Direct measurement of the 2H(α,γ)6Li cross section at energies of astrophysical interest

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1 Direct measurement of the 2H(α,γ)6Li cross section at energies of astrophysical interest NPA V April 3-8, 2011 Eilat, Israel Laboratory Underground Nuclear Astrophysics Alessandro Bellini INFN Genova, Italy LUNA Collaboration

2 The 2H(α,γ)6Li Nuclear Reaction Introduction BBN prediction Li 7 Li 6 BBN prediction Li has been found in non negligible quantities in very old low metallicity halo stars unexpectedly high amount (2-3 orders of magnitude compared to available BBN network predictions - NACRE) Li has the next-highest predicted primordial abundance after D, 3He, 4 He and 7Li 6 Challenge: what is the origin of observed 6Li in old halo star? Primordial? Pre-galactic? Exotic origin? 6 Primordial 6Li production 2 H(α,γ)6Li

3 The 2H(α,γ)6Li Nuclear Reaction Status of Art S-factor of the D+α reaction as a function of the interaction energy [taken from F. Hammache et al., Phys. Rev. C 82, (2010)] Direct measurements: does not exist at BBN energies; only above 1 MeV (Robertson et al., 1981) and around the resonance at 711 kev (Mohr et al., 1996). At energy of astrophysical interest only indirect measurements using Coulomb dissociation exist (Kiener et al., 1991; Mukhamedzhanov et al., 1995; Hammache et al., 2010). All the estimates are still differing by more than one order of magnitude. Expected cross section of few pico-barns At LUNA a direct measurement at energies of astrophysical interest is possible!

4 LUNA Underground Laboratori Nazionali del Gran Sasso Rlab=σIpερNAV/A Ip~mA; ε~10%; ρ~μg/cm2 pb < σ < nb events/month < Rlab < events/day Low coss section condition poor signalto-noise ratio improve yields or reduce background. Advantages of going LNGS: Natural shielding of about 1400 m of rocks (4000 m w.e.) Muon flux reduced by a factor 106; neutron flux reduced by a factor 103 (referring to the surface)

5 LUNA Underground Laboratori Nazionali del Gran Sasso Underground Pb-Cu shielding and Radon box 3 orders of magnitude!!! Detectors can be shielded passively with proper Pb-Cu shield as on surface, but underground passive shielding is more effective since μ and neutron fluxes, that create secondary γ s in the shield, are suppressed. The decaying 222Rn and its daughters produce secondary γ radiation. A popular solution of this problem is to house the detector in a box with a small overpressure of flushing nitrogen.

6 LUNA Underground Facility Experimental Apparatus LUNA II ACCELERATOR Installed in 2000 VMAX = 400 kv High beam intensity: 350 μa protons 300 μa α particles High stability High energy resolution ΔEbeam 100 ev

7 LUNA Underground Facility Experimental Apparatus Windowless differentially pumped gas target Adjustable target tickness (D2 target, 18 cm long, pressure range mbar) Good isotopical purity High stability over long run periods HpGe detector (135% relative efficiency) Located in close geometry (distance from target: 5mm) Simulated total gamma efficiency in the ROI is 2.5% Calorimeter (made of copper) for beam intensity measurement (termic power measurement)

8 LUNA Underground Facility Experimental Apparatus Lead passive shielding Radon-box PE shielding Tmeas= 4.8 d

9 Beam Induced Background Reactions induced by α beam D(d,n)3He & D(d,p)3H Neutron interaction with setup (inelastic scattering) produce a γ backgound Neutron production control nedeed LNGS Scientific Commitee neutron production limit neutrons 10 sec e.g. 130 μa α beam at 0.5 mbar D2 gas target pressure Experimental set-up adjustments: reduction of diffused deuteron mean free path

10 LUNA Underground Facility Experimental Apparatus D d, n He n produced = p produced D d, p H 3 3 ; p measured p produced = Si D(d,n)3He & D(d,p)3H (cross section known with high precision) Ebeam = 360 kev, Ibeam = 289 μa, PD2 = 0.2 mbar, Tmeas = 2.48 d Si detector 1500 μm Al window Peltier cooling

11 Expected Signal by Simulations γ spectra simulation Scattering Chamber scheme Extended target 2 2 H(α,γ)6Li Q = 1.46 Mev H(α,γ)6Li ROI kev E = E cm Q E Doppler E Recoil (@ 400 kev)

12 Data Taking Best Experimental Conditions Maximum beam energy: Ebeam = 400 kev (Ecm = 133 kev) Signal P Beam Induced Background Current limits vs D2 pressure (neutron production) P2 D2 pressure: P = 0.2 mbar Maximum beam current (Ibeam = 260 μa) Measurements from October 21st 2010 to November 1st 2010, 75% duty cycle ~ 200 h

13 Data Taking Measured Spectra Eα= 400 kev, Ibeam= 260 μa, PD2= 0.2 mbar Laboratory Background

14 Data Taking Measured Spectra 63 Eα= 400 kev, Ibeam= 260 μa, PD2= 0.2 mbar Cu (1547 kev) 2 H(α,γ)6Li ROI 471±8 counts day Laboratory Background Cu 65 (1623 kev)

15 Spectra Analisys H(α,γ)6Li ROI 2 Laboratory background Measured Spectra counts 76±4 day counts 471±8 day counts 44±2 day (Calculated using existing indirect data of Hammache et al.) Expected Signal 1 Signal-to-Noise ratio 10

16 Spectra Analysys Copper Peaks kev Cu? HpGe detector and calorimeter

17 Spectra Analysys Copper Peaks Ebeam=400 kev Ebeam=360 kev Signal counting rate reduction by 25%

18 Spectra Analysys Copper Peaks Ebeam=360 kev Ebeam=400 kev kev

19 Beam Induced Background Measurements Beam Induced Background: E4He beam= 400 kev dominant part of the spectra to be measured with high precision H(3He,p)4He 2 Allow to measure only beam induced backgroun without the signal Simulations show a very similar diffused deuterium distributions E3He beam= 370 kev Using 3He beam same beam induced background is expected TO BE DONE!!!

20 Work in progress Challenging direct measurement due to the beam induced background produced by neutrons to measure with the highest precision The 2H(3He,p)4He could be used to subtract the beam induced background High statistic measurements with 3He and 4He are in progress.

21 Thanks 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, F. Terrasi, H.P. Trautvetter, D. Trezzi