= Hofstadter nuclear electron scattering experiment
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<Robert Hofstadter>'s Stanford installation selected <Electrons> up to $190\,\mathrm{MeV}$. <Magnetic spectrometer> scans counted shielded Lucite <Cherenkov radiation> pulses per monitor charge. <Carbon-12> at $187\,\mathrm{MeV}$ and $80^\circ$ resolved $185.1\,\mathrm{MeV}$ <elastic scattering> and $180.7\,\mathrm{MeV}$ <inelastic scattering> ($4.43\,\mathrm{MeV}$ excitation). Calibration used forward-beam energy, <hydrogen> <nuclear recoil> and <carbon-12> <energy levels>. Closed entrance slits checked background.<Electron Scattering and Nuclear Structure by Robert Hofstadter>[§III(b)(1), pp. 222–223, Fig. 11; §IV(a), pp. 227–231, Figs. 14–17]{ref}
\Image[/nuclear-electron-scattering-beam-and-spectrometer.png]
{title=Selecting a scattering angle and an outgoing electron momentum}
{description=Original functional schematic: separate scattering and bending planes.}
The key distinction is between an <Electron> counter and an <energy> measurement. Here the counter registers particles admitted by the <magnetic spectrometer>; changing its <magnetic field> selects another outgoing <momentum>. A peak is therefore reconstructed from a scan, rather than read as one pulse whose height equals the scattered <Electron> <energy>. Dividing counts by monitored incident <electric charge> compensates for changes in beam intensity. A peak's height alone is insufficient: its area and the instrument's accepted <momentum> interval determine the selected yield.
An <inelastic scattering> peak corresponds to a different final nuclear <quantum state>, so including it in the <elastic scattering> yield would answer a different structural question. Conversely, finite <energy> resolution can hide sufficiently nearby excitations beneath an apparent <elastic scattering> peak. Background subtraction, target-loss corrections, angular acceptance and counter efficiency all enter before counts become a <differential scattering cross-section>. A dip in uncorrected counts is therefore not automatically a zero of a <nuclear charge form factor>.
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