All of us probably know what neutrons are, or have at least heard of them back in physics class. Yet these little bundles of quarks are much more than just filler inside an atom’s nucleus. In addition to being an essential part of making matter as stable as it (usually) is, free neutrons can be used in a variety of manners.
From breaking atoms apart (nuclear fission), to changing the composition of atoms by adding neutrons (transmutation), to the use of neutrons in detecting water and inspecting materials, neutrons are an essential tool in the sciences, as well as in medicine and industrial applications. This has meant a lot of development toward the goal of better neutron sources. While nuclear fission is an efficient way to get lots of neutrons, for most applications a more compact and less complicated approach is used, some of which use nuclear fusion instead.
In this article we’ll be taking a look at the many applications of neutron sources, and these neutron sources themselves.
The Humble Neutron
Strictly defined, a neutron is a hadron, just like a proton. What this means is that they both are composite particles which consist out of two or more valence quarks. In the case of both the proton and neutron we find that they contain three quarks. This makes them the baryon sub-type of hadrons. The difference is that a neutron has two down quarks and one up quark, whereas a proton has two up quarks and one down quark. Confusing as this may sound, it’s essential to understanding how the sub-atomic world affects everything around us.
Outside of a nucleus, a neutron is unstable, with a half-life of approximately 10 minutes and 11 seconds, after which it beta decays into a proton. One of the neutron’s down quarks emits a W– boson which then decays into an electron and an anti-neutrino:
Or using the quark notation:
About 1 in 1,000 of these beta decays also produce gamma radiation, which is a form of internal bremsstrahlung. This occurs when the emitted beta particle (the electron) interacts with the (positively charged) proton.
Generally a Bit Unstable
Inside a nucleus a neutron isn’t necessarily stable, either. A nuclide (collection of bound neutrons and protons) forms a quantum mechanical system, which may or may not form a stable energy state. Essentially, if there is a lower energy state available within the nuclide, neutron decay will occur. An example of this is carbon-14 (6 protons, 8 neutrons) which decays to the more stable nitrogen-14 (7 protons, 7 neutrons).
During bound neutron decay we see the same process as for free neutron decay. This is distinct from inverse beta decay and electron capture, two processes through which a proton in a nuclide can transform into a neutron. The latter type being useful for our purposes, as we will see in the next section.
Generating Free Neutrons at Will
Since neutrons cannot exist for extended periods of time outside of a nuclide, this means that free neutrons have to be generated where they are needed. The easiest way to accomplish this is to take an isotope which exhibits neutron emission as part of its radioactive decay chain, such as californium-252 or berylium-13:
The obvious disadvantage of using a radioactive isotope as neutron source is that it cannot be turned off, and will emit fewer and fewer neutrons per time unit as more of its nuclides settle into a new state which will be either stable or not emit neutrons when they decay further. A partial solution here is the use of photodisintegration, whereby a gamma ray is used to excite a nuclide to the point where it emits a neutron. For example in the case of the only stable form of berylium:
This interestingly leads us to another fascinating source of neutrons: nuclear fusion of deuterium and tritium (D-T):
The relative ease of using (cheap) D-T fuel in a simple inertial electrostatic confinement (IEC) device (like a fusor) has led to them to become a popular neutron source for both scientific research and medicine, allowing for a device that generates neutrons at will, and at a constant rate over the device’s lifetime.
Neutrons on Mars
The Curiosity rover which has been doing science on Mars for years now has an instrument on board which is called the DAN, which is short for Dynamic Albedo of Neutrons. It uses a neutron beam aimed at the ground from about 80 cm height. The neutrons which are scattered back to its sensors after interacting with the soil provide information about the soil’s contents, specifically its moisture content. This is caused by the interaction of neutrons and hydrogen.
This DAN instrument uses an ING-10K neutron tube-based pulse neutron generator, manufactured by VNIIA (a Rosatom company). It emits 107 neutrons per pulse, rated for 107 pulses during a 3-year lifespan. These neutron generators generally use D-T fusion, using a linear accelerator to accelerate deuterium, tritium, or a combination thereof into a metal hydride target, which also contains these isotopes. With enough energy to overcome the coulomb barrier, these isotopes’ nuclides will fuse and neutrons are emitted.
