  The electronic energy deposited on nuclear emulsions due to C ions of 5 --
200~keV and Kr ions of 5 -- 600~keV are evaluated and compared with those due
to fast ions for design and construction of fine grain nuclear emulsion for
directional dark matter searches. Nuclear quenching factors and the electronic
LET (linear energy transfer), the specific electronic energy deposited along
the ion track, are evaluated. The so-called core and penumbra of heavy-ion
track structure is modified for understanding the track due to recoil ions
produced by dark matter candidate, WIMPs, striking nucleus in the AgBr crystal
of nuclear emulsion. The very heavy recoil ions, 100 -- 180~keV Pb ions,
produced in $\alpha$-decay are also studied. In addition, the track structures
due to proton ions of 25 -- 80~keV are evaluated to consider the influence of
background neutrons in underground laboratories.
