Suppose a ${ }_{88}^{226} \mathrm{Ra}$ nucleus at rest and in ground state undergoes $\alpha$-decay to a ${ }_{86}^{222} \mathrm{Rn}$ nucleus in its excited state. The kinetic energy of the emitted $\alpha$ particle is found to be $4.44 \mathrm{MeV} .{ }_{\$ 6}^{222} \mathrm{Rn}$ nucleus then goes to its ground state by $\gamma$-decay. The energy of the emitted $\gamma$-photon is $\_\_\_\_$ keV , [Given: atomic mass of ${ }_{88}^{226} \mathrm{Ra}=226.005 \mathrm{u}$, atomic mass of ${ }_{86}^{222} \mathrm{Rn}=222.000 \mathrm{u}$, atomic mass of $\alpha$ particle $=4.000 \mathrm{u}, 1 \mathrm{u}=931 \mathrm{MeV} / \mathrm{c}^2, \mathrm{c}$ is speed of the light]