Associated Particle Imaging (API) is a nuclear imaging technique that uses neutrons and gamma rays to create images of the inside of objects, allowing the elemental composition of materials to be accurately measured. It is used in various applications, from detecting explosives and illicit drugs to nuclear materials and diamonds.

However, uncertainties inherent to the technique can limit its use in applications that require high spatial resolution.

Now, researchers from the Accelerator Technology & Applied Physics (ATAP) Division at Berkeley Lab have developed a method for correcting distortions from center-of-mass (COM) movement in API. The work could extend API to measure the carbon content in soil from acre-sized fields on agricultural land at depths down to 30 centimeters. This would allow it to be used by farmers and landowners to claim credits for carbon sequestration activities, making it a powerful tool in the fight against climate change.

API works “by firing neutrons at a volume of soil, which collide with atoms in a target material, in our case soil, and produce gamma rays,” says Arun Persaud, a staff scientist and deputy of ATAP’s Fusion Science & Ion Beam Technology Program, who led the research.

“By measuring the energy and timing of the gamma rays, and the timing and position of co-created alpha particles, the technique can create a three-dimensional map in real-time of carbon and many other elements, such as iron, aluminum, silicon, oxygen, in soil samples with centimeter resolution.”

Soil imaging with neutrons can give a quick, detailed look at the amount and distribution of carbon (and certain other important elements) in soil without disturbing the soil or plant roots. (Credit: Berkeley Lab)

The technique, he adds, utilizes the inelastic scattering of neutrons produced in deuterium-tritium (DT) fusion reactions to create three-dimensional images that show the distribution of carbon (or other elemental) isotopes within a soil sample, which can then be used to estimate the total carbon content in the volume of soil. He says that to achieve this, these reconstructed images must be accurate.

“In API, the neutrons and associated alpha particles from the DT reaction travel in diametrically (180 degrees) opposite directions in the COM system. However, this angle is slightly less than 180 degrees in laboratory experiments.” This shift in angle is due to the ‘COM movement’ and must be corrected and accounted for to produce an accurate reconstructed image.

“Furthermore,” he continues, “the ions creating the DT reaction have different momenta due to the energy loss of the particles in the neutron production target; consequently, their COM velocities vary, which adds to the uncertainty in the final reconstructed positions and impacts the accuracy of the images.”

To address COM movement in the API system, the researchers used a system comprising an alpha particle detector, a gamma ray detector, and a neutron generator (consisting of an ion source, an acceleration gap, and a neutron-production target, which was made from a thin layer of titanium placed on a copper backing). They used a microwave-driven plasma to generate an ion beam and fired it at the titanium/copper target.

To reconstruct element locations in API, the team recorded the timestamps, energies, and alpha particle positions of the coincident alpha particle/gamma-ray pairs to reconstruct the inelastic scattering locations of the neutrons.

“To find the location of the scattering center, we first calculated the alpha particle travel time from the alpha particle position and the known alpha velocity,” explains Persaud. “We then calculated the velocity vector of the alpha particle in the COM system using an averaged COM velocity, and, using the gamma-ray arrival time, we can then calculate the scattering location.”

To quantify the COM contributions, they simulated the energy loss of ions in the titanium target. This allowed them to analyze the COM corrections in the reconstructed images and to identify sources of uncertainties in these corrections.

Persaud says the work showed a shift introduced by the COM movement in the API systems was “significant” but can be corrected. “The uncertainties affecting the reconstructed images’ accuracy were smaller than other sources of errors. The study helped us better understand the accuracy of the reconstructed images in API and what ultimate resolution limit is achievable.”

Commenting on the research, ATAP Division Director Cameron Geddes said: “The imaging improvements demonstrated by Arun and his team have the potential to significantly advance our ability to measure and monitor the carbon content of soil as well as applications from national security to law enforcement.”

The Department of Energy supported the work presented here through the Laboratory-Directed Research and Development Program as Part of its Carbon Negative Initiative.

Learn More

C. Egan, A. Amsellem, D. Klyde, B. Ludewigt, and A. Persaud. “Center-of-Mass Corrections in Associated Particle Imaging,” in IEEE Transactions on Nuclear Science, 2023, https://doi.org/10.1109/TNS.2023.3313873