New measurements of the 7Be(n, α) 4He and 7Be(n, p)

7Li reaction cross sections from thermal to keV neutron energies have been performed at CERN/n TOF. High purity 7Be material was produced at the Paul Scherrer Institute (PSI) and implanted at CERN/ISOLDE, demonstrating the feasibility of neutron measurements on samples produced at radioactive beam facilities. The cross sections are significantly higher than previously determined, at low energies. The new estimate of the 7Be destruction rate based on these new results yields a decrease of the predicted cosmological 7Li abundance, insufficient to provide a viable solution to the Cosmological Lithium Problem (CLiP). The two n TOF measurements can finally rule out neutron-induced reactions as a potential explanation of the CLiP, leaving all alternative physics and astronomical scenarios still open

Barbagallo, M.

Cosentino, L.

Damone, L. A.

Mastromarco, M.

Mengoni, A.

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DOI 10.1393/ncc/i2019-19120-0Colloquia: EuNPC 2018IL NUOVO CIMENTO 42 C (2019) 120Exploring neutron channel solutions at CERN/n TOF for theCosmological Lithium ProblemA. Mengoni(1)(2), L. A. Damone(3)(4), M. Barbagallo(3)(5),M. Mastromarco(3)(5), L. Cosentino(6), F. Käppeler(7), N. Colonna(3),P. Finocchiaro(6) and The n TOF Collaboration(∗)(1) Agenzia Nazionale per le Nuove Tecnologie, l’Energia e lo Sviluppo Economico Sostenibile(ENEA) - Bologna, Italy(2) Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Bologna - Bologna, Italy(3) Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Bari - Bari, Italy(4) Dipartimento di Fisica, Università degli Studi di Bari - Bari, Italy(5) European Organization for Nuclear Research (CERN) - Geneva, Switzerland(6) INFN Laboratori Nazionali del Sud - Catania, Italy(7) Karlsruhe Institute of Technology, Campus North, IKP - 76021 Karlsruhe, Germanyreceived 5 February 2019Summary. — New measurements of the 7Be(n, α)4He and 7Be(n, p)7Li reac-tion cross sections from thermal to keV neutron energies have been performed atCERN/n TOF. High purity 7Be material was produced at the Paul Scherrer In-stitute (PSI) and implanted at CERN/ISOLDE, demonstrating the feasibility ofneutron measurements on samples produced at radioactive beam facilities. Thecross sections are significantly higher than previously determined, at low energies.The new estimate of the 7Be destruction rate based on these new results yields adecrease of the predicted cosmological 7Li abundance, insufficient to provide a viablesolution to the Cosmological Lithium Problem (CLiP). The two n TOF measure-ments can finally rule out neutron-induced reactions as a potential explanation ofthe CLiP, leaving all alternative physics and astronomical scenarios still open.1. – IntroductionMost of the parameters of the ΛCDM universe (cold dark matter with a cosmologicalconstant), defining a standard model for big bang cosmology, are presently known witha precision below one percent [1]. These parameters are essential to setup the conditionsfor the synthesis of the first few light nuclear species during the big bang nucleosynthesis(∗) www.cern.ch/ntofc© CERN on behalf of the n TOF CollaborationCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 A. MENGONI et al.(BBN). In particular, the baryon to photon number density ratio η = nb/nγ , has beendeduced from the most recent measurement of the cosmic microwave background (CMB)radiation with unprecedented accuracy. The recent evaluation of η, provided by thePlanck Collaboration [1], stands at 6.13 ± 0.03 × 10−10 (1).The BBN yield of light nuclei is very sensitive to η (baryon contents of the universe)and for a long time the agreement between observed and predicted primordial abundancesof Deuterium and, to a minor extent, of 3He and 4He, has been used as a baryometer,i.e. to fix the value of η. With the precise determination of η from CBM observations,the situation is now reversed: from an independently determined value of η, BBN is usedto verify the concordance between