1. n_TOF Annual Collaboration Meeting
n_TOF EAR-1 Simulations Neutron fluence | Spatial profile | Time-to-energy
A. Tsinganis (CERN/NTUA), V. Vlachoudis (CERN),
C. Guerrero (CERN) and others
n_TOF Annual Collaboration Meeting
Lisbon, December 13-15, 2011

2. Outline Neutron fluence Spatial profile Time-to-energy conversion
Geometry
Methodology
Changes & improvements
Spatial profile
The beam interception factor
Time-to-energy conversion
Moderation length
Comparison: the “t0 offset” and the “λ(E) relation”
Conclusions
Details on neutron fluence simulations and a preliminary discussion on energy calibration can be found in relevant talks from the October 2011 Analysis Group Meeting at:
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

3. Neutron fluence

4. Geometry Geometry implemented in FLUKA Simulation of target area only
Demineralised water setup
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

5. Simulations: FLUKA + MCNPX
Simulations performed combining two codes:
FLUKA (dev. version)
MCNPX 2.6
Why?
FLUKA
Well-benchmarked high energy models, BUT…
Group-wise treatment of neutrons <20MeV (260 groups)  information on resonant absorption dips is not detailed
MCNPX
Point-wise neutron cross sections, BUT…
Less accurate high energy models
See talk by Marco (analysis group meeting Nov. 2010) for comparison of FLUKA and MCNPX results (
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

6. Simulations: FLUKA + MCNPX
FLUKA is used to simulate the proton beam and production of neutrons inside the lead target
With the use of a modified MGDRAW routine:
Neutrons >20MeV scored at beginning of beam tube and dumped to file
Neutrons falling below 20MeV are “stopped” and dumped to file
Geometry (incl. materials) exported to MCNPX using FLAIR
MCNPX input file automatically generated adding necessary cards
FILES card, tally, NPS card
Dump file of neutrons <20MeV read by MCNPX SOURCE routine to continue the history, taking advantage of the point-wise cross sections
Neutrons scored on the same plane (modified TALLYX routine)
Finally, FLUKA (>20MeV) and MCNPX (<20MeV) results merged
Quantities scored
Coordinates
Directional cosines
Energy
Time (since primary proton)
Weight
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

7. Neutron propagation Very small solid angle  prohibitive CPU time
2cm
180m
Very small solid angle  prohibitive CPU time
Propagation of neutrons to EAR-1 performed off-line with external routine accounting for:
Tube and collimator geometry
Misalignments
Statistics need to be “artificially” improved
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

8. Neutron propagation Initial cut Detection surface selected
Rcut ≈ 10m
θ = 30
EAR-1
Rmax = 2cm
Initial cut
Neutron emission assumed isotropic within this angle
Assumption holds for small angles: 30 chosen (conservatively) after tests
Neutrons falling outside r = Rcut are discarded
Detection surface selected
Position along beam
Size (radius, Rmax)
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

9. Neutron propagation Rmax = 2cm
Several instances of each neutron are emitted towards the detection surface, “scanning” the whole area with a defined step
Accounting for different tube diameters and collimators
“Ideal” collimation
Any neutron that hits a tube or collimator is discarded
Does not account for scattering on beam-line elements
The energy and position of the neutrons that reach the EAR are used to determine the flux and the spatial profile
Appropriate normalisation of results
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

10. Improvements & investigation
Gravitational effect added
Relevant below 1eV
Geometry corrections
Rotation of neutron window
Expected deformation of moderator window
Detailed comparison with technical drawings
Investigation of various parameters
Collimation, misalignments
Comparison with older simulations and experimental data
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

11. Results Present results 1.45x108 protons run
~ 3y of CPU time on EET cluster
Statistical error ≤2% in 1eV-3GeV region at 600 bins per energy decade
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

12. Spatial profile & beam interception factor

13. Spatial profile
The energy and position of the neutrons that reach the EAR are used to determine the spatial profile
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

14. Spatial profile
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

15. Beam interception factor
How much – and what part – of the beam hits a sample of radius R?
Dependent on energy and geometry
Different samples / geometries studied
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

16. Beam interception factor
BIF calculated for 1, 2 & 3cm diameter
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

17. Beam interception factor
Influence of gravity
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

18. Beam interception factor
Normalising at 5eV…
…differences in the shape and the 5eV-to-thermal ratio emerge
Note: simulations performed for the demineralised water setup: this could affect the results below ~1eV
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

19. Beam interception factor
Beam-line alignment
Tested for 3 cases
Realistic collimation setup
Aligned
“Ideal” alignment
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

20. Beam interception factor
Sample alignment
x- and y-offsets of 2mm
Changes in the shape and the 5eV-to-thermal ratio!
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

21. Beam interception factor
Comparison with BIF extracted from XY-MGAS data
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

22. Time-to-energy conversion

23. The problem…
How do we reconstruct the neutron energy from the measured time-of-flight?
Protons hit the lead target
Neutrons enter the tube after following an unknown path inside the target and other materials during an unknown time interval
 using the measured TOF will lead to an incorrect estimate of the neutron energy
Different approaches to the problem…
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

24. The moderation length
Effective moderation length evaluated as: v: velocity, tmod: moderation time
The moderation time is an experimental unknown, but it is known in the simulations
We can therefore study the behaviour of λ over the full energy range
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

25. The effective moderation length
The λ(E) distribution extracted from the simulations
For each energy bin, the position of the maximum and the mean are plotted
The proton pulse width (7ns rms) is accounted for
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

