Neutron Radiation, Emission and Scattering

Neutron radiation typically refers to the movement and emission of free neutrons and can subsequently include how they will interact with matter. Free neutrons are typically created by emitting or ejecting a neutron from the nucleus of an atom.

Neutrons are electrically neutral particles contained within the atomic nucleus. They have no innate charge, unlike electrons or protons. This neutrality allows them to penetrate deeply into matter without significant electromagnetic interaction, making them valuable probes for investigating the structure of materials.

In terms of charge, neutrons are neutral. However, they have a magnetic moment which enables them to interact with magnetic fields within materials, providing insights into magnetic phenomena. Unlike photons, neutrons have mass and interact via the strong nuclear force, enabling them to induce nuclear reactions.

 

How Are Free Neutrons Created?

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In order for neutrons to penetrate a material, they need to be released from a nucleus. This neutron is then known as a free neutron. Free neutrons are primarily generated through nuclear reactions. For example, in nuclear fission, a heavy nucleus splits releasing neutrons as byproducts. Additionally, certain radioactive decay processes, particularly those involving alpha particle emission, can induce nuclear reactions that subsequently release free neutrons.

Fission

Nuclear fission produces two or more lighter nuclei, known as fission products, along with the release of a significant number of fast neutrons. The exact number of neutrons emitted per fission event varies depending on the specific fissioning isotope.

A crucial parameter characterizing fissioning isotopes is their "neutron multiplicity," which refers to the statistical distribution of the number of neutrons emitted per fission event. For instance, the isotope Californium-252 (²⁵²Cf) has a mean neutron multiplicity of 3.8 neutrons per fission, indicating that on average, 3.8 neutrons are released in each fission event of ²⁵²Cf.

Radioisotope and Photoneutron Sources

Neutrons can also be generated through various nuclear reactions. One of these mechanisms involves alpha decay induced reactions, often denoted as (α,n) reactions. In these reactions, alpha particles (helium nuclei) emitted during radioactive decay can interact with other nuclei. If the alpha particle possesses sufficient energy, it can be absorbed by the target nucleus, exciting it to a higher energy state. This excited nucleus subsequently de-excites by emitting a neutron.

Here is a common example of an (α,n) reaction involves the interaction of alpha particles from Americium-241 with Beryllium. Americium-241 is a strong alpha emitter, and when combined with Beryllium, it serves as a convenient source of neutrons. However, the neutron yield from this source is relatively low, with approximately seventy neutrons produced for every million alpha particles emitted by Americium-241.

Another method for producing neutrons involves photoneutron reactions. In these reactions, high-energy gamma rays interact with certain nuclei, such as Beryllium or Deuterium (hydrogen-2), leading to the emission of neutrons. However, this process requires gamma rays with sufficiently high energies, typically originating from nuclear decay processes.

Controlled Generators (DD/DT)

Achieving nuclear fusion artificially to create a net energy gain remains a significant research challenge. However, it's important to note that controlled fusion reactions can be achieved in laboratory settings.

By accelerating deuterium nuclei (hydrogen-2) towards each other (D-D fusion) or towards tritium nuclei (hydrogen-3) (D-T fusion) using high voltages (typically several hundred thousand volts), it's possible to overcome the electrostatic repulsion between the positively charged nuclei. This allows them to fuse together, forming a heavier nucleus (usually an isotope of helium), releasing a significant amount of energy. A significant amount of that kinetic energy is carried by a neutron.

These fusion reactions produce neutrons with characteristic energy levels: about 3 MeV for D-D and 14 MeV for D-T reactions.

Devices that utilize these controlled fusion reactions to produce neutrons are known as "neutron generators." A key advantage of these generators is their on-demand nature. Neutron emission only occurs when the generator is actively powered. This "on-off" capability distinguishes them from radioactive neutron sources, which continuously emit neutrons.

Spallation

Free neutrons can also be produced through a process called spallation. In spallation, a nucleus is hit by a very high energy particle, which essentially breaks the nucleus apart. This involves aiming high energy protons towards target made of heavy metal, such as Tungsten. This processing requires energetic protons, typically around 1 GeV, created by accelerating them to 84% of the speed of light within a proton accelerator. 

After being hit by the high energy proton, neutrons are ejected from the heavy metal target in pulses which can be easily controlled, produce a measured high energy neutron beam. This is much cheaper than producing neutrons from singular fission reactions, but much more expensive than producing these from a fission reaction chain or nuclear reactor. This technique is used for producing neutrons at some leading nuclear research facilities.

