Birth of a Tantalizing Atom

Hoping to discover the origins of the rarest natural isotope, scientists have exposed the world's supply of tantalum-180 to conditions akin to those in dying stars. But the solution to the mystery remains tantalizingly out of reach.

Nobody knows how tantalum-180, which amounts to only 0.012% of all naturally occurring tantalum--itself the rarest element--is created. None of the known decay pathways that build atomic nuclei lead to the isotope. And that's not the only oddity of this atom. Any other atomic nucleus lives in its ground state, the configuration with the least possible energy; if it gets a jolt of energy, it emits a photon in a matter of nanoseconds, returning to that ground state. But tantalum-180 naturally resides in an excited state. This is because the excited state has nine units of angular momentum--a measure of the nucleus's spin--so it must shed eight extra units of angular momentum to reach its ground state, explains physicist Andreas Zilges at the Technical University of Darmstadt in Germany. Unloading so much energy in one photon is so unlikely that it happens to half of all excited tantalum-180s only once every 1000 trillion years or so. A tantalum-180 in its zen state lasts only hours before breaking down to hafnium.

This property is a perfect tool for understanding the isotope's origins, reasoned a German-U.S. group of physicists. They leased from Oak Ridge National Laboratory in Tennessee the world's stock of tantalum-180--about 6.7 milligrams in 150 milligrams of tantalum oxide, produced by separating the isotope from natural tantalum. Figuring that, like other heavy elements, tantalum-180 is forged in the death throes of stars, the scientists recreated such conditions by irradiating the tantalum with high-energy gamma rays, which knocked some of the excited nuclei into an even higher energy level. From that state, the nucleus could shed several photons, each carrying angular momentum away, allowing it to settle into its ground state. The team then counted how many ground-state nuclei decayed away, which gave them a measure of how many Ta-180 nuclei had emitted photons, and thus how sensitive Ta-180 is to high-energy radiation.

The team's measurements, described in the 20 December Physical Review Letters, show that gamma rays from a dying star would destroy most of the Ta-180 as it forms, unless the tantalum was whisked to the cooler outer regions of the star. Since Zilges believes that process can't account for the natural abundance of tantalum in the universe, he suspects that tantalum-180 might form from some exotic decay of an excited state of another element, hafnium.