HW6-11

# HW6-11 - 5-7 10 How many atoms are there in a 1.20 MBq...

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5-7 10. How many atoms are there in a 1.20 MBq sourceof (a) 24Na and (b) 238U? Solution: Because A == >'N we have N(atoms) = A(Bq)j>.(s-l). (a) For 24Na we find from Table A.4 that Tl/2 = 14.96 h = 5.385 X 104 s. Then>.. = In2/TI/2 = 1.287 x 10-5 s-1 Thus the number of atoms of 24Na for a 1.20 MBq source is 1.200 X 106 1.287 X 10-5 A N=-:x= = 9.32 X lWo atoms. (b) For 238U we find from Table A.4 that T 1/2 = 4.468 X 109 Y = 1.410 X 1017 s. Then>. = In 2/Tl/2 = 4.916 X 10-18 S-1 Thus the number of atoms of 238U for a 1.20 MBq source is N = ~ -1.200 X 106 ). -4.916 X 10-18 = 2.44 X 1023 atoms. 11. A very old specimen of wood contained 1012 atoms of 14C in 1986. (a) How many 14c atoms did it contain in 9474B.C.? (b) How many 14c atoms did it
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