This paper provides guidance for determining the neutron activation profile of core drill samples taken from the biological shield of nuclear reactors using gamma spectrometry measurements. Thus, it provides guidance for selecting a model of the right form to fit data and using least squares methods for model fitting. The activity profiles of two core samples taken from the biological shield of a nuclear reactor were determined. The effective activation depth and the total activity of core samples along with their uncertainties were computed by Monte Carlo simulation. •The paper provides guidance for determining the activity profile of core drill samples.•Effective activation depth and total activity are computed by Monte Carlo simulation.•Exponential fit is the simplest way for evaluating the activity profile of core samples.