The world of physics can be strange, very strange indeed. And every once in awhile something happens during an experiment that throws a wrench into the neatly constructed model of how everything works and causes all assumptions to be reassessed.
Such a thing happened to the physicists at CERN, home of the Large Hadron Collider (LHC) in Geneva, Switzerland during December 2010.
Astonished theorists forced to return to the drawing board
The unexpected result of a mundane fission reaction caused jaws to drop in the particle physics laboratory. Amazed scientists recorded what was expected to be a normal fission of mercury-180. When the experiment was complete the scientists discovered that instead of the expected symmetric reaction—two equal fragments of atomic nuclei—an asymmetrical reaction occurred that turns accepted theory on its proverbial head.
The physics of nuclear fission has been known for many decades. It entails the splitting of a heavy nucleus into two nuclei with less weight and mass. Each time a heavy nucleus is split it should produce two equal nuclei...at least that's the theory. And the theory seemed to work fine, until the experiment at CERN revealed there was something terribly flawed about that assumption.
Although some types are fission reactions are known—and expected—to produce asymmetric reactions, such as splitting the nucleus of uranium and a handful of associated elements, it has been assumed that such asymmetry is present only within a narrow band of associative elements and isotopes.
Yet the experiment with mercury-180 disproved that assumption.
During the experiment, conducted by physicists using CERN's On-Line Isotope Mass Separator (ISOLDE) radioactive beam facility, the relationship between two parts of nuclear fission were explored: the macroscopic and microscopic components.
The results, published in the Physical Review Letters, were totally unexpected.
Shooting uranium with a proton beam
At the ISOLDE facility, the team of physicists fired a proton beam at a target composed of uranium. After firing the beam, lasers and a special magnetic field were employed to carefully separate thallium-180 ions from the confused cloud of expanding, diverse nuclei created by the proton beam smashing into the uranium.
To facilitate the analysis of the released nuclei, the ions were shepherded onto a carbon foil where they stuck. There the were ions transformed during a process of nuclear decay. Some of the mercury-180 atoms created by the process fissioned and the results were captured with two silicon wafers that were positioned in front of and behind the carbon foil forming a sort of layered sandwich.
At that point, all the researchers assumed the results of the split nucleus would be symmetrical, as accepted theory predicted that. Yet instead, to their astonishment, two spikes occurred in the readings. Instead of splitting into two nuclei of zirconium-90, the mercury-180 split into asymmetrical nuclei: one of ruthenium-100 and the other krypton-80.
One of the researchers, particle physicist Peter Möller of the Los Alamos National Laboratory, had previously created a model of the nucleus that predicted that mercury-180 would undergo asymmetric fission. Yet when that actually transpired even he was at a loss as to how it occurred.
A new door
Some scientists are thrilled over the unexpected results. If the theoretical model was wrong, as the experimental results clearly imply, then finding the right model can have real world benefits.
Understanding the actual properties of fission reactions with exotic elements can improve the safety and efficiency of future nuclear reactors and open up a new door into the sometimes mystifying world of particle physics.