Low background materials and fabrication techniques for cables and connectors in the Majorana Demonstrator

Abstract

The Majorana Collaboration is searching for the neutrinoless double-beta decay of the nucleus 76 Ge. The Majorana Demonstrator is an array of germanium detectors deployed with the aim of implementing background reduction techniques suitable for a tonne scale 76 Ge-based search (the LEGEND collaboration). In the Demonstrator, germanium detectors operate in an ultra-pure vacuum cryostat at 80 K. One special challenge of an ultra-pure environment is to develop reliable cables, connectors, and electronics that do not significantly contribute to the radioactive background of the experiment. This paper highlights the experimental requirements and how these requirements were met for the Majorana Demonstrator, including plans to upgrade the wiring for higher reliability in the summer of 2018. Also described are requirements for LEGEND R&D efforts underway to meet these additional requirements

The Majorana Demonstrator readout electronics system

“…Above the cold plate, custom connectors [21] are needed to meet M D 's background specifications. Electrical contact springs in commercially available connectors typically contain beryllium-copper (BeCu), which has unacceptably high radioactivity.…”

The MAJORANA DEMONSTRATOR Readout Electronics System

“…On the vacuum side of the flange, the signal cables are soldered to D-Sub sockets using low-radioactivity solder (one for the central conductor, and one for the shield), which are then inserted to the D-Sub receptacles on the flange. Above the cold plate, custom connectors [21] are needed to meet M D 's background specifications. Electrical contact springs in commercially available connectors typically contain beryllium-copper (BeCu), which has unacceptably high radioactivity.…”

Ultra-low radioactivity Kapton and copper-Kapton laminates

“…Determinations of Ca and P were performed using a Thermo iCAP7600 ICP Optical Emission Spectrometer (OES) Duo (Thermo Scientific, Waltham, MA) equipped with standard quartz sample introduction. The instrument was calibrated using dilutions of 1000 ppm single element standards (Inorganic Ventures, Christianburg, VA) in 2% HNO 3 . Calcium was calibrated from 5-500 ng/g and phosphorus from 0.1-10 µg/g.…”

“…Signal sensors and their associated cabling and readout electronics are often a significant contributor to the radioactive background budget of rare-event experiments such as searches for neutrinoless double beta decay or the direct detection of dark matter. [1][2][3][4][5][6][7]. Circuitry and cables are typically composed of two major components -the conductor (typically copper) and insulator.…”

“…While extremely radiopure copper can be obtained [8], commercial Kapton is not a very radiopure material, with measured contamination levels of roughly 1400 ppt 238 U (1.7×10 4 µBq/kg) [9], leading to high-radioactivity components. Low-background experiments therefore have to either limit the amount of Kapton used to the absolute minimum necessary [1], or use other materials [3,4,10] that, while more radiopure, do not have all the advantageous properties of Kapton. Additionally, since Kapton is an industry standard for electronics, the use of alternative material often requires custom-made components, which can increase costs, risk, and production time.…”