Monte Carlo photon codes: MCG and MCP

Cashwell, E.D.; Neergaard, Jon R.; Everett, Cath; Schrandt, R.G.; Taylor, William M.; Turner, G.D.

doi:10.2172/4480451

Cited by 17 publications

(22 citation statements)

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“…The photo-atomic data in the ENDF/B-VI format are based on the evaluated photon data library (EPDL97) [42]. However, the work supporting this report observed that the sections of code describing the form factor evaluation of coherent and incoherent scattering had not been updated since their precursor code, MCP, was written in 1973 [43]. This legacy code results in significant inaccuracies in photon spectra computed by MCNPX when simulating geometries that are typical of NRF experiments [44].…”

Section: Appendixmentioning

confidence: 92%

Nuclear Resonance Fluorescence for Safeguards Applications

Ludewigt¹,

Quiter²,

Ambers³

2011

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Executive SummaryIn nuclear resonance fluorescence (NRF) measurements, resonances are excited by an external photon beam leading to the emission of gamma rays with specific energies that are characteristic of the emitting isotope. The promise of NRF as a non-destructive analysis technique (NDA) in safeguards applications lies in its potential to directly quantify a specific isotope in an assay target.This report addresses the assessment of NRF-based methods for safeguards applications in the context of related studies at LBNL, LLNL, and BNL. Our FY10 effort was comprised of three tasks: the study of the non-resonant scattering background and its simulation with MCNPX, analysis of our previously performed NRF transmission experiment, and the assessment of NRF for safeguards applications.While the recent correction of the treatment of Rayleigh scattering in MCNPX resulted in much better agreement with some experimental data, the photonuclear processes, which are important contributors to the elastic scattering background at higher energies, are still not included. Our analysis showed that calculations based on ENDF form factors as currently implemented in MCNPX could underestimate the elastic scattering cross section by as much as a factor of ten for a uranium target at photon energies above 2 MeV. Even larger discrepancies, up to several orders of magnitude, are possible for lighter elements such as zirconium. It would be desirable to at least include nuclear Thomson scattering in MCNPX simulations for NRF studies.The transmission experiment, using a bremsstrahlung beam and a target of comparable thickness to a spent nuclear fuel assembly, demonstrated sensitivity to a 238 U content of 1%. However, the precision was count rate limited. The data obtained in this experiment indicated notch refill that could change the measured NRF rates by up to 5% for the worst case. A correction based on MCNPX modeling has been implemented in the analysis.NRF-based methods were assessed for three potential safeguards applications: the isotopic assay of spent nuclear fuel (SNF), the measurement of 235 U enrichment in UF 6 cylinders, and the determination of 239 Pu in mixed oxide (MOX) fuel. Given the small integrated nuclear resonance cross sections, the main challenge in these application, albeit to a varying degree, lies in accruing of sufficient counting statistics in an acceptable measurement times for achieving the desired uncertainties.Pu isotopic masses in SNF could precisely be determined in transmission measurements using bremsstrahlung sources, but such measurements would require up to 100's of hours, a very intense bremsstrahlung source, and a very large array of fast detectors with high energy resolution. Quasimonoenergetic photon sources such as Laser Compton scattering sources could potentially enable greatly improved performance. As an example, assuming a photon source with a 1 keV energy spread, an intensity of 6x10 8 ph/eV/s, and operating continuously or at very high pulse rates, the measurement time would be on ...

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Section: Appendixmentioning

confidence: 92%

Nuclear Resonance Fluorescence for Safeguards Applications

Ludewigt¹,

Quiter²,

Ambers³

2011

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“…The photo-atomic data in the ENDF/B-VI format are based on the evaluated photon data library (EPDL97) [49]. However, the work supporting this report observed that the sections of code describing the form factor evaluation of coherent and incoherent scattering had not been updated since their precursor code, MCP, was written in 1973 [50]. This legacy code results in significant inaccuracies in photon spectra computed by MCNPX when simulating geometries that are typical of NRF experiments [51].…”

Section: Rayleigh Scattering In Mcnp D1 Introductionmentioning

confidence: 92%

“…The derivation and use of momentum transfer values in MCNP has been poorly documented. MCP and MCNP user manuals describes q in units of cm −1 ; [48,50] however, the data tabulated in MCNPX are in units ofÅ −1 [52]. The legacy problems in elastic scatter come from the fact that MCNP has historically had a fixed array of q-values for which coherent scattering form factors, C(Z, q) are defined.…”

Section: Rayleigh Scattering In Mcnp D1 Introductionmentioning

confidence: 99%

Nuclear Resonance Fluorescence to Measure Plutonium Mass in Spent Nuclear Fuel

Ludewigt¹

2011

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Executive SummaryThe Next Generation Safeguard Initiative (NGSI) of the U.S. Department of Energy is supporting a multi-lab/university collaboration to quantify the plutonium (Pu) mass in spent nuclear fuel (SNF) assemblies and to detect the diversion of pins with non-destructive assay (NDA) methods. Nuclear Resonance Fluorescence (NRF) has been investigated as one of 14 NDA techniques included in the NGSI study. In contrast to many of the other, older techniques, NRF had not previously been considered for nuclear safeguards or similar applications and basic aspects including nuclear data, modeling and simulation tools, measurement methods, and instrumentation needed to be researched.Nuclear resonance fluorescence is the process by which a nucleus is excited to a specific state by the absorption of a photon, then subsequently de-excites to the ground state by the emission of one or more γ rays. The energies of the γ rays are characteristic of the specific state that underwent NRF and therefore are characteristic of the isotope. The emitted NRF γ rays are detected at backwards angles where the backgrounds from elastically and inelastically scattered photons are lowest. Two distinct assay methods, the backscatter and the transmission methodsare analyzed in this report. In both cases a beam of energetic photons excites nuclear resonances of the isotopes of interest (IOI) within the SNF. In the backscatter assay geometry, the emitted NRF γ rays detected at backwards angles are a measure of the amount of a specific isotope in the SNF. In transmission assay, a detection system down-stream of the SNF measures the excess attenuation of resonant-energy photons due to NRF in the SNF assembly. The detection system consists of a thin sheet composed of the isotope of interest, called the transmission detector (TD), and a detector array that measures the NRF γ rays emanating from the TD. Given the low concentrations of the Pu isotopes in SNF and the relatively small integrated nuclear resonance cross sections, the main challenge for achieving precise and accurate measurements lies in accruing sufficient counting statistics in an acceptable measurement time.The goals of this study were to design and model suitable NRF measurement methods, to quantify capabilities and corresponding instrumentation requirements, to investigate possible implementations, and to evaluate the prospects and potential of the technique for the assay of SNF assemblies, i.e., for the measurement of their Pu isotopic content.Analytical models were developed that enabled calculation of NRF signals for a variety of backscatter and transmission geometries. Extensive simulations were performed using the computer code, Monte Carlo N-particle eXtended (MCNPX) to calculate photon scattering effects, i.e., to determine scattered, non-resonant background and the notch refilling in transmission measurements. We found that MCNPX underestimated the non-resonant elastic scattering at backwards angles by multiple orders of magnitude mainly due to a flawed treatment of ...

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“…For the photoelectric effect, the ensuing primary and secondary fluorescence are both explicitly simulated for Z P 31 using the method by Everett and Cashwell (1973). For the incoherent and coherent scattering interactions, the electron's binding effects are being accounted for using the method by Cashwell et al (1973). Secondary electrons are not simulated, and their energy is assumed to be locally deposited.…”

Section: Physics Modelmentioning

confidence: 99%