  The superconducting order parameter of the noncentrosymmetric superconductor
LaRhSn is investigated by means of low temperature measurements of the specific
heat, muon-spin relaxation/rotation ($\mu$SR) and the tunnel-diode oscillator
(TDO) based method. The specific heat and magnetic penetration depth
[$\lambda(T)$] show an exponentially activated temperature dependence,
demonstrating fully gapped superconductivity in LaRhSn. The temperature
dependence of $\lambda^{-2}(T)$ deduced from the TDO based method and $\mu$SR
show nearly identical behavior, which can be well described by a single-gap
$s$-wave model, with a zero temperature gap value of $\Delta(0)=1.77(4)k_BT_c$.
The zero-field $\mu$SR spectra do not show detectable changes upon cooling
below $T_c$, and therefore there is no evidence for time-reversal-symmetry
breaking in the superconducting state.
