Answer
The electrons are at the lower limit in energy for useful probes.
Work Step by Step
The diameter of a copper nucleus, for example, is about $8.6 \mathrm{fm},$ just a little larger than the de Broglie wavelength of a $200-\mathrm{MeV}$ electron. To resolve detail, the wavelength should be smaller than the target, ideally a tenth of the diameter or less. $200-\mathrm{MeV}$ electrons are perhaps at the lower limit in energy for useful probes.