observed and predicted abundances of 4He, D, 3Heand 7Li. Hence, the insurgence of the cosmological Lithium problem (CLiP). In fact, asignificant discrepancy of a factor 3÷ 4 appears between the BBN predication of the 7Liyield and the inferred primordial abundance from observations. With a severe constrainton the value of η provided by the Planck observations, the reason for such a discrepancyremains to be explained.Considering that over 95% of the 7Li abundance is the result of the 7Be+e− →7Li+νeelectron-capture decay, a change in the production and/or destruction rate of 7Be dur-ing BBN is the essential ingredient for a different outcome of the cosmological Lithiumabundance. The n TOF Collaboration decided, therefore, to explore the possibility ofa neutron channel solution for the CLiP by measuring the rate for two most importantdestruction mechanisms of 7Be during the BBN.2. – 7Be(n, α)4He and 7Be(n, p)7Li cross section measurements at n TOFThe 7Be(n, α)4He reaction cross section has been measured using two samples of 7Bematerial placed in the neutron beam of the EAR2 experimental area of the n TOF facilityat CERN, Geneva. The 18 GBq samples, ≈ 1.4 μg each, were sandwiched between 3× 3cm2 140 μm silicon strip detectors which, combined with the low duty-cycle of the n TOFneutron beam, allowed strong rejection of background events due to, amongst others, γ-rays from the sample activity and protons from the (n, p) reaction channel. A full accountof this experiment is provided in reference [2]. The (n, γα) cross section, leading to thetwo 2+ excited states in 8Be at 16.63 and 16.92 MeV, has been deduced in the energyrange from thermal, En = 0.0253 eV, to ≈ 10 keV, and is attributable to the captureof s-wave neutrons and the successive 2α breakup. The cross section can be describedby a direct radiative capture (DRC) process as well as by the tail of the 2− resonancestate just above the neutron separation energy of 8Be. In order to derive the total (n, α)reaction process, the measured cross section has to be complemented by the calculationof the transition probabilities to the other α-emitting lower bound states in 8Be. Thishas been done in the framework of the DRC model, producing the s-wave componentof the neutron capture by 7Be. The direct breakup of 8Be into two α particles, dueto the negative parity states in 8Be have been deduced from an R-matrix analysis ofthe (α, αn) reaction channels, recently confirmed by the measurement of Kawabata etal.. [3]. The resulting total (n, α) cross section can be integrated over the energy rangeof interest for BBN network calculations in a proper temperature grid and the results(1) numerically, once the CMB temperature is fixed at T = 2.7255◦K, this value is derived frommeasured baryon density Ωbh2, through the relation: η × 1010 = 273.8 × Ωbh2.EXPLORING NEUTRON CHANNEL SOLUTIONS AT CERN/n TOF ETC. 30.00010.0010.010.110.001 0.01 0.1 1 100.1 1 10 100reactionrate[cm3 /s/molex109 ]T9kT [keV]Wagoner (1969)presentanalytical0123456780.001 0.01 0.1 1 100.1 1 10 100reactionrate[cm3 /s/molex109 ]T9kT [keV]Cyburt (2004)presentFig. 1. – Reaction rates NA〈σv〉 for the 7Be(n, α)4He (left) and 7Be(n, p)7Li (right), shown incomparison to rates used before the n TOF measurements. The gray bands around the analyticalcurves indicate the evaluated uncertainties on the rates, while the vertical band indicates thetemperature of interest in BBN network calculations.can be represented accurately by the following expression of the reaction rateNA〈σv〉 = a0(1 + a1T 1/29 + a2T9 + a3T3/29 + a4T29 + a5T5/29 +(1a)+a6T 39 + a7T7/29 + a8T49 + a9T9/29 + a10T59 )(1b)in units of cm3/s/mole when a0 = 4.810 × 105, a1 = −0.226, a2 = 5.301, a3 = 11.249,a4 = −18.940, a5 = 13.539, a6 = −0.133, a7 = −0.591, a8 = −1.144, a9 = 0.731 anda10 = −0.094(2).The 7Be(n, p)7Li measurement [4] has been performed using sample material extractedfrom the water cooling system of the SINQ spallation source at the Paul Scherrer Insti-tute, PSI at Villigen. 