The choice of regions of interest (ROIs) in the annihilation gamma line spectrum, which define the S and W parameters commonly used in the study of vacancy-type defects in Si, has been optimized by simulations using theoretical Doppler-broadened spectra. The figure of merit used is the difference between the S(W) parameter and its value in bulk Si expressed as multiple of the standard deviation in S(W). The optimum ROIs depend on the crystalline orientation of the Si and the energy resolution of the gamma ray detector used. Experimental data on B+-implanted Si were found to be consistent with the simulation results, which can therefore be used reliably to guide further experimental work. (C) 2002 Elsevier Science B.V. All rights reserved.