UCN

With TRIUMF’s Ultracold Neutron (UCN) facility, scientists are exploring one of the great mysteries in physics by making the most precise measurements ever of neutrons. 

Following the discovery of the Higgs Boson in 2012, the biggest gap in the Standard Model is its inability to explain the asymmetry between matter and antimatter in the universe. It appears that conditions in the early universe favoured the production, or survival, of matter over antimatter. If there had been absolute symmetry and equal amounts of matter and antimatter, the two would have completely annihilated one another into pure energy. Instead, today we observe a primarily matter universe, including us. A key to understanding this matter-antimatter asymmetry could lie with the neutron’s subtle electrical characteristics. A neutron consists of three quarks: two down quarks each with a charge − ¹⁄₃ e (for a total negative charge of – ²⁄₃ e); and one up quark with a charge of + ²⁄₃ e. As a result, the neutron has an overall neutral charge, and thus its name. 

However, depending on the arrangement of its quarks, the neutron could exhibit poles with slightly greater positive or negative charge, creating a neutron electric dipole moment (nEDM). A dipole moment is a measurement of the separation of two opposite electrical charges.  If this exists in neutrons, it will be infinitesimal and enormously difficult to precisely measure. For example, considering an imaginary spherical neutron the size of the Earth, physicists predict that the poles of any nEDM would be separated by much less than the width of a hair, resulting in an extremely weak nEDM. However, what’s key is that many beyond-Standard Model theories that predict matter-antimatter asymmetry also predict a nEDM.

The grand challenge is that precisely measuring the nEDM requires cooling neutrons to almost absolute zero, about .003 Kelvin (-273.147°C), in order to contain and manipulate them.

Thus, TRIUMF’s nEDM experiment will be a critical test of beyond-Standard Model theories, using tiny particles to potentially solve one of the biggest questions in the cosmos. 

Creating ultracold neutrons requires first creating a high number of fast neutrons and then using sophisticated cryogenic techniques to slow them down to ultracold. (Temperature is a measurement of how quickly particles are moving.) 

In nature, neutrons are usually bound within atomic nuclei. So, the first UCN step is to create free neutrons. Based in TRIUMF’s Meson Hall, the UCN facility uses a new, dedicated beamline of high-energy (480 MeV) protons produced by TRIUMF’s main cyclotron fired into a block of tungsten to produce a stream of free neutrons. 

Tungsten is used because it’s neutron-rich, each nucleus contains 74 protons and, on average, 110 neutrons. The cyclotron-produced protons hit and shatter the tungsten nuclei, breaking them up into smaller pieces, including large numbers of fast neutrons. 

To slow the neutrons to ultracold is a complex three-step process, the first two of which are forms of thermal cooling in which the neutrons are slowed the way billiard balls would be by hitting other objects, in the neutrons’ case, nuclei. 

To start, the fast neutrons spewed from the tungsten target are sent through room- temperature heavy water cooling them to about 300 K (27°C). Next, the neutrons are sent through frozen heavy water, or liquid deuterium, cooling the neutrons down to about 20 K (-253°C). 

However, this still isn’t cold enough for TRIUMF’s neutron experiments. The third and final stage involves a more complex, quantum mechanically-mediated form of cooling. The cold neutrons are immersed in superfluid helium at about 0.9 K. Here, through quantum mechanical interactions, the neutrons emit phonons, a variety of quasi-particle analogous to a sound wave, losing energy in the process. This creates ultracold neutrons at about 0.003 K (-273.15 °C). 

From fast neutrons emerging from the tungsten target at more than 10,000 kilometers-per-second, the three-step cooling slows them to less than 8-meters per second, or about the speed of an Olympic 100-meter sprinter. 

The ultracold neutrons are now ready to be studied. They are shuttled to the electric dipole measurement container, or cell, where they’re held for about 100 seconds, and EDM measurements are made. A door valve on the EDM cell then opens, and the neutrons are directed into a detector. 

Given the extremely small size of any expected nEDM, it will take several years of measurements, data analysis and research to arrive at a reliable measurement that overcomes inherent measurement limits and errors. 

The UCN facility’s first experiment is searching for the neutron electric dipole moment (nEDM). 

