A method and apparatus are provided for correcting gamma ray data representative of gamma ray energies for spectral degradation. The method and apparatus include degrading reference gamma ray spectra. At least one correction factor is calculated between the degraded gamma ray spectra and the reference gamma ray spectra. The gamma ray data are then corrected using a calculated correction factor. Another method is provided for determining a correction factor for correcting data representative of gamma ray energies for spectral degradation. The method includes disposing a downhole tool in a simulated environment representative of actual downhole conditions, the tool including a neutron source and at least one gamma ray detector. The temperature of at least one of the gamma ray detectors of the tool is then varied while the simulated environment is irradiated with neutrons emitted from the neutron source. Gamma ray energy signals are then detected at the at least one detector in response to gamma rays produced during nuclear reactions between the neutrons and materials in and of the simulated environment. A characteristic of the simulated environment is then determined along with a characteristic of the at least one detector. The determined characteristics of the simulated environment and of the at least one detector are then correlated to determine at least one correction factor.