Tiger (Trans-Iron Galactic Element Recorder) Scientists Stalk Elusive Origin Of Cosmic Rays

To help solve the mystery of the origin of these particles, scientists launched TIGER (Trans-Iron Galactic Element Recorder) at McMurdo Station at 6:30 a.m. EST Dec. 20. Aided by the circumpolar winds that blow this time of year, TIGER has just completed one trip around the South Pole and entered its second lap at 8 p.m. Jan. 2. The 29 million-cubic-foot balloon and its payload float above 99.6 percent of the atmosphere, recording incoming cosmic rays that would otherwise be almost completely absorbed by the atmosphere.

The TIGER team comprises scientists from Washington University (St. Louis), NASA Goddard Space Flight Center, the California Institute of Technology and the University of Minnesota. Principal investigator Bob Binns of Washington University and coinvestigator Eric Christian of NASA Goddard are both in Antarctica for the flight.

TIGER's long balloon flight will enable the team to collect significant numbers of the rare cosmic ray elements heavier than iron (element 26 on the Periodic Table) up to zirconium, which occupies position 40. These heavy atoms, some of which are ultimately accelerated to cosmic ray energies, are likely forged in a supernova explosion. How they achieve their high speeds--between 80 and 99.9 percent the speed of light--and kinetic energy is a major mystery in physics.

One thing most physicists agree on: The supernova explosions that create these atoms probably don't also spew them out at cosmic-ray energies.

"The reason," said University of Minnesota physics professor C.J. Waddington, "is that while the shock wave from a supernova is the only known force that can accelerate atoms to such high speeds, elements heavier than iron tend to form behind the shock wave. After the explosion, these atoms float in interstellar space. Some may be incorporated into new stars and re-released as a stellar flare, but they, too, float around. Eventually, a second supernova explosion occurs, and its shock wave accelerates them to cosmic-ray energies."

The big question is: What happens to them when they're floating around? Do they wander as individual atoms for tens of millions of years, or do they coalesce into dust grains that last for 100 million or even a billion years before the second explosion? That is, does the second blast accelerate the atoms individually, or does it first break up the dust grains, splitting off atoms that become cosmic rays?

To help answer this question, the scientists will examine the relative abundances of certain elements. The ratios of pairs of elements--such as rubidium-37/strontium-38--reflect the environment from which they came.

The key is to examine the abundance ratios of neighboring elements that will vary depending on whether the individual elements can be stripped of their outer electrons (ionized) or condensed into grains. For example, the abundance ratio of rubidium to strontium will depend on whether ionization or the tendency to condense is the determining factor.

This pair of elements was chosen because the partners have similar first ionization potential, implying the outer electrons can be removed with similar degrees of ease. But rubidium and strontium have differing probabilities of being condensed.

"Atoms with low ionization potential can be accelerated more easily and turn up in greater numbers in cosmic rays," said Waddington. "Neighboring pairs of atoms have similar tendencies to become accelerated and to undergo interactions with the interstellar medium. Hence, examining such pairs minimizes the systematic uncertainties."

Whether the atoms are accelerated individually or as dust grains, the acceleration must have occurred a few million years ago. Otherwise, the atoms would have sped right out of the galaxy or been destroyed by interactions with the interstellar medium, Waddington said. But if the experiment yields good measures of composition, "we can tell if we're looking at material that was originally formed millions or billions of years ago," he said. "What the material was when it was accelerated defines what shock waves must be able to do. This will tell us something about supernovas."

TIGER is a forerunner of the ENTICE (Energetic Trans-Iron Composition Experiment) instrument on the HNX (Heavy Nuclei eXplorer) satellite that is being studied for possible launch in a few years. What TIGER learns could help in the design of HNX.

TIGER is funded by NASA.