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Neutron experiment

What are neutrons? Our bodies are actually made of a large number of neutrons, as shown in the figure below. Roughly 70% of the human body is made up of water. A water molecule is composed of one oxygen atom and two hydrogen atoms, an oxygen atom is composed of an oxygen nucleus and eight electrons, and an oxygen nucleus is composed of eight protons and eight neutrons.

What kind of object is a neutron?

The size of a neutron is 0.000000000000001 m (that is, a thousand trillionths of a meter). If the cross-section of a hair were enlarged to the size of the earth, a neutron would have a cross-section about the size of a hair. Neutrons are very small particles, and are known to exhibit strange characteristics whereby they behave like waves, even though they are particles.

The most common experiments conducted using neutrons are scattering experiments; in them, a neutron beam is targeted at the substance (sample) under investigation and the manner in which they fly off (scatter) is measured. A scattering pattern showing the structure of the sample on an atomic or molecular scale is produced due to the wave-like characteristics of the neutrons involved in the scattering. Atomic distributions in crystals and the structures of liquids and gasses can be investigated by carefully analyzing these scattering patterns. Like X-ray radiograph, neutrons can also be used to observe a sample's interior. This experimental method is known as imaging.

Experiments similar to those conducted with neutrons can be performed with X-rays or electron beams; however, information that cannot be obtained using X-rays or electron beams can be extracted with neutrons by taking advantage of their following characteristics:

Neutrons have no electric charge

X-rays and electron beams interact with electrons orbiting the nucleus. There are many electrons in a substance, thus the X-rays and electron beams are immediately scattered and/or absorbed upon entering the sample and cannot penetrate to the sample interior. Meanwhile, neutrons — which have zero electric charge — do not interact with electrons and can subsequently bypass them. For this reason, neutrons have high transmissivity and can be used to study deep inside a sample.

Neutrons can distinguish isotopes

Elements with a small atomic number, such as hydrogen or lithium (referred to as light elements due to the low mass of their atoms), contain a small number of electrons, making them difficult to observe using X-rays or electron beams. Meanwhile, although the responsiveness of neutrons can vary with the type of atomic nucleus (e.g., hydrogen, oxygen), it is not dependent on atomic number. For this reason, neutrons are as sensitive to light elements as they are to heavy elements (e.g., metals). Hydrogen and lithium are particularly favorable observation targets for neutrons. Additionally, even for the same hydrogen nucleus (having one proton), isotopes containing different numbers of neutrons (e.g., hydrogen: one proton; deuterium: one proton + one neutron) each have a different response to neutron beams. These characteristics can be exploited when conducting advanced experiments, in that specific regions of interest can be tracked by measuring at what point the isotopes of these specific atoms have been replaced.

Neutrons are small magnets

Neutrons exhibit a zero electric charge (i.e., are neutral), but weak magnetic properties are induced by self-rotation. For example, to create a powerful magnet, the magnetic characteristics of the electrons and atoms in a substance interior must be determined, and these properties can be evaluated by exposing the material to neutrons. Moreover, neutrons can spin like a top depending on the strength or orientation of the magnetic field generated by the electromagnets; thus, studies investigating the properties of magnetic fields in certain environments (e.g., inside a motor) are also possible.

Neutrons are sensitive to atomic and molecular structure

The wavelength of a neutron is roughly equivalent to the size of an atom. Neutrons scattered in a sample interior generate interference patterns that reflect the arrangement of the constituent atom. We can study the sizes or structures of molecules from these intensity distributions.

Neutrons are sensitive to atomic and molecular dynamics

The atoms and molecules that comprise a sample are never stationary. For this reason, neutrons scattered inside the sample can undergo velocity changes through energy transfers, depending on the atomic or molecular dynamics (this is similar to bunting in baseball-the ball's momentum is impeded by lightly tapping it with the bat, so that the ball rolls slowly). Therefore, we can study the movement of atoms and molecules inside a sample by measuring these velocity changes.

Neutron experiments exploit the above characteristics to research the properties of a variety of substances and materials, including superconductors, battery materials, hydrogen-absorbing alloys, polymer materials, food products, and proteins. A variety of neutron instruments are set up at J-PARC MLF, where we conduct research across a wide range of disciplines, including physics, chemistry, and biology.

Details on neutron experiments

Elastic scattering experiments

This is an experimental method that evaluates the average structures of substances by measuring the scattered neutrons that do not exchange energy with magnetic moments or nuclei within the samples. The structural information of atoms and molecules, as well as that of their aggregates, can be obtained by analyzing the scattering intensity of neutrons depending on the neutron wavelength and scattering angle. According to the specimen morphology and the size of structure to be evaluated, the following experiments are available at MLF.

Single crystal diffraction method (iBIX, SENJU)

This is an experimental method for investigating the arrangement of atoms, molecules, and magnetic moments inside a sample from the diffraction patterns of single crystals. Accurate crystal structure analyses are possible because the diffraction intensity of each spots can be measured separately. Two instruments are available, depending on the crystal unit cell size to be measured.

Powder diffraction method (SuperHRPD, SPICA, PLANET, TAKUMI, iMATERIA)

This is an experimental method for evaluating the structural information of polycrystal samples from their diffraction patterns. This method can be used in crystal structure analyses for samples where large crystals cannot be produced, and/or experiments under various sample environments. Five instruments are available depending on the required resolution and sample environment such as operando-measurement for battery researches, high-pressure for geoscience, and residual stress measurement under loading for mechanical engineering.

