APS News

Scientists Make First Measurement on Ni-78 Half-Life

The half-life of the unstable, exotic nucleus nickel-78 (Ni-78) has been measured for the first time, and was found to be only 110 ms, or about a tenth of a second, according to Hendrik Schatz, a researcher at Michigan State University. Its decay plays a key role in the synthesis of the heavy elements, the understanding of which is one of the 11 Greatest Unanswered Questions in Physics (Discover Magazine, February, 2002). Schatz reported on the most recent experimental results at Michigan State’s National Superconducting Cyclotron (NCSL) during the 2005 April meeting in Tampa.

Physicists believe the heavy elements were built from lighter atoms, such as iron, in supernova explosions billions of years ago, which triggered a chain of nuclear reactions–a process known as rapid neutron capture. How this process takes place is still a mystery. The NCSL is designed to study this question by reproducing the conditions inside supernovas with energetic nuclear collisions.

Ni-78 is known as a "doubly magic" nucleus because it contains a "magic number" of both protons and neutrons–in this case 28 protons, and 50 neutrons–that fill shells in the nucleus.

There are only 10 such nuclei in nature, and Ni-78 has the largest neutron excess. Because the Ni-78 isotope must dispose of so many extra neutrons, it is extremely unstable and does not exist in nature, except briefly in exploding supernovae. The NCSL scientists were able to create the isotope by accelerating a stable isotope of krypton gas to high speeds and then colliding it with a target of beryllium metal. The NCSL is the nation’s premier rare isotope accelerator, capable of shooting 100 billion krypton atoms a second. Even then, Ni-78 is so rare, it only shows up about twice a day.

Ni-78 acts as a kind of valve in the rapid neutron capture process. A shorter half-life would be like opening the valve a little, allowing the process to develop more quickly. Since the NSCL team found that the half-life was substantially shorter than expected, this means nature can produce heavy elements faster than previously thought.

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