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.á 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. áIreland 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.á
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.á 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.á 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.
References
[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.