Reaction mechanism of 12C(e,e'p) reaction at an energy transfer of 60 MeV

T. Tamae

Laboratory of Nuclear Science, Tohoku University,
Mikamine, Taihaku, Sendai 982-0826 Japan


In the photonuclear reaction in the energy region of Eg =60~100 MeV, contributions of meson exchange currents (MEC) are known to be very large. However, there is another discussion that in this region the DKO mechanism contributes to the photonuclear reaction as large as to the quasi-elastic (e,e'p) reaction. Ireland and Steenhoven [1] discussed the contribution of the MEC's in the (g,p) reaction by comparing the quasi-elastic (e,e'p) and (g,p) reactions in the same framework. The DKO calculation for the (g,p) reaction underestimated the data by a factor of 6. They explained this discrepancy from the contribution of the MEC's. Quasi-elastic (e,e'p) and (g,p) data they used for their discussion, however, were obtained under different kinematical conditions, i. e., different energy and momentum transfers. It means the DKO calculation of both reactions cannot always be dealt with the same framework. Very recently there has been another argument [2] that meson exchange current contributions may not be large when treated in a relativistic framework.

The (e,e'p) experiment under the kinematical condition close to the (g,p) experiment is very important to resolve this long-standing problem. We measured the (e,e'p) cross at an energy transfer of 60 MeV and a momentum transfer of 104.1 MeV/c, using a 197 MeV continuous electron beam. The reduced cross section at missing energies between 81.3 and 321.2 MeV/c obtained from the experiment is compared with a distorted wave impulse approximation (DWIA) in reasonable agreement. This result demonstrates a high reliability of the DWIA calculation in this energy region, and supports the discussion that a large difference between the experimental data and the DWIA calculation in the (g,p) reaction is related to the non-nucleonic degree-of-freedom such as meson exchange currents.



[1] D.G. Ireland and G. van der Steenhoven, Phys. Rev. C49 (1994) 2182.

[2] J. I. Johansson and H. S. Sherif, Phys. Rev. 56 (1997) 328.