Optimisation of gamma spectrometry measurements in atmosphere during nuclear emergencies.

During emergencies following a major nuclear accident a prompt and accurate characterisation of the radioactive cloud is one of the main task of any radioactivity monitoring system. This task is usually performed by means of γ-spectrometry High Purity Germanium (HPGe) measurements on atmospheric particulate samples gathered by means of high-volume pumps. The key parameters describing the performances of a monitoring system are the Minimum Detectable Activities (MDAs) of the most relevant radionuclides. These parameters depend on a number of factors related to the efficiency of the available germanium detector, the volume of air filtered by the sampling devices and the γ-decay scheme of each radionuclides as well. Besides the MDAs, another very important characteristic of a monitoring system, especially during an evolving emergency, is its capability of giving reliable results at a given and constant pace. It is therefore important to define the time resolution of the monitoring system, i.e. the minimum time needed to produce the data, namely the activity concentrations of the radionuclides in the atmosphere. The optimization of the measurements procedures are discussed in this work: in particular it is demonstrated that, being t the time resolution of the monitoring system, the lowest MDAs can be achieved with a sampling time given by (2/3)·t and a counting time of (1/3)·t. Finally, the MDAs achievable for a standard monitoring system based on a 30% HPGe detector are calculated for all the most important fission products.