  Due to their large coherent scattering cross section, diamond nanoparticles
(DNPs) are considered as a promising candidate material for a new neutron
reflector. For investigation of scattering cross sections of packed samples, we
have developed a technique for mechanical compression of DNP powder.
Application of 220 MPa allowed us to increase the bulk density from 0.40
g/cm$^3$ to 1.1 g/cm$^3$. The differential cross sections of uncompressed and
packed samples were measured using the high-intensity total diffractometer
instrument NOVA at J-PARC, covering transfer wavenumbers (q) from 0.6 to 100
nm$^{-1}$. The q dependence for the compressed sample agreed with the
theoretical expectation derived from the Born approximation applied to
homogeneous spheres with inclusion of a hard-sphere model to account for the
inter-particle structure, whereas the results obtained from the powder sample
disagreed. This implies that the theoretical description does not well
represent the mesoscopic structure of the DNP powder sample.
