Thursday, April 04, 2019

Vancouver's TRIUMF Lab Bottles Atomic Shrapnel

While visitors and locals flock to Vancouver’s parks for a taste of the region’s famous untamed beauty, at TRIUMF labs another kind of natural exploration is taking place. Nestled among three green spaces, the enormous particle accelerator center might seem a little out of place with its twelve and a half acres of research buildings and radiation warnings. Yet the researchers at TRIUMF work tirelessly to coax some of nature’s deepest secrets out of normally untalkative particles: neutrons.

This photograph of TRIUMF shows the University of British Columbia in the background.
Image credit: TRIUMF
These subatomic particles are always found tucked inside of an atomic nucleus, where they add mass to the atom without contributing any charge. Since their discovery in 1932, physicists have been eager to do what they do best: slice and dice until they can completely characterize the particle with precisely measured constants (and possibly a few equations). The problem is that, in nature, there’s no such thing as a lone neutron; they are always found deep within an atomic nucleus.

TRIUMF physicist Wolfgang Schreyer says that there’s a way around that though.
“To create free neutrons we will have to smash atomic nuclei,” he says with surprising breeziness. As it turns out, the nuclei are bound together by the aptly-named “strong force”, which—on the microscopic scale—acts with roughly 137 times the strength of the more familiar electromagnetic force. To overcome this powerful glue, the collisions must be incredibly high-energy, involving particles flying at speeds approaching that of light.

Fortunately, Schreyer and his colleagues work at a particle accelerator complex. For them, creating these high-energy collisions is just another day at the office; the real challenge emerges after the nuclei are torn apart. Since the atomic nuclei are thrown together with enormous energies, it follows that the particles produced by such collisions are also extremely energetic, making them incredibly hard to contain. “Like a billiard ball shot out of a cannon will smash through a wall, these fast neutrons can penetrate into any material and leave your experiment within a tiny fraction of a second,” Schreyer notes. So much for slicing and dicing.

No, what is needed is a mechanism to calm down the neutrons—and quickly, before they have a chance to escape. One of the most effective methods known to science is surprisingly intuitive: send them into a dense material where they are forced to bounce off of other particles. Just like a fast walker is forced to slow her pace in a crowded venue or a billiard ball slows down each time it glances off another ball, the neutrons can be brought down to a much more reasonable clip simply by shooting them at another material. Of course, it is important that they not get stuck or emerge with too much energy at the other end, so this slowing region must be chosen carefully for the right density and mixture of particle sizes. Although the slightly calmer neutrons still whiz around at several times the speed of a bullet, their relatively cooler nature lends them the moniker “thermal” or “cold” neutrons.

Even so, these neutrons are difficult to handle. Although they may not fly through walls as easily, they are still fast enough that they’re hard to contain, let alone measure. Instead, these cold neutrons need to lose almost all of their energy to become “ultracold”. This is where the bouncing method of cooling fails; thanks to the laws of kinematics, it is impossible for a neutron to transfer all of its energy to anything of a different mass, and it’s nearly impossible to create a wall of perfectly neutron-sized particles. Further complicating the matter, all molecules carry with them some thermal energy corresponding to their temperature, which causes relatively warm (i.e. room temperature) particles to knock into the neutrons and momentarily speed them up again.

This is where the prototype currently being tested at TRIUMF takes the stage. Other collaborations have used a variety of tools—from turbines to solid deuterium—with limited success, typically resulting in less than two dozen ultracold neutrons (UCNs) measured per cubic centimeter. Instead, the source at TRIUMF is one of two worldwide that shoot their cold neutrons through superfluid helium to cool them down even further.

The basic idea is that quantum physics allows cold neutrons to hit several atomic nuclei at once (thanks to the principle of superposition). When that happens, the neutrons release almost all of their remaining energy into a tiny sound wave, a so-called phonon. Since the surrounding helium is so close to absolute zero (0.9 Kelvin at TRIUMF), there is almost no thermal energy that can be transferred back into the neutrons, so they remain ultracold, traveling at only a few meters per second—roughly equivalent to a fast runner. “At this point, they basically act like a gas and can be transported along pipes and stored in bottles,” says Schreyer. While it’s still possible for the neutrons to escape, they can mostly be contained for several minutes. “That is a major step up from only fractions of a second for which we can observe fast, thermal, or cold neutrons.”

Such sophisticated technology takes time to develop. In fact, collaborators at the Research Center for Nuclear Physics in Osaka have spent over a decade pulling together and optimizing the prototype, which was only recently transferred to TRIUMF—but it seems to be worth the effort as the researchers explore early results. While this source itself is not terribly powerful, they are learning enough from it that their next generation source, set to come online in 2021, should produce around ten times more UCNs than any current source.

Granted, this source isn’t the only one to use superfluid helium; so does the Institut Laue-Langevin (ILL) in France. However, at the ILL, the cold neutrons are transported through pipes for some distance before encountering the superfluid helium, resulting in large losses as unruly neutrons escape through the pipe walls. On the other hand, TRIUMF’s helium chamber is right next to the neutron source, so many more neutrons make it to the ultracooling phase. Although it may sound like an obvious solution, researchers have historically considered it to be too difficult to place a supercold chamber so close to the extremely high-energy (i.e. high temperature) neutron source. At TRIUMF, this is realized through a series of rapid expansions, which cools the helium down, and pumping the gases away, which forces warmer helium atoms to boil off.

All in all, this prototype—and the powerful new source that’s slated to follow—promises to help researchers as they continue probing the fundamental laws of nature. For example, the TUCAN collaboration at TRIUMF hopes to measure a miniscule separation of charges housed inside a neutron. “To give an idea what kind of precision is possible with ultracold neutrons,” Schreyer says, “this experiment will be able to measure a separation between two elementary charges of 10-27 cm… a distance one hundred trillion times smaller than the neutron itself.”

An assortment of other flagship experiments are also in the works, some of which are already underway at other UCN sources. “All of these have to collect data for several years to reach their full potential because of the low intensity of current ultracold-neutron sources,” Schreyer asserts. “With our planned next-generation source, we should be able to slash the time needed for any of them.”
Particle physics will never be a total walk in the park—but for researchers who study neutrons, it’s about to get way easier.

—Eleanor Hook

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