To account for the observed matter-antimatter asymmetry of the universe, new sources of CP violation beyond the Standard Model are required. The small amount of CP violation in the Standard Model leads to a very tiny nEDM of about 10⁻³¹ e-cm. However, in several models of physics beyond the Standard Model, extra sources of CP violation are present, and notably, these models often generate nEDM’s at the 10⁻²⁷ e-cm level.  

The current experimental limit of detection for the nEDM is about 3×10⁻²⁶ e-cm. TRIUMF’s nEDM experiment, and other next-generation neutron experiments around the world, aim to constrain the nEDM to roughly the 10⁻²⁷ e-cm level. Critically, this approximately 300 times greater level of nEDM sensitivity will confirm, or reject, a variety of beyond-Standard Model theories.  

UCN measurements of the nEDM use the fact that the neutron has a magnetic dipole moment like a bar magnet. When placed in a magnetic field, the neutron’s spin axis precesses about the direction of the magnetic field the same way that a spinning top precesses about the direction of gravity.  

If an electric field is applied in the same direction as the magnetic field, then, if the neutron has a non-zero EDM, it will change the precession frequency. By using a technique called Ramsey Resonance, the UCN experiment will make very precise measures of the precession frequency, and any change when the electric field is reversed. 

The Neutron Lifetime Experiment 

A second key experiment envisioned at TRIUMF’s UCN facility is ultra-precise measurement of the neutron lifetime. Free neutrons decay with a half-life of about 15 minutes into a proton, an electron, and an electron anti-neutrino. However, at present, there’s a significant discrepancy between the neutron half-life from beam measurements using cold neutrons and bottle experiments using ultracold neutrons. Beam experiments tend to predict a longer neutron lifetime than storage experiments. As a result, the world average neutron half-life calculated by the Particle Data Group has an error inflated by a factor 1.9. (For a detailed listing of neutron measurements see here).

 A more accurate and precise determination the neutron lifetime is important for two key reasons: 

1. The neutron lifetime is an essential parameter for Big Bang nucleosynthesis calculations and is currently the major uncertainty for accurate predictions. 
2. The rate of neutron decay is strongly correlated to the intensity of the weak interaction. At present, the weak force is the least precisely measured of the fundamental constant forces (weak, strong, electromagnetic and gravity). The ability to confine ultracold neutrons will open the possibility for measuring neutron half-lives with much greater accuracy, thus providing an improved understanding of the strength of the weak interaction. 

TRIUMF’s TUCAN (TRIUMF ultracold advanced neutron source) collaboration includes the University of British Columbia, Simon Fraser University University of Winnipeg, University of Manitoba, University of Northern British Columbia, Nagoya University in Japan and Japanese Laboratories, KEK, the High Energy Accelerator Research Organization, RCNP, the Research Center for Nuclear Physics, and the University of Osaka. 

Why ultracold neutrons?   

Ultracold neutrons (UCN) are required for TRIUMF’s neutron experiments because these neutrons are moving so slowly they can be contained in a bottle, and thus precisely measured. 

For a free, fast neutron at room temperature, what to us looks like a solid wall of metal is an open door. For example, if fast neutrons hit a 4-mm thick nickel plate, about 9-in-10 will go straight through without any interaction. This because the solid-looking nickel is actually tiny nickel nuclei separated by vast empty space. The inter-atomic spacing in solid nickel is 10,000 femtometers whereas the nickel nuclei are only about 10 femtometers in diameter. Thus, a neutron has to make a direct hit on a nucleus to be slowed, and this is a rare event.  

However, as the neutron becomes colder, and its speed reduces, it becomes more and more likely to interact with other matter. Quantum mechanically, a neutron exists as both a wave and a particle. Ultracold neutrons have such a long wavelength that they interact with hundreds of nuclei at a time. In the TRIUMF UCN experiments, they are so cold and moving so slowly that they bounce off solid materials. This enables TRIUMF scientists to contain the ultracold neutrons in the same way a gas is held in a metal bottle.  

TRIUMF’s UCN facility is designed to be the world’s flagship experimental facility for ultracold neutrons. It’s a project that’s required a decade-long combination of world-leading vision, international collaboration and technical and scientific expertise. 

The UCN facility is aiming to be the world’s highest-density source of ultracold neutrons. High density is critical to the research because most ultracold neutron experiments are statistics limited—the experiments require enormous numbers of individual measurements to arrive at a statistically reliable result.