The (α, n) cross sections on 17O and 18O between 5 and 12.5 MeV

doi:10.1016/0375-9474(67)90895-0

Nuclear Physics A

Volume 98, Issue 1, 16 May 1967, Pages 25-32

https://doi.org/10.1016/0375-9474(67)90895-0 Get rights and content

Abstract

The excitation functions for the (α, n) reaction on ¹⁷O and ¹⁸O have been measured from 5 to 12 MeV using long counters. Also, the neutron differential cross sections were measured at 9.8, 11.6 and 12.2 MeV, using the Livermore neutron time-of-flight facility. Neutron spectra were obtained every 15° between 3° and 135° with gas targets of enriched ¹⁷O and ¹⁸O. The angular distributions are roughly symmetric about 90°. Small forward peaking is observed at the higher bombarding energies for some of the states, mainly for the ground and first excited states.

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In addition, calculations were performed by using cross section data taken from JENDL/AN-2005 library (Murata et al., 2006) and precompiled data libraries of the SOURCES4C code. Among cross section data in the SOURCES4C and ORIGEN libraries, those concerning α-particle interactions with oxygen are based on Bair's thin- and thick-target measurements (Bair and Willard, 1962; Bair and Haas, 1973; Bair and Gomez del Campo, 1979) for the lower-energy and on Hansen's data (Hansen et al., 1967) for the high-energy range. Table 2 lists the selected set of measured and calculated data for the total neutron yield for different oxide matrix sources used to test the performance of the different cross section libraries.
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During the slow neutron capture process in massive stars, reactions on light elements can both produce and absorb neutrons thereby influencing the final heavy element abundances. At low metallicities, the high neutron capture rate of ¹⁶O can inhibit s-process nucleosynthesis unless the neutrons are recycled via the ¹⁷O( $α, n$ )²⁰Ne reaction. The efficiency of this neutron recycling is determined by competition between the ¹⁷O( $α, n$ )²⁰Ne and ¹⁷O( $α, γ$ )²¹Ne reactions. While some experimental data are available on the former reaction, no data exist for the radiative capture channel at the relevant astrophysical energies.
The ¹⁷O( $α, γ$ )²¹Ne reaction has been studied directly using the DRAGON recoil separator at the TRIUMF Laboratory. The reaction cross section has been determined at energies between 0.6 and 1.6 MeV $E_{c m}$ , reaching into the Gamow window for core helium burning for the first time. Resonance strengths for resonances at 0.63, 0.721, 0.81 and 1.122 MeV $E_{c m}$ have been extracted. The experimentally based reaction rate calculated represents a lower limit, but suggests that significant s-process nucleosynthesis occurs in low metallicity massive stars.
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Citation Excerpt :
As recommended in Ref. (Bair et al., 1979), this accurate measurement was used to normalize the thin-target 17,18O(α,n) cross-section data. As in the SOURCES4C data libraries, the same data for natO(α,n) were also used to adjust the high-energy 17,18O(α,n) cross section measurements of Hansen (Hansen et al., 1967) by a 9.2% reduction, resulting from the integral data for natO(α,n) over the range 5.15≤Eα≤ 8 MeV (Bair et al., 1979). In Fig. 5, one can see that, as the energy increases, the resonance-like structure derived from Bair's experiments is replaced by a smooth, averaged behavior, yielding a cross section with broad fluctuations.
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^†: Work performed under the auspices of the U.S. Atomic Energy Commission.

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Nuclear Physics A

Abstract

References (14)

J. Phys. Soc. Japan

Phys. Rev.

Phys. Rev.

Nucl. Instr.

Nucl. Instr.

Phys. Rev.

Phys. Rev.

Cited by (28)

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(α,n) reactions in oxide compounds calculated from the R-matrix theory

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