Better measurements for better physics

Upgrades at Weapons Neutron Research Facility enable new levels of precision.

By Brian Keenan | December 13, 2021

Neutron Beam Opt
A new spallation target assembly is part of the WNR facility upgrade. Los Alamos National Laboratory

With a half-life of only six days, the radioisotope nickel-56 does not reward procrastination. In preparation for an experimental campaign with the short-lived material, a Los Alamos National Laboratory team of scientists and engineers optimized a neutron beam transport system at the Weapons Neutron Research (WNR) facility. The upgrade, which took 18 months, brings the facility up to a best-in-class standard and allows future experiments on a range of materials.

The team upgraded the facility’s spallation target, a tungsten slug that generates a neutron beam source from within a 40-foot concrete crypt. When combined with the modern metrology infrastructure installed at the facility, the target’s position deep inside that crypt can now be measured with laser trackers for the first time, allowing the target’s absolute position to be measured to +/- 68 microns with 95 percent confidence. “Being able to directly measure the spallation target with laser trackers is a huge development for WNR,” says R&D engineer Brad DiGiovine of the Lab’s Nuclear and Particle Physics and Applications group. “You need to know where these instruments are, and you can’t just go in there and look.”

In a WNR experiment, materials, such as nickel-56, interact with a neutron beam aft er the neutrons have traveled down a flight path in the crypt. A newly designed brass shutter insert and advanced collimation system in the flight path form the neutron beam and keep down unwanted interactions with background neutrons.

At the end of the flight path, another new instrument, the “hot” Low-Energy Neutron-induced Charged-particle chamber, is where researchers measure a material’s nuclear properties as it interacts with the neutron beam.

“This kind of upgrade had never been done at this facility before,” DiGiovine says. “It’s hard to corral neutrons in a beam to get them to go where you want. You really need to precisely constrain the allowable neutron trajectories to keep background down while maximizing the amount of neutrons in the material sample.”

In November 2020, the more precise neutron beam system measured nickel-56’s cross-section (the probability that certain particles will collide and react in certain ways). In nature, nickel-56 is an abundant “seed nucleus”—the starting point for a fusion chain reaction—so better understanding its cross-section is useful for basic science applications and perhaps for researchers whose work focuses on similar, human engineered, fusion chain reactions.

Effective neutron transport relies on precision, and the researchers executed the alignment of the entire experimental system to within 10 microns—approximately one-tenth the thickness of a piece of paper—of the equipment’s ideal position. Although perfection is unattainable, that excellent level of precision attained can be applied to future experiments in a variety of areas.

“The WNR upgrades allow us to optimize our experiments,” says physicist Shea Mosby. “You want your calculation tools to have the best possible physics in them, for whatever it is you are doing. The entire realm of nuclear technology benefits when we get one reaction improved.”