Characterisation of radiation field for irradiation of biological samples at nuclear reactor-comparison of twin detector and recombination methods

Central Laboratory for Radiological Protection is involved in achieving scientific project on biological dosimetry. The project includes irradiation of blood samples in radiation fields of nuclear reactor. A simple facility for irradiation of biological samples has been prepared at horizontal channel of the nuclear reactor MARIA in NCBJ in Poland. The radiation field, composed mainly of gamma radiation and thermal neutrons, has been characterised in terms of tissue kerma using twin-detector technique and recombination chambers. Radiobiological measurements in neutron radiation fields always should be accompanied with careful dosimetric measurement because of large variations in composition and neutron energy. In the case of measurements described here, the main problem was the requirement of precise determination of neutron absorbed dose. It was decided to use two measuring methods-twin-detector and recombination method, both based on the use of ionisation chambers. The application of the same type of detectors greatly simplified and shortened the measuring campaign. Total kerma rate was determined with the same chamber for both methods, whereas the determination of?γ is independent and the measured values could be compared for cross-check of the results. The use of combined recombination method using concepts of RMM and RIQ appeared to be very useful. The value of RIQ for neutrons alone was additionally determined and can be used in further evaluation of radiobiological data. Time needed for the determination of saturation curve makes it possible to observe stability of the beam. It was concluded that the neutron kerma rate was high enough to perform radiobiological irradiations. Stability of the beam was very good and continuous monitoring of the beam intensity was practically needed only for the case of unexpected changes in the reactor operation. (authors)