Use of the MCNPX to calculate the neutron spectra around the GE-PETtrace 8 cyclotron of the CDTN/CNEN, Brazil

The Monte Carlo code MCNPX was used to calculate the neutron spectra in 4 points around the targets of the CDTN/CNEN cyclotron, Belo Horizonte, Brazil, during the production of the 18FDG. Simulated data were compared with experimental data obtained with a Bonner multisphere spectrometry system (BSS) using TLD-600 and TLD-700 and the unfolding codes BUNKIUT, BUMS and NSDUAZ. In general, simulated spectra disagreed with those obtained by experimental means by a factor as high as 14. Measurements performed with a doserate meter in other 3 more shielded points, showed also an overestimation of the ambient dose equivalent rate by a factor as high as 20 in comparison with simulated results. Results are not conclusive and a more refined study is necessary. However, neutron emission rate of the source-term of radiation must be investigated and an special caution must be taken in the experimental measurements, by discriminating of the target selected for the irradiations and utilizing a matrix response suitable for the passive detectors (e.g. TLD) utilized in the experiments, instead of a matrix response (e.g. UTA4) developed for scintillation detectors.