Beinrucker, C.

Erbacher, P.

Fiebiger, S.

Fonseca, M.

Göbel, K.

Heftrich, M.

Heftrich, T.

Krása, A.

Käppeler, F.

Lederer-Woods, C.

Plag, R.

Plompen, A.

Reifarth, R.

Schmidt, S.

Sonnabend, K.

Weigand, M.

KITopen

EPJ Web of Conferences 146, 01014 (2017) DOI: 10.1051/epjconf/201714601014ND2016Neutron capture cross sections of 69Ga and 71Ga at 25 keV andEpeak = 90 keVKathrin Go¨bel1,a, Clemens Beinrucker1, Philipp Erbacher1, Stefan Fiebiger1, Micaela Fonseca1,2,3, Michael Heftrich1,Tanja Heftrich1, Franz Ka¨ppeler4, Antonin Kra´sa5, Claudia Lederer-Woods1,6, Ralf Plag1,7, Arjan Plompen8,Rene´ Reifarth1, Stefan Schmidt1, Kerstin Sonnabend1, and Mario Weigand11 Goethe University Frankfurt, Germany2 LIBPhys-UNL, DF, FCT, Universidade Nova de Lisboa, Portugal3 Universidade Europeia, Laureate International Universities, Portugal4 Karlsruher Institut fu¨r Technologie, Germany5 SCK-CEN, Belgian Nuclear Research Centre, Mol, Belgium6 University of Edinburgh, UK7 GSI Helmholtzzentrum fu¨r Schwerionenforschung, Darmstadt, Germany8 Joint Research Centre (EC-JRC), Geel, BelgiumAbstract. We measured the neutron capture cross sections of 69Ga and 71Ga for a quasi-stellar spectrum atkBT = 25 keV and a spectrum with a peak energy at 90 keV by the activation technique at the Joint ResearchCentre (JRC) in Geel, Belgium. Protons were provided by an electrostatic Van de Graaff accelerator to produceneutrons via the reaction 7Li(p,n). The produced activity was measured via the γ emission of the productnuclei by high-purity germanium detectors. We present preliminary results.1. IntroductionMost of the elements heavier than iron are produced bythe slow (s) and the rapid (r) neutron capture processes.The s-process takes place during stellar He and C burningphases with neutron densities between 108 and 1010 cm−3.The reaction path closely follows the valley of stabilitysince neutron capture times of typically one to ten yearsare much slower than most of the β-decay times of theinvolved nuclei [1].The s-process is composed of the weak and the maincomponent [2]. The main differences lie in the neutron-to-seed ratios, the temperatures as well as the neutrondensities. The main component of the s-process takes placeat about 5 to 25 keV in low-mass asymptotic giant branch(AGB) stars and produces the nuclei with mass numbersabove A ≈ 90 [2]. The weak component of the s-processtakes place in massive stars of more than eight solar massesduring convective helium burning at temperatures of about30 keV and during convective shell carbon burning atabout 90 keV. The time-integrated neutron flux is lowercompared to the main component producing nuclei withmass numbers A ≤ 90.2. The weak s-processThe weak s-process produces most of the s-processisotopes between iron and strontium. The neutron fluencea e-mail: goebel@physik.uni-frankfurt.dein the weak s-process is too low to achieve reaction flowequilibrium, in contrast to the main s-process component.Therefore, a particular neutron capture cross section notonly determines the abundance of the respective isotope(as in the case of the main component), but affects theabundances of all heavier isotopes as well.The uncertainties of the neutron capture cross sectionsof the weak s-process nuclei (60 ≤ A ≤ 90) are higherthan the uncertainties of the main s-proces nuclei (A ≥90). Following the weak s-process path, the cumulateduncertainties affect all isotopes of the weak s-process, up tokrypton and strontium, with possible minor contributionsto the yttrium and zirconium