Ultra-low radioactivity Kapton and copper-Kapton laminates

TL;DR

This study evaluates the radiopurity of Kapton and copper-Kapton laminates, identifying manufacturing additives as contamination sources and presenting significantly cleaner alternatives suitable for ultralow background physics experiments.

Contribution

It identifies the contamination sources in commercial Kapton and introduces alternative, more radiopure materials and laminates for use in sensitive physics applications.

Findings

Commercial Kapton contains significant radioactive contaminants.

Alternative Kapton without DCP shows much lower radioactivity levels.

Copper-Kapton laminates with low radioactivity suitable for ultralow background experiments.

Abstract

Polyimide-based materials, like Kapton, are widely used in flexible cables and circuitry due to their unique electrical and mechanical characteristics. This study is aimed at investigating the radiopurity of Kapton for use in ultralow background, rare-event physics applications by measuring the $^{238}$U, $^{232}$Th, and $^{nat}$K levels using inductively coupled plasma mass spectrometry. Commercial-off-the-shelf Kapton varieties, measured at approximately 950 and 120 pg/g $^{238}$U and $^{232}$Th (1.2$\times$10$^4$ and 490 $\mu$Bq/kg), respectively, can be a significant background source for many current and next-generation ultralow background detectors. This study has found that the dominant contamination is due to the use of dicalcium phosphate (DCP), a nonessential slip additive added during manufacturing. Alternative Kapton materials were obtained that did not contain DCP and were determined to be significantly more radiopure than the commercially-available options with 12 and 19 pg/g $^{238}$U and $^{232}$Th (150 and 77 $\mu$Bq/kg), respectively. The lowest radioactivity version produced (Kapton ELJ, which contains an adhesive) was found to contain single digit pg/g levels of $^{238}$U and $^{232}$Th, two-to-three orders of magnitude cleaner than commercial-off-the-shelf options. Moreover, copper-clad polyimide laminates employing Kapton ELJ as the insulator were obtained and shown to be very radiopure at 8.6 and 22 pg/g $^{238}$U and $^{232}$Th (110 and 89 $\mu$Bq/kg), respectively.