Total scattering method (NOVA)

This is an experimental method for evaluating the structure of a substance with an irregular formation (e.g., liquids or amorphous materials) by measuring neutrons with short wavelengths and high scattering angles. Properties such as the coordination number of atoms can be evaluated by analyzing the interatomic distance distribution in the irregular structure, using a technique known as the PDF method.

Small-angle scattering method (TAIKAN)

This is an experimental method for evaluating nanoscale structures composed of aggregates of atoms and molecules, performed by measuring neutrons with a long wavelength and small scattering angle. A size, morphology, and interparticle distance of polymers, proteins, nanopowders, etc. can be evaluated in the scale of the nm to sub-µm.

Reflectivity method (SOFIA, SHARAKU)

This is an experimental method for evaluating the structures of surfaces and interfaces by measuring the reflectivity of neutrons on thin films on substrates or free liquid surfaces. The refractive index distribution of neutrons can be evaluated not only in the depth direction on the nm to sub-µm scale but also the in-plane direction on the sub-µm to tens of µm scale.

Inelastic scattering experiments

This experimental method evaluates the dynamics of atoms and/or their spin (vibration, relaxation, diffusion, etc.) by measuring energy exchange between neutrons and the specimen on scattering. Information relating to the dynamics inside the substance can be evaluated by measuring the difference in the energies of neutrons before and after scattering precisely. At MLF, the following experiments can be conducted, depending on the energy scale and required resolution for observing the dynamics.

Direct-geometry time-of-flight spectroscopy (4SEASONS, AMATERAS, HRC)

The pulsed neutrons illuminated on the sample are monochromatized by using a rotating body, so-called a chopper. Neutron energies after scattering are determined by the time of flight from the sample to the detectors. This type of spectrometer can measure the dynamics in wide range from the meV to eV, and three instruments covering different energy ranges are available in MLF.

Inverted-geometry time-of-flight spectroscopy (DNA)

Neutrons scattered by the sample are monochromatized by using analyzer crystals. The neutron energies before scattered the sample are determined by the time of flight from the neutron source to the detector. This type of spectrometer with the near-backscattering geometry allows for the investigation of low energy dynamics with the resolution of the μeV scale.

Spin echo spectroscopy (VIN ROSE)

As neutrons have magnetic moment, the spin rotation occurs like a top in a magnetic field (Larmor precession), in which the number of the spin rotation is dependent on the strength of the magnetic field and the time of flight in the magnetic field. This method utilizes the spin rotation as an index of the time of flight after scattering, that is, the number of the rotation of the scattered neutron changes if the change in the neutron velocity happens on the scattering process due to the dynamics of the sample. The sensitivity to the velocity change is quite high under the strong magnetic field, and dynamics at extremely low energy levels (neV scale) can be investigated using this technique.

Imaging / analysis experiments

New analysis and visualization techniques that actively utilize the characteristics of neutrons are emerging and bearing fruit at MLF. These novel experimental methods are being used for the research of cultural artifacts and commercial product development in industry. Experiments based on the methods outlined below are conducted at MLF, depending on the analysis subject and content.

Neutron imaging (RADEN, NOBORU)

This method non-destructively visualizes the structures, etc., inside substances by using the high transmission capabilities of neutrons. This is similar to the neutron-based version of X-ray radiograph; however, neutrons are capable of visualizing light elements such as hydrogen and lithium hardly seen by X-ray, analysis of the wavelength dependencies of neutron transmission allows for the mapping of residual stresses or crystal structures inside substances, and the magnetic moments of neutrons can be used to determine the distribution, intensity, and direction of magnetic fields.

Elemental analysis (ANNRI, RADEN)

Like characteristic X-rays,γ-rays are emitted on the reactions between neutrons and nuclei, in which the γ-ray energy is dependent on the nuclei. This enables us to evaluate element types and its content in a sample quantitatively. Samples can be rapidly and non-destructively analyzed, even if they contain complex constituent elements.

Neutron irradiation (NOBORU)

Cosmic-ray neutrons irradiating onto Earth are known to induce equipment malfunctions through collisions with the semiconductors in electrical equipment. In order to investigate these effects, we can analyze the impacts that radiation damage has on machines, by exposing them to high-energy neutrons (MeV scale).

Neutron test beam port (NOBORU)

Preliminary experiments with neutron beams are essential for the research and developments of novel neutron experiment ideas. Here at MLF, we operate a test beam port to fulfill these requirements, and many new technologies have been developed as a result.

Nuclear physics / fundamental physics with neutrons

Most of the neutron instruments in MLF are used for material sciences; whereas some of them can be used for the research on the nature of the nuclei and neutrons, which is deeply related with fundamental physics. Here at MLF, the following experiments pertaining to nuclear and elementary particle physics are being conducted.

Neutron-nucleus reaction measurement instrument (ANNRI)

Gamma rays can be generated through neutrons being captured by the nucleus. The likelihood of this occurring is known as the "neutron capture cross-section;" here at MLF, the neutron capture cross-section of nuclei can be measured with an extremely high accuracy. The data obtained from these experiments have been used for element synthesis processes in space and the reduction of radioactive waste lifetimes.

Fundamental neutron physics instrument (NOP)

Studying the properties of the neutron will lead to an understanding of the origin of the universe or particle physics. Here at MLF, we operate specialized instruments for experiments that uncover the mystery of the "neutron itself".