Besserer, U

Couture, A

Dababneh, S

Dörr, L

Forssén, C

Görres, J

Haight, R C

Heil, M

Mengoni, A

O'Brien, S

Patronis, N

Plag, R

Reifarth, R

Rundberg, R S

Uberseder, E

Wiescher, M

Wilhelmy, J B

Reifarth, R.

Heil, M.

Plag, R.

Besserer, U.

Couture, A.

Dababneh, S.

D\uf6rr, L.

Forssen, Christian

G\uf6rres, J.

Haight, R.C.

Mengoni, A.

O\u27Brien, S.

Patronis, N.

Rundberg, R.S.

Uberseder, E.

Wiescher, M.

Wilhelmy, J.B.

Chalmers Research

Direct measurement of stellar neutron capture rates of 14C and comparison with the Coulomb breakup method

CERN Document Server

PoS(NIC-IX)223Direct measurement of stellar neutron capture ratesof 14C and comparison with the Coulomb breakupmethodR. Reifarth,a M. Heil,b R. Plag,b U. Besserer,c A. Couture,a S. Dababneh,b L. Dörr,cC. Forssénd J. Görres,e R.C. Haight,a A. Mengoni, f S. O’Brien,e N. Patronis,gR.S. Rundberg,a E. Uberseder,b M. Wiescher,e J.B. WilhelmyaE-mail: reifarth@lanl.gova Los Alamos National Laboratory, Los Alamos, New Mexico, 87545, USAb Forschungszentrum Karlsruhe, IK, P.O. Box 3640, D-76021 Karlsruhe, Germanyc Forschungszentrum Karlsruhe, Tritiumlabor, P.O. Box 3640, D-76021 Karlsruhe, Germanyd Lawrence Livermore Laboratory, Livermore, California, 94550, USAe University of Notre Dame, Physics Department, Notre Dame, IN 46556, USAe CERN, CH-1211 Geneva 23, Switzerlandf Nucl. Phys. Lab., Department of Physics, The University of Ioannina 45110 Ioannina, GreeceThe neutron capture cross section of 14C has been shown to be important for several neutron drivennucleosynthesis scenarios. Due to the high neutron abundance it is expected that the 14C(n,γ)reaction competes strongly with other neutron-induced reactions on 14C. The 14C(n,γ) reaction isalso important to validate (n,γ) cross sections obtained via the inverse reaction by the Coulombbreakup method. In principle, 14C belongs to the few cases where this correspondence can bevalidated in a convincingly clean way. So far, the example of 14C is obscured, however, bydiscrepancies between several experiments and theory. In this contribution we report on a re-analysis of the direct measurements of the 14C(n,γ) reaction presented on the last NIC conference(Vancouver, 2004). The neutron energies used during the experiment ranged from 30 to 800 keV.The earlier presented disagreement between the direct measurements and the Coulomb breakupmethod has been resolved.International Symposium on Nuclear Astrophysics — Nuclei in the Cosmos — IXJune 25-30 2006CERN, Geneva, Switzerlandc© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(NIC-IX)223Neutron capture on 14C0 20 40 60 80 100 120NEUTRON ENERGY (keV)00.81.62.43.244.8NEUTRONS (keV-1 ) kT = 23.3 keV0 100 200 300 400 500 600NEUTRON ENERGY (keV)00.40.81.2NEUTRONS (keV-1 )150 keV500 keVFigure 1: Neutron spectra during the cyclic activations in arbitrary scaled units. The distribution in the leftpanel can be approximated by a Maxwellian averaged distribution for kT =23.3 keV. The distributions in theright panel are typical for a mono-energetic experiment with a wide energy uncertainty.1. IntroductionInhomogeneous big bang models [1] offer a possibility to bridge the mass gaps at A = 5 and 8via the reaction sequence 7Li(n,γ)8Li(α ,n)11B(n,γ)12B(β−)12C [2, 3]. Subsequent neutron captureson 12C and 13C will then lead to the production of 14C, which has a half-life of 5730±40 yr [4].On the time scale of big bang nucleosynthesis 14C can be considered as stable and further proton,alpha, deuteron, and neutron capture reactions on 14C will result in the production of heavier nuclei(A > 20) [2]. Due to the high neutron