Fewer Neutrons Than Expected

A single neutron was removed from the surface of the atomic nucleus Argon-32. Surprisingly few neutrons occupy the outmost shell of this rare isotope.

A beam of Argon-32 was impinged upon a beryllium target. One possible outcome of this collision is the removal of a single neutron from Argon-32. The probability of this reaction to occur is tied to the number of neutrons available in the outmost shell of Argon-32 to yield Argon-31. The residual nucleus Argon-31 is the last particle-bound Argon isotope and its only bound nuclear state is the ground state.

Figure 1: Schematic illustration of the one-neutron knockout process, in which Argon-32 loses a neutron when impinging on a beryllium target.

In weakly bound nuclei, counting the neutrons in such experiments agrees with model predictions within 10%. For stable nuclei, 50-70% of the nucleons predicted by models are found in these orbits. In Argon-32, only 24% of the neutrons expected in this shell are actually found to be in there. In contrast, Oxygen-22 has the same number of neutrons but 10 fewer protons and 70% of the model predictions are realized in nature.

This mismatch sheds light on fundamental correlations, which might depend on the separation energy of the nucleons as illustrated in Figure 2.

Figure 2: Reduction factor RS between predicted and observed nucleons on nuclear surfaces.

This work was supported by the National Science Foundation through grant PHY 0110253.

A. Gade et al., Phys. Rev. Lett. 93 (2004) 042501.

A. Gade
gade at nscl.msu.edu, 517-333-6441.