The resolution funtion of the n_TOF neutron beam

1. The resolution funtion of the n_TOF neutron beam • G. Lorusso, F. Gunsing, C. Carrapico, C. Domingo-Pardo, J.L. Tain, E. Berthomieux and N. Colonna

2. Whatis the ResolutionFunction At alltime-of-flightfacilities, at a fixedtime-of-flightcorresponds a distributionofneutronenergies (or, viceversa, at a fixedneutronenergycorresponds a distributionoftime-of-flight). The shapeof the time-of-flightdistributionfor a givenneutronenergyiscalled “Resolutionfunction” (RF). The RF depends on • characteristicsneutron source (production target, moderator, reflector, etc…) • neutronenergy Once upon a time, some brave people did some hard work, todetermine the resolutionfunctionofn_TOF, from 1 eVto 1 MeV.

3. Why is the RF important The Resolutionfunctionaffects the measuredshapeof a resonance R-matrixanalysiscodes (like SAMMY), determine the resonanceparametersbydeconvoluting the RF (and otherbroadeningeffects) from the measuredresonanceshape. Reactionyield Toextractresonanceparameters (E0, Gn, Gg), itisnecessarytoknow the ResolutionFunctionwithgoodaccuracy (but RF has no effect on the kernelof the resonance). Needto include the correctRF in SAMMY/REFIT, toavoidGIGO effects. Reactionyield 395.3 396.9 400.4 398.6 neutrontime-of-flight (ms) The ResolutionFunctiondepends on the neutronenergy and can beexpressedas a functionoftimeRF(t,En) or distanceRF(d,En)

4. In which energy region is the RF important? Below 1 keV the Doppler broadeningdominates, so thatevenfornarrowresonances the RF plays a minor role (doesnotaffect the shapeof the measuredresonance). Above 1 keV, the ResolutionFunctionbecomesimportant and hastobeproperlyconsidered. C. Massimi, PhDthesis

5. The simulations of the n_TOF resolution The only way tostudysystematically the RF at variousneutronenergiesiswithSIMULATIONS. A check on experimental data can beperformed“a posteriori” (after the RF hasbeenfound). FLUKA CAMOT In the simulations, Dtis the timeelapsedbefore the neutronenters the n_TOF tube (the neutroniscreated at t=0). 10 keV • The ResolutionFunctionfor a givenneutronenergy can beexpressedas a functionoftime or distance. In particular, from the simulationsonegets: • Dt spread in the time-of-flight (due to the timespent in the target/moderator system) • l=Dt vnwherevnis the neutronvelocity at the exitof the target

6. Main trend (from the simulations) FLUKA simulations The resolutionissmall (a few cm in equivalentpath) between 1 eV and 10 keV, whileitislarge at thermal and at high energy. MCNP simulations Pathlength equivalent to the time spent by a neutron of a given outgoing velocity inside the target-moderator system. C. Carrapico et al., RadiationMeasurements and Isotopes 168 (2009) 837

7. The RPI function Include or excludeexponential (X=0 or 1) c2distribution “Rise and fall” exponential The 5 parametersof the RPI function (t, L, A1, A3, A5, t0) depend on the neutronenergy. In SAMMY, the dependenceof the parameters on Enisalreadydefined: Toadapt the RPI functionton_TOFoneneedtofind18 parameters !!! D = Dt vn

8. The parameters of the RPI function • Itwasnotpossibletofind a unique set of (18) parameterscovering the full rangefrom 1 eVto 1 MeV. • Therefore, itwasdecidedto divide the range in 3 parts: • 1 eV – 200 eV (seldomlyused) • 200 eV – 5 keV (mostimportantresonanceregion) • 5 keV – 1 MeV (usedonlyfor some particularisotopes)

9. The n_TOF RPI function The proposedparameterizationhasbeenchecked first on the simulations

10. 500 eV 5 eV 100 eV 50 keV 100 keV 5 keV 800 keV 500 keV 600 keV

11. Validation of the new RPI function on experimental data An experimentalcheckof the ResolutionFunction in the wholeenergyrangeisvirtuallyimpossible. A validation can beperformedfor some specificcases, typicallyabove 1 keV. The validityof the ResolutionFunction can bechecked on some resonancesof the Fe(n,g) reactions(wellknownisolatedresonancesbetween 10 and 200 keV). R-matrixanalysis code SAMMY used, in which the resonanceparameters are keptfixed (only the normalizationisleft free). Measurementdone in 2002, repeated in 2009 and 2010 tocheckif the resolutionchangedwithnewspallation target and 10B-water moderator. Anotherpossibilityistouse238U (narrowresonancesaround 1 keV).

12. 11.2 keV 34.2 keV 59.2 keV 180 keV 80.8 keV

13. The modified RPI function (F. Gunsing)

14. The modified RPI function (F. Gunsing) The modified RPI function (withoptimizedparameters) isnowavailable in SAMMY

15. 1-10 eV 10-100 eV 100 eV -1 keV 1-3 keV 3-10 keV 10-30 keV 30-100 keV 100-300 keV

16. A comparison (at 1 keV) Around 1 keV, the “standard” RPI functioniswiderthan the “modified” one. At higherenergy, itis the opposite. Thereis no finalcomparisonof the “modified” RPI functionwithexperimental data. C. Massimi made a comparisonfor Au data around 1 keV.

17. Conclusions • Forn_TOFPhase 1, the ResolutionFunctionwasstudiedwithsimulations (Fluka, MCNP, Camot, etc…). • TWO functionswereproposed: • the “standard” RPI function (i.e. as a functionoftime) INFN-Bari • a “modified” RPI (asa functionofdistance) CEA-Saclay • Bothseemtoreproducecorrectly the simulations. • The “standard” RPI wasalsovalidated on experimental Fe(n,g) data • BothRFshavebeenextensivelyused in R-matrixanalysisofn_TOF data. n_TOFPhase 2 Accordingtosimulations, the RF for the newspallation target (and for10B-water moderator) shouldnotbemuchdifferentthanbefore. Some testswereperformed last year on Fe data, and the resultsseemedtoconfirmthattherewas no change. A more detailedanalysishastobeperformed. Weneedtocheckif the old RPI function(s) work alsofor n_TOF-Phase2. Otherwise, weneedtofind some brave people toadapt the old RF to the new target/moderator system.