Distorted nuclei spinning to the same beat.

Distorted nuclei spinning to the same beat

"Only occasionally in any area of science is something really unexpected found, but that seems to be the case in nuclear-structure physics just now."

This arresting assertion introduces a report in the July 16 PHYSICAL REVIEW LETTERS concerning the spectra of gamma rays emitted by certain atomic nuclei spinning so rapidly that their shapes becomes extremely elongated. Unexpectedly, different kinds of "superdeformed" nuclei often produce spectra representing virtually identical sequences, or bands, of energy transitions -- despite having different numbers of protons or neutrons.

"I think it's one of the most unusual things we've seen in a long time," says Frank S. Stephens of the Lawrence Berkeley (Calif.) Laboratory, the report's lead author. "As far as I know, nothing in our current understanding explains this behavior. There are lots of reasons why it shouldn't happen."

"It's a very hot topic at this moment," adds John F. Sharpey-Schafer of the University of Liverpool in England. "Why do you get such beautifully identical bands, and why are they so pervasive?"

The typical superdeformed nucleus is the product of an off-center collision between two moderately sized nuclei. The colliding bodies fuse to create a single, whirling entity. If a rapidly spinning nucleus has a mass that falls within a certain, well-defined range, it tends to settle into an elongated shape with a length roughly twice its width (SN: 5/28/88, p.346).

These spinning nuclei slow down in steps, losing energy and angular momenttum by emitting a pair of gamma rays at each step. Filtering and analyzing the signals received by an array of gamma-ray detectors reveals a characteristic spectrum, or band, consisting of as many as 20 equally spaced lines.

Researchers have detected such bands for a variety of superdeformed muclei, and a given mucleus may have as many as six bands, each one representing a different configuration of protons and neutrons but all having the same, extreme deformation. The surprise is that different superdeformed nuclei sometimes produce bands that are strikingly similar.

"That's something we don't expect because just the fact that you've added a nucleon or two nucleons ought to change the moment of inertia [rotational inertia] by quite an appreciable amount," Stephens says. In other words, the additional neutrons or protons should change the mass distribution enough to noticeably alter the way the nucleus spins.

So far, researchers have identified several apparently related sets of bands among dysprosium and related isotopes and at least nine among mercury and lead isotopes. "That doesn't look at all like an accident," Stephens says.

However, the reason why nuclei should behave in this way is "still in the air," says Richard R. Chasman of the Argonne (Ill.) National Laboratory. Although theorists are hard at work, no single, coherent explanation to account for all the results has yet emerged.

Moreover, the quantum-mechanical calculations needed to make predictions about the behavior of nuclei are horrendously complicated. In fact, nuclear physicists usually rely on simpler theoretical models that merely approximate nuclear behavior. Studying superdeformed nuclei is one way to probe the strengths and weaknesses of these models. The unexpected experimental results show that these simplified models miss a potentially important aspect of the way nuclei are organized.

"In terms of the sorts of calculations that we can do, it's very hard to explain the [spectral] similarities down to the level [of precision] to which they're seen experimentally," Chasman says. "The calculations are just a little too crude."

"We have this astoundingly loud and clear signal that we don't understand," Stephens says. "The similarities are so striking, my feeling is that the answer can't elude us for very long."