An innovative technique of harvesting soil gas as a highly efficient source of 222Rn for calibration applications in a walk-in type chamber: part-1

Abstract

The paper describes a novel technique to harvest 222Rn laden air from soil gas of natural origin as a highly efficient source of 222Rn for calibration applications in a walk-in type 222Rn calibration chamber. The technique makes use of a soil probe of about 1 m to draw soil gas, through a dehumidifier and a delay volume, using an air pump to fill the calibration chamber. 222Rn concentration in the range of a few hundred Bq m−3 to a few tens of kBq m−3 was easily attained in the chamber of volume 22.7 m3 within a short pumping duration of 1 h. A new technique referred to as “semi-dynamic mode of operation” in which soil gas is injected into the calibration chamber at regular intervals to compensate for the loss of 222Rn due to decay and leak is discussed. Harvesting soil gas has many important advantages over the traditional methods of 222Rn generation for calibration experiments using finite sources such as solid flow-through, powdered emanation, and liquid sources. They are: (1) soil gas serves as an instantaneous natural source of 222Rn, very convenient to use unlike the high strength 226Ra sources used in the calibration laboratories, and has no radiation safety issues, (2) does not require licensing from the regulatory authority, and (3) it can be used continuously as a non-depleting reservoir of 222Rn, unlike other finite sources. The newly developed technique would eliminate the need for expensive radioactive sources and thereby offers immense application in a variety of day to day experiments—both in students and research laboratories.

Development of a “222Rn incremented method” for the rapid determination of air exchange rate using soil gas

“…As discussed just above, 222 Rn present in soil gas is used as a tracer for the determination of AER. Soil gas is a non-depleting reservoir of 222 Rn and is capable of providing steady concentration flux even when drawn continuously for a long time duration, as demonstrated in the previous publications ( Karunakara et al, 2015 , 2020 ; Shetty et al, 2020 ). The details of soil gas 222 Rn extraction were described in our earlier publications ( Karunakara et al, 2015 , 2020 ; Sudeep et al, 2012 ; Shetty et al, 2020 ).…”

“…Soil gas is a non-depleting reservoir of 222 Rn and is capable of providing steady concentration flux even when drawn continuously for a long time duration, as demonstrated in the previous publications ( Karunakara et al, 2015 , 2020 ; Shetty et al, 2020 ). The details of soil gas 222 Rn extraction were described in our earlier publications ( Karunakara et al, 2015 , 2020 ; Sudeep et al, 2012 ; Shetty et al, 2020 ). A soil gas probe (STITZ, Genitron Instruments, Germany) was inserted inside the ground to a depth of 0.8–1 m since the 222 Rn concentration in soil gas increases exponentially with depth and saturates to an equilibrium concentration at a depth greater than about 0.8–1 m ( Nazaroff and Nero, 1988 ; Al-Azmi and Karunakara., 2007 ; Al-Azmi, 2009 ; Sudeep et al, 2012 ; Katalin et al, 2013 ; Shetty et al, 2019 ; 2020 ; Karunakara et al, 2015 , 2020 ).…”

“…Measurements were performed in 15 workplaces and 11 dwellings using either scintillation or ionization-based monitors, or both instruments simultaneously depending on the availability. The duration of measurement in each location varied from 20 to 30 d. For these measurements, the instruments were operated in diffusion mode in which the 220 Rn does not interfere with 222 Rn measurements since the former is discriminated from entering the detector active volume ( Karunakara et al, 2015 , 2020 ; Sudeep et al, 2012 ; Shetty et al, 2020 ). A good agreement between the measurements recorded using these two types of instruments was ascertained by subjecting the 222 Rn concentration data sets obtained from the instruments to statistical analysis.…”

“…The details of soil gas 222 Rn extraction were described in our earlier publications ( Karunakara et al, 2015 , 2020 ; Sudeep et al, 2012 ; Shetty et al, 2020 ). A soil gas probe (STITZ, Genitron Instruments, Germany) was inserted inside the ground to a depth of 0.8–1 m since the 222 Rn concentration in soil gas increases exponentially with depth and saturates to an equilibrium concentration at a depth greater than about 0.8–1 m ( Nazaroff and Nero, 1988 ; Al-Azmi and Karunakara., 2007 ; Al-Azmi, 2009 ; Sudeep et al, 2012 ; Katalin et al, 2013 ; Shetty et al, 2019 ; 2020 ; Karunakara et al, 2015 , 2020 ). A pump with adjustable flow rates (10–80 L min −1 , depending upon the experimental requirements) was connected to the probe to draw the soil gas and inject it into the workplace as shown in Fig.…”

“…Soil gas was drawn through this arrangement and injected into the indoor workplace. The use of soil gas as a source of 222 Rn has several advantages and was discussed in Karunakara et al (2020) and in brief, they are (i) 222 Rn is naturally available in the soil gas as a source, (ii) soil gas is capable of supplying steady 222 Rn concentration over a long period without any dilution in the concentration, (iii) there is no need of regulatory approvals from national regulatory bodies for the use of soil gas, and (iv) the health risk associated with a short-term injection of 222 Rn into the dwelling is insignificant.

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