In a semiconductor, the number density of intrinsic charge carries at 27$$^\circ$$C is 1.5 $$\times$$ 10¹⁶/m³. If the semiconductor is doped with impurity atom, the hole density increases to 4.5 $$\times$$ 10²²/m³. The electron density in the doped semiconductor is ___________ $$\times$$ 10⁹/m³.

The nuclear activity of a radioactive element becomes $${\left( {{1 \over 8}} \right)^{th}}$$ of its initial value in 30 years. The half-life of radioactive element is _____________ years.