200 GBq were implanted using the 30 keV ≈ 45 nA ISOLDE-GPSseparator and the resonance ionization laser ion source - RILIS - on a suited backing.The measurement was performed at n TOF EAR2 using a silicon telescope (20 and 300μm, 5 × 5 cm2) strip detection device. The telescope system was characterized using anα source as well as a measurement of the well-known 6Li(n, t)4He reaction with the samesetup. Data of the cross section has been derived in the energy range from thermal upto En = 325 keV. Again, the resulting cross section has been integrated over the entireenergy range to produce a reaction rate valid in the proper temperature range of interestfor BBN network calculationsNA〈σv〉 = a0(1 + a1T 1/29 + a2T9 + a3T3/29 + a4T29 + a5T5/29 ) +(2a)+a6(11 + 13.076T9)3/2 + a7T−3/29 e−b0/T9(2b)in units of cm3/s/mole when a0 = 6.805 × 109, a1 = −1.971, a2 = 2.042, a3 = −1.069,a4 = 0.271, a5 = −0.027, a6 = 1.961 × 108, a7 = 2.890 × 107 and b0 = 0.281.(2) with respect to the rate published in [2], this expression includes additional terms in theexpansion, making it valid up to T9 = 10.4 A. MENGONI et al.Table I. – List of the 12th most important reactions used in the BBN network calculations# code index reaction adopted rate1 12 1H(n, γ)D Ando et al. (2006) [6]2 16 3He(n, p)T STARLIB (2016) [7]3 17 7Be(n, p)7Li this work [4] , Cyburt (2014) [8]4 19 7Be(n, α)4He this work [2], Wagoner (1969) [9]5 20 D(p, γ)3He Iliadis et al. (2016) [10]6 24 7Li(p, α)4He STARLIB (2016) [7]7 26 T(α, γ)7Li STARLIB (2016) [7]8 27 3He(α, γ)7Be Iliadis et al. (2016) [10]9 28 D(d, n)3He STARLIB (2016) [7]10 29 D(d, p)T STARLIB (2016) [7]11 30 T(d, n)4He STARLIB (2016) [7]12 31 3He(d, p)4He STARLIB (2016) [7]3. – Implications on the CLiP and conclusionsCalculations of the BBN yields have been performed using an updated version of theAlterBBN code [5], where the 12 most important rates have been updated. Details ofthe reaction rates used are given in reference [4] and in its Supplemental Material. Thereaction rates obtained from the n TOF measurements have been used in combinationwith the set of the most important reactions entering the BBN network (see Table I).In addition to the reaction rates, two other parameters are essential in BBN networkcalculations: the number of neutrino species and the free-neutron half-life. The numberof extra relativistic degrees of freedom (i.e., neutrinos) is constrained by the Planckobservations to be Neff = 2.99± 0.17, in agreement with the prediction of the StandardModel of particle physics Neff = 3.046 (value used in the calculations presented here).As for the neutron half-life, in our calculations we used the mean life evaluated by theParticle Data Group, τn = 880.2 ± 1.0 s [11]. The new estimate of the 7Be destructionrate based on the new results yields a decrease of the predicted 7Li abundance of atmost ∼10% with respect to results obtained with previous rates, insufficient to provide aviable solution to the Cosmological Lithium Problem. This leaves all alternative physicsand astronomical scenarios still open.REFERENCES[1] Aghanim N. et al.(The PLANCK Collaboration), A&A, (2018) arXiv:1807.06209.[2] Barbagallo M. et al. (The n TOF Collaboration), Phys. Rev. Lett., 117 (2016)152701.[3] Kawabata T. et al. (The n TOF Collaboration), Phys. Rev. Lett., 118 (2017) 052701.[4] Damone L.A. et al. (The n TOF Collaboration), Phys. Rev. Lett., 121 (2018) 042701.[5] Arbey A., Comp. Phys. Comm., 183 (2012) 1822[6] Ando S., Cyburt R.H., Hong S.W., and Hyun C.H., Phys. Rev. C, 74 (2006) 025809.[7] Sallaska A.L. et al., ApJ, 207 (2013) 18.[8] Cyburt R.H., Phys. Rev. D, 70 (2004) 023505.[9] Wagoner R.V., ApJS, 18 (1969) 247.[10] Iliadis C. et al., ApJ, 831 (2016) 107.[11] Tanabashi M. et al.(Particle Data Group), Phys. Rev. D, 98 (2018) 030001.

Exploring neutron channel solutions at CERN/n TOF for the

Cosmological Lithium Problem

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Exploring neutron channel solutions at CERN/n TOF for the Cosmological Lithium Problem

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