n_tof EAR-1 Simulations Neutron fluence Spatial profile Time-to-energy

Transcription

1 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 Geometry Methodology Changes & improvements Spatial profile The beam interception factor Time-to-energy conversion Moderation length Comparison: the t 0 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:

3 Neutron fluence

4 Geometry Geometry implemented in FLUKA Simulation of target area only Demineralised water setup

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 (

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

7 Neutron propagation 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

8 Neutron propagation R cut 10m θ = 3 0 EAR-1 R max = 2cm Initial cut Neutron emission assumed isotropic within this angle Assumption holds for small angles: 3 0 chosen (conservatively) after tests Neutrons falling outside r = R cut are discarded Detection surface selected Position along beam Size (radius, R max )

9 Neutron propagation R max = 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

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

11 Results Present results 1.45x10 8 protons run ~ 3y of CPU time on EET cluster Statistical error 2% in 1eV-3GeV region at 600 bins per energy decade

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

14 Spatial profile

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

16 Beam interception factor BIF calculated for 1, 2 & 3cm diameter

17 Beam interception factor Influence of gravity

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

19 Beam interception factor Beam-line alignment Tested for 3 cases Realistic collimation setup Aligned Ideal alignment

20 Beam interception factor Sample alignment x- and y-offsets of 2mm Changes in the shape and the 5eV-to-thermal ratio!

21 Beam interception factor Comparison with BIF extracted from XY-MGAS data

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

24 The moderation length Effective moderation length evaluated as: v: velocity, t mod : 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

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

26 The equivalent t 0 offset Time-energy relation of the n_tof neutron beam: energy standards revisited Summary follows

27 The equivalent t 0 offset The neutron energy can be given as: The effective flight path L can be expressed as: where L 0 is the geometrical length (plus the energy independent term of the moderation length) The moderation length λ is extracted from simulations

28 The equivalent t 0 offset A fit is performed (on the mean value of λ) between 1eV-10 5 ev 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 t 0 offset is found to be approximately -73ns. In reality, it is also t 0 = t 0 (E), but it is not considered important

29 2004 calculations The simulated data from 2004 and the fit that gives t 0 =73ns

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 ~E ½

31 2011 calculations The estimated t 0 value is higher than the old one (165ns) Obviously, neither value can describe the MeV-GeV region

32 Comparison Using the 5900eV Al resonance Estimating centroid with gaussian fit Better agreement using he t 0 value based on the 2011 simulations

33 Extracting the neutron energy from λ(e) Starting again from eq. (1): The effective flight path L can be expressed as: where L geom 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 timeconsuming than the t 0 -offset implementation

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 235 U resonances good in evaluated region (up to 2250eV) Except for eV region! A 55 Mn 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

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)

36 Recommendations for t2e conversion The t 0 -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 t 0 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?

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

38 Internal note Detailed n_tof internal note is being prepared

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

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

44 Geometry: changes & improvements Neutron window Rotated by 45 0, as observed during 2010 alignment campaign Material definition corrected with appropriate Al alloy 2mm 5cm

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 h eq h/2, h << r

46 Geometry: changes & improvements Neutron window Moved 8mm downstream

47 Geometry: changes & improvements Comparison with technical drawings Target, windows Overlay of drawings and simulated geometry now possible

48 Neutron propagation: gravity Gravity can significantly alter the trajectory of low-energy neutrons No significant effect expected above 1-10eV Δy = ½ g t 2

49 Neutron propagation: gravity

50 Neutron propagation: gravity

51 Neutron propagation: gravity Sample neutron trajectories

52 Neutron propagation: gravity Sample neutron trajectories

53 Neutron propagation: gravity Sample neutron trajectories