Gamma ray spectrum:

This value is called the resolution of the photo peak and the detection system.

% Re solution  = FWHM/Peak Position ×100

For a typical NaI (TI) detector, resolution is about 6-8% for 662 keV γ -ray of ¹³⁷ Cs. The resolution is the governing factor in the ability of the detector system to differentiate between the photo peaks of gamma rays of similar energies.  Since denominator is proportional to the γ -ray energy, the resolution will decrease with increasing γ -ray energy. If the radionuclide of interest emits γ -rays having more than one characteristic energy, the gamma ray spectrum will be quite complex.

Figure: Gamma ray spectrum of 137 s with its photo peak and Compton edge. Also shown is the full width at half maximum to express detector resolution.