Nuclear magnetic moment of $^{59}$Cu with on-line $\beta$-NMR on oriented nuclei

The nuclear magnetic moment of the nucleus $^{59}\!$Cu, with one proton and two neutrons outside the closed N = Z = 28 shells, was measured in an on-line experiment combining $\beta$-NMR with low temperature nuclear orientation and with particle detectors operating at a temperature of about 10 K. From the data the center frequency $\nu$(B$_{ext}$ =0) = 209.51 (22) MHz was derived. Using the hyperfine field of Cu in host iron from the literature the result for the moment is $\mu[^{59}$Cu] = + 1.891 (9) $\mu\scriptstyle_\textrm{N}$ , which reveals a large deviation from the proton p$\scriptstyle_{3/2}$ single-particle value. This provides strong experimental evidence for a massive shell breaking at $^{56}\!$Ni.