Nuclear Syrup

A new measurement of how long it takes certain nuclei to fission into large fragments suggests that the “liquid-drop” model of the nucleus should be replaced with a “nuclear syrup”model. Fission is the most dramatic form of radioactivity, when a nucleus loses not merely a small fragment-such as an electron, gamma ray, or an alpha particle-but actually splits in half. The fission of many nuclei has been studied through the years, most famously uranium-235.

As early as 1939 Niels Bohr and John Wheeler tried to model the nature of fission by saying that the nucleus is like a drop of water in which the tendency of the drop to fly apart is checked by the force of surface tension; something like this, they said, kept a nucleus intact until such time as the rapid oscillations of an unstable nucleus became so large that the “surface tension” normally keeping the nucleus together was overcome.

Sometimes as a prelude to fission, the nucleus relieves some of its instability and effectively reduces its internal “nuclear temperature” by flinging out neutrons or gamma rays. In fact, the lifetime for fission has been indirectly measured by observing those cast-off neutrons. The results suggest that the old liquid-drop model was off by a factor of ten or so in predicting lifetimes. Some scientists have begun to think that an additional stickiness in the nuclear substance is at work, which slows up the fission process.

An experiment at Oak Ridge National Laboratory has probed this proposition by creating several fissionable nuclei artificially with heavy-ion beams bombarding a tungsten target; the projectile and target nuclei temporarily fuse together, travel a short distance through the tungsten crystal, and then fission. The spacing of the atoms in the crystal is used as a reference to measure the recoil of the composite nucleus before fission.

According to team member Jens Andersen of the University of Aarhus in Denmark (jua@phys.au.dk, 45-8942-3713), the Oak Ridge experiment suggests that the fission lifetimes are even longer (an additional factor of ten to one hundred) than those derived with the more indirect neutron-emission method. This could imply that the nuclear shape does not oscillate as rapidly as a water droplet would but instead deforms very slowly like a drop of syrup. (Andersen et al., Physical Review Letters, 19 October 2007)