IMAGE BY DANIELS & DANIELS FOUR PLASTIC SQUARES and some wire are fashioned into a flat vacuum chamber.

Energetic protons from deep space continuously bombard our planet and strike atoms in the upper layers of the atmosphere. These collisions produce tiny nuclear explosions, which in turn give rise to every species in the particle zoo--protons, neutrons, electrons, muons, lambda particles, you name it. Of these, only the muons have enough penetrating power to reach the ground unscathed. Still, the flux of these subatomic particles, known as cosmic rays, is surprisingly high: about 200 rain down on each square meter of Earth every second.

With the instrument described here, ambitious amateurs can monitor the intensity of cosmic rays throughout the day, chart their distribution in the sky and learn something about their energies. The detector consists of two large, flat Geiger counters linked together with a simple electronic circuit. Here's how they work. A set of fine wires carries about 1,000 volts or so. This potential creates an enormous electric field (more than one million volts per meter) near each wire. When a cosmic ray enters this space, it strips some of the atoms in the surrounding gas of a few electrons, which then move toward the nearest positively charged wire. On the way, these electrons gain enough energy from the huge electric field to knock more electrons from other gas molecules. These charges also accelerate and collide to release still more electrons, and so forth.

Within just millionths of a second, the few electrons originally liberated by the passage of the cosmic ray trigger an electric avalanche, causing more than a billion negative charges to cascade down onto the wire. This current flows into a capacitor (C1 on the diagram on page 87), which in turn generates a voltage pulse that feeds into the counting circuitry.

Most Geiger counters are filled with a noble gas, usually helium or argon. Both can be found at welder-supply shops. Helium, so useful for filling balloons, can also be obtained cheaply at any party-goods dealer. Ordinary air also works, albeit at a higher operating voltage.

No matter what gas you're using, you must reduce the pressure in the chamber to about seven centimeters of mercury--about 10 percent of atmospheric pressure. The March 1960 and October 1996 installments of this column describe homemade vacuum pumps that should serve nicely. But you can also reduce the chamber pressure with a bicycle pump if you modify it appropriately (consult the Web site of the Society for Amateur Scientists for details).

Begin construction by cutting four pieces (as shown above) from a rigid sheet of plastic that is 3/8 of an inch, or about one centimeter, thick. Using the edge of a small file, carve a series of small notches spaced precisely half a centimeter apart on opposite sides of the piece indicated in the diagram. Next, arrange a length of hefty ¿bus wire¿ (solid copper wire without insulation) as shown. Secure it with tape at the corners and apply tiny dollops of five-minute epoxy between the notches. Also add a liberal amount of epoxy to the wire along the side you've not filed, making sure to leave at least one centimeter around the perimeter untouched to accommodate the piece that fits above.

Use the notches to position the ¿sense wire,¿ bare copper wire that is only 10 thousandths of an inch (about 250 microns) thick. Wrap this fine wire around the square plastic frame, using a steady hand to maintain tension, and hold the ends in place temporarily with duct tape.

Now you must delicately solder the sense wire to the bus wire everywhere they touch. Use a hot soldering iron and plenty of flux. Then attach the sense wire to the frame with a liberal coating of slow-setting (24-hour) epoxy. Once it sets, carefully snip the excess wire just where it emerges from the epoxy to yield a single plane of 29 sense wires. Solder a high-voltage lead to the bus wire. Use epoxy to attach aluminum foil to the top and bottom plastic squares, as shown in the diagram, and install the stopcock and low-pressure gauge to the top piece. Solder ground wires to the aluminum foil. Carve three narrow channels in the middle plastic pieces for the high-voltage and ground wires. (A Dremel tool will work well.)

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