Occasionally, you may encounter problems with your LMI instrument or detector that may be repaired or resolved in the field, saving turnaround time and expense in returning the instrument to us for repair. Toward that end, LMI electronics technicians offer the following tips for troubleshooting the most common problems. Where several steps are given, perform them in order until the problem is corrected. You should keep in mind that the three most common problems are: 1. detector cables, 2. sticky meters, 3. battery contacts.
Note that the first troubleshooting tip is to determine whether the problem is with the electronics or with the detector. A Ludlum Model 500 pulser can be invaluable at this point, because you can simultaneously check high voltage, input sensitivity or threshold, and check the electronics for counting.
Electronics Utilizing a GM Detector, Scintillator, or Proportional Detector
| Symptom | Possible Solution |
|---|---|
| No power (or meter does not reach BAT TEST or BAT OK mark) |
|
| Nonlinear readings |
|
| Meter goes full-scale or "pegs out" |
|
Common Detector Problems
The following are the most common detector problems, by detector category.
GM Detectors
- If the tube has a thin mica window, check for window breakage. If damage is evident, the tube must be replaced.
- Check the HV. For most GM tubes, the voltage is normally 900 Vdc, or 460 to 550 Vdc for "peanut" tubes (Ludlum Model 133 series).
- Input sensitivity is normally 35 mV. If the input sensitivity is too low, the user could see some double-pulsing.
- Wires to the tube may be broken or the crimped connector could have a loose wire.
Scintillator Detectors
- Alpha or alpha-beta scintillators are prone to light leaks. They can be tested for this problem in a dark room or with a bright light. If a light leak is determined, changing the mylar window assembly will usually fix the problem. NOTE: When replacing the window, make sure to use a window made with the same thickness mylar and the same number of layers as the original window.
- Verify that the HV and input sensitivity are correct. Beta scintillators will typically operate at 35 mV input sensitivity (or 2 to 4 mV for the beta channel of an alpha-beta scintillator), while alpha and gamma scintillators operate from 10 to 35 mV. High voltage varies with the photomultiplier tube (PMT) from as low as 600 Vdc to 1400 Vdc.
- On a gamma scintillator, visually inspect the crystal for breakage or humidity leakage. Water inside the crystal will turn it yellow and gradually degrade performance.
- Check the PMT to see if the photocathode still exists. If the end of the PMT is clear (not brownish), this indicates a loss of vacuum which will render the PMT useless.
Proportional Detectors
- Check the HV and input sensitivity settings. In gross counting of alpha particles, gas proportional detectors normally operate at 1250 Vdc and 4 mV threshold. In gross counting of beta or alpha and beta particles, gas proportional detectors normally operate at 1650 Vdc and 4 mV threshold. In simultaneous counting of alpha and beta particles, the HV is normally about 1600 Vdc, the alpha threshold is normally 120 mV, and the beta threshold and window are normally 3.5 mV and 30 mV. Neutron 3He detectors typically require a 2 mV threshold and about 1700 Vdc. Neutron BF3 detectors typically operate at 1750 Vdc and 30 mV.
- Gas proportional detectors need P-10 gas, so check the window for tears or leaks and ensure an adequate supply of gas.
- If the window is torn, frequently the anode wires are also broken and shorting against the detector. Replace broken wires, clean with lacquer thinner, then bake at 200 °F.
- Humidity can be a problem for air proportional detectors. Dry and/or check the desiccants.
Contact Us
We hope these tips will prove to be helpful. As always, please contact our repair department if you have difficulty in resolving a problem, or if you have questions.