Introduction:

The RAD7 is capable of making a direct measurement of thoron gas concentration in air. It does this by counting ²¹⁶Po decays inside the measurement chamber. Thoron, however, has a short half life, 55.6s, so that much of the thoron in the sample will be lost during acquisition if the time from sampling to entering the measurement chamber and then leaving it is as much as a minute or more. It is necessary, therefore, that when measuring thoron the sample acquisition time should be as short as conveniently possible and always the same. Furthermore, for absolute calibrated measurement, exactly the same setup should be used for thoron calibration of the instrument as is used for thoron measurement.

Desiccant

A smoke detector works because the mobility of ions created inside the detector is reduced when they become attached to smoke particles. In a similar way, the mobility of ²¹⁶Po ions inside the RAD7 measurement chamber is reduced in the presence of water molecules. This would reduce the sensitivity of the instrument to thoron. Therefore, for highest efficiency and most precise measurements, it is recommended that desiccant be used to dry the air sample. For thoron measurement, small drying tubes are provided with the RAD7 when originally shipped. These can be refilled with new or regenerated desiccant, or replaced with new tubes when exhausted. Measurement can still be made without desiccant, by using CAPTURE software to download the data and plot readings corrected for humidity.

Thoron Measurement

For a quantitative measurement of thoron it is necessary to use a standardized setup and protocol. The setup assumed in the RAD7 data processing and stated as standard in the manual consists of a small drying tube, used as a wand for sniffing, with a standard input tubing of 36″ (91.4cm) length and ID of 3/16″ (4.8mm), see below. This typically gives the RAD7 a thoron sensitivity about half the radon Sniff sensitivity. The ²¹⁶Po daughter of thoron has only a 145mS half life so the main component in the response time of the RAD7 to a step change in thoron concentration is the time taken to acquire the sample. The response is virtually instantaneous.

Figure 1: Thoron Standard Measurement

Thoron Calibration

To calibrate a RAD7 for thoron it is necessary that the setup be identical to that used for a thoron measurement, that the RAD7 flow rate, V, be the same as that used in a measurement and that the thoron concentration be known at the sampling point. To achieve these criteria is not simple, but it is possible to come very close. The setup is as shown below:

Figure 2: Thoron Calibration

The thoron source is attached close to a T-connector that, itself, is attached close to the end of the small drying tube. The volume of air between the thoron source and the T-connector and between the T-connector and the end of the small tube are sufficiently small that the error from ignoring them is small compared with other uncertaintities in the calibration.

The external pump that is used to inject thoron from the source into the RAD7 sample air flow, at the sampling point, is throttled by the needle valve so that the flow rate through the thoron source , U, is less than the RAD7 sample flow rate, V. When that condition is satisfied, we know that all thoron injected at that sampling point is carried towards the RAD7 and none of it leaks out through the RAD7 fresh air inlet, that will have a fresh air flow rate in to the sampling point of (V – U).

Calculation

If S is the equivalent (actual times emissivity) source strength of the ²²⁴Ra in the thoron source, lambda (λ) the decay constant of thoron and V the RAD7 flow rate, then:

λS = rate of injection of thoron into the RAD7 sampling point, and
λS / V (with consistent dimensions) = Th, the thoron concentration at the sampling point.

If:
S is measured in Bq
λ = 0.0124 s-1
V is flow rate measured in L/min (= V/60 L/s), then

Th = 60. λ.S/V Bq/L = 0.744 S/V Bq/L = 744 S/V Bq/m3

Note that the thoron concentration at the sampling point is inversely proportional to the flow rate, V. Increasing V will increase the RAD7s sensitivity to thoron but may not necessarily increase the reading during calibration.

Conclusion

A ²²⁴Ra thoron source of known strength and known emissivity may be used as described above to calibrate a direct-reading thoron measuring instrument, such as the DURRIDGE RAD7. Because of the significant sources of uncertainty both in the thoron calibration and in thoron measurements due, primarily, to the short half-life of thoron, conservative pessimism should be exercised in assessing the accuracy of the result.

With a precision, nitrogen-cooled gamma spectrometer it is possible to determine the strength of a ²²⁴Ra source. With some care, the same device can be used to determine the emissivity of the source, though this will be a function of humidity in the air stream. By this means, the absolute equivalent strength of the thoron source may be determined.

With such a source it becomes possible to calibrate an instrument for thoron. To use a calibration sensitivity, the instrument setup must be identical to that used in the calibration. Furthermore, the air flow rate must be the same as when the instrument was calibrated, so a flow meter is desired for the most precise work.

With an uncalibrated thoron source, it is still possible to compare the thoron sensitivity of two different instruments and, if the thoron source is fully supported and in equilibrium, to monitor the sensitivity of a single instrument over a period of time.

For most applications, when using thoron as a short-lived tracer, as in locating radon entry points in a building, an absolute thoron calibration is irrelevant in that relative values of thoron concentration are all that are required.