The key to doing microbeam studies is to precisely control and accurately deliver the counted number of particles to a target position. By studying the Columbia University single particle microbeam, this report shows that it can irradiate the targets with a defined number of ,α, particles, and the collimation of beam size according to the resolution of 20 α particles is ~3~4,µ,m which is better than the designed index. Besides, it can be used for cytoplasmic irradiation with a hitting probability nearly 90% and the odds to hitting nucleus during this process is less than 0.8% for most doses. Radiological studies using this facility found that only 20% of irradiated cells were killed by a single ,α, particle, and the dose response curve for nuclear irradiation with exact number of particles was not significantly different from the curve obtained using average particle traversals of broad-field exposure. The mutation data in microbeam studies provide the first demonstration that a single ,α, particle traversal induced mutations in A,L, cells at a level 3X greater than the background value, and the mutation yield increased in a dose dependent manner. This results was quite different from the data obtained from broad field exposure which showed that at high doses, the mutation frequencies decreased.