Experimental Studies of the Mechanism of Photon Absorption on Light Nuclei
- Univ. of Edinburgh, Scotland (United Kingdom)
A measurement of the reaction 12C(γ,pγ') has been made with tagged photons in the range 49.47 < E γ < 70.24 MeV. The relative population of a triplet of states, (7/2-, 1/2+, 5/2+) at Eex ≅ 7 MeV in the residual nucleus 11B, following the reaction 12C(γ,p), has been made by directly resolving their de-excitation γ-ray decay to the ground state of 11B. This measurement constitutes the highest resolution ever achieved in determining the excitation function of the residual nucleus after a (γ,p) reaction (~ 48 keV FWHM at Eex ≅ 7 MeV). Comparison has been made with the data of Kuzin et al, who also measured the relative populations with a resolution of ~ 270 keV at Eex ≅ 7 MeV. The 7/2- state at 6.74 MeV has been observed to be the most strongly excited state of the triplet in agreement with Kuzin et al. Disagreement has been found for the relative strengths of the 1/2+ and 5/2+ states in the triplet. An estimation of the maximum effect of angular correlations between the ejected proton and de-excitation γ-ray has been included in the analysis. This effect increases the disagreement between the measurements. A study of the reaction 12C(e,e'p) has been made in the missing momentum region 260 < pm < 320 MeV/c. A Rosenbluth separation of the reaction cross section has been carried out in parallel kinematics to measure the structure functions WL and WT. The experimental data have been compared with Complete Distorted Wave Impulse Approximation calculations and Random Phase Approximation calculations. The latter when including virtual photon absorption on 2-body currents (Meson Exchange Currents + Isobar Currents) are seen to describe the reduced cross sections better than 1-body current or Complete Distorted Wave Impulse Approximation calculations. For the 1st time a comparison between the 12C(γ,p) reaction and the transverse part of 12C(e,e'p) reaction cross section has been made. This comparison suggests that the mechanism of the 2 reactions converge when both are studied in equivalent kinematics resolving the long standing issue of anomalous excitation of the ~ 7 MeV triplet in 11B, following 12C(γ,p), not seen in the 12C(e,e'p) case. In a further use of the (γ,pγ') reaction the relative population of the 4.44 MeV (T = 0) and 15.1 MeV (T = 1) states in 12C, following 14N(γ,pn), have been measured with tagged photons in the range 50.20 < Eγ < 71.40 MeV. A strong suppression for population of the T = 1 state in the residual nucleus as compared to the T = 0 state was observed. This is consistent with the picture of photon absorption on T = 0 (3S1) p-n pairs in nuclei. A measurement of the 14N(γ,d) reaction has also been made in which the converse population of states in 12C was observed.
- Research Organization:
- Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- AC05-84ER40150
- OSTI ID:
- 921026
- Report Number(s):
- JLAB-PHY-00-108; DOE/ER/40150-4403; TRN: US0801990
- Resource Relation:
- Related Information: Full text contains only the abstract, TOC and list of tables, figures
- Country of Publication:
- United States
- Language:
- English
Similar Records
Nuclear reactions producing /sup 2/He and excited states of /sup 4/He as unbound outgoing systems
Study of Nuclear Reactions with 11C and 15O Radioactive Ion Beams