Elsevier

Physics Letters B

Volume 703, Issue 5, 26 September 2011, Pages 557-561
Physics Letters B

Study of the 6.05 MeV cascade transition in C12(α,γ)O16

https://doi.org/10.1016/j.physletb.2011.08.061Get rights and content

Abstract

The radiative capture reaction C12(α,γ)O16 has been investigated in the energy range E=3.3 to 4.5 MeV. This experiment focused in particular on the cascade transition to the 0+ state at Ex=6.05 MeV in 16O and was performed by detecting the capture γ-rays with a NaI detector array at the windowless 4He gas target of the recoil mass separator ERNA in coincidence with the 16O ejectiles. The 6.05 MeV transition has been considered recently as a component accounting for up to 15% of the C12(α,γ)O16 total cross section at astrophysical energies. The arrangement of the detector array yielded additional information on the γ-ray multipolarity, i.e. the ratio σE2/σE1, and it was found that the 6.05 MeV transition is entirely E2 in the studied energy range. The results for this transition are analyzed in an R-matrix formalism and extrapolated to the relevant Gamow energy of stellar helium burning, E0300 keV. In contrast to a previous analysis, the present extrapolation suggests a negligible contribution from this amplitude, S6.05(300)<1 keVb. Additional data for cascade transitions to excited states at Ex=6.13, 6.92, and 7.12 MeV, respectively, as well as to the ground state were obtained and the corresponding S factors in the studied energy range are given.

Introduction

After stellar hydrogen burning, the core of a star is transformed mainly to 4He. Without the core energy production, the center contracts, and the core temperature, TC, rises. Meanwhile the outer layers of the star expand and the star develops into the Red Giant phase. At TC>108 K the nuclear energy production, due to the synthesis of three α particles to carbon (triple-α process), becomes large enough to sustain the temperature of the core. As a consequence, the contraction stops and the helium burning phase is started. In the first stage of stellar helium burning – since the 12C abundance is very low – the only active process is the triple-α reaction, while later after build up of a significant abundance of carbon the C12(α,γ)O16 reaction is the dominating process. The cross section of the C12(α,γ)O16 reaction at the relevant Gamow energy – E0300 keV – determines the helium burning time scale and, together with the convection mechanism, the abundances of carbon and oxygen at the end of helium burning. The carbon abundance at that stage has important consequences for the subsequent evolution of various astrophysical scenarios, e.g. a direct influence on type II Supernova (SN) nucleosynthesis [1], [2], [3], [4], [5], the maximum luminosity and kinetic energy of type I SN [6], and the cooling sequence of CO white dwarfs [7], [8], [9]. Thus, an experimental determination of the C12(α,γ)O16 cross section in the relevant energy region in the order of 10% or better will improve our understanding of the convection processes and remains an important ingredient for the understanding of stellar evolution.

The cross section of the reaction C12(α,γ)O16 (Q=7.162 MeV) is dominated by E1 and E2 capture processes into the 16O ground state, where the two multipoles appear to be of similar importance at stellar energies. This energy region is not accessible with present experimental techniques and an extrapolation to E0 is necessary. The cross section σ(E) at low energies is typically expressed in terms of the astrophysical S factor [10] defined for this reaction as:S(E)=σ(E)Ee650.35/E where E is in keV. Since the capture cross sections of the E1 and E2 multipoles have different energy dependencies, one must have an independent and precise information on the energy dependence of each multipole cross section.

A series of direct experiments with γ-ray detector arrays (e.g. [11], [12], [13], [14], [15]) were carried out in the past years to constrain the ground state transitions. Additional information can be derived from measurements of the (α,α)-elastic scattering [16], the β-delayed α-decay of 16N [17], [18], [19], and the total cross section with a recoil separator [20].

In addition to the ground state contributions, cascade transitions have to be considered whereas much less data are available. The cascade transitions can proceed through a number of 16O excited states and in particular transitions to the Ex=6.92 MeV (Jπ=2+) and 7.12 MeV (Jπ=1) have been observed in the past [21], [22].

