  We developed a calorimeter with a vacuum container made of superconducting
niobium (Nb) to study monolayers of helium adsorbed on graphite which are
prototypical two-dimensional quantum matters below 1 K. Nb was chosen because
of its small specific heat in the superconducting state. It is crucially
important to reduce the addendum heat capacity ($C_{\rm{ad}}$) when the
specific surface area of substrate is small. Here we show details of design,
construction and results of $C_{\rm{ad}}$ measurements of the Nb calorimeter
down to 40 mK. The measured $C_{\rm{ad}}$ was sufficiently small so that we can
use it for heat capacity measurements on helium monolayers in a wide
temperature range below 1 K. We found a relatively large excess heat capacity
in $C_{\rm{ad}}$, which was successfully attributed to atomic tunneling of
hydrogen (H) and deuterium (D) between trap centers near oxygen or nitrogen
impurities in Nb. The tunnel frequencies of H and D deduced by fitting the data
to the tunneling model are consistent with the previous experiments on Nb doped
with H or D.
