Laser-precision studies of nuclear radii
One of the most fundamental properties of the nucleus is its size. Generally, nuclear radii follow a smooth trend: they gradually increase with the number of constituent protons and neutrons. However, when looked through a magnifying glass, measured radii display local variations, which signal structural changes. To measure such variations, superb precision is needed. Indeed, while the radius of an iron nucleus is very small, about 0.000000000000004 m (or 4 fm), the inter-radii fluctuations are 400 times smaller! To add to the experimental difficulty, many nuclei exist for only a fraction of a second.
To measure such small effects on radii of short-lived isotopes, a novel experimental scheme has been developed at the National Superconducting Cyclotron Laboratory at Michigan State University (MSU). The method involves laser spectroscopy of isotopes produced through a fast in-flight separation followed by gas stopping.
In a recent work published in Physical Review Letters, the charge radius of radioactive 52-Fe (26 neutrons) was determined to be greater than the radius of the heavier 54-Fe isotope with 28 neutrons, and only slightly smaller than that of 56-Fe. Nuclear theory explains the resulting kink in nuclear radii about 54-Fe as a deformation effect: 52-Fe has a football-like shape, while 54-Fe and 56-Fe are spherical. Because the structural variation of radii around neutron-number 28 in the iron isotopes strongly depends on the nature of nuclear interactions, the experimental discovery will have profound impact on nuclear modeling. The novel technique will play an even more essential role in the determination of nuclear sizes at the Facility for Rare Isotope Beams (FRIB) currently under construction at MSU, which will provide unprecedented access to new rare isotopes.