Intrinsic resolution becomes worse with decreasing γ-ray energy because lower-energy γ rays produce fewer light photons per scintillation event, and smaller numbers of light photons result in larger relative statistical fluctuations in their distribution (Chapter 9, Section B.1). As a rule of thumb, intrinsic resolution is proportional to 1/ E, in which E is the γ-ray energy. This follows because the number of scintillation light photons produced, N, is roughly proportional to E and the relative statistical fluctuations in their distribution are therefore proportional to 1/ N. This causes noticeably greater blurring at lower γ-ray energies. An example of the change of intrinsic spatial resolution as a function of γ-ray energy is shown in Figure 14-1.