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n_TOF EAR-1 Simulations Neutron fluence | Spatial profile | Time-to-energy

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. Outline. Neutron fluence Geometry Methodology

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n_TOF EAR-1 Simulations Neutron fluence | Spatial profile | Time-to-energy

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  1. n_TOFEAR-1 SimulationsNeutron fluence | Spatial profile | Time-to-energy A. Tsinganis(CERN/NTUA), V. Vlachoudis (CERN), C. Guerrero (CERN) and others n_TOFAnnual Collaboration Meeting Lisbon, December 13-15, 2011

  2. Outline • Neutron fluence • 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:http://indico.cern.ch/conferenceDisplay.py?confId=154582 EAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy n_TOFAnnual 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 (https://indico.cern.ch/conferenceDisplay.py?confId=114081) 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 2cm • 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 180m 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 Rcut ≈ 10m • 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) θ = 30 EAR-1 Rmax = 2cm 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 energyand 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 samples @184m 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 normalised @5eV 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 afterfollowing an unknown path insidethe target and other materialsduring an unknown time interval •  using the measured TOF willlead 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 fullenergy 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 canequivalently be treated in termsof a time offset. Given eqs. (1) and (2): • Comparing equations (2) and(3), the “t0 offset” is found tobe approximately -73ns. • In reality, it is also t0= t0(E),but it is not consideredimportant 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 2011 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 300-350eV 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 10-100keV • 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_TOFinternal note isbeing 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_TOFγ-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 correctedwith appropriate Al alloy 2mm 5cm 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.

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