The energy resolution of the 47t detector is 14% at 662 keV and 7% at 2.5 MeV gamma-ray energy, the overall time resolution ls 500 ps and the peak efficiency 90% at 1 MeV. This unique combination of attractive features for a gamma-ray detector became possible by the recent availability of !arge barium fluoride crystals with volumes up to 2.5 I. The detector allows to register capture cascades with 95% probability above a threshold energy of 2.5 MeV in the sum energy spectrum.Neutrons are produced via the 7U(p,n)7Be reactlon using the pulsed proton beam of a The combination of short primary flight path, a 10 cm inner radius of the spherical BaF 2 shell, and the low capture cross section of barium allows to discrimlnate background due to capture of sample scattered neutrons in the scintillator by time of flight, leaving part of the neutron energy range completely undisturbed. This feature tagether with the high efficiency and good energy resolution for capture gamma-rays allows to separate the various background components reliably enough, that the capture cross section ratio of two isotopes can be determined with an accuracy of :s: 1.2 %.

The time-dependent radioecological simulation model ECOSYS-87 has been developed to assess the radiological consequences of short-term depositions of radionuclides. Internal exposure via inhalation and ingestion, as well as external exposure from the passing cloud and from radioactivity deposited on the ground, are included in the model. The site-specific parameter values of the model are representative of Southern German agricultural conditions; however, the model design facilitates adaption to other situations. The ingestion dose is calculated as a function of time considering 18 plant species, 11 animal food products, and 18 processed products. The ingestion and inhalation exposure is estimated for six age groups using age-dependent consumption and inhalation rates and age-dependent dose factors. Results demonstrate a pronounced influence regarding the time of year (season) of deposition on the ingestion dose and on the relative importance of the exposure pathways. Model results compare well with activities in foods measured after the Chernobyl accident.

Model predictions for a rapid assessment and prognosis of possible radiological consequences after an accidental release of radionuclides play an important role in nuclear emergency management. Radiological observations, e.g. dose rate measurements, can be used to improve such model predictions. The process of combining model predictions and observations, usually referred to as data assimilation, is described in this article within the framework of the real time on-line decision support system (RODOS) for off-site nuclear emergency management in Europe. Data assimilation capabilities, based on Kalman filters, are under development for several modules of the RODOS system, including the atmospheric dispersion, deposition, food chain and hydrological models. The use of such a generic data assimilation methodology enables the propagation of uncertainties throughout the various modules of the system. This would in turn provide decision makers with uncertainty estimates taking into account both model and observation errors. This paper describes the methodology employed as well as results of some preliminary studies based on simulated data.

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