Simulation MCNP Code

In large samples, the y-ray count rate of a PGNAA system is a multivariable function of the elemental dry composition, density, water, contents, and thickness of the material. The experimental calibration curves require tremendous laboratory work using a large number of standards with well-known compositions. The Monte Carlo simulation code MCNP helps to reduce the experimental standards as described by Oliveira et al. (1997) in their attempt to optimize the PGNAA instrument design for cement raw materials. 

The sensitivity of NAA for a particular element depends on the irradiation parameters (i.e., neutron flux, irradiation, and decay times), measurement conditions (i.e., measurement time, detector efficiency), nuclear parameters of the elements being measured (i.e., isotope abundance, neutron cross-section, half-life, and y-ray abundance). The accuracy of an individual NAA determination usually ranges from 10 to 10 ° grams per gram of sample. Accuracy of a NAA determination is usually between 2% and 10% of the reported value, depending on the element analyzed and its concentration in the sample. 

The lower limit of detection using NAA is given as (Kruger 1971) 

Choosing optimum irradiation, decay, and counting times can improve sensitivity for many of the above significantly. However, some of the elements namely B, Be, Bi, C, Li, N, Nb, Ne, O, P, Pb, S, Si, and T1 are not ordinarily detected by this method. 

Sensitivity (m pfl) Elements in Order of Increasing Atomic Number Z (in Each Row) 

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MCNP Monte Carlo N-Particle code - a computer program for simulating the process of gamma-ray detection, used in gamma spectrometry to generate mathematically efficiency curves. 

Several codes are available for carrying out a direct Monte Carlo simulation of a reactor problem using detailed geometrical models and continuous energy (or very fine group) representation of nuclear data. These can be used to provide reference values and investigate the effects of approximations in deterministic methods. Some widely used Monte Carlo codes are MCNP [4.39], MORSE [4.66] and KENO [4.67], amongst others. 

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