To understand more about neutrinos—tiny, nearly massless particles with no charge—the nEXO experiment is searching for a rare nuclear decay called neutrinoless double beta decay. Double beta decay is a process whereby a nucleus decays into another one and emits two electrons and two antineutrinos. A special version of double beta decay—the “neutrinoless” version—emits only two electrons and no antineutrinos.
Neutrinoless double beta decay is at the boundaries of human knowledge since it has not yet been observed and cannot occur according to the laws of physics as we know them. However, many theories that may explain strange patterns observed in cosmology and particle and nuclear physics predict that this version could occur, but at an unimaginably small rate. Previous experiments have set limits on how rare the decay is, establishing that the half-life of this decay is longer than a million-billion (1015) times the age of the Universe. The nEXO experiment is designed to extend knowledge of this rare decay to a level of 1028 years, or 1019 times the age of the Universe.
A massive experiment measures a long half-life
We won’t have to wait 1028 years to search for neutrinoless double beta decay. The laws of radioactive decay state that nuclei in a large collection are identical, and each one lives and decays independently from the others. For example, in a collection of a million nuclei—each with a million year half-life—half of the nuclei will have decayed in a million years, and about one, on average, decays every year. Additionally, nuclear decays can be individually detected, as long as precautions are taken to separate the decay signal from backgrounds.
nEXO will search for neutrinoless double beta decay in 5,000 kg of the xenon-136 (136Xe) isotope (about 2 x 1028 nuclei). This large number of nuclei allows us to translate the potential observation of a few decays in the ten years of the experiment into a sensitivity that extends our knowledge of neutrinoless double beta decay.
Our design reduces backgrounds
For each nuclear decay, only a small amount of energy is released and available for detection. To protect this small signal from being swamped by other forms of radioactivity, the experiment will be built in SNOLAB, a deep mine. At two kilometers down, the rock overburden protects the entire experiment from cosmic radiation that is too energetic to be stopped by artificial shields.
The next step in reducing radioactive backgrounds is building the detector like an onion, with layers that are progressively lower in natural radioactivity as they get closer to the “inner sanctum”—the xenon located in the center. Each layer shields the radioactivity produced by the outer layers. The rock in the mine contains uranium and thorium, both radioactive elements that exist in the Earth’s crust; this radiation is primarily shielded by water in a large tank. The water itself contains some radioactive contaminants, so those are shielded by a special fluid contained in a sphere around the xenon. The xenon is kept inside the sphere using a very thin cylindrical copper vessel, designed to be as pure and lightweight as possible.
Enriching xenon optimizes the experiment
Of the isotopes of xenon, only 136Xe can potentially undergo neutrinoless double beta decay, so we use xenon that is enriched to 90% in that isotope. This step is achieved by ultracentrifugation—the same technology used to produce uranium-235 for nuclear power plants and weapons—starting from the 9% concentration of 136Xe in natural xenon. nEXO will require a significant enterprise to ultracentrifuge the 5,000 kg of 136Xe needed for the experiment.
To liquefy the xenon, the central part of the detector is cooled to 165 Kelvin, and the volume between the spherical shells is evacuated to provide thermal insulation.
Pinpointing the coordinates of double beta decay
The liquid xenon is not only the source of the potential decays, but also the medium in which the decays would be detected. To achieve this, a large voltage difference (about 50,000 volts) is established between the bottom of the copper cylinder and the top. Because liquid xenon is a good dielectric, no current flows between the electrodes even with this large voltage.
When a double beta decay occurs, the two electrons leave the decay site with substantial kinetic energy and slow down by dissipating their energy, ionizing more atoms of xenon along the way. Clouds of free electrons are produced and are “drifted” towards the positive anode under the influence of the large voltage. This arrangement is called a “time projection chamber,” and properly instrumenting the anode allows measurement of the original electrons’ energy and the location where they were produced. The geometrical structure of the decay is also recorded; this is important because the residual background is mainly due to gamma rays and not electrons, and the gammas deposit energy in patterns that are largely different from those of the electrons produced by the double beta decay. Since the ionization electrons drift in straight lines towards the anode, the point where they land is a direct projection of two of the coordinates of the decay position.
The third coordinate requires more work. Along with ionization, the original electrons produce a flash of light in the liquid xenon. Special photodetectors plaster the inside of the time projection chamber and record the amount of light produced and the exact time at which it was produced. Since the ionization electrons drift relatively slowly, the difference in time between the flash of light and the arrival of the ionization at the anode provides the third coordinate of the decay.
A combination of the amount of light detected and the charge collected at the anode is used to reconstruct the total kinetic energy of the original electrons from the decay. The readout of light and ionization signals is obtained with ultra-sensitive electronics, with custom-made chips (application-specific integrated circuits, or ASICs) designed for the experiment to operate at cryogenic temperature and pure of radioactive elements. Amplified and digitized numbers are continuously transmitted outside of the time projection chamber to conventional computers at room temperature.
An international collaboration
nEXO includes many components beyond what has been described so far, including ultra-high vacuum, cryogenics (we will operate a large liquid nitrogen plant underground), and sophisticated mechanics and electronics. Components for the experiment are fabricated in many places around the world, because, in many cases, they are unique and are procured from wherever they are available.
A consortium of over 150 scientists and technologists from 31 institutions and eight countries are contributing to the design and construction of nEXO. Everybody in the consortium will analyze the data, which is expected to flow over a period of ten years.