A liquid level measurement system employing a radio-isotope is mounted on a tank as shown in the figure. The absorption coefficient of water for the radiation is $7.7 \;m^{-1}$. If the height of water in the tank is reduced from $100\; mm$ to $90\; mm$, the percentage change in the radiation intensity received by the detector, neglecting absorption of the radiation by air, is ______________ $\%$.
