Spontaneous fission rate for sample of U-235 (1mg/cm^2)

Goal: Calculate the fission rate of a bomb grade U235 that has a mass of .91*10^-3 g. Procedure:

   Given- T1/2 = 7.038 * 10^8 year =>7.038 * 10^8 year * 3153600 seconds/year = 2.2195 * 10^16 sec
          m = .91 * 10^-3 g

   Calculations- No = (Na/A) * m = ([6.022 * 10^23 nuclei/mol] / [235 g/mol])* (.91 * 10^-3 g) = 2.33 * 10^18 nuclei
                 [math]lambda[/math] = ln(2)/T1/2 =ln(2)/(2.2195 *10^16 sec) = 3.12 * 10^-17 sec-1