abundance it is expected that the 14C(n,γ)15C reactioncompetes strongly with the other reactions. The 14C(n,γ) reaction is also important to validate (n,γ)cross sections obtained via the inverse reaction by the Coulomb breakup method (see e.g. [5], [6],[7]). Other estimates are given in [8], [9].14C is one of the few nuclei, where the (n,γ) reaction can be measured directly and comparedwith results of Coulomb breakup experiments, in this case 15C(γ ,n)14C. Preliminary data have beenpresented at the last Nuclei in the Cosmos Meeting [10]. In this article we present a re-analysisof these data, which became necessary because of huge dead-time corrections in the Germaniumdetectors used for determining the induced 15C-activity. We present the data with the smallestuncertainties.2. ExperimentA first important step was the independent determination of the sample mass by a calorimet-ric measurement of the decay heat. This measurement was carried out at the Tritium Labor ofForschungszentrum Karlsruhe. The heat production was determined to be 370 ± 4 µW, corre-sponding to 1.25 ± 0.01 Ci or 0.282 ± 0.03 g of 14C. This is more than a factor of 2 less than thespecified value, which had been adopted in the previous activation [11].Neutrons of different energy distributions were produced by bombarding a metallic 7Li targetwith protons of different energies above the (p,n) threshold. The proton beam was provided by theKarlsruhe 3.7 MV Van de Graaff accelerator. The different neutron energy distributions are shownin Figure 1. The corresponding neutron fluxes ranged between 107 and 109 cm−2s−1.2PoS(NIC-IX)223Neutron capture on 14CThe short half-life of 15C of only 2.449± 0.005 s implied the use of the fast cyclic activa-tion technique [12]. The induced activity during each cycle was detected via the characteristic5.297 MeV line (intensity Iγ = 0.68) in the 15C decay using a HPGe detector with a relative effi-ciency of 100%.The detector efficiency was determined with a set of calibration sources and via the 27Al(p,γ)28Sireaction as described in Ref. [13]. The results are shown in Figure 2. Simulations of the γ-ray ef-ficiency using GEANT showed that the energy dependence is slightly different for the setup usedduring the activation (6 mm distance) and during the 27Al(p,γ)28Si experiment (50 cm). This effectwas considered in the analysis. The main difference to our previous analysis was the discoveryof a huge dead time effect during the experiment. Even though the detector was shielded fromthe neutron production target to reduce radiation damage, enough neutrons reached the detector toproduce unstable germanium isotopes in the detector. The 14C powder was previously used duringexperiments with high-energy protons, which activated the thin nickel container. One of the reac-tion products in the container is the long-lived 44Ti, which decays to 44Sc. During the decay of 44Sca γ-ray of 1.15 MeV is emitted. We used the independently measured activity of 44Sc to determinethe necessary dead time corrections of about a factor of 3.Each cycle consisted of an activation time of tbeam = 10 s, the γ-ray detection time tdetector = 10 s(during which the proton beam was switched off), and twice the time for moving the sample be-tween detector and neutron production target twait = 0.8 s. The sample was sandwiched betweentwo thin gold foils, allowing a measurement relative to the well known 197Au(n,γ)198Au cross sec-tion. At the end of each run the activity of the gold foils was determined via the 412 keV γ-rayfrom the 198Au decay (t1/2 = 2.7 d).Figure 3 shows a typical γ-ray spectrum taken during the experiment. Full