  A low-energy electron diffraction (LEED) apparatus which works at
temperatures down to about 100 mK is designed to obtain structural information
of 2D helium on graphite. This very low temperature system can be realized by
reducing the thermal inflow from the LEED optics to the sample which is cooled
by cryogen-free dilution refrigerator. The atomic scattering factor of He is
also estimated using a kinematical model, which suggests that the diffraction
signal from He atom can well be obtained by using a delay-line detector instead
of a fluorescent screen.
