One mole of a monatomic ideal gas undergoes an adiabatic expansion in which its volume becomes eight times its initial value. If the initial temperature of the gas is 100 K and the universal gas constant $\mathrm{R}=8.0 \mathrm{~J} \mathrm{mol}^{-1} \mathrm{~K}^{-1}$, the decrease in its internal energy, in Joule, is $\_\_\_\_$ .