Stabilization of High Concentrations of Nitrogen Atoms in Impurity-Helium Solids

Impurity-helium (Im-He) solids created by injecting gaseous helium with an admixture of nitrogen atoms and molecules into superfluid ⁴He have been studied via electron spin resonance (ESR). We have studied the efficiency of stabilization of N atoms in Im-He samples prepared from nitrogen-helium gas mixtures with different fractions of nitrogen varying from 0.25% to 4%. Some of the observed ESR spectra of N atoms in the Im-He samples are very broad. The highest local concentration of N atoms determined from dipole-dipole broadening of the ESR line is ～8×10²⁰ cm^?3. The highest average concentrations of N atoms in N-N2-He solids were much lower (of order 10¹⁹ cm^?3). The samples with high concentrations of N atoms were stable in liquid helium, and remained stable even after draining liquid helium from the sample at T≤3.5 K.