Various groups around the world are engaged in the compilation and evaluation of decay data for all known and specific radionuclides. Many evaluators operate independently and recommend slightly different values for the same parameter. Even small deviations in the recommended data can have a significant impact on definition of the decay characteristics of radionuclides used as standards in detector efficiency calibrations and subsequent applications. This report presents the results of a Coordinated Research Project on X ray and gamma ray decay data standards for detector calibration and other applications. Recommended half-lives and X and gamma ray emission probabilities are listed in the first part of this report for a carefully selected set of radionuclides and nuclear reactions that are suitable for detector efficiency calibration and other applications. The evaluation procedures used to obtain the recommended values and their uncertainties are detailed in the second part, along with comments on the remaining discrepancies. An appropriate set of suitable X and gamma ray standards have been derived with the expectation that the recommended values will be recognized as international reference standards. --Publisher's description.