Acta Polytechnica doi:10.14311/AP.2013.53.0534 Acta Polytechnica 53(Supplement):534–537, 2013 © Czech Technical University in Prague, 2013 available online at http://ojs.cvut.cz/ojs/index.php/ap THE 2H(α, γ)6LI REACTION AT LUNA AND BIG BANG NUCLEOSYNTHETIS Carlo Gustavino∗ INFN Sezione di Roma, I-00185 Roma, Italy ∗ corresponding author: carlo.gustavino@roma1.infn.it Abstract. The 2H(α, γ)6Li reaction is the leading process for the production of 6Li in standard Big Bang Nucleosynthesis. Recent observations of lithium abundance in metal-poor halo stars suggest that there might be a 6Li plateau, similar to the well-known Spite plateau of 7Li. This calls for a re-investigation of the standard production channel for 6Li. As the 2H(α, γ)6Li cross section drops steeply at low energy, it has never before been studied directly at Big Bang energies. For the first time the reaction has been studied directly at Big Bang energies at the LUNA accelerator. The preliminary data and their implications for Big Bang nucleosynthesis and the purported 6Li problem will be shown. Keywords: LUNA experiment, nuclear astrophysics, Big Bang nucleosynthesis, 6Li abundance, 2H(α, γ)6Li reaction. 1. Introduction In its standard picture, Big Bang nucleosynthesis oc- curs during the first minutes of universe, with the for- mation of light isotopes such as D, 3He, 4He, 6Li and 7Li, through the reaction chain shown in Fig. 1. Their abundance depends on the standard model physics, on the baryon-to-photon ratio η and on the nuclear cross sections of involved processes. Cosmic Microwave Background (CMB) experiments provide the η value with high precision (percent level) [1]. Indeed, the BBN theory makes definite predictions for the abun- dances of the light elements as far as the nuclear cross sections of leading processes are known. The observed abundances of D, 3He, and 4He are in good agreement with calculations, confirming the overall validity of BBN theory. On the other hand, the observed abundance of 7Li is a factor 2 ÷ 3 lower than the predicted abundance (see Fig. 2). The amount of 6Li observed in metal poor stars is unexpectedly large compared to Big Bang Nucleosynthesis (BBN) predictions, about 3 order of magnitude higher than the calculated value (see Fig. 2). Even though many of the claimed 6Li detections may be in error, for a very few metal-poor stars there still seems to be a significant amount of 6Li [2]. The difference between observed and calculated values may reflect unknown post-primordial processes or physics beyond the Stan- dard Model. The leading process to synthesize 6Li is the 2H(α, γ)6Li reaction. The 2H(α, γ)6Li cross section is very small at BBN energies (30 . E(keV) . 400), be- cause of the repulsion between the interacting nuclei. Therefore, it has never been measured experimentally, and theoretical predictions remain uncertain [3]. This process has been experimentally studied only for ener- gies greater than 1 MeV and around the 711 keV [4, 5]. There are two attempts to determine the 2H(α, γ)6Li Figure 1. Leading processes of Big Bang Nucleosyn- thesis. The red arrows show the reactions measured by the LUNA collaboration. Yellow boxes marks stable isotopes. cross section at BBN energies, using the Coulomb dissociation technique [6, 7]. In this approach, an energetic 6Li beam passes close to a target of high nuclear charge. In this way, the time-reversed reaction 6Li(γ, α)2H is studied using virtual photons which are exchanged. The measurements mentioned above are shown in Fig. 3. In the figure, the cross section σ is parame- terized with the astrophysical factor S(E), defined by the formula σ(E) = S(E) e−2πη E . (1) S(E) contains all the nuclear effects and, for non- resonant reactions, it is a smoothly varying function 534 http://dx.doi.org/10.14311/AP.2013.53.0534 http://ojs.cvut.cz/ojs/index.php/ap vol. 53 supplement/2013 The 2H(α, γ)6Li Reaction at LUNA and Big Bang Nucleosynthetis Figure 2. Abundances of 7Li and 6Li as a function of the η parameter. Observations are represented as green, horizontal dashed band. The blue band shows the calculated abundance of 7Li. The calculated abun- dance of 6Li is