Abstract:
The paper describes a novel technique to harvest Rn-222 laden air from soil gas of natural origin as a highly efficient source of Rn-222 for calibration applications in a walk-in type Rn-222 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. Rn-222 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 m(3) 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 Rn-222 due to decay and leak is discussed. Harvesting soil gas has many important advantages over the traditional methods of Rn-222 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 Rn-222, very convenient to use unlike the high strength Ra-226 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 Rn-222, 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.