Newly published result challenges a well-established theory of the nucleus

Published online 24 August 2010 Nature 466, 1034 (2010) doi:10.1038/4661034a

by Eugenie Samuel Reich

In 2002, Oak Ridge physicist Paul Koehler and his colleagues used the neutron beam to measure 'neutron resonances' in each of four different isotopes of platinum. The resonances are particular energies at which the neutrons are especially likely to be absorbed by the platinum nuclei. The motion of protons and neutrons inside the platinum nuclei affects the pattern of resonances. And according to random matrix theory, a mathematical theory that for decades has been crucial for calculating the behaviour of large nuclei, those motions should be chaotic.

Yet, as Koehler and his colleagues report this month in Physical Review Letters (P. E. Koehler et al. Phys. Rev. Lett. 105, 072502; 2010), their analysis of the ORELA data found no sign that the nucleons in platinum were moving chaotically. By looking at the strength of the resonances, rather than just their spacing, the group rejects the applicability of random matrix theory with a 99.997% probability. Instead, the nucleons seem to move in a coordinated fashion. "There's no viable model of nuclear structure that could explain this," says Koehler.

The resolution of the puzzle could have practical implications, as random matrix theory is currently used to estimate the probability that escaping neutrons will collide with nuclei, and from this to calculate the amount of shielding needed in nuclear reactors and stockpiles. "Engineers build in some extra shielding to cover the uncertainty, but if you were building 100 nuclear reactors you'd want the precision," says Gary Mitchell of North Carolina State University in Raleigh, co-author of a recent review article on random matrix theory (H. A. Weidenmüller and G. E. Mitchell Rev. Mod. Phys. 81, 539; 2009).