21.   The earth under our feet and the cosmic rays above our heads constantly expose us to natural background radiation.

22.   The telescope detected unimaginably faint wave patterns, or ripples, in the temperature of the cosmic background radiation.

23.   They are to chart with greater accuracy than ever the cosmic background radiation, the pervasive echo from the Big Bang.

24.   To understand why, return to the opening scene of the story, when the background radiation was set free.

25.   Turning on a Geiger counter, he noted that the low click-clicks indicated everyday background radiation.

26.   While a graduate student, he began to measure background radiation coming from the earth, so it could be distinguished from cosmic radiation.

27.   Why, for example, did the background radiation appear to have precisely the same temperature everywhere?

28.   A spokesman for the Federal Emergency Management Agency, Eugene Brezany, said a slight boost in background radiation is expected any time there is a forest fire.

29.   At the same time, he said, experts also have been unable to link background radiation to health problems.

30.   Experts said the pipes emitted four times normal background radiation, and concluded they were used in a nuclear reactor.