1. The n_TOF neutron flux and resolution function by GEANT4 simulations
CERN, 25 – 27 February 2015
Sergio Lo Meo1,2, Cristian Massimi2,3,
Nicola Colonna4, Federica Mingrone2,3, Gianni Vannini2,3
1 ENEA Research Centre of Bologna (Italy)
2 INFN Section of Bologna (Italy)
3 Physics and Astronomy Dept. Alma Mater Studiorum – University of Bologna (Italy)
4 INFN Section of Bari (Italy)

2. Outline Introduction Spallation Target G4 Physics List Results
Conclusion

3. Introduction
This work had several motivations: - curiosity-driven, to check if Geant4 is able to reliably simulate the spallation process and the neutron production/transport in a large energy range; - need-driven, to develop new tools and competence on simulations of the n_TOF facility, in particular for work related to EAR2 (analysis of the flux and resolution function); - accuracy-driven, to double-check present simulations, now available only by FLUKA (two is always better than one!!!);

4. Fluka Spallation Target Setup

5. G4 Spallation Target Setup
Scoring Plane EAR1
Scoring Plane EAR2
Spallation Target

6. Physics List In Geant4 10.01 version (December 2014) we have used:
FTFP_INCLXX_HP Physics List
FTFP: The Fritiof [1][2] model is used in Geant4 for simulation of the following interactions: hadron-nucleus at Plab > GeV/c, nucleus-nucleus at Plab > GeV/c/nucleon, antibaryon-nucleus at all energies, and antinucleus-nucleus.
INCLXX: we have used the Liège Intranuclear Cascade model INCL++ [3] [4] that is suitable for the simulation of any system where spallation reactions or light-ion-induced reactions play a dominant role. INCL++ is used for reactions induced by nucleons with Plab < 3 GeV/c
HP: the NeutronHP model at low energy to simulate all reactions induced by neutrons using evaluated data libraries (G4NDL)
For De-excitation we have not used the default (G4ExcitationHandler) model of INCL++ but ABLA [5] model that was recognized as one of the best de-excitation model by the IAEA Benchmark of Spallation Models [6]

7. Physics List
The choice of INCL++ and ABLA is also due to previous use that led the n_TOF group of Bologna, along with one of the developers of INCL++ (D. Mancusi) , to publish a work [7] that describes the calculation of cross sections of fission of some actinides and pre-actinides with incident nucleon energies from 100 MeV to 1 GeV
Neutron Tracking Cut modified as follow:
G4NeutronTrackingCut *nCut = new G4NeutronTrackingCut(verboseLevel); nCut->SetTimeLimit( *ns);
All electromagnetic interactions (except Atomic De-excitation) are included as the decays of particles (except Radioactive Decay).
[1] B.Andersson et al. Nucl. Phys. B (1987) 427
[2] B.Nilsson-Almquist, E.Stenlund, Comp. Phys. Comm (1987).
[3] A. Boudard et al., Phys. Rev. C87 (2013)
[4] D. Mancusi et al., Phys. Rev. C90 (2014)
[5] A. Keli´c, M. V. Ricciardi and K.-H. Schmidt, Joint ICTP-IAEA Advanced Workshop on Model Codes for Spallation Reactions, Report INDC(NDC)-0530 (2008) 181.
[6] Benchmark of Spallation Models, organized by the IAEA. Web site:
[7] S. Lo Meo, D. Mancusi, C. Massimi, G. Vannini, A. Ventura “Fission induced by nucleons at intermediate
energies” Nuclear Physics A 993 (2015)

8. Scoring plane at target: events with θ < 2°, propagated to EAR1
Results
Scoring plane at target: events with θ < 2°, propagated to EAR1

9. Scoring plane at target: events with θ < 2°, propagated to EAR1
Results
Scoring plane at target: events with θ < 2°, propagated to EAR1

10. Results Absolute value: G4 simulation 35% higher
Change
Average reduction
1st collimator shifted 1 m 2%
1st collimator: Radius reduction
5 mm (R = 5.5 cm  5.0 cm) 7%
2nd collimator shifted 1 m
2nd collimator: Radius reduction
1 mm (R = 0.9 cm  0.8 cm)
28%
Resampling 10% uncertainty on absolute value

11. Results
Simulation arbitrarily scaled

12. Results
Simulation arbitrarily scaled

13. Results
λ = vtmod
Propagated to EAR1

14. Results λ = vtmod Propagated to EAR1 Energy interval FLUKA
MEAN (cm) - R.M.S (cm)
GEANT4
MEAN (cm) - R.M.S (cm)
1-10 eV
14.1
16.1
11.8 (15.3)
6.9 (12.0) eV
14.8
17.3
12.3 (14.3)
9.0 (12.3) eV
16.2
18.0
14.9 (16.2)
10.0 (14.8)
1-10 keV
20.4
21.9
17.5 (21.0)
13.6 (18.5)
keV
31.9
32.2
29.0 (35.3)
25.0 (31.5)
keV
51.6
38.3
51.4 (56.4)
37.1 (38.7)

15. Results
Numerical Resolution function
TEST using 197Au data from 2012

16. Results Numerical Resolution function
TEST using Fe and 238U data from 2011 measurement campaign

17. Results
Numerical Resolution function

18. Conclusion
- Geant4 seems to reproduce n_TOF flux  further tests are foreseen; - Geant4 seems to reproduce neutron moderation time inside neutron-producing target  Effect due to the propagation on the Lambda values can be investigated; - The result can improve the accuracy of the resolution function.

19. Sergio Lo Meo - sergio.lomeo@enea.it
Cristian Massimi -

20. Flux Vs target rings

21. Flux Vs target rings

22. EAR 2
EAR2
Simulation arbitrarily scaled

23. EAR 2
λ = vtmod
Propagated to EAR2

24. EAR2 – Resolution function
n+238U

25. EAR2 – Resolution function
n+238U

26. EAR 1 – 10B content
1.28% 
1.38% B

27. Resampling 1st collimator target neutrons R = 5.5 cm 2nd collimator
protons
EAR1 θ
2nd collimator
R = 0.9 cm
1st collimator
R = 5.5 cm
2.85 m
θ < 2° m
2.0 m m
~ 5x106 protons  1 neutron
1/2 week CPUs = 106 protons !!!

28. Resampling 1st collimator target neutrons R = 5.5 cm 2nd collimator
protons
EAR1
2nd collimator
R = 0.9 cm
1st collimator
R = 5.5 cm
Θ’ < 0.128°

29. Al
3.6 cm
H2O
1 cm Pb
1 cm
H2O grigliata
1.4 cm
3.5 cm
1 cm
Acqua Borata
4.0cm
5 cm
15 cm
10 cm 70 cm 40 cm
58 cm Pb
Al (griglia)
1.5 cm Al
0.7 cm Al
0.3 cm
5 cm
5.4 cm
40 cm
H2O
0.7 cm
H2O
0.3 cm Al
0.4 cm Al
0.3 cm Z

30. 1.5 cm
1.1 cm 60 cm
9.45 cm

31. Sergio Lo Meo - sergio.lomeo@enea.it
Cristian Massimi -