A method of detecting and identifying radioactive materials by processing a gamma ray spectrum acquired from a target, the method comprising: acquiring the gamma ray spectrum from the target as the target passes through a detection zone; determining whether the gamma ray spectrum of the target belongs to a first class or to a second class of gamma ray spectra using optimal loading coefficients, wherein the optimal loading coefficients have been obtained using Fisher linear discriminant analysis by: computing a between-class scatter matrix and a within-class scatter matrix for the gamma ray spectra in the first class and the second class; and computing the optimal loading coefficients from the generalized eigenvector corresponding to the largest generalized eigenvalue of the between-class and within-class scatter matrices, wherein the determining whether the gamma ray spectrum of the target belongs to the first class or to the second class further comprises: - computing a distance between the gamma ray spectrum of the target projected by the optimal loading coefficients and the gamma ray spectrum of the second class projected by the optimal loading coefficients; and - determining that the gamma ray spectrum of the target belongs to the first class when the computed distance is above a threshold distance; wherein the first class comprises at least one artificial gamma ray spectrum being one of (i) a constant value with an additive Gaussian noise component, and (ii) a quasi-linear spectrum with an additive Gaussian noise component wherein the second class comprises gamma ray spectra of naturally occurring radioactive materials, NORMs; and generating an output signal dependent on the determining. acquiring the gamma ray spectrum from the target as the target passes through a detection zone; determining whether the gamma ray spectrum of the target belongs to a first class or to a second class of gamma ray spectra using optimal loading coefficients, wherein the optimal loading coefficients have been obtained using Fisher linear discriminant analysis by: computing a between-class scatter matrix and a within-class scatter matrix for the gamma ray spectra in the first class and the second class; and computing the optimal loading coefficients from the generalized eigenvector corresponding to the largest generalized eigenvalue of the between-class and within-class scatter matrices, computing a between-class scatter matrix and a within-class scatter matrix for the gamma ray spectra in the first class and the second class; and computing the optimal loading coefficients from the generalized eigenvector corresponding to the largest generalized eigenvalue of the between-class and within-class scatter matrices, wherein the determining whether the gamma ray spectrum of the target belongs to the first class or to the second class further comprises: - computing a distance between the gamma ray spectrum of the target projected by the optimal loading coefficients and the gamma ray spectrum of the second class projected by the optimal loading coefficients; and - determining that the gamma ray spectrum of the target belongs to the first class when the computed distance is above a threshold distance; - computing a distance between the gamma ray spectrum of the target projected by the optimal loading coefficients and the gamma ray spectrum of the second class projected by the optimal loading coefficients; and - determining that the gamma ray spectrum of the target belongs to the first class when the computed distance is above a threshold distance; wherein the first class comprises at least one artificial gamma ray spectrum being one of (i) a constant value with an additive Gaussian noise component, and (ii) a quasi-linear spectrum with an additive Gaussian noise component (i) a constant value with an additive Gaussian noise component, and (ii) a quasi-linear spectrum with an additive Gaussian noise component wherein the second class comprises gamma ray spectra of naturally occurring radioactive materials, NORMs; and generating an output signal dependent on the determining.