Portal monitors, are instruments for external contamination measurement. They are used for monitoring of personnel, which exit a nuclear facility. Portal monitors are usually last line of defence against contaminated people and equipment leaving the station. These detectors are able to measure simultaneously beta and gamma contamination. The total body surface including hands, head and feet is measured by many beta gas flow proportional counters and several large additional gamma scintillation detectors, which are positioned in the breast area (left and right side). Large scintillation detectors (plastic or liquid) are installed in order to be able to detect the passage of low levels of gamma activity. As they are only sensitive to gamma radiation, they are able to detect incorporated gamma activity if present. In this way, if some internal contamination exists, the worker is immediately sent for whole-body counting and the dose assessment is much more accurate.