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Title: Correlated Neutron and Gamma-Ray Emissions in MCNP6

Author(s): Rising, Michael EvanSood, AvneetTalou, PatrickKawano, ToshihikoStetcu, Ionel

Intended for: 2014 ANS Winter Meeting and Nuclear Technology Expo,2014-11-09/2014-11-13 (Anaheim, California, United States)

Issued: 2014-12-04 (rev.1)

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2014 ANS Winter Meeting, Anaheim, CA

U N C L A S S I F I E D

Slide 1

Correlated Neutron and Gamma-ray Emissions in MCNP6

1Michael E. Rising, 1Avneet Sood, 2Patrick Talou, 2Ionel Stetcu and 2Toshihiko Kawano

1XCP-3 Division, Los Alamos National Laboratory 2T-2 Division, Los Alamos National Laboratory

ANS Winter Meeting

Anaheim, CA November 9-13, 2014

Operated by Los Alamos National Security, LLC for NNSA U N C L A S S I F I E D


Outline

n  Introduction

n  Background

n  Secondary Particle Event Generators •  CGMF •  FREYA

n  Numerical Results •  Average Quantities •  Neutron and Gamma-ray Correlations

n  Conclusions

n  Future Work

Slide 2



Introduction

n  Application of interest •  Global security and nuclear nonproliferation •  Detection of special nuclear material (SNM) •  Warhead measurement campaign (WMC) •  Passive and active interrogation techniques •  Coincident neutron and photon leakage

n  Key issues •  Average nuclear data quantities are insufficient •  Cannot predict correlated signatures of shielded SNM

n  Approach to obtain predictive capability •  Use transport code MCNP for modeling neutrons and photons •  Need microscopic fission event information •  Fission event generators are under development •  Implement in MCNP and compare to experiments

Slide 3



Background

n  In tabulated nuclear data libraries (i.e. ENDF/B-VII.1): •  Average secondary neutron and photon information can be available

—  Average multiplicity, —  Average spectrum, —  Average energy-angle spectrum,

•  Generally, high-dimensional distributions of secondary particles are unavailable —  Multiplicity distribution, —  Multiplicity-dependent emission spectra, —  Multiplicity-dependent energy-angle emission spectra, —  Neutron-neutron, neutron-photon and photon-photon correlations —  Too much data to tabulate!

n  Default MCNP uses average quantities •  Consider this a nuclear data “variance reduction” technique •  Good for integral quantities, like flux and effective multiplication •  Bad for studying detailed particle emission physics

Slide 4

χ (E)ν

χ (ν,E)χ (ν,E,θ )

P(ν )

χ (E,θ )



Secondary Particle Event Generators

MCNP6.1.1 contains two event generators:

n  LLNL Fission Library1

•  Spontaneous, neutron-induced and photo-fission •  Fission Reaction Event Yield Algorithm (FREYA)2 isotopes:

—  Spontaneous: 238U, 240Pu, 244Cm and 252Cf —  Neutron-induced: 233U, 235U and 239Pu

•  When available, FREYA generates secondary neutrons and photons

n  Cascading Gamma-ray Multiplicity (CGM)3 - LANL

•  Generates secondary particles from a variety of reactions •  No fission! (CGMF under active development)

1J.M. Verbeke, C. Hagmann, and D. Wright, Lawrence Livermore National Laboratory, UCRL-AR-228518 (2014). 2R. Vogt and J. Randrup, Phys. Rev. C, vol. 84, pp. 044612-1-14 (2011). 3T. Kawano, P. Talou, M.B. Chadwick, and T. Watanabe, J. Nucl. Sci. Tech., 47 (5), 462-69 (2010).

Slide 5



Secondary Particle Event Generators: CGMF

n  CGMF is a superset of CGM with an added fission reaction capability

n  Fission fragments are sampled from a joint probability distribution function of mass (A), charge (Z) and total kinetic energy (TKE)

n  Uses Hauser-Feshbach statistical theory of nuclear reactions

n  Neutron / photon competition is treated during evaporation from fission fragments

n  Monte Carlo is used to sample each step in the de-excitation process

Slide 6

4B. Becker, P. Talou, T. Kawano, Y. Danon, and I. Stetcu, Phys. Rev. C, 87, 014617 (2013).



