Study on U/Th residual radioactivity in acrylic from surface treatment

TL;DR

This study develops a sensitive method to measure ultra-low levels of U/Th radioactivity in acrylic, crucial for low-background experiments like neutrino observatories, and evaluates surface treatments to minimize contamination.

Contribution

A new measurement technique for sub-ppt U/Th in acrylic and an optimized surface treatment protocol for low-background applications.

Findings

Developed a method to measure U/Th at <10^-12 g/g in acrylic.

Identified effective surface treatments to reduce acrylic radioactivity.

Provided guidelines for acrylic preparation in low-background experiments.

Abstract

Acrylic is widely used as material for the target container in low background experiments due to its high light transparency and low intrinsic radioactivity. However, its surface can be easily contaminated during production, so careful treatment of the surface is essential to avoid direct contamination of the target. The Jiangmen Underground Neutrino Observatory will use about 600~t of acrylic to build the spherical vessel of 35.4~m in diameter for a 20~kt liquid scintillator (LS). Since acrylic will contact the LS directly, the cleanliness of the its surface is quite important for the radiopurity of the LS. A new method for measuring the radioactivity of $^{238}$U and $^{232}$Th in acrylic to sub-ppt ($<10^{-12}$~g/g) was developed, and it is crucial for the acrylic radioactivity screening in this study. We performed many background tests on different surface treatments, and the recommended procedure for the treatment of acrylic to achieve low radioactivity and high light transparency could be applicable to other low background experiments.