Lead blocks may catch nuclear killer.

Spectacular explosions in which heavy atomic nuclei spontaneously burst into neutrons and protons have long puzzled scientists. Scientists first noticed these "star events" while using thick photographic emulsions to detect cosmic rays. But the cause for these rarely seen bursts has remained unknown.

Now, Russian and U.S. scientists report that they may have found a way to solve this mystery. Instead of studying emulsions, Thomas E. Ward of the Department of Energy in Washington, D.C., and his colleagues at the Khlopin Radium Institute in St. Petersburg are using neutron detectors.

The scientists are watching for star events in lead blocks in the St. Petersburg subway, as well as in a street-level Russian lab. Regardless of the blocks' location, the scientists have seen bursts made up of some 120 neutrons, which is about the number in a lead nucleus. The bursts occur on average once per day.

Because it takes so much energy to explode an atomic nucleus, the researchers suspect an outside force. Either ordinary cosmic ray particles are reacting with the nuclei in an unknown way or some exotic form of matter, perhaps a weakly interacting massive particle (WIMP), is causing the destruction (SN: 2/26/00, p. 135).

"Whatever this is, it's unusual," says Ward.

Theorists have proposed hard-to-detect WIMPs as one of the candidates for the so-called dark, or missing, matter thought to pervade the universe. Astronomical observations indicate that unobserved matter accounts for more than 95 percent of the universe's mass.