Dear Chailin,
The default minimum particle kinetic energy in FLUKA using the PRECISIOn default is generally 100 keV, except for photons and neutrons. In case of photons it is 33 keV and 1e-5 eV for neutrons.
The thresholds can be lowered using the EMFCUF card to 1 keV for electrons and 100 eV for photons.
You can read more about the EM thresholds here:
Cheers,
David