obtained using the NACRE compilation recommended values (dashed lines). The vertical yel- low band indicates the η parameter as measured by the WMAP experiment. of energy. The exponential term takes into account the coulomb barrier. The Sommerfeld parameter η is given by 2πη = 31.29Z1Z2(µ/E) 1/2. Z1 and Z2 are the nuclear charges of the interacting nuclei. µ is their reduced mass (in units of a.m.u.), and E is the center of mass energy (in units of keV). As Coulomb dissociation measurements strongly de- pends on the theoretical assumptions because nuclear effects are dominant, only a direct measurement of 2H(α, γ)6Li reaction in the BBN energy range can give a reliable base to compute the 6Li primordial abundance. The present papers reports on the first di- rect measurement of the 6Li(γ, α)2H performed by the LUNA collaboration (LUNA – Laboratory for Under- ground Nuclear Astrophysics). The measurement has been performed with the unique underground acceler- ator in the world, situated at the LNGS laboratory (LNGS – Laboratorio Nazionale del Gran Sasso). The “Gran Sasso” mountain provides a natural shielding which reduces the muon and neutron fluxes by a factor 106 and 103, respectively. The suppression of the cos- mic ray induced background also allows an effective suppression of the γ-ray activity by a factor 102 ÷ 105, depending on the photon energy [8]. 2. Experimental set-up Figure 5 shows the experimental set-up used for the 2H(α, γ)6Li reaction. The measurement is based on the use of the 400 kV accelerator, which provides an α beam of high intensity. The α beam impinges a Figure 3. The astrophysical factor of the 2H(α, γ)6Li reaction as a function of the center-of-mass energy. Direct [4, 5] and indirect measurements [6, 7] are reported. The BBN energy region and the energy range studied by LUNA are also reported. windowless gas target of D2, with a typical operat- ing pressure of 0.3 mbar. The signal is maximized by stretching the beam intensity up to about 350 µA and by using a geometry with the germanium detector close to the beam line (2 cm apart), to increase its acceptance. The intrinsic low level of natural back- ground of LNGS is further reduced by means of a 4π lead shield around the reaction chamber and the Ge detector. Everything is enclosed in a radon box flushed with high purity N2, to reduce and stabilize the γ activity due to the radon decay chain. The measurement of the 2H(α, γ)6Li reaction is affected by an inevitable beam induced background. In fact, the 2H(α, α)2H Rutherford scattering induces a small number of 2H(2H, n)3He and 2H(2H, p)3H reactions. While the 2H(2H, p)3H reaction is not a problem in this context, the neutrons produced by the 2H(2H, n)3He reaction (En(cm) = 2450 keV) induce (n,n′γ) reactions in the Ge detector and in the sur- rounding materials (lead, steel, copper), generating a beam-induced background in the γ-ray spectrum, in particular around 1.6 MeV, where the capture transi- tion to the ground state of 6Li is expected. To reduce the neutron production, a tube 16 cm long, with a square cross section of 2 × 2 cm2 is inserted inside the chamber. The tube strongly reduces the effective path for the scattered deuterons and therefore the neutron yield due to the 2H(2H, n)3He reaction is reduced at the level of few neutrons/second. Finally, one silicon detector is faced to the gas target volume to monitor the running conditions through the detection of protons generated in the 2H(2H, p)3H reaction (Ep(cm) = 3022 keV). The measurement of the number of protons detected is strictly related to the number of neutrons produced, since the cross sections of the two 535 Carlo Gustavino Acta Polytechnica Figure 4. Spectra taken with the Ge detector. Blue full line: Beam induced background spectrum at Eα = 400 keV and Ptarget = 0.3 mbar. Grey thin line: laboratory background. Figure 5. Experimental setup. conjugate 2H(2H, n)3He and 2H(2H, p)3H reactions are well known. Figure 4 shows the Ge spectrum at Eα = 400 keV and Pdeuterium = 0.3 mbar. Various transitions due to the interaction of neutrons with the Germanium and the surrounding materials can be identified. 