Decay of the key 92-keV resonance in the 25Mg(p,γ) reaction to the ground and isomeric states of the cosmic γ-ray emitter 26Al

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Decay of the key 92-keV resonance in the 25Mg(p,γ) reaction to the ground and isomeric states of the cosmic γ-ray emitter 26Al © 2020 The Authors. Published by Elsevier B.V. Published version Kankainen, A.; Woods, P.J.; Doherty, D.T.; Albers, H.M.; Albers, M.; Ayangeakaa, A.D.; Carpenter, M.P.; Chiara, C.J.; Harker, J.L.; Janssens, R.V.F.; Lederer-Woods, C.; Seweryniak, D.; Strieder, F.; Zhu, S. Kankainen, A., Woods, P.J., Doherty, D.T., Albers, H.M., Albers, M., Ayangeakaa, A.D., Carpenter, M.P., Chiara, C.J., Harker, J.L., Janssens, R.V.F., Lederer-Woods, C., Seweryniak, D., Strieder, F., & Zhu, S. (2021). Decay of the key 92-keV resonance in the 25Mg(p,γ) reaction to the ground and isomeric states of the cosmic γ-ray emitter 26Al. Physics Letters B, 813, Article 136033. https://doi.org/10.1016/j.physletb.2020.136033 2021 Physics Letters B 813 (2021) 136033 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb Decay of the key 92-keV resonance in the 25Mg(p, γ)reaction to the ground and isomeric states of the cosmic γ-ray emitter 26Al A. Kankainen a,b,∗, P.J. Woods a, D.T. Doherty a,c, H.M. Albers d,1, M. Albers d, A.D. Ayangeakaa e,f, M.P. Carpenter d, C.J. Chiara d,g,2, J.L. Harker d,g, R.V.F. Janssens e,f, C. Lederer-Woods a, D. Seweryniak d, F. Strieder h, S. Zhu d,i aUniversity of Edinburgh, Edinburgh EH9 3JZ, United Kingdom bUniversity of Jyvaskyla, P.O. Box 35, FI-40014 University of Jyvaskyla, Finland cUniversity of Surrey, Guildford GU2 7XH, United Kingdom dPhysics Division, Argonne National Laboratory, Argonne, IL 60439, USA eDepartment of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255, USA fTriangle Universities Nuclear Laboratory, Duke University, Durham, NC 27708-2308, USA gDepartment of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA hSouth Dakota School of Mines & Technology, Department of Physics, Rapid City, SD 57701, USA iNational Nuclear Data Center, Brookhaven National Laboratory, Upton, NY 11973, USA a r t i c l e i n f o a b s t r a c t Article history: Received 15 July 2020 Received in revised form 2 December 2020 Accepted 13 December 2020 Available online 16 December 2020 Editor: B. Blank Keywords: Nuclear astrophysics 26Al γspectroscopy Cosmic γrays The 92-keV resonance in the 25Mg(p, γ)26Al reaction plays a key role in the production of 26Al at astrophysical burning temperatures of ≈100 MK in the Mg-Al cycle. However, the state can decay to feed either the ground, 26gAl, or isomeric state, 26mAl. It is the ground state that is critical as the source of cosmic γrays. It is therefore important to precisely determine the ground-state branching fraction f0of this resonance. Here we report on the identification of four γ-ray transitions from the 92-keV resonance, and determine the spin of the state and its ground-state branching fraction f0=0.52(2)stat (6)syst. The f0value is the most precise reported to date, and at the lower end of the range of previously adopted values, implying a lower production rate of 26gAl and its cosmic 1809-keV γrays. ©2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. The Mg-Al hydrogen-burning reaction cycle shown in Fig. 1 is triggered at temperatures above T≈30 MK. These conditions prevail in hydrogen-burning convective cores of massive mainsequence stars [1,2]. Studies by the INTEGRAL satellite mission have demonstrated that such massive stars are most likely the main source of the cosmic γ-ray emitter 26Al, which is injected into the interstellar medium either from stellar winds in the quiescent burning phase, or by explosive burning in core collapse supernovae [3]. The Mg-Al cycle also occurs, at much higher temperatures (0.1 −0.4GK), in novae, which contribute to the budget of 26Al material. Extinct 26Al is evident in high 26Mg/24Mg isotopic *Corresponding author at: University of Jyvaskyla, P.O. Box 35, FI-40014 University of Jyvaskyla, Finland. E-mail address: anu.kankainen@jyu.fi (A. Kankainen). 1Present address: GSI Helmholtzzentrum für Schwerionenforschung GmbH, D- 64291 Darmstadt, Germany. 2Present address: U.S. Army Research Laboratory, Adelphi, Maryland 20783, USA. abundance ratios found in carbonaceous chondrites [4] and presolar grains [5]. 