A more detailed discussion of the various contributions is given for example in the review of Buchmann and Barnes [23] where a total S factor of S(300)=145 keVb is recommended with an uncertainty range of 25 to 35%.

Recently, the importance of the Jπ=0+ state at Ex=6.05 MeV in 16O was emphasized [24]. This excited 0+ state decays exclusively by e+e transition (E0) to the 0+ ground state and only the primary γ-ray line can be observed. This component was measured with the DRAGON recoil separator at TRIUMF, Canada, in the energy2 region E=2.22 to 5.42 MeV with a high efficiency BGO γ-ray array in coincidence with the observed recoils. The data have been analyzed with an R-matrix calculation and extrapolated to the astrophysical energy region, S6.05(300)=2515+16 keVb. This value is an additional contribution to the S(300) given in [23] and, thus, a non-negligible part of the total S factor at helium burning temperatures. Therefore, this cascade transition deserves a further investigation in order to prove its importance for stellar evolution.

The DRAGON experiment [24] spanned a rather large energy interval and is, due to the coincidence condition, one of the first reliable γ-ray measurements above E>3 MeV in C12(α,γ)O16. In general data at high energies are important for reliable R-matrix extrapolations to astrophysical energies, but the only existing direct γ-ray measurement at such energies so far was published in 1964 [25] where a quoted absolute uncertainty of a factor 2 prevents any definite conclusion. However, the DRAGON measurement [24] suffered from low 12C beam intensities, the moderate energy resolution of the BGO crystals, and the limited acceptance of the separator preventing an independent normalization to the C12(α,γ)O16 ground state transition. Some of these issues could be overcome in the measurement reported here.

Section snippets

Experimental setup

The experiment was carried out at the ERNA (European Recoil Separator for Nuclear Astrophysics) recoil separator at the Dynamitron – Tandem Laboratory of the Ruhr-Universität Bochum, Germany. A 12C beam intensity of typically 5 pμA in the 3+ or 4+ charge state was used in the energy range E=3.3 to 4.5 MeV. Details of the experimental setup are as reported previously [26], [27], [28]. Briefly, a C12 ion beam emerging from the tandem accelerator was focused by a quadrupole doublet, filtered by a

Data analysis

The trigger of the acquisition system was based on the signal from the end-detector. Therefore, only γ-ray events in coincidence with recoil-like signals have been acquired. The selection of the appropriate ΔEE region for the 16O recoils set further strong constraints on the identification of γ-ray events from the C12(α,γ)O16 reaction. The background from accidental coincidences between γ-rays and recoil events in this ΔEE region was estimated from γ-ray-recoil coincidences of the 12C

R-matrix analysis of the 6.05 MeV transition

The 6.05 MeV S factor data have been analyzed in the R-matrix formalism. The corresponding R-matrix code was developed specifically for an analysis of the C12(α,γ)O16 reaction and is based on an alternative parametrization [39] of the original R-matrix theory [40]. The code can use physical resonance parameters [39] and is suited to fit simultaneously the direct γ-ray data, the 16N β-delayed α decay, and the (α,α) elastic scattering, respectively. The details on the code and further results

Summary

In conclusion, the 6.05 MeV cascade transition in C12(α,γ)O16 was studied with the ERNA recoil separator by means of γ-ray-recoil coincidences, delivering background-free γ-ray spectra in the energy region above E3.3 MeV. This energy region gives strong constraints on the extrapolation of the astrophysical S factor for this transition and an R-matrix analysis of the present data yielded an upper limit of S6.05(300)<1 keVb. The large S6.05(300) value found previously [24] is excluded by the

Acknowledgements

The authors thank C. Rolfs and H.-P. Trautvetter for fruitful discussions. This work was supported by DFG (Ro429/35) and INFN.

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    Present address: RUBION, Ruhr-Universität Bochum, D-44780 Bochum, Germany.

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