MONTE CARLO FITTING OF THE AMLI NEUTRON SPECTRUM

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LA-UR-16-27725 MONTE CARLO FITTING OF THE AMLI NEUTRON SPECTRUM

Laboratory David Beddingfield Los Alamos National Laboratory Andreas

1 LA-UR MONTE CARLO FITTING OF THE AMLI NEUTRON SPECTRUM Robert Weinmann-Smith University of Florida, Los Alamos National Laboratory David Beddingfield Los Alamos National Laboratory Andreas Enqvist University of Florida Martyn Swinhoe Los Alamos National Laboratory

2 NOVEL OUTCOMES Characterization of the differences between AmLi sources AmLi R R1 R1 average C-470 C-476 C N-160 N-161 AmLi N-165 N-262 Source N-263 N-432 N-436 N Spectra specific to different series C-series N-series of AmLi sources, and to individual sources Probability (MeV -1 ) Energy (MeV) 2

3 OUTLINE AmLi applications Active neutron interrogation Physics considerations Current AmLi spectra Measurements Source variations Simulations Spectra fitting Limitations 3

APPLICATIONS The IAEA uses an AmLi source in the

compliance with nonproliferation treaties The UNCL

The AmLi spectrum isn t known well enough to allow

4 APPLICATIONS The IAEA uses an AmLi source in the Uranium Neutron Coincidence Collar (UNCL) to verify compliance with nonproliferation treaties The UNCL requires calibration with known uranium samples. The AmLi spectrum isn t known well enough to allow simulated calibrations AmLi source UNCL UNCL with fuel 4

5 PHYSICS CONSIDERATIONS Alphas lose energy traveling through AmO 2 particle of unknown size Energy reduction below Li threshold enhances O contribution Isotope Energy (MeV) LiO Am-241 (α) 5.486, AmO 2 Li-7 (α,n) 4.38 AmO 2 α O-18 (α,n) 0.85 U-238 (n,f) ~1 Unknown Li matrix material affects LiOH neutron production and thermalization LiH LiH 2 Be Intensity Large variation in spectra from each element 1E0 1E Energy (MeV) 5 Li O Be

6 CURRENT SPECTRA Measured single sources Tagziria Obninsk 1998 Birch 1984 Ing 1981 Owen 1982 and more Probability (MeV -1 ) Measured: Tagziria 2003 Obninsk 1998 Calculated: GVZ 1971 Beddingfield 2015 Calculated initial spectra Geiger and van der Zwan 1971 Tagziria 2012 Beddingfield Energy (MeV) Selected AmLi spectra. The high-energy tail is key for a good fit 6

MEASUREMENT OVERVIEW Sources Cf-252 Type 3 A7-series 3 Cf-series AmLi 3 Gammatron C-series 8 Gammatron N-series 12345

7 MEASUREMENT OVERVIEW Sources Cf-252 Type 3 A7-series 3 Cf-series AmLi 3 Gammatron C-series 8 Gammatron N-series Ring Multiplicity Counter (5RMC) gives singles rate by ring Higher neutron energy is required to reach the outer rings 7

8 MEASUREMENT RESULTS VARIATION BETWEEN SOURCES R1 R1 average R1 R1 average Cf-252 R AmLi R A7-866 A7-867 A7-869 Cf-9 Cf-11 Cf-12 Cf-252 Source Cf-252 inner ring ratios Statistical uncert. 0.11% Standard deviation 0.12% Chi-squared 1.41 E C-470 C-476 C N-160 N-161 AmLi N-165 N-262 Source N-263 N-432 N-436 N-437 AmLi inner ring ratios Statistical uncert. 0.01% Standard deviation 0.63% Chi-squared (C) 1.73 E-5 Chi-squared (N) 6.22 E-5 8

9 MEASUREMENT RESULTS TARGETS FOR SPECTRA FITTING R1 R1 average R5 R5 average AmLi R AmLi R C-470 C-476 C N-160 N-161 AmLi N-165 N-262 Source AmLi inner ratios N-263 N-432 N-436 N Ratio C-series N-series R C-470 C-476 C N-160 N-161 AmLi N-165 N-262 source N-263 N-432 N-436 N-437 AmLi outer ratios R R R R Ratios also exist for individual sources 9

