Robust solidXe129longitudinal relaxation times

We find that if solid xenon is formed from liquid xenon, denoted ``ice,'' there is a 10% increase in $^{129}\mathrm{Xe}$ longitudinal relaxation ${T}_{1}$ time (taken at 77 K and 2 T) over a trickle-freeze formation, denoted ``snow.'' Forming xenon ice also gives an unprecedented reproducibility of $^{129}\mathrm{Xe}$ ${T}_{1}$ measurements across a range of 77--150 K. This temperature dependence roughly follows the theory of spin rotation mediated by Raman scattering of harmonic phonons, though it results in a smaller-than-predicted spin-rotation coupling strength ${c}_{K0}/h$. Enriched ice $^{129}\mathrm{Xe}$ ${T}_{1}$ experiments show no isotopic dependence of bulk relaxation mechanisms at 77 K and at kilogauss fields.