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Outline The motivation to study this reaction Experimental setup

Outline The motivation to study this reaction Experimental setup Choice of the measurement parameters Possible data analysis approach GEANT4-simulations Ongoing work and summary. The motivation to study this reaction. comparably small amount of 6 Li has been

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Outline The motivation to study this reaction Experimental setup

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  1. Outline • The motivationtostudythisreaction • Experimental setup • Choice ofthemeasurementparameters • Possibledataanalysisapproach • GEANT4-simulations • Ongoingworkandsummary

  2. The motivationtostudythisreaction • comparably small • amount of 6Li has been • synthesized during Big • Bang nucleosynthesis • productionmainlyby • d(α,γ)6Li • in laterperiodsfurther • depletionand • productionof Li may • haveoccurred • expected: noor variable • Li abundance in stars [takenfrom M. Pospelovand J. Pradler, Annu.Rev.Nucl.Sci. 2010, 60:539-568]

  3. The motivationtostudythisreaction BBN prediction • constant amount of • Li is found in stars • of different metallicity • (which is the content of • heavy elements) • prediction, usingknown • nuclearreactionrates: • threeordersof • magnitudeless6Li • isthis Li primordial? • do wehavewrong • nuclearreactionrates? 7Li 6Li BBN prediction [datafrom M. Asplund et al., Astrophys. J. 644, 229 (2006)]

  4. The motivationtostudythisreaction • Directmeasurements so far: • Robertson et al. 1991, E > 1 MeV • Mohr et al. 1996, aroundthe • resonanceat 0.7 MeV • Recentindirectmeasurements • (high energycoulombbreakup) • byHammache et al. at GSI • GSI workprovidedupperlimits, • due tonuclearbreakupcontribution • directmeasurementat LUNA • ispossible LUNA BBN energyregion 0 0.5 1.0 MeV [takenfrom F. Hammache et al., Phys. Rev. C 82, 065803 (2010)]

  5. Experimental setup • 4He beam on a windowless D2 gas target • beam currentmeasurementbycalorimeter • HPGedetectorforγdetection, Si detectorforprotons pumpingstages 3rd 4∙10-7mbar 2nd 6∙10-7mbar 1st 8∙10-4mbar Accelerator D2 gas inlet Si detector calorimeter Magnet target 0.3 mbar HPGedetector (137%, ULB) tosolid target

  6. Aboutthe experimental setup

  7. Experimental setup The dataaquisitionsystem: • Ortec Maestro fortheHPGeandthesilicondetector • Caen N1728B digitizer („TNT2“) fortheHPGedetector (working in parallel) • calorimeterand gas targetdataarestoredpermanently • Data obtainedsince September 2010: • 0.2 mbar, 400 keV: 200 h, 185 C • 0.3 mbar, 280 keV: 490 h, 540 C • 0.3 mbar, 400 keV: 440 h, 520 C • about 20 daysofnaturalbackgrounddata

  8. Choice of the measurement parameters • Main backgroundsources: • 238U anddaughternuclidesfromthesurrounding rock • 232Th anddaughternuclidesfromdirtandfromleadbricks • but muchmoreimportantisthe beam inducedbackground: 2H(α,α)2H Rutherford scattering • also 2H(2H,p)3H • occurswithsimilarcrosssection • monitoringofneutronproduction 2H(2H,n)3He d+d - reaction

  9. Choice of the measurement parameters TNT2 data, lab backgroundsubtracted, normalized

  10. Choice of the measurement parameters

  11. Possible data analysis approach • Whereisthed(α,γ)6Li gammasignalexpectedtobefound? • verybroadsignal, positiondepends on beam energy • lowexpectedsignalcounting rate in Gedetector (max. 2 counts / hour) • similarnaturalbackground rate insidethefullyshieldedsetup (400 keV beam) Doppler shift (± 16 keV) energyofrecoiled6Li (0.14 keV)

  12. Possible data analysis approach 1549 keV 63Cu(n,n‘γ) 280 keV ROI 400 keV ROI Howcan a yield beextracted? 1623 keV 65Cu(n,n‘γ) TNT2 data, May/June 2011, lab backgroundsubtracted, normalized