10 SIMULATIONS 5RMC MCNP model benchmarked to Cf-252 measurements Chi-squared ring ratio agreement of 9.0E-6 5RMC neutron energy 100-bin monoenergetic response function for AmLi matrix MCNP6 Alpha energy for different particle sizes MCNP6 Li (α,n) neutron energy spectra for different particle sizes SOURCES4C Energy spectra * response function = ring ratios 10

11 SIMULATIONS 0.6 Arb E-4 1E-5 min (calculated) max (calculated) min (extended) max (extended Efficiency T R1 R2 R3 R4 R5 1E-6 1E E Energy (MeV) Energy (MeV) Li (α,n) neutron energy spectrum range between 1 and 7 microns 5RMC response function 11

12 SPECTRA FITTING Minimize chi-squared of simulated and measured ring ratios - adjust oxygen contribution and spectra within expanded limits C-series spectrum N-series spectrum Probability (MeV -1 ) Probability (MeV -1 ) Energy (MeV) Energy (MeV) Parameter C-series N-series Oxygen contribution 6.7% 8.1% Chi-squared fit 1.3 E E-6 12

13 LIMITATIONS Chi-squared value only relates to Cf-252 agreement, no physical meaning MCNP model bias of chi-squared 9.0 E-6, with 1.4 E-6 disagreement between Cf-252 sources We assume the spectra fitting accounts for the unknown AmO 2 particle size, matrix composition, other effects Need to benchmark simulations to UNCL measurements and characterize spectrum effects on U-235 mass in a UNCL 13

14 CONCLUSIONS Characterized the variation in AmLi sources compared to Cf-252 Generation of spectra that are precise enough to distinguish between sources Improved accuracy in modeling AmLi active interrogation systems Measurement of non-calibrated samples Future work find effects on UNCL measurement results 14

15 ACKNOWLEDGEMENTS This work was funded in-part by the Consortium for Verification Technology under Department of Energy National Nuclear Security Administration award number DE- NA This work was funded in-part by the Department of Energy National Nuclear Security Administration Office of Nonproliferation and International Security 15

16 LA-UR MONTE CARLO FITTING OF THE AMLI NEUTRON SPECTRUM Robert Weinmann-Smith University of Florida, Los Alamos National Laboratory David Beddingfield Los Alamos National Laboratory Andreas Enqvist University of Florida Martyn Swinhoe Los Alamos National Laboratory

17 COMPARISON OF SPECTRA Probability (MeV -1 ) Tagziria 2003 Obninsk 1998 GVZ 1971 Beddingfield 2015 Probability (MeV -1 ) C-series N-series Energy (MeV) Energy (MeV) 17

18 SUMMARY SLIDES FOLLOW: 18

19 MONTE CARLO FITTING OF THE AMLI NEUTRON SPECTRUM AmLi sources are used for active interrogation of fresh uranium fuel for treaty verification Characterization of the differences between AmLi sources Cf-252 R R1 (error bars are Cf-252 counting statistics) R1 average (error bars are AmLi Rel. Std. Dev.) AmLi R R1 (error bars are AmLi counting statistics) R1 average (error bars are Cf-252 Rel. Std. Dev.) A7-866 A7-867 A7-869 Cf-9 Cf-11 Cf-12 Cf-252 Source Source C-470 C-476 C-477 Chi-squared agreement - N-160 N-161 N-165 AmLi Source N-262 N-263 N-432 N-436 N-437 Cf E-6 AmLi C-series 1.73 E-5 AmLi N-series 6.22 E-5 19

20 MONTE CARLO FITTING OF THE AMLI NEUTRON SPECTRUM Spectra specific to different series of AmLi sources, and to individual sources Improved accuracy in modeling AmLi Probability (MeV -1 ) C-series N-series active interrogation systems Energy (MeV) Parameter C-series N-series Oxygen contribution 6.7% 8.1% Chi-squared fit 1.3 E E-6 20

M. S. Krick D. G. Langner J. E. Stewart.

LA-UR-97-3427 Title: Energy-Dependent Bias in Plutonium Verification Measurements Using Thermal Neutron Multiplicity Counters Author(s): M. S. Krick D. G. Langner J. E. Stewart Submitted to: http://lib-www.lanl.gov/la-pubs/00412753.pdf