A high-resolution study of the 12C(γ,p) reaction with 49–78.5 MeV tagged photons

doi:10.1016/0375-9474(90)90262-K

Nuclear Physics A

Volume 517, Issue 1, 15 October 1990, Pages 93-107

https://doi.org/10.1016/0375-9474(90)90262-K Get rights and content

Abstract

We present tabulated cross-section measurements for the reaction ¹²C(γ, p) leading to resolved, low-lying, excited states of ¹¹B. The measurements were obtained using tagged photons of 49–78.5 MeV and for mean laboratory proton emission angles from 65° to 115°. The proton excitation energy spectra are dominated by the ground state ( $32^{−}$ ), 2.13 MeV ( $12^{−}$ ), 5.02 MeV ( $32^{−}$ ) and doublet ( $72^{−}$ , $12^{+}$ ) states of ¹¹B. No evidence is found for the population of the 4.45 MeV ( $52^{−}$ ) state. Our analysis demonstrates that both the ground-state and excited-state cross-section data scale with momentum mismatch. The data are qualitatively discussed within the context of the existing (e, e'p) data and direct knockout and quasideuteron models. A consistent and quantitative understanding of the data will require more sophisticated models than are currently available.

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The cross section asymmetry of ${}^{12}C (γ, p_{01}) {}^{11}B$ and ${}^{12}C (γ, p_{2 - 6}) {}^{11}B$ reactions has been studied at the energy range 40–55 MeV, using linearly polarized tagged photons of the MAX-lab facility. The asymmetry of the ${}^{12}C (γ, p_{01}) {}^{11}B$ processes, which assume the one-body mechanism of the reaction, is $Σ \approx 0.82 \pm 0.05$ for photon energies 45–50 MeV. The asymmetry for the ${}^{12}C (γ, p_{2 - 6}) {}^{11}B$ reactions, which produce a maximum at excitation energy $\sim 6 MeV$ , is $Σ \approx 0.53 \pm 0.13$ for a photon energy 49 MeV. It is close to the asymmetry of reaction of the free deuteron photodisintegration, and can be resulted from the two-body mechanism of the photon absorption.
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A high resolution magnetic pair spectrometer for medium energy photons has been used for studies of the exclusive (p,γ) reaction on ¹²C at 98 and 176 MeV. We present angular distributions of the differential cross section for transitions to the ground state and the first three excited states of ¹³N. The distributions from these four states are compared with predictions from a microscopic continuum shell model calculation using a realistic finite-ranged effective interaction with tensor components. In general, reasonable agreement is obtained for reactions to states described by p shell single particle wave functions, but poor agreement is obtained for transitions described by sd shell single particle wave functions. Nevertheless we find, however, that the dominant reaction mechanism is a direct capture process. We also compare the (p,γ) data with existing data on the exclusive (p,π) reaction at 185 MeV revealing notable differences between the two reactions.
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Cross sections have been measured for the reactions ²⁰⁸Pb(γ,p) and ¹²C(γ,p) leading to the low-lying and continuum states in ²⁰⁷Tl and ¹¹B using a high energy-resolution tagged photon beam of 41 to 57 MeV. The data are compared with results obtained with the (e,e′p) reaction in the same recoil-momentum range and with various theoretical calculations. A comparison of the (e,e′p) and (γ,p) data in terms of a reduced cross section versus momentum does not show a systematic overlap of the (e,e′p) and (γ,p) results, i.e. no scaling is observed. Distorted-wave impulse-approximation (DWIA) calculations, of which the input is constrained by the (e,e′p) data, and random-phase approximation calculations (RPA) underestimate the ¹²C(γ,p) data by typically a factor five depending on angle and missing energy. For ²⁰⁸Pb the difference between the calculations and the data for the low-lying states is much less, but an increasing discrepancy between the data and the calculations is found in the continuum. The inclusion of meson-exchange currents (MEC) brings the calculations for both nuclei on average closer to the data, but MEC effects are considerably less important for ²⁰⁸Pb as compared to ¹²C. Finally, a remarkable similarity is observed between the continuum response of the (γ,p) and (e,e′p) reactions on ²⁰⁸Pb despite the difference in the electromagnetic couplings involved.

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¹: Prime Computer R&D, Harston, Cambridgeshire, UK.

²: Physikalisches Institut, Bonn Universität, D-5300 Bonn 1, Fed. Rep. Germany.

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Nuclear Physics A

Abstract

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Radiative proton capture to discrete states in <sup>31</sup>N at 98 and 176 MeV

Photon-induced proton knockout from <sup>208</sup>Pb and <sup>12</sup>C