3. Method The Energy of γ’s coming from D+α reaction strongly depends on the beam energy, through the relationship Eγ = 1473.8 + ECM ± ΔEdoppler. (2) As shown in Fig. 6, the γ-rays energy strongly de- pends on the beam energy. The Region of Interest (RoI) is about 30 keV wide, because of the Doppler broadening. As the γ’s produced the 2H(α, γ)6Li reaction strongly depends on the beam energy, it is possible to extract the signal with a measurement performed in two steps: Figure 6. Simulated full peak detection of γ’s from 2H(α, γ)6Li in the LUNA Ge detector, at different beam energy. Note the Doppler broadening of about 30 keV and the dependence with the beam energy. (1.) Measurement with Ebeam = 400 keV on D2 target. The Ge spectrum is mainly due to the background induced by neutrons. The 2H(α, γ)6Li γ signal is expected in a well defined energy region (1592 ÷ 1620 keV, see Fig. 6). (2.) Same as (1.), but with Ebeam = 280 keV. The background is essentially the same as before, while the gammas from the 2H(α, γ)6Li γ reaction are shifted to 1555 ÷ 1578 keV (see Fig. 6). Figure 7a shows the spectra with Eα = 400, 280 keV, respectively. A counting excess is clearly visible in the Eα = 400 keV RoI. Unfortunately, at Eα = 280 keV the very low reaction yield prevents from any conclusion statistically significant. To verify that the counting excess at Eα = 400 keV is a genuine γ signal com- 536 vol. 53 supplement/2013 The 2H(α, γ)6Li Reaction at LUNA and Big Bang Nucleosynthetis Figure 7. a) Experimental Ge spectra for Ebeam = 400 keV (black line) and for Ebeam = 280 keV (red line); b) experimental Ge spectra for Ebeam = 360 keV (black line) and for Ebeam = 240 keV (red line). T, 〈P 〉, Q are respectively the measurement time, the averaged target pressure, the integrated beam current. The bands indicate the RoI at Eα = 400 keV RoI and Eα = 280 keV a), and the RoI at Eα = 360 keV RoI and Eα = 240 keV b). Note the counting excess visible in correspondence of the 400 keV RoI. As foreseen, the counting excess shifts to the Eα = 360 keV RoI in b). ing from the 2H(α, γ)6Li reactions, the measurement has been repeated by shifting the beam energies to Eα = 360, 240 keV. As shown in Fig. 7b, the counting excess at the higher energy is shifted as expected, even though the worst signal/noise ratio and the shorter measuremet time. 4. Conclusion The cross section of 2H(α, γ)6Li reaction has been measured for the first time at BBN energy. Although the data analysis is still in progress, the LUNA mea- surement excludes a nuclear solution for the purported 6Li problem. The observation of a “huge” amount of 6Li in metal-poor stars must be explained in a dif- ferent way, e.g. systematics in the 6Li observation or physics beyond the Standard Model. In any case, a solid experimental footing is now available to calculate the 6Li primordial abundance. References [1] D.N. Spergel, et al., 2007, ApJS, 170, 377 [2] See proceedings of “lithium in the Cosmos”, 27-29 february, Paris [3] L. Marcucci, K. Nollett, R. Schiavilla, and R.Wiringa, Nucl. Phys. A 777, 111 (2006) [4] P. Mohr et al., Phys. Rev. C 50, 1543 (1994) [5] R. G. H. Robertson et al., Phys. Rev. Lett. 47, 1867 (1981) [6] J. Kiener et al., Phys. Rev. C 44, 2195 (1991) [7] F. Hammache et al., Phys. Rev. C 82, 065803 (2010), 1011.6179 [8] A. Caciolli et al, Eur. Phys. J. A 39, 179–186 Discussion Maurice H.P.M. van Putten — Could the excess of 6Li in Pop III stars be due to inhomogeneities in primor- dial Li production due to density fluctuaction? That is 6Li formation is a binary reaction product with a rate proportional to the density squared, and Pop III stars form selectively out of initially overdense region. Carlo Gustavino — In my opinion it is difficult to explain the purported 6Li excess in this way, because the barionic density determines not only the 6Li abundance but also the amount of all the other primordial isotopes. In particular, deuterium abundance is very sensitive to the barionic density, but observations are in good agreement with calculations. 537 Acta Polytechnica 53(Supplement):534–537, 2013 1 Introduction 2 Experimental set-up 3 Method 4 Conclusion References