Beyond controlled accelerator environments, spallation also occurs naturally in Earth's atmosphere. High-energy cosmic rays interacting with atmospheric nuclei, such as nitrogen and oxygen, induce spallation reactions. This process generates a cascade of secondary particles and produces cosmogenic nuclides like carbon-14 and beryllium-10. These nuclides play crucial roles in radiocarbon dating and geological studies, highlighting the ongoing impact of cosmic ray spallation on our planet.

Neutron Interactions with Matter

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Due to their lack of electric charge, neutrons interact with matter differently compared to charged particles. Charged particles interact primarily through Coulomb forces with the electrons of the medium. Neutrons do not experience significant electromagnetic interactions, so they can penetrate deeply into materials without significant energy loss. They primarily interact with matter through scattering and nuclear interactions.

Elastic Scattering

“Fast” neutrons, with high kinetic energy, scatter elastically with other particles in a medium. In elastic scattering, the neutron collides with the nucleus, transferring some of its kinetic energy to the nucleus. This causes the neutron to slow down, while the nucleus recoils with increased kinetic energy. The kinetic energy imparted to the recoiling nucleus can subsequently lead to ionization or other charged particle interactions within the medium. 

When the neutron possesses very high energy, the energy transferred to the recoiling nucleus during the collision can be sufficient to excite the nucleus to a higher energy level. This excited nucleus then de-excites by emitting gamma-ray photons, resulting in a significant loss of kinetic energy for the neutron.

“Slow” or “thermal” neutrons, possessing low kinetic energy, also undergo frequent scattering interactions with nuclei. However, due to their low energy, they typically transfer only a small amount of energy to the nucleus in each collision. This energy is often dissipated as heat within the medium, contributing to the "thermalization" of the neutron population, where the neutrons slow down to thermal energies.

These scattering processes play a crucial role in the behavior of neutrons within materials, influencing their penetration depth, energy distribution, and overall interaction with the medium. These scattering events and measuring how neutrons scatter from the nuclei within a material can give you interesting structural information abut a sample. In fact, this underpins the fundamental principles of neutron scattering techniques such as Small-Angle Neutron Scattering (SANS).

Radiative Capture

At low kinetic energies, neutrons can undergo a process known as radiative capture, where a target nucleus absorbs a free neutron, increasing its mass number by one. This absorption results in an excited state of the nucleus. To return to a more stable state, the excited nucleus releases the excess energy in the form of one or more gamma-ray photons.

The likelihood of a particular nucleus capturing a neutron varies significantly. This probability is quantified by the neutron capture cross-section. Materials with a high neutron capture cross-section are frequently used as neutron moderators, employed in nuclear reactors to slow down fast neutrons, increasing the likelihood of further fission reactions. Also, materials with high neutron capture cross sections are used to absorb unwanted neutron backgrounds and reduce radiation levels in sensitive detector settings.

Boron is a well-known example of an element with a high neutron capture cross-section:

Furthermore, some radiation detectors use neutron capture reactions to detect certain particles. For instance, the Super-Kamiokande experiment in Japan has a large volume of water containing dissolved gadolinium (a material with a very high neutron capture cross-section). When antineutrinos interact with the water molecules, they can induce inverse beta decay:

This reaction produces a positron and a neutron. The gadolinium nuclei capture these neutrons leading to the emission of gamma rays. These gamma rays (and their specific energies) can be detected by the experiment. This provide a crucial signature for the detection of antineutrinos.

Neutron Energy Transfer Within a Material

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Neutrons interact with matter differently compared to charged particles and photons, but their distance travelled through a material still follows an exponential probability distribution:

where I(x) is the intensity (number of neutrons) at a depth 'x' within the material, I(0) is the initial intensity of the neutron beam, and 'S' is the macroscopic cross-section, a parameter that represents the probability of a neutron interacting with the material through various mechanisms, including scattering, radiative capture, and other nuclear reactions.

This equation indicates that the number of neutrons surviving within the material decreases exponentially as they penetrate deeper. The macroscopic cross-section, 'S', is a crucial parameter that determines how neutron will move within a material. This cross-section will depend on the properties of the material, such as its density and atomic composition, as well as the energy of the incident neutrons.

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