Automatic Energy Scanning by Target Self Charging for High Precision Measurements of Rapidly Varying Cross Sections

An instrument was developed to trace excitation curves of nuclear reactions induced by charged particles at low energies with a precision of the order of 50 eV. Energy scanning periods of the order of one second were achieved, with total variations ranging typically from 2 KeV to 20 KeV. Good linearity was obtained with beam currents down to about 10-8 A. Applications to the measurements of the cross section of the reaction O18(p, α)N15 near the narrow resonances at 628 and 1766 KeV at two angles simultaneously are described. Results are presented on the energy straggling of protons through 150 Å thick foils, showing the asymmetry effects predicted by theory, using the narrow resonance at 1165 KeV in the same reaction. The basic idea consists of controlling the scanning speed by the beam current itself, using target self charging. The fluctuations of the current do not therefore, by definition, impair scanning uniformity. The device is thus especially convenient for Van de Graaff type accelerators. An extension of the method for scanning parameters other than energy is described. An example is given, showing high precision automatic angle scanning in channeling experiments, where electrostatic deflection of the beam is used. Steps of 3 minutes of arc were achieved without any moving part in the system.