Estimation of background interference in prompt-gamma neutron activation using MCNP

Prompt-gamma neutron activation (PGNA) is used to measure total-body nitrog en and hydrogen in humans. Background interference in the gamma spectra ari ses from both subject and shielding. A Monte Carlo simulation program (MCNP 4B2) was used to examine the neutron and gamma signals in the PGNA system ( (AmBe)-Am-241 source). N and H peak regions were assessed in the presence a nd absence of calibration phantoms. The simulations suggested extracorporea l H peak contributions of up to 30%, depending on subject body habitus. Mos t N background could be attributed to detector pileup events. The MCNP resu lts allowed us to improve shielding design and develop background correctio n algorithms to improve measurement precision.