Suppose a ²²⁶₈₈Ra nucleus at rest and in ground state undergoes α-decay to a ²²²₈₆Rn nucleus in its excited state. The kinetic energy of the emitted α particle is found to be 4.44 MeV. ²²²₈₆Rn nucleus then goes to its ground state by γ-decay. The energy of the emitted γ photon is _____ keV. [Given: atomic mass of ²²⁶₈₈Ra = 226.005 u, atomic mass of ²²²₈₆Rn = 222.000 u, atomic mass of α particle = 4.000 u, 1 u = 931 MeV/c², c is speed of the light]