energy, singleescape as well as double escape peaks of the 5.3 MeV line are clearly visible above the background.In order to reduce systematic uncertainties, the time dependence of the 15C decay during the 10 scounting period, has been monitored. The decay curve is compared in Figure 4 with a fit of aconstant background and the exponential decay law with 2.449 s half-life. Within the statisticaluncertainties the measured activity follows the expected time dependence.3. ResultsThe results and a preliminary comparison with results from Coulomb-breakup experimentsare shown in Table 1 and Figure 5. The results of this measurement are not for mono-energeticneutrons, but for a broad distribution of neutron energies. This holds true in particular for the23.3 keV cross section measured, which is in fact a Maxwellian Averaged Cross Section (MACS)at kT = 23.3 keV. The position of this point in Figure 5 however is at 23.3 keV. A more realisticcomparison between the methods is done in Table 1, which contains a comparison done by foldingthe differential cross sections derived from the RIKEN data [7] (solid line in Figure 5) with theneutron spectra shown in Figure 1.The determining contributions to the uncertainty estimation come from counting statistics (2-8%), the γ-ray detection efficiency (5%), and the determination of the neutron flux (2-10%). Allother uncertainties are smaller than 2%.3PoS(NIC-IX)223Neutron capture on 14C1000 3000 5000 7000γ-RAY ENERGY (keV)02.55DETECTION EFFICIENCY (%)set of calibration sourcesAl(p,γ), full energyFigure 2: The γ-ray efficiency of the HPGe detector used during the cyclic activation. The efficiency wasmeasured using calibration sources and the 27Al(p,γ)28Si reaction. The red and blue solid curves correspondto normalized efficiency simulations using GEANT with 50 cm and 6 mm distance between sample anddetector. The extrapolation from low to high energies is slightly smaller for the 6 mm case.0 1000 2000 3000 4000 5000 6000γ-RAY ENERGY (keV)100101102103104105106107108COUNTS1.15 MeV (44Sc)4.8 MeV (14C, SE)5.3 MeV (14C, FE)Figure 3: Measured γ-ray spectrum after 22 h of cyclic activation during the run corresponding to neutronenergies of kT = 23.3 keV. Clearly visible above the background are the 3 peaks corresponding to the decayof 15C at 5.3 MeV (full energy peak), 4.8 MeV (single escape peak), and 4.3 MeV (double escape). Thepeak from the decay of 44Sc at 1.15 MeV is also marked.4PoS(NIC-IX)223Neutron capture on 14C0 2 4 6 8 10TIME [s]050100150200250300COUNTS PER 0.25 smeasured activity of the 5.3 MeV linefit: e-t/2.449s + backgroundFigure 4: Activity of the 5.297 MeV line from the decay of 15C as a function of time. The solid linecorresponds to the half-life of 15C. The first four points, which are below the fit, were take while the samplewas not yet in front of the detector and are not included in the actual fit.Compared to the result of the previous activation with kT = 23.3 keV [11] we find agreementonly if if the new sample mass is taken into account and a similar dead-time problem like during ourexperiment is assumed. The present values are in agreement with data from two Coulomb breakupstudies [6], [7]), while they disagree from a third study ([5]. A more comprehensive comparisonwith the available data will be published in a separate paper.Table 1: Cross sections of the 14C(n,γ)15C reaction measured via activation technique. The last columnshows a preliminary comparison with the results from the Coulomb breakup method.Energy cross section Uncertainty Ratio(keV) (µb) (%) CD / Activation23.3 (MACS) 7.1 7 1.01150 10.7 11 1.05500 17.0 9 1.01AcknowledgmentsWe would like to thank E.