Set your operating voltage at the center of the plateau.

Once you've built and tested two identical chambers, it's easy to construct a cosmic-ray telescope. Just align the two chambers and flip the switch to the A-and-B position, which counts just the events that trigger both detectors. Because particles produced by radioactive decay don't have enough energy to pass through both plastic boxes, your telescope will now show only cosmic rays. This equipment affords many opportunities for research. Position the chambers close together to detect daily and seasonal variations in the flux of cosmic rays. Or place the detectors farther apart to restrict the angular acceptance of the telescope. This maneuver allows you to measure the flux coming from a given direction and to observe how the rate depends on elevation angle and azimuth. By placing material between the two chambers, you can screen out low-energy cosmic rays. Muons lose about two million electron volts (MeV) of energy for each centimeter of water they pass through. A brick, which is about two times as dense as water, will extract about 4 MeV for each centimeter of thickness. You can use this effect to investigate the energy spectrum of the more feeble muons impinging on your detector. And you can detect the immense ¿air showers¿ that very energetic protons spawn by comparing results from two telescopes situated about 100 yards (or meters) apart. With a little imagination and effort, you will surely make some fascinating discoveries.

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Further Information:

The Society for Amateur Scientists will offer a kit for this project until January 2002. The package contains only the various electronic components required (apart from the pedometer) and a spool of fine sense wire. The cost is $30. To order, call the society at 401-823-7800. For an ongoing discussion about this project, surf over to www.sas.org and click on the Forum button. You can write the society at 5600 Post Road, #114-341, East Greenwich, RI 02818. To purchase Scientific American's CD-ROM containing every article published in this department through the end of 1999 (more than 1,000 projects in all), consult www.tinkersguild.com or dial toll-free: 888-875-4255. Erratum: Mercury's freezing temperature was incorrectly given in the Amateur Scientist for December 2000. The correct value is -38.9 degrees; the corresponding output voltage in the table on page 104 should read -0.365 volt.

This article was originally published with the title Counting Particles from Space.

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