“Pathfinder” in James Fenimore Cooper’s frontier tale of the same title demonstrated his exceptional marksmanship by aiming his rifle into a flock of gulls flying overhead. He was able to time his bullet to arrive exactly where and when the paths of two gulls crossed. A team at the National Superconducting Cyclotron Laboratory, Michigan State University has performed a similar feat in the subatomic world. They studied collisions in which the radioactive nucleus magnesium-28, propelled at an energy of 2.3 GeV, loses two protons in an encounter with a light target, beryllium-9. From the reaction probability and the energy of the outgoing residue, the group could show that the interaction involved only two protons while the remaining 26 particles were untouched. This is a typical feature of reactions that in nuclear physics are referred to as “direct”. They selectively excite a simple property, here the single-particle motion of just a pair of protons. Direct reactions are an important diagnostic for understanding the motion inside nuclei. Elaborate theories have since been developed that show how the internal motion of the two protons governs the reaction probabilities. Several experiments at the NSCL have exploited this reaction to study rare neutron-rich nuclei at the limit to stability such as magnesium-32, silicon-42, and more.
This work was supported by the National Science Foundation through grants PHY 0110253.
D. Bazin, B.A. Brown, C.M. Campbell, J.A. Church, D.C. Dinca, J. Enders, A. Gade, T. Glasmacher, P.G. Hansen, W.F.Mueller, H. Olliver, B.C. Perry, B.M. Sherrill, J.R. Terry and J.A. Tostevin, New direct reaction: Two-proton knockout from neutron-rich nuclei, Phys. Rev. Lett. 91 (2003) 012501.
gade at nscl.msu.edu, 517-333-6441