With any radiation detector only some of the emissions from a source or surface are detected. In order to estimate the activity, consideration must be given to what is present but not detected.

The active layer of the source is shown in red

A: Particle/photon emitted from surface (may or may not interact in detector)

B: Particle/photon absorbed in surface grime  (see Table 2)

C: Particle/photon absorbed in active layer (see Table 1)

D: Particle/photon back scattered from substrate (see Table 1)

E: Particle/photon absorbed in substrate.

Figure 4 Schematic of emission processes from the surface of a source

Figure 4 shows what may happen to emissions that originate in the active layer of a source.  Assumptions need to be made and an account needs to be taken of the number of emissions that could not possibly be registered by the detector or in the case of (D), emissions that add to the instrument response and potentially cause an increased estimate of activity. Table 1 shows how some of these effects can be taken into account.

The surface effects illustrated can be categorised into 4 main components:

• Geometry

4π geometry describes the emissions that occur at all angles from the nucleus. 2π geometry describes those emissions from a source that are emitted in the direction of the detector. In the case of a perfect surface, half of the emissions go up and half of them go down.

• Backscatter

The effects of backscatter contribute to the observed reading, particularly if the contaminated surface is a high density material (for example, some floors or stainless steel workbenches). However in practice, no correction is made for this because it only affects photons and high energy betas significantly and it results in an over estimate of the activity. See Table 1.

• Absorption in the active layer

When assessing alpha or low energy beta contamination, consideration must be given to particles that never leave the surface, i.e. self absorption. A reliable estimate of this effect can only be made if the active layer is small in relation to the range of emissions. See Table 1.

• Absorption in surface grime

When assessing alpha or low energy beta contamination, consideration must be given to any coatings (e.g. paint or varnish) or any material that may be deposited on the contaminated surface (e.g. polish or dust) as these will absorb a proportion of the emissions. See Table 2.

By applying a correction for these surface effects to the surface activity calculated for 'ideal' contamination of this radionuclide, it is possible to estimate a more realistic value for the activity of the real contamination.

However, if the 'ideal surface' 2π detector efficiency is used, any action levels set will be too high.

Good practice online modules

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• Practical radiation monitoring
  
• Practical radiation monitoring units
• Radiation monitoring strategy
• Choice of radiation instrument 
• Radiation monitoring techniques  
• Estimating surface radioactivity
• Radiation instrument calibration
• Uncertainties in radiation measurement 
• Troubleshooting in radiation monitoring