Chirality crops up in nuclear physics
Feb 2, 2001
Mirror images crop up throughout nature. Many biological molecules occur in identical but left- and right-handed forms - a phenomenon known as chirality. Some subatomic particles - such as fermions - are also chiral because they can exist in two forms that are identical apart from the direction of their spin. However, scientists have long believed that the nuclei of atoms are too symmetrical to exist as left- and right-handed versions. But now Krzysztof Starosta of the State University of New York at Stony Brook and colleagues have shown that some nuclei are asymmetrical enough to have equal but opposite forms (K Starosta et al 2001 Phys. Rev. Lett. 86 971).
The nucleons - neutrons and protons - that make up a nucleus 'pair up' in a structure analogous to the electrons in an atom. Certain nuclei have an odd number of neutrons and protons, leaving two 'spare' nucleons orbiting the 'core' independently. If these nucleons orbit the same axis that the core spins around, the whole system is highly symmetric - and mirror images could not exist. But when the nucleus becomes 'potato-shaped' - it has a different diameter in all three directions - the nucleons split up and move in orbits that are perpendicular to each other and to the spinning core. The resulting three vectors of angular momentum can be oriented with respect to each other in two distinct ways - corresponding to two possible resultant spins for the nucleus - and Starosta's team believes this is the key to the chiral nuclei.
To test the theory, Starosta and colleagues created samples of caesium, lanthanum, praseodymium and promethium - which all have the 'spare nucleon' structure - using heavy-ion induced nuclear reactions. The nuclei are in a wide range of excited energy states and emit gamma rays as they fall to lower energy states. Starosta's team noticed that the nuclei emitted pairs of gamma rays with slightly different energies but the same amount of angular momentum. The best explanation for this tiny discrepancy is that left- and right-handed versions of the same nuclei produced the signals. The energies were found to be too similar to originate from completely different decay processes. "The role of chirality may also be important in other many-body systems", Reiner Kruecken, a team member, told PhysicsWeb, "and we hope our work will inspire such investigations".