  The bound beta-decay (BoB) of neutron is also known as the two-body neutron
decay, which is a rare decay mode into a hydrogen atom and an anti-neutrino.
The state of neutrino can be exactly inferred by measuring the state of
hydrogen atom, providing a possible pathway to explore new physics. However,
this rare decay mode has not yet been observed so far since it was predicted in
1947. The challenge in observing this decay is not only that its cross section
is extremely low, equivalent to about branching ratio of the order of $10^{-6}$
of the three-body decay, but also that the final-state hydrogen atom is neutral
and has extremely low kinetic energy, which cannot be effectively detected. In
this study, we propose a microcalorimeter-based scheme for measuring the
kinetic energies of hydrogen atoms produced from BoB of ultracold neutrons,
which has a great advantage in terms of accuracy of the energy measurement. In
this study, first, several important issues that require rigorous
considerations for the decay measurements and possible solutions are discussed.
Then, the requirements of the neutron flux and the appropriate structure design
of the microcalorimeter are present by theoretical calculations. In short, this
paper outlines our proposed novel experimental scheme for observing the BoB
mode, addressing the possible solutions to all the necessary problems.
