Abstract
In an adjoining publication, we demonstrated the novel technique to harvest soil gas of natural origin as a highly efficient source of 222Rn for calibration applications in a large volume 222Rn calibration chamber. Its advantages over the use of conventional high strength 226Ra sources, such as the capability to serve as a non-depleting reservoir of 222Rn and achieve the desired concentration inside the calibration chamber within a very short time, devoid of radiation safety issues in source handling and licensing requirements from the regulatory authority, were discussed in detail. It was also demonstrated that stability in the 222Rn concentration in large calibration chambers could be achieved within ± 20% deviation from the desired value through a semi-dynamic mode of injection in which 222Rn laden air was periodically pumped to compensate for its loss due to leak and decay. The necessity of developing a theory for determining the appropriate periodicity of pumping was realized to get good temporal stability with a universally acceptable deviation of ≤ ± 10% in the 222Rn concentration. In this paper, we present a mathematical formulation to determine the injection periods (injection pump ON and OFF durations) for the semi-dynamic operation to achieve long term temporal stability in the 222Rn concentration in the chamber. These computed pumping parameters were then used to efficiently direct the injection of soil gas into the chamber. We present the mathematical formulation, and its experimental validations in a large volume calibration chamber (22 m3). With this, the temporal stability of 222Rn concentration in the chamber was achieved with a deviation of ~ 3% from the desired value.
Highlights
In an adjoining publication, we demonstrated the novel technique to harvest soil gas of natural origin as a highly efficient source of 222Rn for calibration applications in a large volume 222Rn calibration chamber
Testing and calibration of 222Rn detectors in large volume walk-in type calibration chambers are carried out, mainly by three operational modes: (1) dynamic mode—in which 222Rn rich air from the source is continuously pumped to the chamber through the inlet and simultaneously maintaining the outlet in the open condition to attain the concentration levels[3,4,5], (2) static mode—in which 222Rn laden air is filled once to achieve desired initial concentration value and the experiment is performed during the decay of 222Rn5–7, and (3) semi-dynamic mode—in which, initially 222Rn is injected into the chamber to obtain the desired value and switched over to a pulsed mode of injection so that concentration is maintained within a certain band of the desired value
Higher deviation than the acceptable limit was because the periodicity of pumping was selected (1) by monitoring the concentration in the chamber using active monitors which may have some delayed response, (2) based on the knowledge of soil gas 222Rn concentration and the chamber volume, and (3) total 222Rn outflow from the chamber[8] and not based on a theoretical model involving the functional behaviour of various parameters affecting the 222Rn concentration profile in the calibration chamber
Summary
Inhalation of 222Rn, 220Rn and its decay products accounts for more than half of the annual effective dose from radiation sources of natural origin. We present a mathematical formulation to determine the exact injection periods (Pump ON and OFF durations) for 222Rn laden air for the semi-dynamic operation to achieve good long-term temporal stability in the 222Rn concentration in the chamber. These computed pumping parameters are used to efficiently direct the injection of soil gas into the chamber. The experimental validations of the theoretical model and achievement of very good stability in 222Rn concentration in the chamber are discussed here
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