abundances [1,3,4]. Accuratepredictions require the cross sections to be known with anaccuracy of at least 5% for all involved nuclei in the weaks-process [5].3. The case of galliumGallium is mostly produced by the weak s-process inmassive stars. Recent simulations [3] show that galliumis the most abundant s-element at the end of shellcarbon burning. So far, there is only a time-of-flightmeasurement with a sample of natural gallium [6]. Othernuclei (81Br, 75As, 74Ge) measured within the sameexperimental campaign show large deviations from morerecent results [7]. For 71Ga two discrepant results fromintegral measurements at 25 keV are published [8,9].The cross sections should be remeasured to solve thediscrepancies. Data for carbon shell burning in massivestars at 90 keV are highly desired.c© The Authors, published by EDP Sciences. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0(http://creativecommons.org/licenses/by/4.0/).EPJ Web of Conferences 146, 01014 (2017) DOI: 10.1051/epjconf/201714601014ND2016Table 1. Parameters for the simulation of the neutron spectra with PINO [11].Proton energy Peak energy of Lithium target Ga activation sampleneutron spectrum Thickness Radius Distance to Lithium Radius(1912 ± 2) keV 25 keV 27.5µm 3 mm 1.0 mm 6 mm(1920 ± 2) keV 90 keV 1.15µm 3 mm 9.7 mm 6 mmFigure 1. Path of the s-process between zinc and arsenic. Theneutron capture cross sections were measured at 25 keV and90 keV by activation for the isotopes 69Ga and 71Ga, which aremarked in yellow.Figure 2. Simulated neutron spectrum with a peak energy at25 keV [11]. The parameters for the simulation are given inTable 1. The red dashed line shows the corresponding Maxwell-Boltzmann distribution for kBT = 25 keV.4. ActivationThe activation was carried out at the Joint ResearchCentre (JRC) in Geel, Belgium. An electrostatic Van deGraaff accelerated protons to the necessary energy toproduce neutrons via the reaction 7Li(p,n). The neutronflux was monitored by a B4F ionisation chamber. Samplesof natural gallium positioned between two gold monitorswere irradiated with neutrons with two different energyspectra: (a) a quasi-stellar spectrum with kBT = 25 keVand (b) a spectrum with a peak energy at 90 keV [10]. Thebeam parameters and the properties of the lithium and thegallium samples are given in Table 1. The neutron spectrawere simulated with PINO [11] and are shown in Figs. 2and 3.5. AnalysisThe produced activity was determined by γ -ray spec-troscopy with high-purity germanium detectors. In the caseof 69Ga we used the γ lines with energies of 176 keVand 1039 keV from the decay of 70Ga, in the case of71Ga we used the γ lines 601 keV, 629 keV, 835 keV,Figure 3. Simulated neutron spectrum with a peak energy at90 keV [11]. The parameters for the simulation are given inTable 1. The red dashed line shows the corresponding Maxwell-Boltzmann distribution for kBT = 90 keV.894 keV and 1051 keV from the decay of 72Ga. Higher-energy γ lines were not considered since the efficiency wasonly determined up to about 1330 keV using a calibrationsource.The cross section is calculated byσ = Nproducedn · Nsample (1)where Nproduced is the number of produced nuclei, n isthe integrated neutron flux, and Nsample is the number ofsample nuclei.The number of sample nuclei was determined by thesample mass. The number of produced nuclei is calculatedfrom the counts in the γ line with several correctionfactors:Nproduced = CIγ ·  · κ · fdt · fb · fdw · fcasc (2)where C is the number of counts for one γ line, Iγ isthe corresponding γ intensity, and κ is the self-absorption.The factor fdt corrects for the deadtime, fb for the decaysduring the activation, fdw for the decays before and afterthe measurement, and fcasc for the decay cascades.6. Preliminary resultsThe analysis of the activation experiment gives the ratioof the neutron capture cross sections of 69Ga and 197Au aswell as of 71Ga and 197Au. The results are shown in Table 2.The neutron capture cross sections at kBT = 25 keVcan be calculated from the ratios using the MACS of197Au(n,γ ) at 25 keV (683 mb), which was taken fromRef. [12]. Table 3 compares the results to the MACSsfrom KADoNiS v0.3 [7], which were renormalized withthe 197Au cross section value from Ref. [12]. The measuredvalue for 69Ga is about 30% higher than the evaluatedvalue, the measured value for 71Ga is about 20% lower.2EPJ Web of Conferences 146, 01014 (2017) DOI: 10.1051/epjconf/201714601014ND2016Table 2. Ratios of neutron capture cross sections for kBT =25 keV and for the neutron spectrum with Epeak = 90 keV.Neutron spectrum for25 keV 90 keVσ69Ga/σ197Au 0.31 0.29σ71Ga/σ197Au 0.18 0.17Table 3. Neutron capture cross sections at kBT = 25 keV. The197Au(n,γ ) MACS at 25 keV (683 mb) was taken from Ref. [12].The MACS from KADoNiS v0.3 was renormalized with thisvalue.σ MACSmeasured KADoNiS v0.3 [7,12]69Ga 212 mb 161 mb71Ga 123 mb 145 mbThis project was supported by EFNUDAT, ERINDA, theEuroGENESIS project MASCHE, HIC for FAIR and BMBF(05P15RFFN1).References[1] R. Reifarth, C. Lederer, F. Ka¨ppeler, Journal ofPhysics G Nuclear Physics 41(5), 053101 (2014)[2] P.A. Seeger, W.A. Fowler, D.D. Clayton, TheAstrophysical Journal, Supplement 11, 121 (1965)[3] M. Pignatari, R. Gallino, M. Heil, M. Wiescher,F. Ka¨ppeler, F. Herwig, S. Bisterzo, The Astrophysi-cal Journal 710, 1557 (2010)[4] M. Weigand, T.A. Bredeweg, A. Couture, K. Go¨bel,T. Heftrich, M. Jandel, F. Ka¨ppeler, C. Lederer,N. Kivel, G. Korschinek et al., Physical Review C92(4), 045810 (2015)[5] Z.Y. Bao, H. Beer, F. Ka¨ppeler, F. Voss, K. Wisshak,T. Rauscher, Atomic Data and Nuclear Data Tables76, 70 (2000)[6] G. Walter, KfK Bericht 3706 (1984)[7] I. Dillmann, R. Plag, F. Ka¨ppeler, T. Rauscher,Proceedings of the workshop ”‘EFNUDAT Fast Neu-trons - scientific workshop on neutron measurements,theory & applications”’, Geel, Belgium (2009)[8] R. Anand, M. Jhingan, D. Bhattacharya, E. Kondaiah,Nuovo Cimento 50A, 247 (1979)[9] G. Walter, H. Beer, F. Ka¨ppeler, G. Reffo, F. Fabbri,Astronomy and Astrophysics 167, 186 (1986)[10] R. Reifarth, M. Heil, C. Forsse´n, U. Besserer,A. Couture, S. Dababneh, L. Do¨rr, J. Go¨rres, R.C.Haight, F. Ka¨ppeler et al., Physical Review C 77(1),015804 (2008)[11] R. Reifarth, M. Heil, F. Ka¨ppeler, R. Plag, NuclearInstruments and Methods in Physics Research A 608,139 (2009)[12] C. Massimi, B. Becker, E. Dupont, S. Kopecky,C. Lampoudis, R. Massarczyk, M. Moxon,V. Pronyaev, P. Schillebeeckx, I. Sirakov et al.,European Physical Journal A 50, 124 (2014)3

Neutron capture cross sections of $^{69}$Ga and $^{71}$Ga at 25 keV and e$_{peak}$ = 90 keV

https://publikationen.bibliothek.kit.edu/1000076029/15099286

Neutron capture cross sections of $^{69}$ Ga and $^{71}$ Ga at 25 keV and e $_{peak}$ = 90 keV

Abstract

Similar works

Full text

Available Versions

KITopen

Neutron capture cross sections of 69^{69}69Ga and 71^{71}71Ga at 25 keV and epeak_{peak}peak​ = 90 keV

Abstract

Similar works

Full text

Available Versions

KITopen

Neutron capture cross sections of $^{69}$ Ga and $^{71}$ Ga at 25 keV and e $_{peak}$ = 90 keV