Design of a high-resolution ion pulse ionization chamber for 222Rn decays detection in air

TL;DR

Radon in ambient air poses health risks and requires accurate, timely monitoring. The authors design a double-cylinder coaxial impulse ionization chamber (CIPIC) that yields energy-proportional signals from alpha decays and spectroscopically resolves $^{222}\mathrm{Rn}$, $^{218}\mathrm{Po}$, and $^{214}\mathrm{Po}$. The chamber volumes of $7.7$ L and $8.8$ L enable high statistics, achieving spectroscopic resolutions of $2-3\%$ and measurement uncertainties below $5\%$ in $15$ minutes at $50$ Bq/m$^3$, with SI-traceable calibration via PTB. Long-term tests demonstrate stable decay tracking (e.g., a $^{222}\mathrm{Rn}$ half-life of $4$ days) and reliable performance at low concentrations (e.g., $24$ Bq/m$^3$ with $\pm 2$ Bq/m$^3$ uncertainty). This work provides a practical, SI-traceable approach for fast, direct radon monitoring in indoor and environmental settings.

Abstract

Radon is a naturally occurring radioactive gas that contributes significantly to human radiation exposure and must be controlled to avoid concentrations harmful to health. The paper presents an impulse-proportional ionization chamber that is suitable for the direct measurement of low radon concentrations in ambient air and achieves spectroscopic resolutions of 2-3%. This accuracy can be achieved through two novel principles. Firstly, the double-cylindrical, coaxial design of the IC allows for efficient, nearly complete detection of $α$-radiation. Secondly, a customized measurement method for spectroscopic evaluation was developed to discriminate between the proportions of 222Rn, 218Po, and 214Po and extract their concentrations. Particular attention was paid to the energy resolution of the detection system by suppressing the effects of acoustic and vibration noise on the detector's operation. The high spectral resolution of the developed ionization chamber, with working volumes of 7.7l and 8.7l, enables measurements with uncertainties of less than 5\% at 15-minute measurement times in ambient air with $50\,\mathrm{Bq/m^3}$ radon activity.

Figures (7)

• Figure 1: CAD drawing showing a cut-through of the cylindrical impulse-proportional ionisation chamber (CIPIC I)
• Figure 2: CIPIC II, its multi-wire cathode and the housing of the chamber
• Figure 3: Signal path from detector to digital acquisition
• Figure 4: Artificially generated stress pulses (left) sampled with our signal path and the resulting histogram of energy pulses (right) in the ratio of 1:2:3
• Figure 5: Histogram of a long-term measurement of 4915836 counts within 60 days at approx. $\mathrm{100\,Bq/m^3}$