Impact of flow rate on sensitivity of semiconductor type thoron monitor

• Thoron sensitivity of all monitors varies with flow rate of gas.
• A theoretical model has been proposed for the variation of sensitivity of the monitor with respect to the flow rate of gas.
• An optimum flow rate of gas through the monitor has been obtained.
• The monitor is compared with standard thoron measurement procedure.

The commercially available continuous thoron monitor, RAD7, applies electrostatic collection of charged thoron progeny followed by alpha spectrometry with a semiconductor detector. Because of the short half life of thoron, the sensitivity of this monitor should vary with the flow rate of gas through it. In the work, direct correlation has been demonstrated between a theoretical model and the measured monitor sensitivity. Comparative thoron measurements were carried out using scintillation cells and double-filter method. It was found that the RAD7 thoron sensitivity increased with increasing flow of gas, reaching a maximum value 33.33 × 10−6 m3 s−1 and decreased thereafter. To obtain improved thoron concentration accuracy at the standard 10.83 × 10−6 m3 s−1 flow rate of the RAD7 monitor, a correction factor was estimated.