Secondary Particle Event Generators: FREYA

n  FREYA is LLNL’s fission event generator

n  In MCNP6, it is accessible through LLNL Fission Package

n  The LLNL Fission Package includes more tabulated and fitted data used for lesser known isotopes FREYA can’t presently handle

n  FREYA uses a Monte Carlo Weisskopf approach •  Neutrons emitted by sampling from Weisskopf spectrum •  After neutrons are done emitting, gamma rays are emitted from residual energy

n  Computationally more efficient than Monte Carlo Hauser-Feshbach

Slide 7



Numerical Results

n  Implementation of CGMF into MCNP6 is underway

n  Standalone and integrated code comparison needed for verification

n  CGMF behavior is studied to understand trends and correlations

n  Validation experiments will be needed to provide feedback to models

n  For the following numerical results: •  CGMF results are based on standalone code •  FREYA used through LLNL Fission Package in MCNP6 •  Default MCNP6 behavior is shown

n  Average energy, multiplicity and gamma ray spectrum

n  Secondary gamma ray spectrum

Slide 8



Numerical Results: Average Quantities

n  Thermal neutron incident on 239Pu

n  For MCNP6 and FREYA: •  Secondary neutrons from

reaction •  Secondary gamma rays

from , , etc.

n  For CGMF (standalone): •  Secondary neutrons and gamma rays

from only

n  Discrepancies due to : •  Multiplicity lower •  Average energy higher •  Peak at 5.5 MeV causes bump

n  Check again after integration

Slide 9

Quantity MCNP6 FREYA CGMF

⌫̄N 2.8725 2.8723 2.9764⌫̄� 2.0 7.2973 9.1797

hEN i (MeV) 2.1034 2.0003 2.0327hE�i (MeV) 0.9949 1.0327 0.7728

10-6

10-5

10-4

10-3

10-2

10-1

100

0.1 1 10

Gam

ma-

ray S

pec

trum

(1/M

eV)

Outgoing Energy (MeV)

MCNP6FREYACGMF

(n,γ )(n, f )

(n, f )

(n, f )

(n,γ )



Numerical Results: Neutron and Gamma-ray Correlations

n  Neutron – gamma ray energy and multiplicity correlations

n  Analyzing CGMF results for n(th)+239Pu fission reaction

n  Use to verify implementation into MCNP

n  This physics needs to be validated with experiment

Slide 10

0

2

4

6

8

10

12

14

0 5 10 15 202.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

Ener

gy (

MeV

)

Neu

tron M

ult

ipli

city

Gamma-ray Multiplicity

Average Energy / Neutron

Total Neutron Energy / Fission

Average Energy / Gamma ray

Total Gamma-ray Energy / Fission

0

2

4

6

8

10

12

14

16

18

0 1 2 3 4 5 6 7 84.0

6.0

8.0

10.0

12.0

Ener

gy (

MeV

)

Gam

ma-

ray M

ult

ipli

city

Neutron Multiplicity

Average Energy / Neutron

Total Neutron Energy / Fission

Average Energy / Gamma ray

Total Gamma-ray Energy / Fission



Conclusions

n  Many applications are in need of high fidelity physics models

n  Fission event generators are under active development

n  When implemented in MCNP a predictive capability may be possible

n  SNM signature detection with code validation will soon be possible

n  Future work: •  Finish implementation and improvements to fission event generators in MCNP •  Develop verification tests for all these new features •  Need experimental measurements to compare against •  Validate the new physics features with experiment •  Compare against MCNP-PoliMi and other specialized codes

Slide 11



Questions?

This work was supported by the Office of Defense Nuclear Nonproliferation Research & Development (DNN R&D), National Nuclear Security

Administration, US Department of Energy.

Slide 12

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