26Al has a long-lived ground-state (T1/2=0.7My) with a spinparity Jπ=5+, and a short-lived (T1/2=6.35 s) 0+isomeric state at 228 keV. At high temperatures the effective lifetime of 26Al in the stellar plasma can be reduced significantly by thermal communication between the two states [6–8]. At temperatures below 0.4 GK, however, the thermal equilibrium is not established and the two states operate effectively as separate nuclear species in the reaction network [9]. The ground-state β-decay of 26Al feeds dominantly the first excited state of 26Mg, which de-excites and produces the 1.8-MeV γ-ray flux observed in satellite missions such as INTEGRAL. The superallowed Fermi β-decay of the isomeric state 26mAl, on the contrary, feeds ≈100% the ground state of 26Mg and does not contribute to the observed flux. When considering the astrophysical production of 26Al, it is therefore critically important to know the fraction of the reaction yield that populates the ground state, known as the ground-state branching fraction f0. https://doi.org/10.1016/j.physletb.2020.136033 0370-2693/©2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. A. Kankainen, P.J. Woods, D.T. Doherty et al. Physics Letters B 813 (2021) 136033 Fig. 1. The Mg-Al hydrogen-burning reaction cycle. The arrows indicate the reaction flow via proton captures and β-decays. The 1809-keV γray follows the β-decay of the 26Al ground state (Jπ=5+, T1/2=0.7My) highlighted in red. The bypass routes related to 26mAl (Jπ=0+, T1/2=6.35 s) are shown in blue. The black squares indicate stable isotopes and β-unstable nuclei are shown in white. The 25Mg(p, γ)26Al reaction plays the main role in the production of 26Al in the Mg-Al cycle, and more generally, in determining the relative abundance of Mg/Al [10]. Considerable experimental efforts have gone into determining the 25Mg(p, γ)26Al reaction rate at low astrophysical energies where reactions on individual resonances play the dominant role (see Ref. [11]for a recent example and references therein). In the critical temperature region ≈100 MK, the rate is almost entirely dominated by a single resonance at 92 keV, corresponding to an excitation energy of 6399 keV in 26Al. Underground measurements of the 25Mg(p, γ)26Al reaction performed by the LUNA collaboration at Gran Sasso have reported a first resonance strength value for this state [12]. However, Strieder et al. [12] comment that the ‘ground-state branching fraction is an important input parameter for the reaction rate’ (for the 92-keV resonance), but ‘available literature information’ is ‘contradictory’ (see Ref. [12]for a more detailed discussion). In fact, no γ-decay branches are listed for the 6399-keV excited state in the most recent data compilation [13]. Endt and Rolfs analyzed in detail spin-parity assignments of states near the proton threshold energy of 26Al relevant for the 25Mg(p, γ)26Al reaction and assigned 2−to the 6399-keV state [10]. This assignment is also given in the compilation of Endt [14]. The more recent 26Al data compilation [13]also allows Jπ=1+as a possibility. In the present paper, measurements of four γ-decay branches from the 6399-keV state are reported for the first time. These are used to derive the ground-state branching fraction and assign the spin of the resonance. The experiment was performed using the ATLAS accelerator facility at Argonne National Laboratory. A 10-MeV 3He beam was used to bombard an ≈840 μg/cm2-thick 24Mg target for approximately three days producing 26Al nuclei via the single proton evaporation channel. De-excitation γ-rays were detected with the Gammasphere detector array, which has a near spherical geometry [15,16]. Energy and efficiency calibrations were performed with the standard 152Eu and 60Co calibration sources. In order to improve the energy calibration at the highest energies, an additional 6.129-MeV line in 16O, from the 13C(α, n)16O reaction, was also used. The trigger on Gammasphere was set to accept γ−γ coincidence data. Fig. 2presents the γ-decay level scheme showing the four identified γ-ray transitions from the 6399-keV state in 26Al, and their connection to known lower-lying states in 26Al [13]. Fig. 3shows the γ-ray energy spectra including these four transitions obtained by gating on known lower-lying