nEXO

Based at SNOLAB, the international nEXO (next Enriched Xenon Observatory) collaboration’s main goal is to solve a key mystery in neutrino science: whether a neutrino is its own antiparticle. To achieve this, the experiment is searching for the first detection of a theorized form of extremely rare radioactive decay, neutrinoless double beta decay. 

 nEXO is one of five TRIUMF-SNOLAB collaborations involving some of the world’s most advanced experiments in neutrino science (SNO+, HALO), and dark matter detection (DEAP-3600, SuperCDMS). SNOLAB is the deepest underground laboratory in North America, based two kilometers under Sudbury, Ontario in order to shield experiments from cosmic ray background. 

Neutrinos are the second-most abundant particles in the cosmos, after photons, but neutrinos are enormously difficult to detect because they very rarely interact with other matter. Trillions of neutrinos pass through your body every second.  

The 2015 Nobel Prize in physics was co-awarded to SNOLAB’s predecessor, the Sudbury Neutrino Observatory (SNO) collaboration, for the co-discovery that neutrinos oscillate between different forms on their journey from Sun to Earth. Yet, much about neutrinos remains unknown. The Standard Model assumes that, like photons, neutrinos are massless, but neutrino oscillation implies that neutrinos have a distinct mass. Thus, physicists think that understanding neutrino nature may be a window into beyond-Standard Model physics. 

The nEXO experiment is tackling the neutrino challenge using an indirect way of determining neutrino’s deeper nature: making the first measurement of the putative and extremely rare neutrino-less double beta decay. The nEXO experiment will use a detector made of five tons of the highly purified radioactive isotope liquid xenon-136 (¹³⁶Xe), which double beta decays to barium-136 (¹³⁶Ba).

The observation of neutrino-less double beta decay would be a major discovery of new physics, confirming that the neutrino is its own anti-particle, and measuring the rate of this decay would provide an indirect absolute measurement of neutrino mass. 

The experiment is extending the limits of detection technologies, and in so doing could provide a variety of important commercial spin-offs, including high-speed light sensors for self-driving vehicles. 

The nEXO collaboration involves 115 researchers from 20 institutions in seven countries: the United States, Switzerland, Germany, South Korea, Russia, China and Canada. The project is presently in the design, construction and testing stage, expected to begin data collection in 2025. 

nEXO is a scaling-up of the Enriched Xenon Observatory (EXO-200) experiment based at the U.S. Department of Energy’s Waste Isolation Pilot Plant facility in New Mexico. With the expanding of the experiment and its move to SNOLAB, TRIUMF joined the nEXO collaboration in 2014. 

nEXO will have three key components: a cryogenically-cooled liquid xenon source of double beta decays; specialized photon and charge detectors; and shielding to isolate the experiment from background particle noise both from the environment and the experimental apparatus itself.  

Neutrinoless double beta decay

To attempt to detect a neutrinoless double beta decay, nEXO will use five tons of cryogenic, liquid xenon, enriched to 80% in the isotope ¹³⁶Xe, making nEXO the world’s largest collection of ¹³⁶Xe. The xenon will be contained in a barrel-shaped vessel, about 1.5 meters in diameter. 

nEXO will be 50 times larger than EXO, and this is crucial to its potential success. The half-life of ¹³⁶Xe double beta decay is extremely long, more than a billion times the age of the universe, and the predicted rate of putative neutrinoless double beta decay is even longer. Thus, nEXO requires a large volume of ¹³⁶Xe to increase the probability that such a decay might occur. 

In normal double beta decay two neutrons in a nucleus each decay to a proton by emitting a beta particle (a high-energy electron) and an anti-neutrino. Thus, each nucleus emits two beta particles and two neutrinos. EXO-200 used a 200-kilogram prototype detector to make the first detection of ¹³⁶Xe double beta decay. However, some theories predict that in very rare instances a nucleus will decay through neutrinoless double beta decay.  

The detection of such a decay would provide an indirect window on neutrinos. One of the key questions in neutrino physics is whether neutrinos are Majorana or Dirac particles. Dirac particles have distinct anti-particles, for example, there’s the proton and antiproton, electron and positron. However, Majorana particles are their own antiparticle.  

