13.7 An unknown amount of radioactive gold is deposited as a thin layer on the ends of two rods of gold. The two rods are then joined to form a specimen having a planar source of radioactive gold (Au") atoms at the origin x = 0. After diffusion for 100 hours at 920C, the distribution of Au' is as shown below. Calculate the self-diffusion coefficient of gold in pure gold, based on the data at 0.3 mm and 0.6 mm as indicated in the plot of the relative concentration of gold. Thin layer of Au* Relative concentration of radioactive gold, Au* -1.5 -12 09 -06 -03 0 0.3 06 09 1.2 1.5 Distance, x, mm