De Smet, L

Heyse, J

Van Gils, J

Vermote, S

Wagemans, C

CERN Document Server

PoS(NIC-IX)085Experimental determination of the 41Ca(n,α)38Arreaction cross section as a function of the neutronenergyLiesbeth De Smet∗a, C. Wagemans a, J. Heyse b, S. Vermote a and J. Van Gils caUniversity of Gent, Dept. of Subatomic and Radiation Physics,Proeftuinstraat 86, B-9000 Gent, BelgiumbIRMM-EC-JRC, Neutron Physics Unit, Retieseweg 111, B-2440 Geel, BelgiumcSCK•CEN, Boeretang 200, B-2400 Mol, BelgiumE-mail: liesbeth.desmet@gmail.com, cyrillus.wagemans@UGent.beThe 41Ca(n,α)38Ar reaction cross section has been studied with resonance neutrons at the linearaccelerator GELINA of the IRMM in Geel (Belgium) and has been determined up to 45 keVusing the time-of-flight technique. A dozen resonances have been identified and for most of themthe area, the total width, and a value for Γn/Γp could be determined. From the obtained crosssection data the Maxwellian averaged cross section (MACS) has been calculated by numericalintegration.International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IX25-30 June 2006CERN∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(NIC-IX)085Determination of the 41Ca(n,α)38Ar reaction cross section Liesbeth De Smet1. IntroductionIn 1978 already Woosley et al. [1] predicted that the 41Ca(n,α) reaction strongly dominatesover the 41Ca(n,γ) reaction at stellar temperatures of importance for s-process nucleosynthesisin stars. This has been confirmed in more recent theoretical works (S. Goriely et al. [2] andT. Rauscher et al. [3]). To verify the theoretical Maxwellian averaged (n,α) cross section values(MACS), a dedicated measurement of the 41Ca(n,α)38Ar reaction cross section as a function of theneutron energy has been performed.2. Experimental setupThe 41Ca(n,α)38Ar reaction was studied at the GELINA neutron time-of-flight facility of theIRMM. Two different measurement campaigns were performed to define the reaction cross section.One at an 8.5 m long flight path and a second one at 30 m to extend the energy range. For themeasurement at 8.5 m, a Frisch gridded ionization chamber with methane as detector gas was used,while a 100 µm thick surface barrier detector was used at the 30 m long flight path. For the moment,a new measurement at 30 m is ongoing, this time using an ionization chamber. Therefore, the resultsfrom the 30 m measurement are only preliminary and the results presented in this contribution arebased on the measurement at 8.5 m.For neutron induced charged particle reactions on 41Ca, both, (n,p) and (n,α) reactions arepossible. However, the settings of the ionization chamber were adjusted to detect only α0- andα1-particles. For all the measurements, the accelerator was operated at a repetition frequency of800 Hz and the electron bursts had a width of 1 ns. To remove neutrons from previous bursts, acadmium overlap filter at 8.5 m and a boron overlap filter at 30 m were permanently used. Thetime dependent background was determined in a separate measurement by putting black resonancefilters such as Au, Co, Mn, Rd and W in the neutron beam.The 41Ca sample used for the measurements was prepared at the IRMM by suspension spray-ing of CaF2 in methanol on an aluminium foil. This resulted in a 41CaF2 sample with 81.69%enrichment [4] containing (3.36 ± 0.44) × 1017 41Ca atoms on a 6 × 5 cm2 effective area. For theneutron flux determination 10B layers containing (7.17 ± 0.13) × 1019 atoms were used.The total observed counting rate YCa(En) for a neutron induced reaction as a function of theneutron energy is:YCa(En) = εCa NCa σCa(En) ϕ(En) + Y BGCa (En), (2.1)where εCa is the detector efficiency and NCa the number of atoms in the 41Ca sample