We have examined the feasibility of a new online nanoscale diffusion measurement method using a radioactive Li-8 tracer by computer simulations. We have found that the detection limit of the lithium diffusion coefficient can be improved to a low value of 1 x 10(-12) cm(2)/s by detecting alpha particles emitted at a small angle relative to a sample surface that is irradiated with a low-energy Li-8 beam of about 8 keV. (C) 2013 The Japan Society of Applied Physics