Everyman's Atomics

For amateur atom fans, a group of scientists at Brookhaven National Laboratory offered last week a "continuous cloud chamber." Based on a ten-year-old but neglected idea, it is simple enough, say the Brookhaven men, to be built and operated by the teacher of a high-school science class.

The standard Wilson cloud chamber for making atomic particles visible is a rather complicated apparatus. It contains moist air and a movable diaphragm or piston to rarefy it suddenly. This action cools the air by expansion and makes it "supersaturated" with water vapor which will condense into water droplets if given proper nuclei to condense upon. Fast-moving atomic particles provide the nuclei by ionizing (electrifying) the normally neutral atoms of the air. So particles (e.g., cosmic rays from outer space) that pass through the cloud chamber become visible as thin white trails of water droplets.

The trouble with such complicated chambers is that they operate only for a brief period after the air has been rarefied. Brookhaven's simple chamber works continuously. It consists of a metal plate standing on a layer of dry ice and covered with black velvet for better visibility. On top of the plate sits an open-ended glass cylinder anywhere from 5 to 17 inches in diameter. A second metal plate with a layer of felt cemented to its under side is placed on the cylinder. On top of the whole apparatus is a tray of water at room temperature.

To put the chamber in operation, the felt under its cover is saturated with methyl (wood) alcohol. The alcohol vapor diffuses downward, becoming colder as it approaches the dry ice at the bottom. At some point in its downward motion it makes the air supersaturated. In this sensitive layer, cosmic rays or other fast-moving atomic particles leave trails that show up in a flashlight beam as brilliant white streaks.

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