used. σCa(En)is the differential neutron induced cross section to be determined and ϕ(En) represents the neutronflux. The time dependent background Y BGCa has been determined as a function of the time-of-flight tby fitting a function Y BGCa (t) = atb +c through the counting rates in the black resonance regions andhas been subtracted from the counting rate YCa(t). An identical relation for the flux counting rate,in our case the 10B(n,α)-counting rate, is adopted and dividing them gives:σCa(En) =εBεCaYCa(En)−Y BGCa (En)YB(En)−Y BGB (En)NBNCaσB(En). (2.2)2PoS(NIC-IX)085Determination of the 41Ca(n,α)38Ar reaction cross section Liesbeth De Smet1 10010203040X 10X 10s[b]Neutron Energy [keV]Ca(n, )  Ar41 38a14 16 18 20 22051015s[b]Neutron Energy [keV]Measurement 8.5 mMeasurement 30 mVoigt fitFigure 1: Left: The 41Ca(n,α)38Ar cross section measured at 8.5 m.Right: Voigt fits to the measured 41Ca(n,α) cross section data for some resonances (measurement at 8.5 m).The cross section data measured at 30 m are preliminary.For the measurements with the ionization chamber as well as with the surface barrier detectorthe 41Ca(n,α) reaction and the 10B(n,α) reaction have been measured in the same experimentalconditions, so εBεCa = 1 (detection geometry equals 2pi). The known 10B(n,α) reference cross sectionσB(En) is taken from the ENDF/B-VI database.3. Results and resonance analysis3.1 Fitting procedureFigure 1 shows the 41Ca(n,α)38Ar cross section in the neutron energy region from 600 eV to50 keV. The resonances are fitted with Voigt shapes to determine the energy position, the area andthe total level width. Such a Voigt function is a convolution of a Breit-Wigner function and a Gaussfunction. The Gauss shape represents the experimental broadening, which can be calculated and istaken fixed during the fit. The results of these Voigt fits are listed in the left part of table 1. ωα is theresonance strength and is proportional to the area. The resulting Voigt fits are shown in figure 1 forthe resonances at 14.73 keV and 19.43 keV. The preliminary data from the 30 m measurement areplotted with dots in figure 1. It is clear that the resolution is much better, confirming the existenceof a resonance at 18.45 keV. Clearly, this better resolution will lead to a more precise determinationof the level widths.3.2 Spin assignments-wave (` = 0) neutrons impinging on 41Ca, ground state Ipi = 72−, will populate the Jpi = 3−and Jpi = 4− states of the compound nucleus 42Ca, whereas p-wave (` = 1) neutrons can populatestates from Jpi = 2+ through Jpi = 5+. The formed states decay to the ground (Jpi = 0+) or firstexcited state (Jpi = 2+) of 38Ar by emitting α0- or α1-particles, respectively. The smallest allowedangular momenta of the emitted α-particles are tabulated in table 2.3PoS(NIC-IX)085Determination of the 41Ca(n,α)38Ar reaction cross section Liesbeth De SmetEnergy [eV] Γ [eV] Area [b.eV] ωα [eV] ωp [eV] [7] ΓnΓp1090 ± 70 30 ± 154490 ± 10 40 ± 20 3400 ± 300 3.73 ± 0.33 7.4 8.07 ± 0.716170 ± 60 260 ± 26 11350 ± 9509725 ± 75 125 ± 100 170 ± 60 0.40 ± 0.14 9.2 0.70 ± 0.2514725 ± 75 175 ± 140 2500 ± 300 9.00 ± 1.08 2.1 68.57 ± 8.2418450 ± 300 a19430 ± 150 420 ± 100 5800 ± 600 27.55 ± 2.86 1.6 275.5 ± 28.623080 ± 200 600 ± 200 400 ± 150 2.26 ± 0.85 2.8 12.90 ± 4.8426100 ± 800 b29700 ± 700 1250 ± 1000 250 ± 100 1.82 ± 0.73 2.4 12.10 ± 4.8533700 ± 700 1000 ± 600 550 ± 220 4.53 ± 1.82 5.7 12.72 ± 5.0938850 ± 1200 2000 ± 1200 900 ± 300 8.55 ± 2.86 2.9 47.16 ± 15.79a The resonance at 19.43 keV is a double resonance with a first level at 18.45 keV.b Too low counting statistics, so the area has not been fitted.Table 1: Left part: Resonance parameters obtained from the Voigt fits.Right part: ΓnΓp for the resonances