26. The equivalent “t0 offset”
“Time-energy relation of the n_TOF neutron beam: energy standards revisited”
Summary follows…
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

27. The equivalent “t0 offset”
The neutron energy can be given as: The effective flight path L can be expressed as: where L0 is the geometrical length (plus the energy independent term of the moderation length)
The moderation length λ is extracted from simulations
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

28. The equivalent “t0 offset”
A fit is performed (on the mean value of λ) between 1eV-105eV following E-½
The moderation process can equivalently be treated in terms of a time offset. Given eqs. (1) and (2):
Comparing equations (2) and (3), the “t0 offset” is found to be approximately -73ns.
In reality, it is also t0= t0(E), but it is not considered important
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

29. 2004 calculations
The simulated data from 2004 and the fit that gives t0=73ns
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

30. 2011 calculations
Shape of data in the same region is quite more complex due to the resonance dips
After several tests (removing the dips and tightening the energy range) the data can be fitted with an equation ~E½
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

31. 2011 calculations
The estimated t0 value is higher than the old one (165ns)
Obviously, neither value can describe the MeV-GeV region
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

32. Comparison 2004-2011 Using the 5900eV Al resonance
Estimating centroid with gaussian fit
Better agreement using he t0 value based on the simulations
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

33. Extracting the neutron energy from λ(E)
Starting again from eq. (1): The effective flight path L can be expressed as: where Lgeom is the geometrical length, thus giving a new estimate for the energy: In general:
The correct energy can be determined iteratively, based on the λ(E) relation extracted from the simulations
Very quick convergence (2-3 Newton-Raphson iterations), but still more time- consuming than the t0-offset implementation
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

34. Extracting the neutron energy from λ(E)
The calculated λ(E) relation has been tested (by Diego Tarrio, USC) in the analysis of PPAC data (using the mean value)
Position of 235U resonances good in evaluated region (up to 2250eV)
Except for eV region!
A 55Mn resonance is present in the flux at this energy
Behaviour in 1MeV-hundreds of MeV seems OK (using 232Th(n,f) and 238U/235U(n,f) data) (preliminary check)
Graphs by Diego Tarrio, USC
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

35. Extracting the neutron energy from λ(E)
Why do we have this problem?
Because of the way the simulations are performed
In fact, we are considering more intermediate material than we should (the neutron window)
The dips in λ(E) are more pronounced than they should
We score here (after the neutron window)
We reduce to here
(after the moderator window)
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

36. Recommendations for t2e conversion
The “t0-offset” approach
Valid in specific energy range
Completely wrong above keV
Ignores dips in the flux  validity is compromised at those energies
Use of the t0 value from 2011 simulations (165ns) is more appropriate
Using the λ(E) relation
Valid at any energy
Reduced validity at energies corresponding to flux dips
More CPU-intensive
Must be used for analysis in the MeV region (fission measurements)
Comments?
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

37. Conclusions Neutron flux simulations completed DONE
Spatial profile of neutron beam
Final configuration (beam-line alignment etc.) DONE
Calculate beam interception factor DONE
Compare with XY-MGAS results DONE
Understand discrepancies PENDING
BIF can be calculated for specific sample diameters and positions UPON REQUEST
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

38. Internal note Detailed n_TOF internal note is being prepared
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

39. Conclusions Neutron flux simulations completed DONE
Spatial profile of neutron beam
Final configuration (beam-line alignment etc.) DONE
Calculate beam interception factor DONE
Compare with XY-MGAS results DONE
Understand discrepancies PENDING
BIF can be calculated for specific sample diameters and positions UPON REQUEST
Prepare internal note IN PROGRESS
Borated water setup
Find equivalent B(OH)3 concentration, run simulations PENDING
Create repository of n_TOF simulations and related files (external programmes etc.) for EARs 1&2 PENDING
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

40. Don’t go away….
Simulations pertaining to the better understanding of the n_TOF γ-flash, its effect on EAR-1 detectors and on EAR-2 planning and operation to be presented on Thursday
EAR-1 Neutron Flux Simulations
n_TOF Analysis Group Meeting – CERN, October 4-5, 2011 | A.T.

41. The end EAR-1 Neutron Flux Simulations
n_TOF Analysis Group Meeting – CERN, October 4-5, 2011 | A.T.

42. extra slides

43. Strange feature
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

44. Geometry: changes & improvements
Neutron window
Rotated by 450, as observed during 2010 alignment campaign
Material definition corrected with appropriate Al alloy
5cm
2mm
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

45. Geometry: changes & improvements
Moderator window
Expected deformation at operating pressure: 1.3mm sagitta (according to design report)
Equivalent (equal volume) increase in moderator thickness: 0.65mm
heq ≈ h/2 , h << r
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

46. Geometry: changes & improvements
Neutron window
Moved 8mm downstream
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

47. Geometry: changes & improvements
Comparison with technical drawings
Target, windows…
Overlay of drawings and simulated geometry now possible
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

48. Neutron propagation: gravity
Gravity can significantly alter the trajectory of low-energy neutrons
No significant effect expected above 1-10eV
Δy = ½ g t2
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

49. Neutron propagation: gravity
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

50. Neutron propagation: gravity
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

51. Neutron propagation: gravity
Sample neutron trajectories
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

52. Neutron propagation: gravity
Sample neutron trajectories
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

53. Neutron propagation: gravity
Sample neutron trajectories
EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy
n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.