Spatial distribution of the neutron and gamma spectra incident onto the external neutron converter at the RB reactor

For experiments in neutron dosimetry, radiation protection and some research work in biology, it is necessary to have sources of fast neutrons in larger experimental space. Therefore, at the RB reactor in Vinca Institute was built the external neutron converter (ENC). Determination of spatial distribution of neutron and gamma spectra is very important for the investigations mentioned above. Computation of these spectra: (1) in the centre of the RB reactor core; (2) escaped from the core through the RB tank outer wall and entering the external neutron converter, and (3) at the ENC exit, is presented in this paper. This calculation is performed by using the well-known MCNP sup T sup M Monte Carlo transport code. Modern data libraries (based on the ENDF/B-VI) are used for the code. The minimal approximations in 3D model of geometry of the problem are introduced. Computation is divided in two runs. In the first run, calculation of neutron and gamma spectra in the RB reactor centre and escaped from the RB tank and entering the external neutron converter is done. The second run is divided in two subruns: (1) neutron transport through the ENC taking into account generation of gamma rays in the ENC, and (2) transport of the incident gamma rays through the ENC. Total gamma spectrum at the exit of the ENC is the sum of the spectra obtained in the subruns (1) and (2). Neutron spectrum calculated by MCNP code at the ENC exit is in good agreement with the measured one (Fig. 5), but the spatial distribution and spectra of gamma rays and neutron entering the ENC (Fig. 2-4) are neither calculated nor measured in the experiments performed up to now. (author)