From the fitting of the peaks, one obtains the frequency of the incoming rmf photons. Next calculate the energy of these photons by multiplying by hbar (E=freq*hbar). Since this is at resonance, the energy of the photons corresponds to the hyperfine splitting g*B*µ_n. Now B is composed from the applied external field B0 and the induced field dB. From equating the energies B can be determined and by substracting the known B0, dB can be determined.