Dynamic Proton Polarization in AlK Alum and AlNH4Alum

Abstract

Over 50% proton polarization has been attained in three sulfate alum crystals containing dilute ${\mathrm{Cr}}^{3+}$ ions, at 1 K and \ensuremath{\sim}19.5 kG, with \ensuremath{\sim}\textonequarter{}-W microwave power. We measured polarization growth-rate curves, polarization as a function of magnetic field, proton line shapes and widths, and proton spin-lattice relaxation times. The theory of dynamic polarization for well-separated allowed and forbidden transitions applies to the alums. To obtain high proton polarization, the alums require about three times as much microwave power as lanthanum magnesium nitrate (LMN). A pure AlK alum had a very long proton relaxation time (1.3\ifmmode\times\else\texttimes\fi{}${10}^{4}$ sec at 0.96 K), making it a promising crystal for "storing" polarization. AlN${\mathrm{H}}_{4}$ alum should make useful polarized targets for low-energy scattering experiments, because it contains 1.97 times as much hydrogen by weight as LMN.