High flux neutron generators are used in a wide range of research and development techniques, such as neutron radiography, neutron activation analysis, radiation survivability testing, and radiation detector calibration, all of which require high neutron fluxes. Historically, nuclear fission reactors in labs owned and operated by colleges and universities have been the primary high-flux neutron source for these purposes. However, the number of research reactors in operation has been steadily declining over the years, as they are being shut down due to old age or regulatory reasons with no new facilities under construction to replace them. Phoenix’s high-yield, high-flux neutron generators have been designed as an alternative neutron source.
What Is a High Flux Neutron Generator?
A high flux neutron generator is a neutron generator designed not only to create large amounts of neutrons, but also to maximize the amount of neutrons delivered through a specific area. For many applications, such as neutron radiography, knowing that a neutron generator can produce a high neutron flux is more important than simply knowing that it has a high neutron yield.
Phoenix’s DD (deuterium-deuterium) neutron generators are capable of powering a thermal flux system with relatively large cavities (~100×11 cm) with uniform thermal flux as a high as 108 n/cm2/sec.
What Is Neutron Flux?
The total neutron yield of a neutron source, or the amount of neutrons produced per second by the source, is not always the most valuable metric for determining a neutron source’s output. Neutron flux, on the other hand, shows how many neutrons pass through a given region of space per second (neutrons per square centimeter per second). This is a critical distinction in many applications such as thermal neutron imaging, fast neutron imaging, and radiation effects testing. In many applications only a very small amount of the total neutron yield (namely, neutrons traveling in the correct direction, and in thermal neutrons’ case, the neutrons at the correct energy level) counts toward the effective neutron flux.
What Is Neutron Fluence?
Neutron fluence is a term related to neutron flux, but frequently confused with it. It is common for people looking for neutron generators to confuse yield, flux, and fluence.
If you run a neutron generator for one minute, the neutron flux shows how many neutrons pass through a certain area in one second. The neutron fluence shows how many neutrons pass through that area over the course of the entire minute. And the neutron yield shows how many neutrons are generated in total in one second.
Neutron Yield: neutrons per second
Neutron Fluence: neutrons per square centimeter
Neutron Flux: neutrons per square centimeter per second
What’s the Difference Between Thermal Neutrons and Fast Neutrons?
Your typical neutron has an energy of 1 MeV or more. High-energy neutrons such as those used for fast neutron imaging in Phoenix’s system have up to 16 MeV. Even higher energies of neutrons are used for purposes such as certain space applications. Sometimes, though, you need to have neutrons that have been slowed down to a much lower energy state. Because these neutrons have so little energy (roughly 0.025 eV, nearly ten thousand times less energy than an average neutron) that they have reached a state of thermal equilibrium, we call them thermal neutrons.
Different neutron energies and temperatures have their own unique applications. Useful neutron temperatures for various purposes include:
Are All of Phoenix’s Neutron Generators High Flux?
All of Phoenix’s neutron generators are high-yield neutron generators. Are they all high flux neutron generators as well?
All of Phoenix’s neutron generators are high flux neutron generators capable of projecting a consistent neutron flux across a wide area, making them ideal neutron sources for applications requiring a larger irradiation area, like neutron imaging and radiation effects testing.
Not all neutron generators that are high-yield are high flux. Conversely, not all neutron generators that are high flux are also high-yield. Some neutron generators are designed to provide a high neutron flux, but only over a very small area and for a brief period of time, without providing a high neutron yield; some neutron generators are designed to provide a high yield but cannot focus a large amount of neutrons into a given area efficiently.
The distinction between high yield and high flux is an important one to make. A neutron generator with a low neutron yield may be able to produce a high neutron flux, but only in a very small area. If you need a massive flux for neutron imaging, for example, you need that flux to be more or less consistent over the entire object you’re imaging and not just in a circle with a diameter of a few millimeters.
