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Abstract. The 12C(e, e'p)llB~ reaction has been measured in parallel and anti-parallel kinematics over a p.~ range of -120 q) and anti-parallel (p < q) kinematics, an effect observed only for proton knock-out from 1~C. In reference [3] van der Steenhoven et al. interpreted this discrepancy as a possible signature of the enhancement of the transverse form factor of a proton bound in the nuclear medium. Because of the great importance of the question of nuclear medium effects, it is of interest to measure again this reaction at the facility in Mainz with its 100%-duty-cycle accelerator MAMI.The relevant parameters of the experimental setup in Mainz have been discussed in reference [4], describing an 160(e,e'p) experiment. There we concluded that the systematic uncertainty in the (e,e'p) cross section was 8% (adding up linearly the different contributions) with * E-mail address! offerman @kph.uni-mainz.de the main contribution coming from the target thickness (4%). Now we estimate a target thickness uncertainty of 3%, thereby arriving at a total systematic uncertainty of 7%. The target thickness was determined by measuring its weight and area This value was checked through elastic electron scattering from 12C. Data acquisition and analysis proceeded along the same lines as in the 160 experiment. In table I the central kinematical settings are given. Note that settings 1 and 2 were performed in parMlel kinematics while in setting 3 data was accumulated in anti-parallel kinematics. In settings 1 and 3 maximum solid-angle acceptances were used in both electron (5.6 msr) and proton (28 msr) spectrometer. Setting 2 has a large kinematical overlap with setting 1. In order to check the luminosity and the large proton acceptance we changed the target thickness and reduced the proton solid angle to 15 msr (see table I).In Figure 1 the measured distorted momen...

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