Neutron Generators

What Is a Neutron Beam?

By August 27, 2020September 8th, 2020No Comments

We use this photograph a lot on our website:

Ion Beam

It is a very striking and beautiful image, but it is not a neutron beam, as one might assume. Rather, this image shows the high-energy ion beam we use to create neutron radiation. What you are seeing in the above image is actually superheated plasma from an ion source. At such high temperatures, the plasma, comprised of deuterium ions, creates a stunningly ethereal bluish-violet glow.

Actual neutron beams, though, aren’t much to look at. Like most forms of radiation, neutron radiation isn’t visible to the naked eye, and so it isn’t quite so easy to get an awe-inspiring, captivating look at neutron radiation (and therefore, neutron beams).

Your typical colorful photographs of stars, galaxies, and nebulae have been altered and color-corrected to depict X-rays, gamma rays, ultraviolet light, radio waves, and other forms of radiation on the electromagnetic spectrum within the range of our eyesight, since very only a tiny fraction of the light astronomical objects give off can actually be seen with our eyes.

Since neutrons aren’t a part of the EM spectrum, the approach we use to colorize photographs from the Hubble Space Telescope doesn’t quite work. Neutron beams are invisible, but that doesn’t make them any less potent.

What Is A Neutron Beam?

A cyclotron used to produce neutron beams for radiation therapy at the University of Washington uses a collimator to focus and limit the beam

A cyclotron used to produce neutron beams for radiation therapy at the University of Washington uses a collimator to focus and limit the beam

A neutron beam is a focused stream of neutrons. However, neutron emitters tend to release neutrons in all directions instead of just in a focused neutron beam, so special technology has to be employed to wrangle the neutrons into shape. By using neutron-absorbing material around the emission source and providing a single aperture, a fraction of the neutron output can be emitted in a single direction. These focused neutron beams much more useful for industrial and research applications.

For many industrial uses for radiation, the radioactive emissions of your source need to be corralled into a neat and orderly beamline—the neater and more orderly the neutron beam, the better. In radiography, for example, the more disorderly your radioactive output is, the blurrier and more indistinct the resulting image will be. In this example, the neutrons traveling outward need to be traveling as parallel to each other as possible. This requires more special neutron-absorbing and focusing technology.

Our Technology to Focus Neutrons

Creating focused neutron beams is actually quite challenging, even compared to creating the neutrons themselves. In order to focus radiation into a beam and carefully manage the neutron beam divergence to ensure maximum effectiveness, we use a device called a collimator, or beam limiter.

Phoenix's neutron imaging system uses collimators to ensure detailed, crisp neutron images meeting ASTM Category I quality standards

Phoenix’s neutron imaging system uses collimators to ensure detailed, crisp neutron images meeting ASTM Category I quality standards

A collimator narrows and focuses a beam of particles and waves. Collimators work as a filter for radiation, allowing parallel streams to pass through while blocking streams that approach at an angle. Although only a trickle of the total radiation produced will make it through, this is the price you pay for a crisper and sharper—and thus, usually, more useful—beamline.

Because in these situations, only the focused fraction of the total neutron output is actually used, the total neutron output of a neutron source ceases to be a useful metric. Instead, the important metrics are neutron flux, which measures how many neutrons pass through a given area (neutrons per square centimeter) and neutron fluence, which measures how many neutrons pass through a given area over a certain length of time (neutrons per square centimeter per second).

For focusing neutron radiation, we use a collimator of our own design made out of proprietary neutron-absorbing material. Only parallel streams of neutrons, or at least as parallel as possible, pass through the collimator without being impeded. How parallel the beamline is depends on the ratio between the collimator’s length and the diameter of its aperture.

Our Technology to Moderate Neutron Temperature

One important quality of a neutron beam is its temperature—how much energy the neutrons traveling in that beam have.

The temperature of a given free neutron depends on the nature of the fusion reaction it was born from. For example, a deuterium-deuterium or DD fusion reaction produces free neutrons with average energies of around 2.5 MeV (mega-electron-volts). A deuterium-tritium or DT fusion reaction produces neutrons of around 14 MeV.

