Radiation: Atomic Signals from Silver

Since late last year, the silver-short U.S. has been forced to mint silverless "sandwich" quarters and dimes containing a central layer of copper between two thin slices of copper-nickel alloy. Now another Government agency has suggested a more direct solution: find more silver. To aid prospectors, U.S. Geological Survey scientists have designed and successfully tested a "silver snooper," a device capable of locating silver deposits buried as deep as three feet below the ground. By shooting a stream of neutrons into the earth, the snooper turns the silver temporarily radioactive, causing it literally to signal its presence.

Silver atoms that are caught in the penetrating beam capture neutrons and briefly become unstable isotopes, emitting gamma rays that can be recorded by the snooper's scintillation counter. Since silver isotopes, like radioactive atoms of other elements, have their own characteristic half life—or rate of decay—and emit gamma rays at a specific energy level, the snooper's detectors can distinguish them from atoms of other elements in the area that have also been made radioactive by the neutron beam.

The power to produce the snooper's stream of neutrons comes from a simple gasoline engine that runs a primitive type of particle accelerator. A beam of deuterium (or heavy hydrogen) particles emitted by the generator is directed against deuterium absorbed in a titanium target. As the deuterium particles collide, they release neutrons that are channeled into a beam that can cover a two-square-foot area of ground. The entire device, including the recording instruments, is small and light enough to be carried in the back of a Jeep. It has already been given trials in the field.

In one such test, the snooper successfully detected silver ore particles that scientists had "seeded" in the ground at Greenbelt, Md. In another test, near Mineral, Va., the snooper determined that natural deposits of silver in an old zinc-mining area were too small to exploit economically. Geological Survey Physicist Frank Senftle, who headed the group that developed the snooper, believes that commercial models can be available for use as early as this fall, at a cost of between $25,000 and $35,000 per unit.

For prospectors who find that price too steep, the Survey scientists are already developing a hand-held "baby snooper" that is expected to cost about $3,000. It will shoot low-energy X rays at the ground, causing silver on the surface to fluoresce, and will measure the fluorescence on a scintillation counter. Senftle sees the baby snooper as the silver equivalent of the inexpensive Geiger counter, which leads uranium prospectors directly to their quarry.