National Physical Laboratory

Direct measurement of surface contamination

manMegaphonesDetect contamination by listening to the audio output of the monitor. Concentrate on moving the detector over the surface of interest at the appropriate speed and probe to surface spacing - important because the majority of contamination detectors are  extremely fragile and the surface-to-probe spacing can be critical.

Direct contamination monitoring provides a reasonable assessment of the contamination level, but for more accurate measurements there may be a need to take samples of the surface for laboratory analysis. (Not covered  by this module)

Points to consider:

1. Nature and location of the contamination

The measurement will be affected by the following:

  • Surface absorbency
  • Presence of water/grease/dirt/paint/polish on the surface
  • Type and energy of radiation
  • Time constant of instrument
  • Distribution of potential contamination
  • Other radiation sources in the vicinity.  There may be a need to remove sources or shield an area before the contamination monitoring is started.

Note:
See module on 'Estimating surface activity for examples of how it affects instrument reading'.

2. The averaging area

For body measurements, contamination should be averaged over an area of 300 cm2 or less, except when doses to skin are being assessed when the averaging area is then 1 cm2.

For other surfaces, contamination should be averaged over an area of 1000 cm2 or less.

3. The detector to surface distance of measurements

Alpha and low energy β emissions will only be detected if the probe to surface distance is not greater than a few millimetres.

Where this is impossible to achieve because the surface is uneven, consider indirect options such as dry or wet wipes.

Where the expected contaminant has a low range, the probe must be held close to the surface being monitored – a distance of 3 mm is standard but is difficult to achieve with any accuracy.  Never let the probe touch the surface as this might cause it to become contaminated.

Where contamination is expected to be low level and of sufficient energy, one practical solution...

... to the problem is to use a spacer or gloved finger to regulate the distance of the probe from the surface, providing the area is smooth.  Hold the probe so that one gloved finger at the trailing edge of the probe touches the surface, supporting the probe at about 10 mm above it.  The probe is then moved over the area of interest but always with the supporting finger trailing so that it only touches areas of the surface that have already been monitored.  Take care not to touch any surfaces that are obviously contaminated.  Check regularly that contamination is not building up on the glove.  This method would not be possible if attempting to measure alpha  and low level beta contamination due to their low range.

The monitoring distance can be increased for a contaminant of energetic β particles or Χ, γ rays. The practical effect of this is to increase the averaging area.

4. The speed of monitoring

Monitoring speed, that is the speed at which a probe can be moved over a surface to ensure reliable assessment of contamination levels, needs to be carefully assessed prior to any survey. Many factors affect monitoring speed. 

Speed of measurement with a contamination monitor

Monitoring speed, that is the speed at which a probe can be moved over a surface to ensure a reliable assessment of contamination levels, needs to be carefully assessed prior to any survey.  Many factors affect monitoring speed but in perfect conditions monitoring speeds can be much faster than intuition would suggest.  The reasons for this are given below.

Monitoring for beta contamination

A typical Derived Limit (DL) or maximum acceptable level for β contamination is 4 Bq cm-2.  A typical probe will have an active area of 100 cm2 and a response of 10 to 20 s-1 Bq-1 cm2 (dependent on β energy) to b radiation.  Hence the probe response to a DL will be 40 to 80 s‑1.

Assume a ratemeter is a recycling scaler, i.e. one that counts for a fixed period, displays the answer in counts per second and then restarts, and displays the answer at the end of each counting period.  If the counting time is n seconds, the probe width is x cm and the permitted averaging area is A cm2 then the maximum monitoring speed is given by A/nx.

If typical values of n =2, x =7 and A=1000 are taken, then the maximum speed is 70 cm s-1, which is very fast indeed.

If a large area of 1000 cm2 were to be monitored and the maximum acceptable level for β contamination spread evenly over it was 4 Bq cm-2, the activity that must be present to exceed this limit would be (4 x 1000) = 4000 Bq, or 4 kBq. The probe, having a response of 10 – 20 s-1 Bq-1 cm2 would undoubtedly produce a clear response. On the other hand, 4 kBq could be localised in a small area.  Although the probe passes rapidly over this area, the concentration of activity is such that the probe will still respond.

Monitoring for alpha contamination

A typical derived limit, or maximum acceptable level, for a contamination is 0.4 Bq cm-2. A typical probe will have an active area of 100 cm2 and An alpha response of 16 s-1 Bq-1 cm2.  Hence the probe’s response to a derived limit of alphas will be about 6 s-1 (0.4 x 16). This is significantly more than the normal background rate (<1 s-1).

A good surveyor will pause on each beep when working at low levels. If the expected countrate at the maximum acceptable level is 6 s-1 then the chance of  'no beeps' being heard  is very small.  Therefore the whole 1000 cm2 could be swept in one second and the speed would be 100 cm s-1, based on a 10 cm probe width.  The probe would either pass over 400 Bq uniformly distributed across 1000 cm2, or pass over a small area in which 400 Bq of α contamination was concentrated (i.e. the equivalent of a small α source).  Either way, it is almost certain the probe will register the presence of a contamination.

However, if the derived limit is reduced to 0.04 Bq cm-2 then the maximum acceptable average count rate drops to 0.6 s-1.  To guarantee at least one count in each period, a 10 second count is required on the area of interest to give a mean of 6 counts. If the averaging area is only 300 cm2 (e.g. the averaging area when monitoring the human body) the sweep speed drops to virtually nothing, 300/10x10, or 3 cm s-1.  In practice this will require the surveyor to do a stationary reading for 10 seconds and then move one probe dimension for the next reading and so on, in order to ensure that a contamination levels are less than 0.04 Bq cm-2.

In conclusion, monitoring speeds are very dependent on the derived limits. If the derived limit is set so that the probe response to the derived limit is equivalent to the probe/ratemeter background count then monitoring to clearance levels will be painfully slow. If, however, the derived limits are set so a good response can be expected from the probe then clearance monitoring can be surprisingly quick.

5. Skin/clothing material being monitored

Care should be taken when attempting to assess α or low energy β contamination as complete absorption of the radiation could occur in the top few layers of skin or within the fibres of a material.

The absorption rate is affected by:

  • Particle size
  • Clothing material.

Please also read module ‘Estimating surface activity’.

6. Recording measurements

  • All relevant measurements required by the RPA or RPS should be recorded.
  • If short lived radionuclides are being measured, the time of measurement will also be important.
  • If decontamination is carried out or contamination removed, it may be necessary to provide pre- and post- monitoring results.
  • Staff who carry out monitoring must be aware of the action levels determined for each area and know what to do if these levels are exceeded (as determined when planning the monitoring strategy)

Before leaving the area

When leaving the potentially contaminated area remove any protective clothing and monitor yourself in accordance with local rules.

Carry out post-survey instrument check

Immediately after completing the measurement carry out post-survey checks.

If any of the above checks indicate a problem, the survey may need to be repeated.

 

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