-P. Knaetsch, D. Roller, and W. Seith for their support at the Karl-sruhe Van de Graaff accelerator. This work was partly supported by the Joint Institute for NuclearAstrophysics (JINA) through NSF Grants Nos. PHY-0072711 and PHY-0228206.5PoS(NIC-IX)223Neutron capture on 14C0 0.1 0.2 0.3 0.4 0.5 0.6NEUTRON ENERGY [MeV]05101520CROSS SECTION  14 C(n,γ) [µb] Direct Capture, derived from RIKEN dataGSI (CD)MSU (CD)RIKEN (CD)This measurement (Activation)Figure 5: Comparison between this measurement and cross section estimates based on Coulomb dissocia-tion experiments (CD) on 15C. The results of this measurement are not for mono-energetic neutrons, hencethe quite large deviation at 23.3 keV.References[1] J. H. Applegate, C. J. Hogan. Relics of cosmic quark condensation. Physical Review D (Particles andFields) 31(12) (1985) 3037.[2] J. H. Applegate, C. J. Hogan, R. J. Scherrer. Cosmological quantum chromodynamics, neutrondiffusion, and the production of primordial heavy elements. Astrophysical Journal 329(2) (1988) 572.[3] R. A. MALANEY, W. A. FOWLER. LATE-TIME NEUTRON DIFFUSION ANDNUCLEOSYNTHESIS IN A POST-QCD INHOMOGENEOUS OMEGAB=1 UNIVERSE.ASTROPHYSICAL JOURNAL 333(1) (1988) 14.[4] F. Ajzenberg-Selove. Energy levels of light nuclei A=13-15. Nucl. Phys. A 449 (1986) 1.[5] A. Horváth, J. Weiner, A. Galonsky, F. Deák, Y. Higurashi, K. Ieki, Y. Iwata, A. Kiss, J. Kolata,Z. Seres, J. Schwarzenberg, H. Schelin, S. Takeuchi, S. Typel, R. Warner. CROSS SECTION FORTHE ASTROPHYSICAL 14C(n,g)15C REACTION VIA THE INVERSE REACTION. Ap. J. 570 (2002)926.[6] U. Datta Pramanik, T. Aumann, K. Boretzky, B. Carlson, D. Cortina, et al. Coulomb breakup of theneutron-rich isotopes 15C and 17C. Phys. Lett. B 551 (2003) 63.[7] T. Nakamura. Neutron Capture Cross Section of 14C Studied by Intermediate-Energy CoulombDissociation. In M. Terasawa, S. Kubano, T. Kishida, T. Kajino, T. Motobayashi, K. Nomato, eds.,Origin of Matter and Evolution of Galaxies (2005), 155.[8] M. Wiescher, J. Görres, F.-K. Thielemann. Capture Reactions on 14C in Nonstandard Big BangNucleosynthesis. Ap. J. 363 (1990) 340.6PoS(NIC-IX)223Neutron capture on 14C[9] N. K. Timofeyuk, D. Baye, P. Descouvemont, R. Kamouni, I. J. Thompson. C-15-F-15 chargesymmetry and the C-14(n,gamma)C-15 reaction puzzle. PHYSICAL REVIEW LETTERS 96(16)(2006) 162501.[10] R. Reifarth, M. Heil, R. Plag, U. Besserer, S. Dababneh, L. Dörr, J. Görres, R. C. Haight, F. Käppeler,A. Mengoni, S. O’Brien, N. Patronis, R. S. Rundberg, M. Wiescher, J. B. Wilhelmy. Stellar neutroneapture rates of C-14. NUCLEAR PHYSICS A 758 (2005) 787C.[11] H. Beer, M. Wiescher, F. Käppeler, J. Görres, P. E. Koehler. A Measurement of the 14C(n,γ)15C CrossSection at a Stellar Temperature of kT = 23.3 keV. Ap. J. 387 (1992) 258 .[12] H. Beer. Capture cross section measurements of krypton and xenon isotopes and the fundamentalparameters of the s-process. Ap. J. 375 (1991) 823.[13] A. Anttila, J. Keinonen, M. Hautala, I. Forsblom. Use of the 27Al(p,g) 992 keV resonance as ag-intensity standard. NIM 147 (1977) 501.7

Direct measurement of stellar neutron capture rates of $^{14}C$ and comparison with the Coulomb breakup method

Direct measurement of stellar neutron capture rates of $^{14}C$ and comparison with the Coulomb breakup method

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Direct measurement of stellar neutron capture rates of 14C^{14}C14C and comparison with the Coulomb breakup method

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Direct measurement of stellar neutron capture rates of $^{14}C$ and comparison with the Coulomb breakup method