A system has been developed by SÚRO in the collaboration with the company Envinet, a.s. (now NUVIA) for the large-scale monitoring of accidental radioiodine thyroid dose in population. Six gamma-ray spectrometry scintillation units equipped with collimators are connected to at least two PC stations for registration of measured subjects and operation of the units. Barcode and RFID readers built in the measurement units facilitate matching the registered subject and measurement result in this multi-unit detection system. In order to achieve up to thousand subjects to be counted within one day, two- or tree-minute measurement time is sufficient for the detection of 100 Bq of I-131 in thyroid in non-elevated ambient background. The hardware and software of the system is fully adapted also for the measurement of children. The capacity of the system has been verified in a field exercise.

In the case of an accidental release of I-131, a system for large-scale monitoring of the population for the radionuclide intake is necessary, which has been recognized by various national and international initiatives. In the Czech Republic, the monitoring is ensured by SÚRO (National Radiation Protection Institute) within the frame of the national Radiation Monitoring Network. For a proper assessment of the radiation dose to a thyroid in population, the monitoring system should have the capability of measuring adult as well as child subjects across a wide range of ages.

The system intended in SÚRO for the large scale monitoring of radioiodine activity in a thyroid consists in a transportable set of six collimated scintillation gamma-ray spectrometers linked as a portable instruments with personal computer-based software. The hardware and software ensure

• registration of people for measurement,
• measurement semi-control,
• gamma-ray spectra evaluation, and
• result recording and further processing.

Three trained persons are required for operation the system during the monitoring process. With two minute counting time the detection level of 100 Bq can be expected for I-131 in non-elevated ambient background. The thyroid monitoring system is shown in the Figure 1.

Two field exercises were organized by the Czech Fire & Rescue Service that plays the key role in the system  of emergency preparedness.

The exercises provided required information on

• real time of the deployment of the system for measurement
• throughput in number persons that can be measured per time unit
• reaction of the system to power cut-off
• effort demanded on the operators, etc.

The presence and behaviour of non-professional figurants has revealed variety unexpected issues and has extended the number of situations that should be considered and trained in advance. Under uncomplicated conditions, around 100 persons can be counted for radioiodine activity in thyroid per hour. The necessity to inform the measured person about the measurement result may require that another operator should be in place and communicate with measured persons about the results.

ACKNOWLEDGEMENTS: The work is funded by the project of the Ministry of the Interior of the Czech Republic  VF20162016050.