Science: 72 Inches of Bubbles

At Berkeley, Calif., one of the world’s biggest, most complex and most dangerous scientific instruments was ready for full operation for the first time. Its name was a tongue twister: the liquid hydrogen bubble chamber, designed and built by the University of California’s Radiation Laboratory. In the next week or so, a beam of antiprotons from Berkeley’s great 6 billion-volt Bevatron will pass through a pipe 200 ft. long, enter an odd-looking building and strike into a glass-topped metal bathtub containing 150 gal. of liquid hydrogen. As the antiprotons travel through the liquid, they will make slender, scratchlike trails of hydrogen bubbles. These trails, lasting but a fraction of a second, are the reason for the massive. $2,000,000 instrument; scientists around the world hope that photographs of the trails will reveal the innermost secrets of matter.

Glass & Golf. The first bubble chamber, invented in 1953 by Dr. Donald Glaser of the University of Michigan, was a glass tube filled with ether at a temperature that would make it start to boil when pressure was suddenly reduced. If high-energy particles (e.g., protons from a cyclotron) are shot into the ether at the right moment, lines of bubbles form on their trails, thus showing where the particles go and how they interact with atoms in the ether. When Inventor Glaser delivered his classic paper at a Washington physics convention. Physicist Luis Alvarez, associate director of the Radiation Lab, was not in the audience. He was at the White House delivering a strobo-scopic gadget he had invented to improve President Eisenhower’s golf game. But Alvarez knew about the Glaser paper, and had plans for improvements. The best liquid to use, he thought, was not ether; it was pure liquid hydrogen, which contains no carbon or oxygen atoms to confuse researchers.

Liquid hydrogen is rugged stuff to fool with, so cold (boiling point: —252.7° C. at atmospheric pressure) that steel cracks on sudden contact. It must be elaborately refrigerated or it will flash into vapor. Even a small leak is highly explosive. The 150 gal. in Berkeley’s chamber have the explosive power of 1,500 lbs. of TNT.

Alvarez’ first model was all glass and only 2 in. in diameter. When it worked, he gradually increased his chambers to 2 in., 4 in., 10 in., and each step multiplied the difficulties until the laboratory blossomed with safety devices. Yet the 10 in. chamber spotted tricks of Bevatron particles that might have been missed by years of work with more primitive instruments—and only whetted Alvarez’ desire for more.

Mechanized Dinosaur. With the promise of $2,000,000 from the Atomic Energy Commission, Alvarez and his team went to work on a chamber 6 ft. long. The difficulties became fantastic. The electromagnet surrounding the chamber had to weigh 200 tons. The great machine had to be movable, but wheels were too unstable. Instead, it was given four massive feet on which it could be walked around like a mechanical dinosaur. Leak detectors were installed everywhere to watch for escaping hydrogen; 104 alarm circuits inside the machine flash lights, ring bells and honk horns at the slightest hint of trouble. In a serious emergency (e.g., the failure of the refrigeration system) the entire stock of liquid hydrogen can be dumped through a pipe down a canyon and into a spherical tank. If all precautions fail, a hydrogen explosion may not wreck the whole apparatus. The top of the building is made of mint green plastic, is designed to blow off easily, allowing the blast to spend its force in empty air.

Last week, in trial runs with a pi meson beam, the whole weird contraption worked precisely. And over the glass cover, a stereoscopic camera takes pictures of meson tracks every twelve seconds, gathering 1,000 times more information for U.S. science than with the most sophisticated of earlier instruments.