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The beam from the Yale University Electron Accelerator has been used in systematic studies of the nuclear charge distributions of Ca 40 and Ca 48 . The beam energy has been varied between 20 and 60 MeV, and the angular distributions include angles between 70° and 150°. Elastic electron scattering has been used to obtain the rms radius of Ca 40 , and a value for the difference between the Ca 40 and Ca 48 radii. The results indicate that this difference is not as large as that predicted by the A 1/s rule, in agreement with the Stanford electron scattering work and the Chicago and CERN experiments on the spectra from muonic atoms. Inelastic scattering experiments also have been performed, yielding results for the reduced transition probability B(EL T ) and the transition radius R tr for the following states: Ca 40 , 3.73-MeV (3-), 3.90(2+), 6.94(2+, 3-); Ca 48 , 3.83-MeV (2+) and 4.51-MeV (3-).

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The nuclear rms charge radii measured by low energy electron scattering at Darmstadt are summarized. Improvements in the experimental equipment and method permitted a redetermination of the 12C radius which yielded R m (12C) = 2.462 + 0.022 fm. This value has been used to recalibrate the radii measured relative to izC.Cross sections for elastic electron scattering at low energies (< 65 MeV) are very often measured relative to 12C since absolute measurements in this energy range are extremely difficult. The values of the rms charge radii obtained from these measurements are hence calibrated on the 12 C radius. Any change in this number necessitates a recalibration.So far, most of the radii measured at Darmstadt depend on the 12C radius Rm02C)=2.395+0.028 fm given by Bentz [1,2]. This value is the weighted mean of the result obtained by Engfer and Tfirck [3], Rm0EC)= 2.42_+ 0.04 fm, and that of Bentz' own measurements, Rm(12C) --= 2.387 fin. The experiments of Engfer and Tiirck were done on foils of polyethylene ((CH2)n) and polystyrene ((CH),) whereas Bentz used gaseous targets of methane (CH4) and hydrogen.Subsequent measurements of R,~02C), either by electron scattering [4--6] or on muonic atoms [7] yielded values which were consistently larger (about 2~o) than Bentz' result. Similar deviations were found between the 14N and 160 radii measured by Bentz [1, 2] and other authors [4,7]. This discrepancy has been removed now by a careful reexamination of the experimental method [8] together with new experiments on ~2C, 14N and 160 [8,9].An improved calculation of multiple scattering corrections [8] yielded slightly greater radii for these nuclei but could not account for the whole discrepancy. Other effects, e.g. the density change in the target gas due to beam heating [10], did not significantly influence the results too. Most of the deviation was found to stem from an erroneously applied real gas correction factor. If all corrections are properly taken into account Bentz' value for the a2C radius becomes Rm02C)=2.45 fro, in good agreement with the results of the other authors [4][5][6][7].

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