Neutrons in Medicine
Neutron tubes are similar to the colliding beam fusion concept, along with that of the aforementioned fusor and the polywell in that they use the principle of IEC to achieve fusion. The fusor uses two spherical grids, using opposing charges to accelerate the isotopes, whereas the polywell does essentially the same, but seeks to eliminate these physical grids, to increase the efficiency of the fusion reaction.
Regardless of the exact configuration, such IEC devices have found significant interest in the field of medical
isotopes, specifically molybdenum-99 (99Mo). This particular isotope is the precursor to metastable technetium-99m (99mTc) — its decay product — before the 99mTc decays to 99Tc with a half-life of 6.01 hours. Here, 99mTc is crucial in medicine as a radioactive tracer, seeing regular use for imaging studies of the body as it emits clearly detectable, 104 keV gamma rays.
Unfortunately, the main source of 99Mo is from a small number of fission reactors in which uranium-235 targets are bombarded with neutrons. With these reactors suffering extended downtime due to maintenance and replacement reactors having been cancelled, a shortage of 99Mo is becoming critical. This has led to alternatives being explored, one of which is the use of IEC fusion devices as the source of neutrons.
A promising approach by a company called Phoenix LLC uses a linear accelerator fusion device to generate neutrons which irradiate a uranium target, causing it to fission and generate the 99Mo and other isotopes. The 99Mo can then be separated and transported to hospitals in a technetium-99m generator as is already commonly done every day. This 99Mo production is supposed to start in 2021 with commercial scaling projected for 2022, according to Phoenix.
The imaging of objects using neutrons is very similar to that of using X-rays, with the main difference being the way
that they produce an image. With X-rays, the resulting image depends on the density of the materials encountered, so that the final image depends on how much the X-rays were attenuated. With neutron imaging, the interaction with the materials determines how many neutrons will reach the sensor and what their physical (molecular) properties are. The result is something similar to an X-ray, but with important differences due to how neutrons interact with the object relative to X-rays, as the image to the right illustrates.
In neutron imaging, after the neutrons have been produced, they have to be slowed down to the desired speed for the imaging. The speed of the neutrons will affect the penetration depth and the final imaging results, allowing for fine-tuning the process. The use of neutron imaging ranges from inspecting finished products — including welds, cast parts, turbine blades, nuclear fuel rods and high-precision parts in industry and beyond — to proposed uses like detecting explosives, such as in a war zone.
Distinct from neutron imaging is neutron activation analysis (NAA), which is essentially what the Curiosity’s DAN module does.
The use of nuclear fusion-based neutron generators is becoming more common for neutron imaging and related, because of the potential for smaller, more efficient devices. The US military’s NEMESIS (Neutron-Emitting Mobile Explosives Sensing and Identification System) program is one example of this, which may enable the use of small devices that can detect explosives like improvised explosive devices (IEDs) and landmines with great ease, even for tricky metal-free landmines that metal detectors and ground-penetrating radar struggle to detect.
Involved in the NEMESIS program is the aforementioned Phoenix LLC, which expresses their belief that such NAA as well as neutron imaging devices can in the future be used for not only dangerous tasks, but also more routine ones such as bridge inspections and presumably inspections in avionics. According to Phoenix, most effort currently goes into improving the detection algorithms, and to make the equipment more rugged and economical.
Beyond Science Fiction
Although a lot of this may sound rather fantastical, such as the ability to ‘look’ inside the ground to find any buried explosives there, or to see fractures inside a turbine blade or weld, one can see this as the progression from commonly used technologies like X-rays. Whereas X-rays and their kin were found to be easy enough to produce during the early 20th century and beyond, the process of producing a large number of neutrons in an efficient manner that did not require the use of a nuclear fission reactor has held progress back for decades.
Fusion devices have many advantages, with maintenance being fairly uncomplicated as well. For non-sealed fusion devices, a continuous supply of deuterium-tritum (or deuterium-deuterium) fuel is supplied, with the device running maintenance-free except for the swapping out of components of the generator that become radioactive due to neutron activation. These components fall under low- to medium-radioactive waste, akin to what laboratories and hospitals produce, making for easy disposal.
While the prospect of a handheld scanning device that uses a neutron source to perform environmental analysis is still a while off, neutron imaging stands a good chance of improving life in a lot of ways, much like X-rays have done.