Nuclear magnetic moment ofAs69from on-line β-NMR on oriented nuclei

A precise value for the magnetic moment of the $^{69}\mathrm{As}$ $5/{2}^{\ensuremath{-}}$ ground state has been obtained from nuclear magnetic resonance on oriented nuclei (NMR/ON) using the NICOLE $^{3}\mathrm{He}$-$^{4}\mathrm{He}$ dilution refrigerator setup at ISOLDE/CERN. The NMR/ON signal was observed by monitoring the anisotropy of the $^{69}\mathrm{As}$ \ensuremath{\beta} particles. The center frequency $\ensuremath{\nu}[{B}_{\mathrm{ext}}=0.0994(10)T]=169.98(9)$ MHz corresponds to $\ensuremath{\mu}[$$^{69}\mathrm{As}$$]=+1.6229(16){\ensuremath{\mu}}_{N}$. This result differs considerably from the $\ensuremath{\pi}{f}_{5/2}$ single-particle value obtained with g factors for a free proton but is in reasonable agreement with the value obtained with effective g factors and with values from a core polarization calculation and from calculations in the framework of the interacting boson-fermion model. Assuming a single exponential spin-lattice relaxation behavior a relaxation time ${T}_{1}^{'}=10(25)$ s was observed for $^{69}\mathrm{As}$$\underset{\ifmmode\bar\else\textasciimacron\fi{}}{\mathrm{Fe}}$ at a temperature of about 20 mK in a magnetic field $B=0.1$ T.