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New cross section measurements for the reaction ¹⁶0(p,γ)¹⁷F with an extrapolation to astrophysical energies Hall, Thomas Hammond

Abstract

Improved absolute cross section and angular distribution measurements have been made for the direct radiative capture of protons by ¹⁶0. Gamma ray yields have been obtained at four center of mass energies from 1.288 to 2.404 Mev for direct capture to the d⁺[sub 5/2] ground state of ¹⁷F and at seven center of mass energies from 0.795 to 2.404 Mev for direct capture to the s⁺[sub 1/2] first excited state. Angular distribution data have been obtained for both transitions for three angles at 0.778 Mev and four angles at each of 1.289, 1.840 and 2.306 Mev in the center of mass. In addition, separate measurements have determined differential cross sections to ± 0.5% for the elastic scattering of protons by ¹⁶0 at 171°.5 in the center of mass from the energy region of pure Rutherford scattering to 2.0 Mev bombarding energy. The direct capture cross section was normalized to the elastic scattering cross section by a technique which allowed simultaneous observation of both reactions. This was made feasible by the development of transmission targets consisting of a layer of WO₃ sputtered onto a self-supporting gold backing. A large volume Ge(Li) detector was used to observe the gamma rays and to provide accurate measurements of the gamma ray yields. The present measurements agree to within 5% with the most recent cross section calculations of Chow (1973). The reaction ¹⁶0(p,γ)¹⁷F is important in stellar astrophysics through the role it plays in the CN0 bi-cycle. Its cross section at stellar energies affects the relative abundances of the catalyst nuclei carbon, nitrogen and oxygen in those stars which are burning hydrogen by this cycle of reactions. By lending confidence to, and providing an accurate normalization for the direct capture calculations, the present measurements allow a reliable extrapolation of the theory to stellar thermal energies. For center of mass energies below 100 Kev, the contributions to the astrophysical S-factor from capture to the l/2⁺ first excited state and the 5/2⁺ ground state of ¹⁷F can be expressed as S [sub (l/2⁺ state)] (E) = (9.245 – 0.0329E + 0.00007E2) ± 4.1% Kev barns S [sub (5/2⁺ state)] (E) = (0.302 + 0.0002E) ± 6.8% Kev barns. The large energy dependence of S [sub (1/2⁺ state)], in particular its dramatic increase with decreasing energy in the region of thermal energy, arises from the low binding energy of the l/2⁺ first excited state. Consequently, the ¹⁶0(p,γ)¹⁷F reaction rate at stellar thermal energies is determined almost entirely by direct capture to this state.

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