A method and apparatus for calibrating readings of a downhole tool are provided. The method including collecting data representative of gamma ray photon energies detected by a downhole tool during the well-logging, a spectrum of the gamma ray photon energies detected by the downhole tool having a derivative with respect to the gamma ray photon energies. The method and program comprise calibrating the readings of the downhole tool using at least one predetermined correction factor based on the derivative of the spectrum of the gamma ray photon energies detected by the downhole tool. A computer-readable program storage device and a computer programmed to perform a set of instructions are also provided.