National Physical Laboratory

Consider the following characteristics of an instrument

manMegaphones

1.      Audio output

Gives an instant indication of radiation or contamination level allowing the surveyor to concentrate on guiding the probe, rather than looking at the display.

2.      Sensitivity

The instrument must be capable of making meaningful measurements at the lowest decision level as determined by the expected background count rate and the expected response to the radiation of interest.

For example:

For example, for alpha contamination, the lowest useful count rate is approx 4 s‾. At this level the chance of there being zero counts in any one second is low. Typically, when surveying for alpha contamination, each count is treated as suspicious. The surveyor will pause, and see whether there is a genuine elevated count rate or whether the click was one of the rare background events.

3.      Time Constant

For operational use, the time constant should not exceed about 4 seconds. This allows the surveyor to pause, wait about 12 seconds and then take an eye average over the following 10 seconds or so.

4.      Statistical fluctuation

This has to be balanced against the time constant.

  • Short time constant will give a quick response but a high level of fluctuation. This makes it difficult to record an eye average.
  • Long time constant will give a relatively fixed reading, but this demands that the surveyor waits a long time between readings and eye averages over a long period.

For any monitoring process, 3 counts per second represents about the minimum level where both an acceptable time constant and an acceptable level of statistical fluctuation can be obtained.

Note:

Note that achieving this balance is intrinsically more difficult with digital instruments. The best digital instruments check whether the recorded count rate has genuinely changed or whether it could represent the same average level. If it appears to have changed then the indication will move to the new value. If it does not then it will be added into the average count rate calculation.

5.      The smallest source – detector distance anticipated

A simple rule in the measurement of dose rate is to try to work no closer than 3 times the maximum detector dimension from the source (due to the inverse square law).

For example:

  • if using a Geiger Muller detector which is 50 mm in length , then reliable readings will be obtained down to a 150 mm source to detector centre distance.
  • if the detector is a 400 cm3 ionisation chamber, with a depth and width of about 80 mm, then the instrument will start to under-read for a source to detector centre distance less than 240 mm.
6.      The permitted averaging area.

Any detector will average over its volume. If a large detector is exposed to a narrow radiation beam, the indication will be much lower than the dose rate in the beam. The best that can usually be done is to choose a suitable detector and then define the averaging area as its area. This approach favours the use of GM tubes.

7.      Fail to danger

ONLY USE EQUIPMENT WITH ‘FAIL TO DANGER’ PROTECTION

Some  instruments may inappropriately display zero when exposed to high dose-rate fields; this could have catastrophic consequences.

It is essential that any instrument type/instrument, has been tested up to the maximum intensity of radiation  it could reasonably encounter to ensure that it continues to function correctly. Failure to danger at high dose rates is unusual in any but the oldest European equipment, but can be a problem with USA and former Soviet Union produced GM instruments in particular.


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