Energy deposition on nuclear emulsion by slow recoil ions for directional dark matter searches

The electronic energy deposited on nuclear emulsions by C ions of 5--200 keV and Kr ions of 5--600 keV is evaluated and compared with those deposited by fast ions for designing and fabricating fine-grain nuclear emulsions for directional dark matter searches. The nuclear quenching factor and the electronic linear energy transfer, which refers to the specific electronic energy deposited along the ion track, are evaluated. The so-called core and penumbra of heavy-ion track structure is modified to understand the track caused by recoil ions produced by dark matter candidate, i.e., weakly interacting massive particles, striking a nucleus in the AgBr crystal of nuclear emulsion. Very heavy recoil ions, 100--180 keV Pb ions, produced in $\ensuremath{\alpha}$ decay are also studied. Furthermore, the track structures due to protons of 25--80 keV are evaluated to consider the influence of background neutrons in underground laboratories.