MCNPX Simulations: Discrepancies in Spallation Experiments

1. MCNPX simulations of spallation experiments Mitja Majerle majerle@ujf.cas.cz

2. Outline • Phasotron and EPT experiment • Simulations • Disagreement between experiment and simulation : • Experimental uncertainties • MCNPX code • Other applications of MC simulations

3. Dubna experiments • Phasotron • Bare, lead target + 660 MeV protons • Activation detectors longitudinally, samples

4. Dubna experiments • EPT • Complex setup, energies from 0.7-2.5 GeV • Activation detectors, SSNTd, samples

5. Dubna experiments +results from radial detectors, SSNTd, samples…

6. MCNPX simulations • Geometry is implemented in MCNPX • Incident particles are directed to the setup • Neutrons are counted at the places of the detectors EPT SETUP PHASOTRON SETUP

7. Neutron spectra, cross-sections • XS for convolution are calculated with the combination of TALYS and MCNPX codes

8. Comparison exp/simPhasotron experiment Longitudinal detectors Beam monitors

9. Comparison exp/simEPT experiment Activation detectors - radial SSNTd Energy range : 10-100 MeV Energy range : 200-1000 MeV

10. Comparison exp/simEPT - (n,g) reactions • EPT has neutron reflector – polyethylene (most neutrons back to the system and induce (n,g) reactions) • (n,g) product Au-198 reliably tells us about the number of produced neutrons • Production rates of Au-198 are very well predicted by MCNPX

11. Comparison exp/sim • Disagreement in the range 10-100 MeV • Total number of neutrons is ok (n,g) • Activation detectors are NOT ok • SSNTd are ok again • What are the possible reasons ? • Experimental uncertainties • Partially wrong code • ?

12. Experimental uncertainties From the seminary in 2005 : • INFLUENCE OF THE SETUP PARTS • simplifications of the setup description • different parts of the setup • SYSTEMATIC ERROR (not accurately known exp. conditions) • beam geometry • reactions with protons • inserted detectors • ACCURACY OF SIMULATION • intra-nuclear cascade model used in calculations • PARAMETERS OF THE SETUP • the number of produced neutrons (spallation, fission, ..) • k (criticality)

13. Polyethylene, Cd layer • The spectra were taken inside the 1st and 3rd gap. • No influence on HE neutrons. absorption done by238U resonance capture

14. The wooden plate • Wooden plate under the target(1+2cm,0.5kg/l). • Without box. • Detectors from top to bottom. • Asymmetry 5% => negligible wood influence.

15. Aluminum and iron holders, upper iron plate • Two simulations with and without Al, Fe components. The results do not differ outside the limits of statistical error - (HE 3%, LE 10%) • The upper iron plate reduces the number of neutrons for 2%.

16. Beam profile • Simulations with 3mm, 3cm homogenous beams and with a beam with gaussian profile (FWMH=3cm). • Differences only for few percents. • Not important.

17. Beam displacement • Beam displaced for 3,5,8, and 10 mm. • Differences between results up to tens of %Displacement must be measured as accurately as possible !

18. The influence of detectors on neutron field • Metal plate on top reduces the number of neutrons only for 2%. Our detectors are much smaller. • Golden strap (2mm, 4mm) in the first gap has no influence on detectors in other gaps. • Only 0.1 mm thick golden strap is an obstacle for thermal neutrons : it can reduce the production rates of reactions with thermal neutrons inside the same gap for 20%.

19. Intra-Nuclear Cascade models • In MCNPX are 3 models (above 150 MeV): • Bertini • CEM • Isabel • The differences are up to 30% (standard, our detectors).

20. Total uncertainty: 50% Setup description < 10% Experimental uncert. ca 20% Differences in models ca 30% Total for SSNTd : >>50%

21. Comparison exp/simPhasotron experiment Longitudinal detectors Beam monitors

22. Comparison exp/simEPT experiment Activation detectors - radial SSNTd Energy range : 10-100 MeV Energy range : 200-1000 MeV

23. Systematical error in analysis ? • HPGe detectors simulations (efficiencies, cascade coefficients) • Good knowledge of processes • Consistent results • Little chance to explain the discrepancies…

24. Is MCNPX wrong ? • Calculations with simplified setup were repeated in FLUKA code (Maxime Oden) • The same, wrong trends exp/sim were obtained (presented on Prague Physics summer school)

25. Is MCNPX wrong ?Spallation experiments on thin targets • Recent experiments with protons directed on thin targets • Leroy, Ledoux • Trebukhovskiy, Yurevich • Meigo… • Some of their simulations show that there are exp/sim discrepancies in the region 20-80 MeV

26. .. or is something wrong with our experiments ? • Repeat one experiment, which we believe that is good and see if we get the same results • Perform experiments with changed setup (without blanket, uranium) .. not very likely, so ..

27. Other work connected to ADS simulations Blanket made of ENH, BaF rods and lead • Polish subcritical setup, based on MARIA reactor (Gael de Cargouet) EK-10 rods Beam from horizontal channel of the reactor

28. GAMMA-MD • Pb target • Graphite block • 2.33 GeV deuterons

29. Conclusion • Simulations unify experimental data from different experiment well, but they are not precise everywhere • Maybe we can confirm exp/sim discrepancies from completely different experiments • That many simulations require a lot of processor power (CESNET) Thank you for your attention.