|
Symptom |
Possible cause |
Action |
| 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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