Scientists deduce cosmic ray mystery

It has taken nearly 100 years of detective work to partially solve the cosmic ray mystery.

Last Updated: Jan 02, 2013, 10:14 AM IST

Washington: It has taken nearly 100 years of detective work to partially solve the cosmic ray mystery.

Now scientists feel they are finally close to a complete solution.

A WUSTL-led team is currently in Antarctica lofting a two-ton instrument the size of a pool table into the polar vortex to catch more cosmic rays.

What is the Super-TIGER experiment looking for?

If scientists could just order the data they wanted from a catalog, they’d ask for the abundances of all of the naturally occurring elements in the periodic table and all of their isotopes.

But when they go into the field to collect cosmic rays, this is not what they get. Out of every 100 cosmic rays they intercept, only 1 will be the nucleus of an element heavier than helium.

But most of the information about cosmic-ray origins has been gleaned from that 1 percent. So the most desirable quarry is also the most elusive quarry.

Even though Super-TIGER’s predecessor, TIGER, flew for a record-breaking 31.5 days in 2001, its detector was struck by only about 300 particles of the elements between zinc and zirconium.

That’s only about 10 particles per element, which didn’t give the scientists a very good measure of their relative abundance.

“Look at the size of those error bars,” said W. Robert Binns, PhD, research professor of physics and Super-TIGER principal investigator, pointing to a graph of the data with a grimace of chagrin.

Super-TIGER, which is much bigger than TIGER, should catch nearly eight times cosmic rays, if it can only stay up as long. That would give the scientists much better “statistics.”

While scientists are nearly certain supernova blasts are the acceleration engine, nobody is sure exactly how this works. But Binns and Martin Israel, PhD, professor of physics at Washington University and co-investigator on Super-TIGER, think a clue is beginning to emerge from the cosmic ray data.

It has to do with gas and dust. There isn’t much of anything in space, but it isn’t empty. There is a little gas -- about one gas atom per cubic centimeter -and a sprinkle of dust -- not dust as in dust bunnies but tiny grains of stuff like sand or ice.

The data are suggesting that when the interstellar medium is accelerated, the dust somehow gets the jump on the gas. An acceleration mechanism that would make this distinction has been proposed. It predicts that heavier volatiles (the gas) should have higher cosmic-ray/solar system ratios than lighter ones, but that the refractories (the dust) would not display mass dependence.

“However, our TIGER data indicate similar (but not identical) mass dependence for both the volatiles and refractories,” said Israel.

“So one of the main things we are looking for with Super-TIGER is improved statistics for the heaviest elements, so that we can pin down the refractory mass-dependence,” he added.