  We study the energy relaxation times ($T_1$) of superconducting transmon
qubits in 3D cavities as a function of dielectric participation ratios of
material surfaces. This surface participation ratio, representing the fraction
of electric field energy stored in a dissipative surface layer, is computed by
a two-step finite-element simulation and experimentally varied by qubit
geometry. With a clean electromagnetic environment and suppressed
non-equilibrium quasiparticle density, we find an approximately proportional
relation between the transmon relaxation rates and surface participation
ratios. These results suggest dielectric dissipation arising from material
interfaces is the major limiting factor for the $T_1$ of transmons in 3D cQED
architecture. Our analysis also supports the notion of spatial discreteness of
surface dielectric dissipation.
