fission_radiation_yield_report

fission_radiation_yield_report - UCRL-AR-228518 Simulation...

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Unformatted text preview: UCRL-AR-228518 Simulation of Neutron and Gamma Ray Emission from Fission Jerome M. Verbeke, Chris Hagmann, Doug Wright Lawrence Livermore National Laboratory January 5, 2009 Contents 1 Data and physics models 2 1.1 Neutron number distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Neutron energy distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 Gamma-ray number distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.4 Gamma-ray energy distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2 User manual 12 2.1 Limitations of the fission library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.2 Fission library interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.3 Geant4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.4 MCNPX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.5 COG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 References 20 Contact info: wright20@llnl.gov, 925-423-2347 1 1 Data and physics models This paper describes a general-purpose and extensible software library to accurately simulate neutron and gamma-ray emission from fission reactions (both spontaneous and neutron induced). This was motivated by detailed statistical studies of fission chains in multiplying media. This model is data-driven and incorporates all available multiplicity measurements found in the literature. Empirical models are employed whenever multiplicity data are not available. Essentially no data are available for the correlations between the neutrons and gammas, so this model samples these distributions independently. By default, this model effectively scales the multiplicity data to match the average multiplicity value ( ) found in external evaluated data libraries. At present the gammas and neutrons are emitted isotropically. The data and empirical models are described in detail in the following subsections. This model has been incorporated into the latest release of Geant4 and has been submitted for inclusion in future versions of MCNPX and COG. The standalone version of the software library can be downloaded from http://nuclear.llnl.gov/simulation . 1.1 Neutron number distribution Based on reasonable assumptions about the distribution of excitation energy among fission fragments, Ter- rell [1] showed that the probability P of observing neutrons from fission can be approximated by a Gaussian-like distribution n = P n = 1 2 integraldisplay + 1 2 + b e t 2 2 dt (1) where is the average number of neutrons, (set to 1.079) is the width of the distribution, and b is a small correction factor ( b < . 01) that ensures that the discrete probability distribution has the correct average...
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fission_radiation_yield_report - UCRL-AR-228518 Simulation...

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