How We Make Neutrons
Phoenix’s high flux neutron generators create neutrons using an accelerator and isotopes of hydrogen. In particular, we use deuterium, which is hydrogen with an added neutron in its nucleus, and tritium, which is hydrogen with two extra neutrons.
Our compact accelerator produces an intense beam of deuterium ions, which have been stripped of their electrons to give them a positive electrical charge. Since the ion beam has a positive charge, we can use electromagnets to accelerate the ions at high speeds, coax the beam into shape, and control its diameter as we fire it directly into a target.
In neutron generators, the target is a solid or gaseous material containing large amounts of deuterium or tritium. Solid targets are made up of various metals layered together and heavily laced with the hydrogen isotope of choice. Gas targets, on the other hand, can consist solely of deuterium or tritium, making them much more efficient for promoting DD and DT reactions. While some of our high-yield neutron generator systems use solid targets, our highest yield systems use gas targets. The neutron output from a solid target can be ten times weaker than a gas target for the same beam current and energy!
When the ion beam collides with the target, the deuterium ions in the beam collide with the deuterium or tritium atoms in the target with enough energy that they fuse into new elements. Every fusion reaction leaves behind free neutrons, subatomic remnants of the original atoms which no longer have a place in the resultant element, imbued with leftover energy from the reaction. These free neutrons comprise the neutron radiation our neutron generators produce.
The free neutrons exit the neutron source at various energy levels ranging from their neutron birth energy (the energy imparted to them in the reaction) to birth energy plus acceleration energy (if the neutrons are traveling forward) or birth energy minus acceleration energy (if the neutrons bounce backward). In a DD reaction, for example, the band of neutron energy can be 2.5 MeV ± 300 keV.
To create low-energy thermal neutrons, we connect a moderator to the neutron generator filled with material that will slow the neutrons produced by our ion beam. When the neutrons enter the moderator, they run into heavy water (water in which every molecule contains deuterium instead of hydrogen), which absorbs energy from the neutrons as they collide with molecule after molecule. By the time the neutrons exit the moderator, they have lost most of their energy and cooled to thermal equilibrium, making them ideal for applications such as neutron imaging.
What Is a High Flux Neutron Generator Used For?
Neutron generators optimized for high neutron flux are important for any application involving thermal neutrons, as well as applications such as neutron radiography which only use a small fraction of the total neutron yield.
High Flux Neutron Generators for Neutron Radiography
A high neutron flux (as well as a high yield) is necessary for both thermal and fast neutron imaging. In neutron imaging, all the neutrons used to create the image need to be traveling in the same direction with little variance in order to create a sharp, clear image.
In order to make the neutrons in the imaging beamline as parallel as possible, neutron radiography systems must use a collimator which restricts the passage of neutrons, filtering out ones that exit the source at odd angles and enforcing a uniform direction. Because so few of the neutrons produced by the system end up being at all useful to the process, having a high flux to start with is vital.
The Phoenix Neutron Imaging Center uses high flux neutron generators to power both a fast neutron imaging and thermal neutron imaging system. Our system is the first compact, accelerator-based neutron imaging system in the world that can produce ASTM Category I images. ASTM Category I images are the highest image quality level specified by ASTM E545, the gold standard for defining the quality of neutron radiographs.
High Flux Neutron Generators for Radiation Survivability Testing
Producing a high neutron flux is important for radiation survivability testing and damage assessment, in which items such as radiation shielding for nuclear reactors and electronics for spacecraft and satellites are blasted with radiation in order to test how well they hold up. For any system that operates in a high-radiation environment, especially in fields such as aerospace, defense, and energy, knowing how much radiation an item can take before failure and understanding what radiation damage looks like on these items is crucial.
Phoenix’s high flux neutron generator technology offers unprecedented abilities to perform radiation survivability testing and damage assessment. By coupling its gaseous-target, deuterium-tritium (DT) neutron generator with a subcritical LEU assembly, it is possible to perform steady state irradiation testing with performance exceeding all existing solutions.