  The unconventional multi-gap superconductivity in elemental Pb were reported
previously by surface sensitive tunneling experiments, as well as predicted by
several theory works. To obtain bulk evidence for such multiple gap behavior,
the thermodynamic critical field $B_{\rm c}$ was measured along three different
crystallographic directions ([100], [110], and [111]) in a high-quality Pb
single crystal by means of muon spin rotation/relaxation. No difference in
temperature evolution of $B_{\rm c}$ for all three directions was detected. The
average reduced gap $\alpha=\Delta/k_{\rm B}T_{\rm c}=2.312(3)$ ($\Delta$ is
the zero-temperature gap value and $T_{\rm c}$ is the transition temperature)
was further obtained by employing the phenomenological $\alpha-$model. Our
results imply that the elemental Pb is an isotropic superconductor with a
single energy gap.