  13. Possible data analysis approach • Beam inducedbackground • subtractionapproach • But: • bothspectratobesubtractedhavetobenormalized (toincludethe same beam • inducedbackground) • due to different neutronenergyspectra, thenormalizationfactor • depends on thegammaenergy 1623 keV 65Cu

  14. Possible data analysis approach • choiceofseveral „flat“ regionstocalculate beam inducedbackgroundratiosalongthespectrum 200 600 1000 1400 1800 2200

  15. Possible data analysis approach „Flat“ regions: (with May/June 2011 data; naturalbackgroundissubtracted)

  16. Possible data analysis approach Plot of flat regioncontents, normalizedbychargeandregionwidth

  17. Possible data analysis approach • the beam inducedbackgroundratiodepends on the • regionenergy!

  18. Possible data analysis approach • How to find theenergydependenceofthe beam inducedbackground? • Basic idea: • trying fit functions on theplottedregioncontentratios • weightingtheresultsusingtheχ²/DoFvalue • calculating a normalizationfactorfor Eγ= 1580 keV • (between 280 keVand 400 keV ROIs)

  19. Possible data analysis approach

  20. Possible data analysis approach • Yield calculation: • 400 keV ROI: 1589.6…1621.5 keV 280 keV ROI: 1549.7…1580.6 keV • hasbeendoneforbothmeasurementcampaigns in 2011 • delivers positive yieldsforboth beam energies in spring • delivers negative resultsornoyieldforthe fall measurements •  Isthebackgroundshapeinsidethe ROIs not stable in time?

  21. Possible data analysis approach 0.3 mbar TNT2 data 400 keVand 280 keV rawspectra sumofcounts in 200…4000 keV normalizedby time Counts/s fall 2011 spring 2011 Charge (C)

  22. Possible data analysis approach Analysisconstraints

  23. GEANT4-simulations • measurement • andsimulation • are in good • agreement! • simulationsdone • by Z. Elekes

  24. GEANT4-simulations The beam inducedbackgroundlevelhasdoubledwithin 1000 C of beam charge. Hypothesis: As the gas targetpressurehas not beenchanged, thiseffectis due todeuteriumimplantation in metalsurfaces: • targetcollimator • steeltubealongthe beam (was alwaysintendedtostopdeuterons) • beam calorimeter • Howaffects a changingneutronsourcegeometrythemeasuredγ-spectra?

  25. GEANT4-simulations beam inducedbackgroundshape depends on the neutronsourcegeometry!

  26. GEANT4-simulations 0.3 mbar Maestro dataofsilicondetector, compared withsimulationresults (by P. Corvisiero)

  27. Ongoingwork The next beam time isscheduledfor March and April 2012. Intentionsandgoals: • exchangingdeuteratedsetupcomponents • measurementwith an AmBeneutronsourcetocomparewithsimulationresults • increasingamountofd(α,γ)6Li data • Si detectorresolutionandrecalibrationworks • furtherworktounderstandthebackground in theγ-detector • dataanalysis

  28. Summary • nodirectmeasurementoftheastrophysical S-factoratlowenergiesyet • newdatacouldanswer still open questionsaboutthe6Li origin in ouruniverse • the LUNA experimentat LNGS isabletomeasureverylowcrosssections • a possibledataanalysisapproachfor a small, broadsignalhasbeendeveloped • the beam inducedbackgroundneedstobestudiedfurther • GEANT4 simulationsarehelpful

  29. Thank you for your attention! The LUNA collaboration: A. Bellini, D. Bemmerer, C. Broggini, A. Caciolli, P. Corvisiero, H. Costantini, Z. Elekes, M. Erhard, A. Formicola, Zs. Fülöp, G. Gervino, A. Guglielmetti, C. Gustavino, Gy. Gyürky, G. Imbriani, M. Junker, A. Lemut, M. Marta, C. Mazzocchi, R. Menegazzo, P. Prati, V. Roca, C. Rolfs, C. Rossi Alvarez, E. Somorjai, O. Straniero, F. Strieder, T. Szücs, F. Terrasi, H.P. Trautvetter, D. Trezzi This workissupportedby DFG (BE 4100/2-1).

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