New Portable Camera Simultaneously Images Gamma Rays and Slow Neutrons

A compact imaging device that can picture gamma rays and slow neutrons at the same time has been designed and tested entirely through detailed computer simulations, according to a study published in EPJ Techniques and Instrumentation. The system, called GN-Vision, is conceived as a handheld instrument for nuclear safety and security work, where inspectors may need to locate hidden radioactive material that emits both types of radiation simultaneously. Because the device exists so far only as a conceptual design validated by Monte Carlo modelling, its reported performance figures describe simulated capabilities rather than measurements from a working prototype.

The central idea is to combine two well-established imaging principles in a single instrument. Gamma rays are imaged with a Compton camera, which uses two position-sensitive detection planes and electronic collimation to reconstruct the direction of incoming photons. Neutrons, by contrast, are imaged with a passive pin-hole collimator attached to the first detection plane, working much like a pin-hole camera. The design builds on an earlier Compton imager called i-TED, developed within the European Research Council project HYMNS, and replaces the crystal in the first detection layer with a material that can tell neutrons and gamma rays apart.

That material is a scintillation crystal known as CLYC-6, a caesium lithium yttrium chloride compound enriched to 95 percent in the lithium-6 isotope. Slow neutrons reaching the crystal are captured by lithium-6 nuclei in a reaction that produces an alpha particle and a triton, which together deposit about 3.2 megaelectronvolts of energy in the crystal. Because these charged particles travel only tens of micrometres before stopping, the interaction point can be localized far more precisely than the long tracks of Compton electrons from megaelectronvolt gamma rays. The crystal also discriminates gamma rays from neutrons by pulse shape discrimination, a technique that classifies each light pulse according to its temporal profile.

The proposed prototype pairs a 50 by 50 by 10 millimetre CLYC-6 scatterer with an absorber plane of four lanthanum chloride crystals, each 50 by 50 by 25 millimetres, coupled to pixelated silicon photomultipliers and read out with electronics originally developed for time-of-flight positron emission tomography. The pin-hole collimator is made of polyethylene enriched in lithium-6, a low-density material that is nearly transparent to gamma rays above a few hundred kiloelectronvolts but strongly absorbs slow neutrons. Lithium-6 was also chosen because its neutron capture reaction emits no gamma rays, avoiding an artificial background in the Compton images.

They simulated three point-like neutron sources placed 20 centimetres from the device and separated by 5 centimetres, generating 100 million neutrons per source at energies from thermal values up to 1 kiloelectronvolt. Experimental effects, including realistic energy and position resolutions and low-energy thresholds previously characterized for the i-TED detector, were folded into the simulations.

The simulated neutron images show a clear trade-off between contrast, resolution and efficiency. Increasing the collimator thickness from 10 to 30 millimetres raised the peak-to-background ratio for 1-electronvolt neutrons from a factor of 2.2 to nearly a factor of 10, and the simulations indicate that at least 40 millimetres of lithium-6 enriched polyethylene would be needed to keep reasonable contrast for neutrons beyond 100 electronvolts. Shrinking the pin-hole diameter from 5 to 1 millimetre improved the image resolution from 24 to 17 millimetres full width at half maximum, but at the cost of losing half the detection efficiency. Overall, the device achieved a spatial resolution of about 20 millimetres, corresponding to roughly 6 degrees, which the authors compare favourably with the 9 to 30 degree resolutions reported for other compact dual imagers sensitive only to fast neutrons.

The intrinsic neutron imaging efficiency came out at roughly 2 times 10 to the minus 3 for energies from thermal up to 1 electronvolt, with absolute imaging efficiencies close to the value of 10 to the minus 4 reported for large liquid-scintillator arrays. The authors attribute this advantage to the lithium-6 capture reaction, whose thermal cross section of 940 barns grows as neutron velocity falls. They note that the pin-hole geometry remains the limiting factor and suggest that coded-aperture masks adapted for neutron detection could improve efficiency in future designs.

The gamma-ray channel was tested separately in simulations of monoenergetic sources between 100 kiloelectronvolts and 1 megaelectronvolt. The passive neutron collimator absorbs a fraction of incoming gamma rays, ranging from 40 percent for a 40-millimetre collimator at 100 kiloelectronvolts down to 10 percent for a 20-millimetre collimator at 1 megaelectronvolt, reducing Compton efficiency in the same proportion, but it did not degrade the image itself. For two point sources of 500-kiloelectronvolt gamma rays placed 200 millimetres away, the reconstructed images reached a resolution of 33 millimetres full width at half maximum, or 9 degrees, whether or not the collimator was present. The intrinsic gamma imaging efficiency was around 10 to the minus 2, which the authors report as higher than other compact dual imagers in the literature, and reducing the gap between the two detection planes from 30 to 5 millimetres nearly doubled the imaging efficiency at the expense of angular resolution.

To bridge the gap between idealized monoenergetic tests and a realistic inspection, the team simulated a scenario in which GN-Vision faces a polyethylene container hiding a sample of californium-252, a spontaneous fission source that emits both neutrons and gamma rays and whose neutron spectrum resembles that of materials such as uranium and plutonium. Containers of 10 by 10, 15 by 15 and 20 by 20 centimetres were modelled at 50 centimetres from the device. After partial moderation in the polyethylene, neutrons below 100 electronvolts, within the imaging range of GN-Vision, made up 29 percent of the emerging spectrum for the smallest container and 57 percent for the largest, underscoring why sensitivity to slow neutrons matters when material is deliberately hidden behind hydrogen-rich shielding that blinds fast-neutron detectors.

In the simulated dual images of the 15 by 15 centimetre container, the gamma-ray image localized the emitting source precisely, while the neutron image preserved the source information in a thermalized, broadened form that traced the dimensions of the moderating container itself. The authors present this as a validation, at the simulation level, of the concept for identifying special nuclear material hidden in a neutron moderator. They caution, however, that while the gamma-imaging capability inherits a technology readiness level of 6 from the i-TED predecessor, the neutron imaging capability has only just been conceptually proven in this work, the device has been patented and is still under development, and the experimental integration of neutron-gamma discrimination with the compact readout electronics is only now entering first laboratory tests, with full development and field measurements expected to require several more years of research.

Subject of Research: Simulation-based conceptual design of a portable dual gamma-ray and slow-neutron imaging device for nuclear inspections

Article Title: Simultaneous Gamma-Neutron Vision device: a portable and versatile tool for nuclear inspections

Article References: Lerendegui-Marco, J., Babiano-Suárez, V., Balibrea-Correa, J., Caballero, L., Calvo, D., Ladarescu, I., & Domingo-Pardo, C. (2024). Simultaneous Gamma-Neutron Vision device: a portable and versatile tool for nuclear inspections. EPJ Techniques and Instrumentation, 11(1), Article 2. https://doi.org/10.1140/epjti/s40485-024-00108-w

Image Credits: AI Generated

DOI: 10.1140/epjti/s40485-024-00108-w

Keywords: GN-Vision, Compton camera, neutron imaging, gamma-ray imaging, CLYC-6 scintillator, lithium-6 collimator, Monte Carlo simulation, Geant4, nuclear security, special nuclear material, californium-252, i-TED