Activation Measurement of theHe3(α,γ)Be7Cross Section at Low Energy

Abstract

The nuclear physics input from the 3 He(α, γ) 7 Be cross section is a major uncertainty in the fluxes of 7 Be and 8 B neutrinos from the Sun predicted by solar models and in the 7 Li abundance obtained in big-bang nucleosynthesis calculations. The present work reports on a new precision experiment using the activation technique at energies directly relevant to big-bang nucleosynthesis. Previously such low energies had been reached experimentally only by the prompt-γ technique and with inferior precision. Using a windowless gas target, high beam intensity and low background γ-counting facilities, the 3 He(α, γ) 7 Be cross section has been determined at 127, 148 and 169 keV center-of-mass energy with a total uncertainty of 4 %. The sources of systematic uncertainty are discussed in detail. The present data can be used in big-bang nucleosynthesis calculations and to constrain the extrapolation of the 3 He(α, γ) 7 Be astrophysical S-factor to solar energies. The 3 He(α, γ) 7 Be reaction is a critical link in the 7 Be and 8 B branches of the proton-proton (p-p) chain of solar hydrogen burning [1]. At low energies its cross section σ(E) (E denotes the center of mass energy, E α the 4 He beam energy in the laboratory system) can be parameterized by the astrophysical S-factor S(E) defined as. The 9.4 % uncertainty [3] in the Sfactor extrapolation to the solar Gamow energy (23 keV) contributes 8 % to the uncertainty in the predicted fluxes of solar neutrinos from the decays of 7 Be and 8 B [4]. The interior of the Sun, in turn, can be studied [4,5] by comparing this prediction with the data from neutrino detectors [6,7], which determine the 8 B neutrino flux with a total uncertainty as low as 3.5 % [7].Furthermore, the production of 7 Li in big-bang nucleosynthesis (BBN) is highly sensitive to the 3 He(α, γ) 7 Be cross section in the energy range E ≈ 160-380 keV [8], with an adopted uncertainty of 8 % [9]. Based on the baryon-to-photon ratio from observed anisotropies in the cosmic microwave background [10], network calculations predict primordial 7 Li abundances [11] that are significantly higher than observations [12,13]. A lower 3 He(α,γ)7 Be cross section at relevant energies may explain part of this discrepancy. The3 He(α,γ) 7 Be (Q-value: 1.586 MeV) reaction leads to the emission of prompt γ-rays, and the final 7 Be nucleus decays with a half-life of 53.22 ± 0.06 days, emitting a 478 keV γ-ray in 10.44 ± 0.04 % of the cases [14]. The cross section can be measured by detecting either the induced 7 Be activity (activation method) or the prompt γ-rays from the reaction (prompt-γ method). Previous activation studies [15,16,17,18] cover the energy range E = 420-2000 keV. Prompt γ-ray measurements [15,19,20,21,22,23,24] cover E = 107-2500 keV, although with limited precision at low energies.The global shape of the S-factor curve is well reproduced by theoretical calculations [25,26]. However, the slope has been questioned [26] for E ≤ 300 keV, where there are no high-precision data. Furthermore, a global analysis [3] ind...

Big Bang 6 Li nucleosynthesis studied deep underground (LUNA collaboration)

“…For this purpose, 3 He( α, γ ) 7 Be reaction rate evaluated by Kontos et al [32] is used, which is within 1.5% of the slightly updated de Boer et al [33] and Takács et al [34] rates. The excitation function adopted in that work closely tracks the LUNA data on 3 He( α, γ ) 7 Be [35,36] , which are the only recent experimental data on this reaction at energies below 300 keV, most relevant for BBN. 6 Li reaction from the present work.…”

“…[ cm 3 Therefore, by using the Kontos et al rate in essence the 6 Li/ 7 Li isotopic ratio is determined by the ratio of two LUNA experimental S-factors in similar setups: The present 2 H( α, γ ) 6 Li data on 6 Li production, and the previous 3 He( α, γ ) 7 Be data [35,36] on 7 Be → 7 Li production. Using the Kontos et al rate [32] , 7 Li/H = (5.2 ± 0.4) ×10 −10 is found, 15% higher than when using CF88.…”

Isospin effects on astrophysicalS-factors

“…Since one has factored out the strong energy dependence of σ(E) due to the barrier penetrability, the S-factor could be approximated by a smooth energy dependence in the absence of low-energy resonance. A lot of effort has been made to extract the S-factor in the low energy region in particular for transfer reactions experimentally [3,4,5,6]. Although to know the low energy S-factor for radiative capture reactions, still one needs the extrapolation with theoretical formulae [7].…”

“…We will make use of it, if there is no experimental data available at lower energies. Within this simple approach we have estimated the S-factor ratios for the proton and deuteron induced reactions with positive Q-values on the nuclei D, 6 Li, 7 Li, 9 Be, 10 B and 11 B. All the investigated reactions are listed in Table I and II.…”

“…Using eqs. (5)(6) and assuming that the correction coming from the CN decay is given by their respective Q-values and temperatures, the ratio of the S-factor is expressed as [21]:…”