I'm not fully aware of what you intend to investigate or to see. Is it that you want to see emission of photons of nuclear excited states ? Or do you look for atmic excitations as in the classic X ray fluorescence experiments ? In the first case be aware that nuclear states can only be excited if all quantum numbers match and also the exciting photons have an energy corresponding to the energy gap between ground state and excited state (resonance experiment) like Moessbauer. That is barely the case. Even if you do resonant scattering with photons (from a source) the emitting atom will suffer recoil and the emitted energy is reduced due the recoil energy. With a Co source you could theoretically only excite states in 60Ni which is the daughter of 60Co after beta decay. But also only if you account for the recoil energy. You can do other excitations with gammas like rotational or vibrational states of even gamma disintegration but here you would rather need bremsstrahlung up to 10 or 11 MeV as available from electron accelerators or to particle induced gamma emission.

If you want to excite atomic levels that is fluorescence light from electron holes in the atom you have to deal with the effect that the cross section for the photoelectric effect as a source for vacancies in the electron orbitals for 1MeV gammas has dropped some orders of magnitude compared to the keV regime. 60Co is emitting basically 2 distinct gammas at approx 1.17 MeV and 1.34 MeV. The propability to excite atomic states below 100 keV is therefore almost zero and not 1%

Thanks Joerg for your comments and sorry for delay response, its really helpful to me.  I am sorry for the short description in the question, however I was investigating the Eu3+ doped A2B2O7 nanoparticles and trying to see the Eu3+ fluorescence under gamma excitation. As I mentioned before I was using 120 kV x-Ray source to see the Eu3+ fluorescence however I was expecting to see something weak under Co-60 (0.5 mCi) source too. Definately, gamma absorption cross-section should be lower compared to that of X-Ray. Based on that I may need gamma source with higher activity. With your experience in this area, could you please suggest some more idea in this area. If you can share some papers, it will be helpful too.

Hi, I suggest you to read the work of my colleagues: Lecavalier et al. Chem. Commun., 2013, 49, 11629-11631. DOI: 10.1039/C3CC46209A