This International Standard is applicable to a portable or transportable photon spectrometry assembly using a high purity germanium (HPGe) detector to survey, in situ, generally at 1 m above ground level, areas in the environment for discrete radionuclides. Such equipment is used to make rapid assessments of activity levels and corresponding free air exposure rates from photon emitting radionuclides. Such measurements may be used to develop guidance for subsequent follow-up action, for example including radiological asessments, sampling and monitoring programmes.