observed in this work which correspond to resonances observed by Meijerand Van Gasteren [7].Wagemans et al. [5] measured the 41Ca(nth,αi) reaction cross section with thermal neutronsat the ILL in Grenoble. This thermal cross section is the sum of the tails of all s-wave resonances.From the fitted total natural line width and from the fitted area, the contribution from each resonanceto the thermal cross section can be calculated. If this calculated value is higher than the measuredthermal cross section, the resonance is taken as a p-wave. On the basis of these calculations itis concluded that only the resonances around 1.1 keV and 10 keV may be s-wave. Although thenatural line width could not be determined for the resonance at 1.1 keV, it can be deduced fromthe resonance area that its thermal contribution in case of an s-wave will not exceed the measuredthermal cross section. The pulse height spectra (anode and cathode) for the resonance at 1.1 keVreveal that this is a transition only to the first excited state of 38Ar. Taking into account the spinassignments for s-wave neutrons, the only possibility is that this resonance corresponds to a 4−state of 42Ca. On the contrary, the 10 keV resonance consists of (n,α0) as well as (n,α1) transitions.With the help of table 2, it is concluded that the 10 keV resonance is a 3− state in 42Ca. All theother resonances are considered to be p-wave. Because (n,α0) and (n,α1) transitions are observedin all of them, these resonances are most probably 2+ states as 3+ and 5+ states are ruled out byparity constraints and the 4+ state is less probable because the higher orbital momentum of theα-particle reduces the penetrability through the centrifugal barrier.In principle, interference effects between two neighbouring resonances with the same spin canreduce the contribution to the thermal cross section, leaving open the possibility of a different spinassignment for some resonances. This would lead to a non 1/v-behaviour of the cross section atlow neutron energy. The experimental cross section obtained in this work shows some possible4PoS(NIC-IX)085Determination of the 41Ca(n,α)38Ar reaction cross section Liesbeth De SmetJpi 38Ar Jpi 42Ca compound nucleuss-wave p-wave3− 4− 2+ 3+ 4+ 5+0+ (α0) 3 p.f. 2 p.f. 4 p.f.2+ (α1) 1 3 0 2 2 4p.f. = parity forbiddenTable 2: Smallest allowed angular momenta of the emitted α-particles from the formed compound nucleus42Ca in the case of impinging s- or p-wave neutrons.interference effects between the resonances at 4.5 keV and 6.2 keV and between 14.7 keV and thedoublet around 19 keV [6].3.3 Determination of the partial level widthsThe only additional information on 42Ca compound levels in the covered energy range wasobtained in a proton induced reaction on 41K by Meijer and Van Gasteren [7]. They reported a listof ωp = (2J +1)ΓpΓα0/Γ values for the measured 41K(p,α0)38Ar reaction. The Γp-value from theirwork corresponds with p0-decay in the case of neutron induced reactions on 41Ca. In the case of aneutron induced reaction on 41Ca only p0- and p1-decay to 41K is possible, so Γp = Γp0 + Γp1 .However, the probability that the p1-particles penetrate the Coulomb barrier is very low sincetheir energy is only 0.21 MeV. Therefore, it can be assumed that Γp = Γp0 in a neutron inducedmeasurement. Consequently, the Γp from the measurement of Meijer and Van Gasteren [7] isequal to the Γp from this work. By dividing the fitted resonance strength ωα obtained in this work(ωα = g ΓnΓαΓ = (2J+1)16 ΓnΓαΓ and is proportional to the resonance area), by the reported ωp-value ofMeijer and Van Gasteren, a value for ΓnΓp is obtained. The values for ΓnΓp for the resonances observedin this work which correspond to resonances