transitions. Transitions from other known levels near or above the proton threshold in 26Al are also indicated in these spectra. We note that in Figs. 3(b) and (c), the peak including the transition from the 6399-keV state is too broad to be fitted as a single peak. The peak Fig. 2. γ-ray transitions and their branchings observed from the 6399-keV state together with the gating transitions (in bold). The transitions going dominantly to the ground state and the isomer are shown in blue and red, respectively. The transitions observed in this work from the 6398.64(21)-keV [13]state have uncertainties of around 2 keV (see Table 1) and result in Ex=6398.0(20)keV. Other intense γ-ray transitions (Iγ≥10% in [13]) are also indicated for the relevant states. The fraction of transitions populating the ground state either directly or via cascades from each level (bg.s.) is based on the intensities from Ref. [13]. widths were fixed from the isolated single peaks in the same energy region. The double-peak fit yields a better agreement with the data, and is also required to match the known 4604.5-keV transition from the 6364-keV state in Fig. 3(b). Angular-distribution analyses were performed following the procedure outlined in Ref. [17]. Coincidence matrices were sorted where the energies of γ-rays detected at specific Gammasphere angles, Eγ(θ), measured with respect to the beam axis, were incremented on one axis, while the energies of γ-rays detected at any angle, Eγ(any), were placed on the other axis. Gates were placed on the Eγ(any) axis to clean the spectra and the intensities of the transitions of interest were extracted. As a consistency check, we compared the relative branch intensities of two transitions from the 6610-keV level (shown in Fig. 3(b) and (c)) with known literature values [13] and found them to be in good agreement. Fits to the measured yields at each angle were then performed. The standard angular distribution function W(θ) = a0[1 +a2P2(cos θ) +a4P4(cos θ)], where P2and P4are Legendre polynomials, was fitted to the data. The extracted coefficients, a2 and a4, contain information on the multipolarity of the transitions. Figs. 4(a)-(b) show angular distributions for the two most intense transitions (both to 2+states) from the 6399-keV state, both well fitted as J=0 transitions. For comparison, in Figs. 4(c) and (d) fits are shown for known J=0 and J=2 transitions, respectively. J=1(dipole) transitions have a negative a2parameter, and therefore exhibit an opposite trend compared to J=0 and J=2(quadrupole) transitions (see e.g. Fig. 1 in Ref. [17]). The data support a J=2 assignment for the 6399-keV level, consistent with 2−assignments deduced in Refs. [10,14], where for example transfer-reaction angular-distribution data support negative parity. We henceforth proceed on the basis that the spin-parity of the 6399-keV state is 2−. This assignment rules out significant direct γ-branches to the ground and isomeric states, as high multipolarity transitions would be required. This is important for the ensuing analysis, as the present experiment is not sensitive to single γ-ray transitions since it requires γ−γcoincidences. 2 A. Kankainen, P.J. Woods, D.T. Doherty et al. Physics Letters B 813 (2021) 136033 Fig. 3. Transitions observed from the 6399-keV state. Gates at γ-ray energies of (a) 1012 keV, (b) 1342 keV, (c) 2743 keV, and (d) 4724 keV have been applied to observe transitions to 2+states at 2069 keV, 1759 keV, 3160 keV, and 5142 keV, respectively. The peaks have been labelled with excitation energies corresponding known excited states in 26Al. Fig. 4. Angular distributions for (a) 4327-keV transition from the 6399-keV state to the 2+state at 2069 keV, (b) 3239-keV transition from the 6399-keV state to the 2+state at 3160 keV, (c) a known J=0 transition (4431 keV, 2−→2069 keV, 2+), and (d) a known J=2 transition (3402 keV, 5+→417 keV, 3+). Table 1lists the four observed transitions from the 6399-keV state in 26Al, and their relative intensities. These intensities are combined with the known branching ratios from the lower-lying states [13]in the observed decay cascades shown in Fig. 2, to deduce a ground-state branching fraction of 0.52(2), where the error is mainly statistical, stemming from the determined peak areas and uncertainties in the known