[Paper Review] nEXO Pre-Conceptual Design Report

NEXO Collaboration, Kharusi, S. Al+172 more|arXiv (Cornell University)|May 28, 2018

Neutrino Physics ResearchPhysics and Astronomy7 references79 citations

TL;DR

This pre-conceptual design report outlines a tonne-scale, xenon-based search for neutrinoless double beta decay using about 5000 kg of Xe-136 enriched to 90%, aiming roughly 10^28 year sensitivity with a monolithic, ultra-clean liquid xenon detector leveraging EXO-200 experience.

ABSTRACT

The projected performance and detector configuration of nEXO are described in this pre-Conceptual Design Report (pCDR). nEXO is a tonne-scale neutrinoless double beta ($0νββ$) decay search in $^{136}$Xe, based on the ultra-low background liquid xenon technology validated by EXO-200. With $\simeq$ 5000 kg of xenon enriched to 90% in the isotope 136, nEXO has a projected half-life sensitivity of approximately $10^{28}$ years. This represents an improvement in sensitivity of about two orders of magnitude with respect to current results. Based on the experience gained from EXO-200 and the effectiveness of xenon purification techniques, we expect the background to be dominated by external sources of radiation. The sensitivity increase is, therefore, entirely derived from the increase of active mass in a monolithic and homogeneous detector, along with some technical advances perfected in the course of a dedicated R&D program. Hence the risk which is inherent to the construction of a large, ultra-low background detector is reduced, as the intrinsic radioactive contamination requirements are generally not beyond those demonstrated with the present generation $0νββ$ decay experiments. Indeed, most of the required materials have been already assayed or reasonable estimates of their properties are at hand. The details described herein represent the base design of the detector configuration as of early 2018. Where potential design improvements are possible, alternatives are discussed. This design for nEXO presents a compelling path towards a next generation search for $0νββ$, with a substantial possibility to discover physics beyond the Standard Model.

Motivation & Objective

• Motivate a next-generation 0νββ search by scaling up to a tonne-scale LXe detector.
• Present a base detector design and performance expectations as of early 2018.
• Leverage EXO-200 experience and xenon purification to minimize backgrounds.
• Assess materials, feasibility, and risks for a large, ultra-low-background detector.
• Discuss potential design improvements and alternative options within the pCDR base design.

Proposed method

• Adopt ultra-low background liquid xenon technology validated by EXO-200.
• Utilize a monolithic and homogeneous detector geometry to maximize active mass.
• Scale active xenon mass to ~5000 kg enriched to 90% in Xe-136.
• Apply xenon purification techniques to suppress internal backgrounds.
• Assess external background contributions and rely on material assays and R&D results from prior experiments.
• Present a base design with discussion of possible improvements and alternatives as of 2018.

Experimental results

Research questions

• RQ1 What is the projected half-life sensitivity for a tonne-scale Xe-136 0νββ search using ~5000 kg of enriched xenon?
• RQ2 How does increasing the active mass in a monolithic liquid xenon detector impact background-dominated sensitivity compared to current experiments?
• RQ3 What are the dominant background sources, and can external sources be mitigated to achieve the stated sensitivity?
• RQ4 What materials, assays, and design choices are required to realize a reliable 0νββ search at the tonne scale?
• RQ5 What alternative design improvements or options could further enhance performance within the base design constraints?

Key findings

• Projected half-life sensitivity of approximately 10^28 years.
• Detector uses ~5000 kg of xenon enriched to 90% in Xe-136.
• Background is expected to be dominated by external sources of radiation.
• Sensitivity gain primarily from increased active mass and a monolithic, homogeneous detector, aided by R&D and purification techniques.
• Most required materials have been assayed or have reasonable estimates of properties, mitigating construction risk.
• The design represents a base configuration with discussion of potential improvements and alternatives as of early 2018.