Dee Voltage Calibration
It is sometimes needed to determine the actual peak voltage reached by RF accelerating electrodes (so-called "Dees" in cyclotrons) either to characterize a new system or to check and reset the calibration of built in diagnostics which may have drifted over time (e.g. pick-up probes or forward RF power indicator). Measuring the voltage of a high Q, low capacitance resonator by direct means is highly unpractical as the measuring device upsets the system's parameters: an indirect method is preferred.
Principle of Measurement
Electrons escaping (from) the surfaces of RF structures are accelerated (under vacuum) in the RF field and emit X-rays when hitting a surface (bremsstrahlung). The maximum kinetic energy gathered by these electrons corresponds to the maximum (peak) electrode voltage therefore the endpoint energy of the bremsstrahlung spectrum provides a measure of the peak voltage.
Our spectroscopy chain consists of a high resolution X-ray detector, a pulse shaping amplifier, a Multi-Channel Analyzer and data acquisition and analysis software. The picture below illustrates one actual voltage measurement and the system calibration using a small radioactive source.
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