Smashing debut for novel atomic nuclei.

Smashing debut for novel atomic nuclei

Researchers have identified six new isotopes in the debris resulting from high-speed collisions between krypton ions and nickel target. The discovery of these rare atomic nuclei -- particularly arsenic-65 -- provides the first experimental evidence to support a key step in a theoretical model that describes how a neutron star generates intense bursts of X-rays.

"This is the first time that arsenic-65 has been identified," says graduate student Michael F. Mohar of Michigan State University in East Lansing. "It's an astrophysically important result." Mohar and his co-workers report their discovery in the March 25 PHYSICAL REVIEW LETTERS.

Working at Michigan State's National Superconducting Cyclotron Laboratory, the researchers directed a high-energy beam of krypton-78 ions into a nickel target. The interaction produced a spray of atomic nuclei, which then passed through a detector for identification.

The six newly identified isotopes -- arsenic-65, gallium-61, germanium-62, germanium-63, bromine-69 and strontium-75 -- lie very near a somewhat ill-defined, theoretically derived boundary known as the proton-drip line. This boundary marks the point at which the repulsive forces between protons in a nucleus become so great that no more protons can be added to create a new nucleus. An added proton would simply "drip off."

The new isotopes survive for at least 150 nanoseconds -- the time it takes them to travel the detector's length. "What we've done so far is simply identify the isotopes," Mohar says. "We are now planning experiments to actually measure their decay half-lives. This is what the astrophysicists need."

Arsenic-65 in particular occupies a strategic position in a hypothetical chain of nuclear reactions taking place on the surface of a neutron star that has collected material from a nearby, hydrogen-rich star. In this model, the energy required to drive the thermonuclear explosion necessary to produce an X-ray burst would come from a special sequence of nuclear fusion reactions in which nuclei capture protons to create new isotopes.

Evidence that arsenic-65 exists and has a sufficiently long half-life would remove a potential bottleneck in this proton-capture process. "Whether or not arsenic-65 exists affects how much energy you get out in an X-ray burst ... and the time between bursts," says astrophysicist Stanford E. Woosley of the University of California, Santa Cruz, who with a colleague first proposed the proton-capture pathway for generating X-ray bursts.