The example uses a dual purpose probe which is slightly unusual, but commercially available from a mainstream manufacturer to estimate the activity of contamination from a mixture of ¹⁴C and ^99mTc. The probe combines a thin aluminised melinex window with a thin sheet of CsI scintillator. It is intended mainly for life sciences use, where both low energy betas, such as ¹⁴C, and low energy X or gamma emitters, such as ^99mTc, are in use. Note that this probe is very different to conventional beta scintillation probes, which are often incorrectly referred to as “beta/gamma” probes; the latter have a very low gamma detection efficiency.

The probe is supplied with a cover (absorber) that has a thickness of 1 mm plastic.  A measurement is first made with no cover present and the reading noted,  this measures the emissions from both radionuclides. The measurement is then repeated with the cover in place and the reading noted again. The cover absorbs all of the beta particles from ¹⁴C. For the ^99mTc, a fraction of the 18‑21 keV X-rays will be absorbed and also, to a much lesser extent, the 140 keV gammas. 

For this example, it is assumed that the overall effect of the absorber is to reduce the ^99mTc emissions reaching the detector probe by an amount which equates to a 15% reduction in the reading. This reduction could either be estimated theoretically or experimentally, if facilities are available. Hence we have:

Count rate observed from open window (no cover)2000 s ^-1
Count rate observed with cover (i.e. due to ^99mTc alone)1200 s ^-1

From manufacturer’s data for this detector:
IR(A) for ¹⁴C (no cover)          = 3.5 cps per Bq cm^-2
IR(A) for ^99mTc (no cover)      =  12 cps per Bq cm^-2

From the assumptions made above, the following figures can be adopted for the cover in place:
IR(A) for ¹⁴C (with cover)      = 0.0 cps per Bq cm^-2
IR(A) for ^99mTc (with cover)  =  0.85 x 12 cps per Bq cm^-2 = 10.2 cps per Bq cm^-2

The activity for ^99mTc is calculated from the reading with the cover in place:
1200 / 10.2 = 118 Bq cm^-2

Hence, the ^99mTc contribution to the instrument reading with no cover in place would be:

= 118 x 12      = 1416 cps

And therefore the ¹⁴C contribution to the reading with no cover in place would be:

2000 – 1416    =   584 cps

And the activity of ¹⁴C =  584/3.5 =   167 Bq cm^-2

Greasy surface effect on instrument response

Assume a thin layer of grease covers the surface being measured and that the grease will absorb 40 % of the ¹⁴C beta emissions (see table 2). It can also be assumed that the grease absorbs 5% of the ^99mTc emissions.  The instrument readings will be affected and we now observe:

Count rate observed from open window (no cover, grease)                             1698 s ^-1
Count rate observed with cover (i.e. due to ^99mTc alone, grease)                   1140 s ^-1 

The instrument response factors, corrected for the effects of the grease are:
IR(A) for ¹⁴C (no cover, grease)          = 0.6 x 3.5 cps per Bq cm^-2 = 2.1 cps per Bq cm^-2
IR(A) for ^99mTc (no cover, grease)      = 0.95 x 12 cps per Bq cm^-2 = 11.4 cps per Bq cm^-2

IR(A) for ¹⁴C (cover, grease)       = 0.0 cps per Bq cm^-2
IR(A) for ^99mTc (cover, grease)   =  0.85 x 11.4 cps per Bq cm^-2 = 9.7 cps per Bq cm^-2

Using these values gives a ^99mTc activity from the covered reading of:
1140/9.7 Bq cm^-2 = 118 Bq cm^-2

The instrument reading with no cover in place due to ^99mTc alone would then be 118 x 11.4 cps = 1345 cps.
This gives a ¹⁴C contribution to the no cover reading of 1698 – 1345 cps = 353 cps.
This then leads to a ¹⁴C activity of 353/2.1 Bq cm^-2 = 168 Bq cm^-2