  In this article, as an extension of our study on the angular distribution of
the recoil flux of WIMP-scattered target nuclei, we demonstrate a possibility
of determining the mass of incident halo WIMPs by using or combining
"ridge-crater" structures of the angular recoil-energy spectra with different
target nuclei observed in directional direct Dark Matter detection experiments.
Our simulation results show that, for a WIMP mass of only a few tens GeV, the
stereoscopic angular recoil-flux distributions of both of light and heavy
target nuclei would have a (longitudinally) "ridge-like" structure. However,
once the WIMP mass is as heavy as a few hundreds GeV, the angular recoil-flux
distributions of heavy target nuclei would in contrast show a (latitudinally)
"crater-like" structure.