observed by Meijer and Van Gasteren [7] are listed intable 1.The partial widths may be calculated through combining the expressions for ωp and ωα men-tioned above, and the expression for the total width Γ = Γn + Γγ + Γp + Γα . Five parameters (J, Γn,Γγ , Γp and Γα ) have to be fixed with only three equations at our disposal. This means that it is notpossible to solve this system of equations unambiguously. For this reason no partial level widthscould be determined. Additional measurements leading to the same compound nucleus 42Ca arenecessary to determine the partial widths.4. Astrophysical implicationsThe 41Ca(n,α) MACS values are obtained by numerical integration of the determined crosssection and are shown in figure 2. In this work the cross section is obtained up to an energy ofapproximately 45 keV and because a resonance at an energy Eres has its maximum contribution atkT = 0.5 Eres, the obtained MACS values above 22 keV are lower limits. In figure 2 a comparison ismade with the values obtained from the MOST [2] and the NON-SMOKER [3] codes. In the energyregion covered by the experiment, two stellar temperatures are of interest in s-process network5PoS(NIC-IX)085Determination of the 41Ca(n,α)38Ar reaction cross section Liesbeth De Smet0 10 20 30 40 500.00.40.81.21.62.0MACS [b]kT [keV]This workNON-SMOKERMOSTFigure 2: The 41Ca(n,α0+α1)38Ar MACS values obtained by numerical integration of the obtained crosssection data. A comparison is made with theoretical values.calculations: 8 keV and 25 keV. For 8 keV, it is clear from figure 2 that both theoretical modelsunderestimate the MACS value obtained in this work. At 25 keV, the MOST value is in agreementwith the measured MACS value, but still, the NON-SMOKER value is significantly lower.The implementation of these much higher experimental MACS values in nucleosynthesis net-work calculations will among others affect the 36S abundance. 36S is believed to be synthesizedmainly during the weak s-process in massive stars, in particular during the convective He-coreburning (at 8 keV and 25 keV) followed by convective C-shell burning (at 86 keV). To checkthis hypothesis, nucleosynthesis network calculations in the mass region between 28 ≤ A ≤ 42are necessary. The 41Ca(n,α) reaction plays a role in this network as it recycles to 36S via thefollowing reaction chain: 41Ca(n,α)38Ar(n,γ)39Ar(n,α)36S. Although the 39Ar(n,α)36S channel isless important than the 36Cl(n,p)36S channel for the production of 36S, it is included in the stellarmodels. As a second part of the weak s-process occurs at temperatures around 86 keV, the addi-tional measurement on a 30 m flight path at the GELINA facility will provide the complete MACSinformation needed to check the hypothesis.References[1] S. Woosley et al., At. Data and Nucl. Data Tables 22 (1978) 371.[2] S. Goriely et al., http://www-astro.ulb.ac.be/Html/ncap.html.[3] T. Rauscher et al., http://quasar.physik.unibas.ch/∼tommy/reaclib.html.[4] C. Wagemans, Nucl. Instr. & Meth. in Physics Research A282 (1989) 4.[5] C. Wagemans, R. Bieber, H. Weigmann and P. Geltenbort, Phys. Rev. C57 (1998) 1766.[6] M. Moxon, private communication.[7] R. J. Meijer and J. J. M. Van Gasteren, Nucl. Phys. A148 (1970) 62.6

Experimental determination of the $^{41}Ca(n,\alpha)^{38}Ar$ reaction cross section as a function of the neutron energy

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Experimental determination of the 41Ca(n,α)38Ar^{41}Ca(n,\alpha)^{38}Ar41Ca(n,α)38Ar reaction cross section as a function of the neutron energy

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Experimental determination of the $^{41}Ca(n,\alpha)^{38}Ar$ reaction cross section as a function of the neutron energy