γ-ray transition intensities. A key additional systematic uncertainty relates to possible weak unobserved branches from the 6399-keV level. In order to explore this, we consider the effect of excluding the weakest observed branch from the 6399-keV level, for which the decay sequence predominantly leads to the ground state. The effect of its removal is to reduce the derived ground-state branching fraction value by an amount of 0.06. Since other possible branches are likely to be weaker than this, and not necessarily feeding dominantly either the ground or isomeric state, we consider ±0.06 to be a reasonable estimate of this systematic uncertainty, giving a value for f0=0.52(2)stat (6)syst. 3 A. Kankainen, P.J. Woods, D.T. Doherty et al. Physics Letters B 813 (2021) 136033 Table 1 Observed transitions from the 6399-keV state and their relative intensities Iγ. The transitions were gated with the dominant γ-ray transitions (1342 keV, 1012 keV, 2743 keV and 4724 keV) originating from the final states at energies Efwith spinparity Jπ f, and a fraction bg.s.of transitions populating the ground state (either directly or via cascades). Eγ(keV) Iγ(%) Ef,Jπ f[13]bg.s.(%) 4638.0(21) 12.7(7) 1759.034(8), 2+98.00(17) 4326.9(20) 51.2(23) 2069.47(3), 2+21.6(9) 3238.7(20) 35.5(17) 3159.899(13), 2+79.2(11) 1256.4(13) 0.69(12) 5141.68(6), 2+93.8(32) In total 100(2) 52(2) The 92-keV resonance strength value measured at LUNA for the 25Mg(p, γ)26Al reaction is ωγ =2.9(6) ×10−10 eV [12]. For the production of 26gAl in reaction-rate calculations, this needs to be multiplied by f0and the electron screening enhancement factor, fesc, calculated to be 1.25 in Ref. [12]. Although not observing discrete γ-ray transitions from the 92-keV resonance, Strieder et al. performed a simulation analysis on their BGO summing crystal data to adopt a value for f0=0.6+(2) −(1)[12], which is consistent with the more precise value reported here. Prior to the LUNA measurement, the most recent analyses of the low-energy 25Mg(p, γ)26Al reaction rate had assumed values of 0.61 [18] and 0.85 [10]for f0of this state, with no error given. An even earlier study of the 24Mg(3He, pγ)26Al reaction [19,20]reported a 3+assignment for a state at 6400(3) keV and a value for f0=0.8(2) (in Table 1 of Ref. [20]). The spin assignment is in disagreement with the present study, as are the branch intensities (e.g. the main gamma branch observed here dominantly feeding the isomer was not observed in Ref. [19]), and these data were not adopted in subsequent data compilations [13,14]. In summary, four γ-ray branches have been identified for the key 92-keV resonance in the low-energy 25Mg(p, γ)26Al reaction. These data have been used to determine the spin of the resonance, and to derive the most precise value yet for the ground-state branching fraction f0; the latter is required for the accurate calculation of the production of the cosmic γ-ray emitting species, 26gAl. The value for f0is the lowest yet reported, although broadly consistent with previous adopted values. It would imply a lower production rate of 26gAl and consequently a less intensive cosmic 1809-keV γ-ray flux than the previous calculations of the Mg-Al cycle. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by The U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-O6CH11357. This research used resources of ANL’s ATLAS facility which is a DOE office of Science User Facility. The support from STFC under grant ST/J00006X/1 is gratefully acknowledged. A.K. acknowledges the funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 771036 (ERC CoG MAIDEN) and Academy of Finland grant No. 275389. This work was supported by The U.S. Department of Energy (DOE), Office of Nuclear Physics, under Grants No. DE-FG02-97ER41033 (TUNL), DEFG02-97ER41041 (UNC) and DE-FG02-94ER40834 (UMD). C.L.W. acknowledges support from the Austrian Science Fund (FWF): J3503. We thank Gianluca Imbriani for discussions related to the nucleosynthesis of 26Al. References [1] A. Palacios, G. Meynet, C. Vuissoz, J. Knödlseder, D. Schaerer, M. Cerviño, N. Mowlavi, New estimates of the contribution of Wolf-Rayet stellar winds to the galactic 26Al, Astron. Astrophys. 429 (2) (2005) 613–624, https://doi .org / 10 .1051 /0004 -6361 :20041757. [2] M. 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