Precise Measurement and Control of Radon Progeny on Detector Surfaces

Abstract

In low-background particle physics experiments, surface deposition of radon progeny presents a significant background challenge. To characterize this contamination, a high-sensitivity surface $α$-activity measurement system was developed, which employs a 3$\times$3 Si-PIN array operating in vacuum to perform $α$-spectroscopy on samples. The system was calibrated using Poly(Methyl MethAcrylate) (PMMA) plates exposed to a controlled high-radon atmosphere, achieving an energy resolution of 2.09 \% for 5.30~MeV $α$ particles and a one-day measurement sensitivity of 1.27~$μ$Bq/cm$^2$ for $^{210}$Po surface activity. Using this system and a self-built high radon concentration chamber, the deposition behavior of radon progeny on PMMA surfaces was investigated. Results indicate a non-monotonic dependence on exposure time, a significant enhancement of deposition with increasing negative surface electrostatic potential, and a strong modulation by ambient humidity. This paper details the apparatus design, calibration, and experimental study of radon progeny deposition dynamics on PMMA surfaces.

Figures (12)

• Figure 1: Schematic Diagram of $^{222}$Rn decay chain.
• Figure 2: A picture of the Surface $\alpha$ Activity Measurement System.
• Figure 3: A detailed view of the sample chamber.
• Figure 4: Sample waveforms recorded by the oscilloscope.
• Figure 5: A photograph of the radon chamber.