How do I convert F4:n tally results to number of particles?

I have electron source with 4.9E8 particles, and F4:n tally (size 3x3 cm^2) which gives me result 3.4E-9 n/cm^2. When I multiply 3.4E-9x4.9E8x9 the result is15,12  particles in my detector, but...

13 November 2015 7,323 3 View