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Possible cause


No spectrum in MCA, but some counts present Low activity, wrong display scaling Alter display to log
Improperly set ADC Check digital offset, LLD & ULD on ADC, and conversion gain
Improperly set amp: peaks outside range Check amplifier output with 'scope'; adjust gain to give signal of proper amplitude
No count in MCA Source activity too high Check high rate LED on preamp; move source away from detector
Pulses are of wrong polarity Check amplifier pulses are positive
No amplifier output No input to amp Check preamp output with scope to confirm (50mV/cm)
Check power supply cable to preamp
Amplifier faulty Check settings and connections
Put pulse generator pulse through preamp test input
No preamp output at working bias. High count rate LED flashes Source activity too high Move source away from detector
Excessive detector leakage current Decrease bias in 100V steps until output pulses appear
No preamp output at any bias. High count rate LED flashes Power supply failure
Preamp failure
All peaks have low energy tail Pole zero correction wrong Turn PZ clockwise to bring tailing edge up to baseline; vice versa for a high energy tail
Poor charge collection HV too low, check HV unit output
Shaping time too short; increase time constant or use gated integrator
All peaks have high energy tail to baseline Pole zero correction wrong Turn PZ counterclockwise to bring trailing edge down
Pile up Increase source-detector distance
Switch in PUR
True coincidence summing Increase source-detector distance
Poor resolution in gamma peaks. Wobbling baseline on oscilloscope Baseline restorer not working Check BLR; optimise threshold
Poor resolution in gamma peaks and pulser. Sinusoidal baseline oscillation Noise on baseline Eliminated by filtering or isolating
RF pickup Use common mode rejection
Use extra EMI shielding
Reroute cabling
Move VDU away
Move/replace HV bias supply
Ground loop Ensure one effective common ground
Check/replace cabling
Poor resolution. Ragged baseline on scope with one or more of the following: positive spikes, negative spikes, square positive pulses, square negative pulses Breakdown across surface of detector or an insulator Check value of bias supply
Reduce bias in 100V stages until baseline returns to normal
Wandering peaks Amplifier gain drift Repair/replace amplifier
Other faulty electronics Check cables/connectors for intermittent connection of signals or ground 
Detector-preamplifier connection loose Tap gently on detector to induce shift
Clean preamplifier contacts if possible

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