National Physical Laboratory

Common failure modes for monitors

Man wonderingGeneral Problems

  • Poor battery contact, leading to erratic operation. It is important that only one type of battery is used, as battery lengths vary considerably between types and manufacturers brands. Fitting a short-lived battery after a long-lived one may lead to poor contact. Flat spring connectors are more prone to failure than coil spring designs. Battery contact corrosion is also a problem in old or badly maintained units, especially where batteries have leaked.
  • Cables and connectors may also cause problems. The PET connector, which is standard on UK instruments, and the SHV connector are not strong enough to support the weight of a probe on a regular basis. Damage can lead to intermittent contact or high background count rates. Cables can also be damaged by pulling them too tight, cutting them on sharp corners or closing them in instrument carrying case lids.
  • Conventional meters can also be damaged by impact. The glass can smash or the free movement of the needle can be restricted. Impact can damage instrument cases and circuit boards and can also generate cracks in glass to metal seals in Geiger Muller detectors.
  • Internal structure of photomultiplier tubes is vulnerable to damage, as is the optical coupling with the scintillator. It is important to use the correct optical coupling liquid as otherwise this can lead to many problems not least incorrect sensitivity.
  • Contamination of monitors can lead to contamination of the user and the spread of contamination generally. The normal indication that an instrument is contaminated is a high background count rate. This is a particular problem when monitoring for a contamination, where a very low background is essential.

Specific problems

  • Ion chambers
    The air inside an ion chamber must be kept dry. If the desiccant becomes exhausted then the background tends to increase, (either positive or negative), and the level of fluctuation increases. The desiccant should be dried according to the manufacturer’s instructions. The instrument should also be dried by placing it in a warm (+40 ºC) dry place for several hours.
  • End-window Geiger Muller detectors
    These are very vulnerable to damage by contact with anything that is at all rigid, such as tools, swarf and grass stems. The detectors can implode and replacement of the whole detector is the only option. The conductive coating (DAG) may flake or inadvertently get removed by solvents, this will affect the performance of the detector.
  • Thin-window alpha and beta scintillation detectors
    Damage to these generally results in a light leak, leading to high background count rates or whistling noises. The foil can be replaced relatively easily. Photo- multiplier tubes are susceptible to magnetic interference.
  • Sealed thin-window proportional counters
    These do not implode, but gradually lose their sensitivity if punctured. Total replacement is the only option but this is expensive.
  • Sodium iodide scintillation detectors
    Window damage leads to light leakage or the formation of yellow patches where water has entered (which effectively increases the energy threshold and reduces the sensitivity). Impact can result in shattering of the crystal, leading to a very marked loss of sensitivity. Photo-multiplier tubes are susceptible to magnetic interference.

 

Arrow Right Arrow Left

3 of 4