EFFICIENT MONTE-CARLO SIMULATION OF O-16 NEUTRON-ACTIVATION AND N-16 DECAY GAMMA-RAY DETECTION IN A FLOWING FLUID FOR ONLINE OXYGEN ANALYSIS OR FLOW-RATE MEASUREMENT

A Monte Carlo code named MCNAFF has been developed and tested for flow rare measurement and general composition determination of a flowing f luid by neutron activation analysis. Specifically, oxygen determinatio n in a flowing fluid is treated, including simulating the emission and transport of neutrons in the fluid, the activation of O-16 to N-16, t he subsequent flow and dispersion of the N-16 in the flow channel, the downstream decay of N-16, and the subsequent detection of the emitted decay gamma rays. This code is very efficient, partly because (a) the continuous single history approach has been taken, which follows a si ngle history from emission of a neutron, through the production and de cay of the N-16 and the emission of a characteristic gamma ray, and fi nally to the full energy detection of the gamma ray and (b) the princi ple of forcing can be and is used throughout so that almost every hist ory results in a partial success. The present MCNAFF code is capable o f calculating gamma-ray detection yields per neutron emitted to the sa me accuracy as an approach by Perez-Griffo, Block, and Lahey, which nu merically solves the partial differential equations for modeling parti cle dispersion and diffusion and calculates separately by Monte Carlo both the neutron absorption and gamma-ray detection process. The MCNAF F code is estimated to be about two orders of magnitude faster and sho uld be more convenient to use because all calculations are accomplishe d in a single step.