If neutrinoless double beta decay is observed it would prove that neutrinos are Majorana particles, which would help explain their low mass, and would provide a measurement of it, since the rate of neutrinoless double beta decay is theoretically related to the square of the neutrino mass. 

Detectors

nEXO will detect double beta decays using unique, TRIUMF-designed high-speed hard UV detectors.  

When ¹³⁶Xe double beta decays, it emits two beta particles. These high energy electrons knock electrons from other xenon atoms creating a tiny trail of free electrons and ionized xenon atoms. Some of these electrons recombine with the ions, and excited atoms return to their ground state they emit vacuum ultraviolet (VUV) light, a wavelength of light termed “vacuum” because it doesn’t pass through air. Some of the electrons remain free and travel through the xenon to the detector under the influence of an electric field. Separate detectors monitor these VUV light and charge signals, providing precise measurement of double beta decay events.  

Barium Tagging

A unique aspect of nEXO is the plan to be the first experiment to use barium-tagging to confirm a double beta decay. The decay of ¹³⁶Xe produces the daughter atom ¹³⁶Ba. TRIUMF, and other members of the nEXO collaboration are developing a state-of-the-art technique to isolate and remove a single atom of ¹³⁶Ba from the five tons of liquid xenon. Tagging the barium would provide a “smoking gun” confirmation of a double beta decay detection, and also provide a way to eliminate background noise, greatly increasing nEXO’s sensitivity. 

Shielding

nEXO will ideally be situated underground at SNOLAB, where a two kilometre rock overburden shields the experiment from most of the high-energy cosmic rays that can cause background events which mimic the signal of interest. nEXO’s inner detector, containing the xenon, will be suspended in the centre of a 12-meter-diameter water-tank. This tank provides about four-meters of water shielding on all sides of the xenon, which reduces the impact of radioactive decays in the surrounding rock on the experiment. In addition, the water tank is instrumented with light sensors to detect the passage of the few high-energy cosmic rays that manage to penetrate so far deep into the Earth. This detection provides a veto against the subsequent signals induced by the cosmic rays.

TRIUMF is providing two key components to the nEXO collaboration: 

High-speed VUV detectors

In collaboration with researchers from the University of Sherbrooke, TRIUMF is designing a unique direct Vacuum Ultra-Violet (VUV) light detector to record decay signals from nEXO’s liquid xenon. It will be the world’s first, high-speed, fully digital, VUV light sensor, capable of nano-second detections. The work builds on TRIUMF expertise developed from contributions to the T2K neutrino experiment, and development of advanced detectors for PET medical imaging. 

One of the key detection challenges with nEXO is that a ¹³⁶Xe decay emits at 175 nanometers, a VUV wavelength. As a result, nEXO can’t use visible light detectors. However, existing VUV detectors such as those used in space-based telescopes are too slow for use with nEXO and have low detection efficiencies of about 20%.  

Thus, TRIUMF is developing high-efficiency, high-speed, built-in-Canada custom VUV-light sensors for nEXO. The detectors will cover about five square meters, with thousands of individual channels.  

TRIUMF scientists are working with collaborators to explore further potential scientific and commercial applications of this high-speed, digital single photon detection technology. This includes potential applications as high-speed sensors for self-driving vehicles.

Barium Tagging 

TRIUMF researchers, in collaboration with researchers from Carleton and McGill universities, are developing the first double beta decay barium-tagging technology in order to confirm a double beta decay. This will involve isolating and extracting a single ¹³⁶Ba atom from five tons of liquid xenon. 

The technology is being developed leveraging the expertise of TRIUMF’s TITAN facility‘s Multi-Reflection Time-Of-Flight Mass Separator. In this mass separator, ions bounce back and forth between electrostatic mirrors. Since the ions have the same energy, the beam separates them out by their differing mass-to-charge ratio. In this way, a ¹³⁶Ba atom with 56 protons, and ¹³⁶Xewith 54 protons, can be separated based on their differing charges, even though they have the same atomic mass.