Answer
$N = 6.0 \times 10^5$
Work Step by Step
The number of electron-hole pair is the energy supplied by the gamma ray. Electron acquires this $E_g$ to make a transition to a conduction band.
The number of electrons that can be excited across the gap is calculated using
$ N = \frac{E}{E_g}$
where
$E_g = 1.10 eV$
$E = 662 keV$
$ N = \frac{ 662 keV}{1.10 eV}$
We obtain
$N = 6.0 \times 10^5$
