Cryogenic safety : a guide to best practice in the lab and workplace

Abstract Flexible printed cables and circuitry based on copper-polyimide materials are widely used in experiments looking for rare events due to their unique electrical and mechanical characteristics. However, past studies have found copper-polyimide flexible cables to contain 400-4700 pg 238 U/g, 16-3700 pg 232 Th/g, and 170-2100 ng nat K/g, which can be a significant source of radioactive background for many current and next-generation ultralow background detectors. This study presents a comprehensive investigation into the fabrication process of copper-polyimide flexible cables and the development of custom low radioactivity cables for use in rare-event physics applications. A methodical step-by-step approach was developed and informed by ultrasensitive assay to determine the radiopurity in the starting materials and identify the contaminating production steps in the cable fabrication process. Radiopure material alternatives were identified, and cleaner production processes and treatments were developed to significantly reduce the imparted contamination. Through the newly developed radiopure fabrication process, fully-functioning cables were produced with radiocontaminant concentrations of 20-31 pg 238 U/g, 12-13 pg 232 Th/g, and 40-550 ng nat K/g, which is significantly cleaner than cables from previous work and sufficiently radiopure for current and next-generation detectors. This approach, employing witness samples to investigate each step of the fabrication process, can hopefully serve as a template for investigating radiocontaminants in other material production processes. 

pdf/ultra-low-radioactivity-flexible-printed-cables-2orzk926z6.pdf

Ultra-low radioactivity flexible printed cables

Flexible printed cables and circuitry based on copper-polyimide materials are widely used in experiments looking for rare events due to their unique electrical and mechanical characteristics. However, past studies have found copper-polyimide flexible cables to contain 400-4700 pg $^{238}$U/g, 16-3700 pg $^{232}$Th/g, and 170-2100 ng $^{nat}$K/g, which can be a significant source of radioactive background for many current and next-generation ultralow background detectors. This study presents a comprehensive investigation into the fabrication process of copper-polyimide flexible cables and the development of custom low radioactivity cables for use in rare-event physics applications. A methodical step-by-step approach was developed and informed by ultrasensitive assay to determine the radiopurity in the starting materials and identify the contaminating production steps in the cable fabrication process. Radiopure material alternatives were identified, and cleaner production processes and treatments were developed to significantly reduce the imparted contamination. Through the newly developed radiopure fabrication process, fully-functioning cables were produced with radiocontaminant concentrations of 20-31 pg $^{238}$U/g, 12-13 pg $^{232}$Th/g, and 40-550 ng $^{nat}$K/g, which is significantly cleaner than cables from previous work and sufficiently radiopure for current and next-generation detectors. This approach, employing witness samples to investigate each step of the fabrication process, can hopefully serve as a template for investigating radiocontaminants in other material production processes.

pdf/ultra-low-radioactivity-flexible-printed-cables-1l02shv3.pdf

Cryocooled cold trap system for the SuperCDMS dilution refrigerator

Potential of thermal protection improvements in large-scale cryostat development

The Super Cryogenic Dark Matter Search (SuperCDMS) experiment is a direct detection dark matter experiment intended for deployment to the SNOLAB underground facility in Ontario, Canada. With a payload of up to 186 germanium and silicon crystal detectors operating below 15 mK, the cryogenic architecture of the experiment is complex. Further, the requirement that the cryostat presents a low radioactive background to the detectors limits the materials and techniques available for construction, and heavily influences the design of the cryogenics system. The resulting thermal architecture is a closed cycle (no liquid cryogen) system, with stages at 50 and 4 K cooled with gas and fluid circulation systems and stages at 1 K, 250 mK and 15 mK cooled by the lower temperature stages of a large, cryogen-free dilution refrigerator. This paper describes the thermal design of the experiment, including details of the cooling systems, mechanical designs and expected performance of the system under operational conditions.

/pdf/the-cryogenics-design-of-the-supercdms-snolab-experiment-3yjukl1kfh.pdf

The cryogenics design of the SuperCDMS SNOLAB experiment

The detectors of the Super Cryogenic Dark Matter Search experiment at SNOLAB (SuperCDMS SNOLAB) will operate in a seven-layered cryostat with thermal stages between room temperature and the base temperature of 15 mK. The inner three layers of the cryostat, which are to be nominally maintained at 1 K, 250 mK, and 15 mK, will be cooled by a dilution refrigerator via conduction through long copper stems. Bolted and mechanically pressed contacts, flat and cylindrical, as well as flexible straps are the essential stem components that will facilitate assembly/dismantling of the cryostat. These will also allow for thermal contractions/movements during cooldown of the sub-Kelvin system. To ensure that these components and their contacts meet their design thermal conductance, prototypes were fabricated and cryogenically tested. The present paper gives an overview of the SuperCDMS SNOLAB sub-Kelvin architecture and its conductance requirements. Results from the conductance measurements tests and from sub-Kelvin thermal modeling are discussed.

Thermal conductance modeling and characterization of the SuperCDMS-SNOLAB sub-Kelvin cryogenic system

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The cryogenics design of the SuperCDMS SNOLAB experiment

Thermal conductance modeling and characterization of the SuperCDMS-SNOLAB sub-Kelvin cryogenic system