Protons and neutrons are the basic constituents of nuclei and each particle type has its own magnetic property, termed the magnetic dipole moment. The overall magnetic moment of a nucleus is a reflection of a specific arrangement for the protons and neutrons, which can be explored in a very sensitive way by studying mirror nuclei, which differ only by the simple exchange of proton and neutron number. Data for magnetic moments of mirror nuclei near stability abound, however, there are only scant data available on magnetic moments of nuclei farther from the stability line, where larger proton-to-neutron ratios may produce new and unusual magnetic effects. As an example, nuclei near the proton binding limit (proton drip line) are expected to show evidence of Coulomb effects in the nuclear medium, since the proton carries a positive charge. The simple sum of the magnetic moments of mirror partners can be compared with expectations of nuclear structure models, which consider the nuclear force to be charge independent (neutrons and protons are treated identically). Deviations of experimental moment values from theoretical expectations may be a reflection of the more complex nature of exotic nuclei. We searched for such deviations from theoretical expectations in the magnetic moment of the Potassium-35 nucleus, which has 3 more protons (19) than neutrons (16) and a proton separation energy of only 80 keV.
Studies of magnetic moments are completed at the NSCL using the Nuclear Magnetic Resonance (NMR) technique on nuclei that are unstable against beta emission (acronym beta-NMR). The NMR technique is very similar to Magnetic Resonance Imaging (MRI) methods employed in medical diagnoses. The beta-NMR technique is 1014 times more sensitive than both the traditional NMR and MRI approaches because of the detection of outgoing beta particles from the radioactive samples. The resonance curve obtained by monitoring the beta particle counting rates as a function of applied radiofrequency to the Potassium-35 nuclei is presented in the figure. The abrupt change in the counting ratio observed at 600 kHz reveals the precession frequency of the Potassium-35 magnetic moment in an externally applied magnetic field, and this frequency is directly proportional to the magnetic moment value, which was deduced as (+)0.392 nuclear magnetons.
The sum of the Potassium-35 magnetic moment with the known magnetic moment of its mirror partner Sulfur-35 (16 protons and 19 neutrons) agrees surprisingly well with the results obtained for the more stable mirror moments near mass number 35. Therefore, although Potassium-35 lies very near the proton drip line, its magnetic properties still remain similar in nature to those nuclei nearest the valley of stability.
Reference T.J. Mertzimekis et al., Physical Review C in press
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