Different temperatures of neutron beams are useful for different applications, because how much energy the neutron has can influence how it reacts with materials. For example, thermal neutrons, which have energies of around 0.025 eV (about ten million times less energy than DD neutrons!), can have a larger neutron absorption cross-section than neutrons that have more energy. Fast neutrons, which have temperatures between one and twenty MeV, are critical for jump-starting fission reactions due to their higher energies. There are various categories of different temperatures of neutrons ranging from cold (anything colder than thermal neutrons) to ultrafast (neutrons moving at relativistic speeds), but fast and thermal neutrons are some of the most important for industrial applications.

In order to turn the fast 2.5 MeV neutrons coming out of a DD neutron generator into thermal neutrons, we employ a device called a moderator to slow the neutrons down. A moderator consists of neutron-slowing material. In many moderator setups the material used is heavy water. Heavy water molecules are just like normal water molecules, except with deuterium atoms in place of hydrogen. One property of heavy water is that when neutrons in the neutron beam collide with the deuterium within, they lose energy. This decreases their temperature, and after countless collisions, the neutrons in the neutron beam exit the moderator at the proper temperature.

What Are Neutron Beams Used For?

Neutron beams have a wide variety of applications in industrial testing and research. Neutron radiation has unique properties compared to other forms of radiation, which create unique opportunities in many fields ranging from destructive and non-destructive testing to fusion research to even archaeology and art history.

Neutron Beams and NDT (Non-Destructive Testing)

A comparison of a 100kV X-Ray, 190 kV X-ray, and neutron Image of Fuel Injectors.

A comparison of a 100kV X-Ray, 190 kV X-ray, and neutron Image of Fuel Injectors.

Neutron beams are especially useful in nondestructive testing, most notably for neutron imaging/radiography. A collimated neutron beamline can be used to create radiographic neutron imaging of an object’s internal structure, just as can be done with a beam of X-rays or gamma rays. Because neutrons have special properties compared to electromagnetic radiation, neutron radiography provides a unique perspective compared to X-ray and gamma radiography and can be used to identify defects and flaws that other forms of testing might miss. Neutron radiography can be done with thermal or fast neutrons. Traditionally, researchers performed neutron imaging using research reactors as the neutron source, but Phoenix has developed other neutron source options in the form of accelerator based D-D and D-T neutron generators.

Neutron Beams and NAA (Neutron Activation Analysis)

One particular wide range of applications for neutron beams has to do with a physical phenomenon known as neutron activation. When a neutron beam passes through a material, the material’s atoms could absorb some of the neutrons, gain extra energy, and then shed this extra energy by releasing a mild burst of radiation.

By analyzing the radiation produced by neutron activation, you can actually determine a material’s composition very precisely. For this reason, NAA (neutron activation analysis) sees use not only in industrial non-destructive testing and materials research, but also in areas such as art history, archaeology, geology, and agriculture. Neutron beams can even be used to detect explosives and narcotics!

Other Applications

In addition to NDT methods such as neutron imaging/neutron radiography, neutron beams can also be employed for destructive testing methods, such as radiation hardening and survivability testing. These tests involve bombarding materials with ionizing radiation, such as neutron radiation, in order to see how much radiation the materials can withstand before physical flaws begin to manifest. Radiation exposure can cause materials to become brittler or lose tensile strength, for example, and electronics can fail to operate properly.

Neutron scattering is another powerful research application of neutron beams. In neutron scattering, researchers explore how atoms are arranged in a material by firing a neutron beam through it and observing the way the beam diffracts in transit through the material. Neutron scattering uses cold or ultracold neutrons, which have temperatures far lower than thermal neutrons and can easily be trapped or scattered within materials. Neutron scattering is frequently used in materials science to investigate hydrogen-rich compounds and biological materials. Another type of neutron scattering, called inelastic neutron scattering, allows researchers to study atomic and molecular phenomena, such as atomic movement and excitation.