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

n_TOF EAR-1 SimulationsNeutron fluence | Spatial profile | Time-to-energy

A. Tsinganis (CERN/NTUA), V. Vlachoudis (CERN),C. Guerrero (CERN) and others

n_TOF Annual Collaboration MeetingLisbon, December 13-15, 2011

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

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

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

http://indico.cern.ch/conferenceDisplay.py?confId=154582

http://indico.cern.ch/conferenceDisplay.py?confId=154582

Neutron fluence

Geometry Geometry implemented in FLUKA

Simulation of target area only Demineralised water setup


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)


https://indico.cern.ch/conferenceDisplay.py?confId=114081

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


Neutron propagation

Very small solid angle prohibitive CPU time Propagation of neutrons to EAR-1 performed off-line with

external routine accounting for:Tube and collimator geometryMisalignments

Statistics need to be “artificially” improved

180m

2cm


Neutron propagation

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

Rcut ≈ 10m

Rmax = 2cm

EAR-1


Neutron propagation

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

Rmax = 2cm


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


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


Spatial profile &beam interception factor

Spatial profile


The energy and position of the neutrons that reach the EAR are used to determine the spatial profile

Spatial profile


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


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

samples @184m


Beam interception factor Influence of gravity


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


Beam interception factor Beam-line alignment

Tested for 3 cases Realistic collimation setup Aligned “Ideal” alignment


normalised @5eV

Beam interception factor Sample alignment

x- and y-offsets of 2mm

Changes in the shape and the 5eV-to-thermal ratio!


Beam interception factor Comparison with BIF extracted from XY-MGAS

data


Time-to-energy conversion

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 will

lead to an incorrect estimate of the neutron energy

Different approaches to the problem…


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


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


The equivalent “t0 offset” “Time-energy relation of the n_TOF neutron beam:

energy standards revisited”

Summary follows…


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


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


2004 calculations The simulated data from 2004 and the fit that gives t0=73ns


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


2011 calculations The estimated t0 value is higher than the old one (165ns) Obviously, neither value can describe the MeV-GeV region


Comparison 2004-2011 Using the 5900eV Al resonance

Estimating centroid with gaussian fit

Better agreement using he t0 value based on the 2011 simulations


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


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)

Extracting the neutron energy from λ(E)


Graphs by Diego Tarrio, USC

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)

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?


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


Internal note

Detailed n_TOFinternal note isbeing prepared


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 PENDINGEAR-1 Simulations: Neutron fluence | Spatial profile | Time-to-energy

n_TOF Annual Collaboration Meeting – Lisbon, December 13-15, 2011 | A.T.

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 Simulationsn_TOF Analysis Group Meeting – CERN, October 4-5, 2011 | A.T.

The end

EAR-1 Neutron Flux Simulationsn_TOF Analysis Group Meeting – CERN, October 4-5, 2011 | A.T.

extra slides

Strange feature


Geometry: changes & improvements Neutron window

Rotated by 450, as observed during 2010 alignment campaign

Material definition correctedwith appropriate Al alloy

5cm2mm


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


Geometry: changes & improvements Neutron window

Moved 8mm downstream


Geometry: changes & improvements Comparison with technical drawings

Target, windows… Overlay of drawings and simulated geometry now possible


Neutron propagation: gravity Gravity can significantly alter the trajectory of low-energy neutrons

No significant effect expected above 1-10eV

Δy = ½ g t2


Neutron propagation: gravity


Neutron propagation: gravity


Neutron propagation: gravity Sample neutron trajectories






Date post:	25-Feb-2016
Category:	Documents
Upload:	glenna
View:	61 times
Download:	2 times

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

Documents