Pulse Response Characteristics and Count Rate of Channel Electron Multiplier High Voltage Power Supply
Channel electron multipliers, also known as continuous dynode multipliers, amplify single electrons through secondary emission in a curved channel coated with semiconducting material. The continuous voltage gradient along the channel accelerates electrons and produces the cascade of secondary emission that amplifies the input signal. The high voltage power supply that biases the channel electron multiplier must provide appropriate pulse response characteristics to handle the pulsed current drawn by the detector at high count rates without degrading the pulse shape or the gain stability.
The channel electron multiplier structure consists of a curved glass or ceramic tube with a semiconducting inner coating that provides both the secondary emission surface and the resistive path for the bias current. A high voltage applied between the input and output ends creates a voltage gradient along the channel. Electrons entering the channel strike the wall, releasing secondaries that cascade down the channel. The curved geometry prevents positive ions from traveling back to the input and causing afterpulsing. The gain depends on the applied voltage and the channel length to diameter ratio.
Each detected particle produces an output pulse of millions of electrons, corresponding to a charge of picocoulombs delivered to the anode in a pulse lasting nanoseconds. At high count rates, these pulses arrive frequently, creating a pulsed current load on the high voltage power supply. The average current equals the pulse charge times the count rate, but the instantaneous current during each pulse is much higher. The power supply must supply this pulsed current without the voltage drooping during the pulse or between pulses.
The pulse response characteristics of the power supply determine the voltage stability during pulsed loading. The output capacitance supplies charge during the pulse, with the voltage droop depending on the pulse charge and the capacitance value. Larger capacitance reduces the droop but increases the stored energy and may affect safety and size considerations. The capacitance must be sufficient to maintain the voltage within acceptable limits at the maximum pulse rate and pulse amplitude.
Recovery time between pulses determines the maximum count rate the system can handle. After a pulse, the voltage must recover to the nominal value before the next pulse arrives to maintain constant gain. The recovery time depends on the output capacitance and the recharge current capability of the power supply. Faster recovery enables higher count rate operation. The power supply bandwidth and the output filter design affect the recovery characteristics.
Gain variation with count rate occurs if the voltage droops at high count rates. The average voltage decreases as the count rate increases if the power supply cannot maintain the voltage under the increased average load. This causes the gain to decrease at high count rates, a phenomenon known as gain depression. The gain depression must be characterized and corrected for quantitative measurements at varying count rates. Power supplies with low output impedance and adequate current capability minimize gain depression.
Pulse pileup occurs when two pulses arrive so close together that they cannot be distinguished as separate events. The time resolution for distinguishing pulses depends on the pulse width and the electronics bandwidth. At high count rates, pileup causes loss of counts and distortion of the measured count rate. The power supply pulse response affects the pulse shape, which affects the pileup characteristics. Stable voltage maintains consistent pulse shape, enabling accurate pileup correction.
Afterpulsing from the channel electron multiplier can cause spurious counts following a true pulse. Afterpulses arise from ion feedback, where positive ions created in the channel travel back to the input and release secondary electrons. The curved channel geometry suppresses ion feedback, but some afterpulsing may still occur. The afterpulse rate depends on the operating voltage and the multiplier condition. The power supply voltage stability ensures consistent afterpulse characteristics.
Linearity of the count rate response measures the accuracy of the measured count rate across the operating range. Ideally, the measured count rate equals the true count rate times the detection efficiency. Nonlinearity from gain depression, pulse pileup, and dead time causes the measured rate to deviate from the ideal. The power supply characteristics affect the gain depression contribution to nonlinearity. Specification of the maximum linear count rate defines the operating range for accurate measurements.
Dynamic range, the ratio of maximum to minimum detectable count rate, depends on the noise level at low rates and the linearity limit at high rates. The minimum detectable rate is set by the dark count rate of the multiplier and the noise in the electronics. The maximum rate is set by the linearity limit. The power supply stability and pulse response affect both ends of the dynamic range through their effects on the detection efficiency and the gain stability.
