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last update: 10/07/07
|Using neutrons to
see waves in a solid
In March 2010, construction was mostly completed on a new neutron
spectrometer at the Japan Proton Accelerator Research Complex (J-PARC);
the new device is the High Resolution Chopper Spectrometer (HRC). The
HRC will explore the hidden motion of atoms within solid materials. Read
here about how the HRC pushes the boundaries of neutron spectroscopy,
and how the state-of-the-art equipment the HRC team developed has made
The atoms within a solid are not entirely still. They vibrate. They also interact with each other. This means that an excitation in one atom can be transmitted to its neighbors, then transmitted to their neighbors, and so on. This moving collective excitation is called a wave.
There are several different types of excitation which can travel through a solid in waves. For example, take lattice vibration in the crystal structure of a solid. The vibrational excitation in a lattice gives rise to wave modes, called phonons. Phonons play a crucial role in determining the physical properties of solids. Another example is a spin wave. Each atom in solid sometimes acts like a tiny magnet, and this characteristic is called spin. The excitation of spin states can also cause waves. The quantized modes of a spin wave are called magnons.
The purpose of the new HRC, which is located at the Japan Proton Accelerator Research Complex (J-PARC), is to measure how waves propagate in solids. It will do so with high resolution, and it has the ability to observe waves with a wide range of energy and momentum. Specifically, the HRC is expected to produce high resolution measurements at energy levels up to a thousand times greater than that of conventional neutron spectrometers installed at research reactors. Additionally, the HRC will be able to make measurements of spin waves on both crystalline samples and powder samples. Ordinary neutron spectrometers are limited to just crystalline samples.
Built in a collaborative effort between KEK and the University of Tokyo, the HRC is the result of the team’s persistence, and their commitment to precision engineering. In particular, KEK Neutron Science Division (KENS) and KEK Mechanical Engineering Center (MEC) have worked together on major improvements to several experimental devices, including large-area aluminum windows, the T0 (pronounced ‘t-zero’) chopper, and the Fermi chopper.
How a neutron chopper works
Neutron scientists produce neutrons by bombarding the neutron target with J-PARC’s high intensity pulsed proton beam. The generated high energy neutrons are moderated to give the lower energy neutrons. Neutrons in each pulse, whose energies ranging from milli-eV to GeV, travel inside neutron mirror guide at different speeds.
In standard neutron scattering, momentum is also transferred either to or from the atoms in the sample. In general, the scattering angle is related to the momentum transfer, which is inversely proportional to the wavelength of the neutrons. This means that for monochromatic neutrons, such as those that come out of the choppers, the momentum transfer can be directly calculated from the angle of scattering. The scattering angle is measured by the detector.
Knowing both energy transfer and momentum transfer of each neutron detected, scientists can produce a map of the number of neutrons on an energy-momentum space. A Fourier transform of the momentum transfer gives spatial information about the atomic structure of the sample. On the other hand, the energy transfer gives information about how strongly neutrons interacted with the atoms. Neutrons interact differently with different types of atoms.
Neutron spectroscopy gives information which complements that given by other spectroscopy methods, such as X-ray spectroscopy. Because neutrons are electrically neutral, they do not interact electrically with the electron clouds surrounding the nucleus. This means that they directly interact with nucleus of the sample atoms, energetically exciting or relaxing them. Neutrons also have spin, which allows them to interact magnetically with the electrons and nucleus of atoms in the samples.
On the other hand, when looking at spin waves in a one-dimensional magnet, a plot of energy transfer versus momentum transfer shows clear evidence of spin waves propagating through the magnet.
High resolution, high energy,
and the first Brillouin zones
The primary goal of the HRC team is to achieve the highest energy resolution in chopper neutron spectroscopy. Currently choppers have an energy resolution of a few percent with respect to the incoming neutron energy. The HRC aims for a 1 percent energy resolution at an energy level of one eV.
However, this is just the beginning. The dream of the HRC team is to access what is called the first Brillouin zone, a space the size of a single cell in a periodic medium. Generally, in order to observe spin waves by inelastic neutron scattering, the sample must be a single crystal with a mass of some tens of grams. Growing such a large crystal is very difficult at best, and impossible for many materials. Samples that cannot be crystallized can only be accurately observed with a spectrometer that has a resolution that allows scientists to access the first Brillouin zone. For a powder sample, spin wave signals get smeared out in higher momentum region due to the random orientation of crystals. By accessing the first Brillouin zone, scientists can observe spin waves that still remain intact.
To access the first Brillouin zone, scientists need to obtain energy and momentum distribution of neutrons that are scattered at very small angles. This is challenging, because the small angle region is close to the neutron beam’s center where there is severe background noise. To cope with this, the HRC setup allows a longer distance between the sample and the detectors for scattering angles within ten degrees in both horizontal and vertical directions. This is so that the team can closely explore the low scattering angle regime.
The team’s ultimate goal is to explore the very high energy region, from sub eV to more than one eV. This is an unexplored region both for neutron spectroscopy and for basic neutron science. “In addition to the eV extension of molecular and material neutron spectroscopy, there are theoretical predictions which have yet to be tested,” says Itoh. “However, these advancements will require a one megawatt proton beam, so it will be a while before we can explore this.”
Designing an ultrafast T0 chopper
Designing and developing the high resolution spectrometer required very high precision engineering. Since 2002, the HRC team has worked with KEK's MEC to conduct the development and rigorous testing of the HRC experimental devices.
“The gap between 50 hertz and 100 hertz is large in terms of mechanical difficulty. The coaxial parts must be aligned within 10 micrometers, and the rotor timing precision needed to be within 5 microseconds,” says Itoh. The completed T0 chopper operated beyond their expectations. The timing was off by less than 1 microsecond. “The T0 chopper pushed the limits of precision engineering in every aspect.”
The team and the MEC also developed aluminum windows for the vacuum chamber and the Fermi chopper. The 1-meter square aluminum windows, placed in contact with the detectors, which sit just outside of the vacuum chamber, needed to be 1-millimeter thick, and yet be strong enough to endure the atmospheric pressure. The Fermi chopper—not to be confused with the T0 chopper—monochromatizes neutrons using a rotating disc with slits in a cylinder which is 10 centimeters in diameter. The Fermi chopper requires the disc to rotate without friction, for which scientists utilized a magnetic rotor bearing. “We now have in-house technology to build the necessary components of the aluminum windows and the Fermi chopper,” says Itoh.
The Helium predicament and prospects
There is in fact a predicament that all members of the neutron experiment community throughout the world find themselves in: a shortage of helium-3. The helium-3 gas detector is the most promising, and best-tested, high precision neutron detector. Due to the security policy currently embraced by the US—the world's sole helium-3 supplier—that inhibits Helium-3 exports for the use beyond security, neutron spectroscopy scientists stand at a crossroad. They can either wait for the policy to change, or develop new detectors.
However, the HRC team has plans to improve the resolution of the spectrometer, even with the partial lack of the detector sections. First, they are developing a design to reduce noise by installing neutron shields. When the beam hits the walls of vacuum chamber, neutrons are scattered inside the chamber, creating noise. Second, in conjunction with MEC, the team is looking for good materials for the slit used in the Fermi chopper. The slit needs to be made of highly effective shielding material to block the high energy neutrons, and also needs to be as thin and strong as possible to allow for the high frequency rotation.
“Even with the difficulty of the missing detectors, we are making solid steps forward. New data is coming out, and we hope that the cutting-edge science will soon become possible at the HRC,” says Itoh.
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