|>Top >Feature Story >this page||
last update: 10/06/09
|NOVA: neutron spectroscopy
for a better environment
The neutron total scattering spectrometer (NOVA)
at the Japan Proton Accelerator Complex is about
to begin searching for hydrogen storage materials.
Learn here about the unique features of the world’s
highest intensity wide-angle spectrometer for
If you ask material scientists what the most studied, yet least understood liquid is, often the answer is water. A water molecule is deceptively simple in structure, composed of one oxygen atom and two hydrogen atoms. The difficulty in imaging such a molecule actually comes from the most abundant element in our universe—the hydrogen atom. X-ray diffraction, the most popular type of spectroscopy, determines atomic structure by looking at how X-rays interact with the clouds of electrons surrounding each nucleus. Since a hydrogen atom has just one electron orbiting its nucleus, its electron cloud is thin, has little effect on X-rays, and so is hard to image using X-ray diffraction.
In the past, the limiting factor for neutron diffraction had been the low intensity of the neutron beams that could be produced at an accelerator. However, with the start of the world’s most intense proton accelerator facility at the Japan Proton Accelerator Complex (J-PARC) in Tokai, neutron spectroscopy is increasingly more accessible and more useful for material scientists, offering a complementary method to the conventional X-ray diffraction.
One of the most important devices at J-PARC for such purpose is the neutron total scattering spectrometer, NOVA. NOVA is a gigantic, five-meter long, three-meter tall, bug-shaped vacuum container with five different detector sections. This device surrounds a sample so as to be able to detect scattered neutrons in all possible directions. NOVA is now almost complete, in good shape to start carrying out experiments late this year.
Materials for hydrogen storage
NOVA’s main ambition is to find an efficient form of hydrogen storage. Hydrogen energy is a promising, clean, energy storage technology in which oxygen and hydrogen are combined to produce energy and nonpolluting water. “Hydrogen atoms are abundant, but are also hard to store,” explains Prof. Toshiya Otomo of KEK, the leader of NOVA group. “To derive sufficient energy from hydrogen gas, the gas must be often stored at a pressure of 300 to 700 hundred atmospheres. You could not casually carry such a heavy, unsafe container on your car.”
“We would not be able to develop a workable material for hydrogen storage by simply mixing and testing. We need to examine what’s going on inside the material at the atomic level. We need to understand the mechanism of hydrogen absorption and desorption in order to develop the most efficient materials,” says Otomo. He and around 30 other members from 7 research institutions in Japan constructed the spectrometer for just such science.
The spectrometer for non-crystalline structure
NOVA will excel in visualizing variety of structures including non-crystalline structures. When hydrogen is absorbed in a crystalline alloy, amorphous—non-crystalline—solid can form, disrupting the regular atomic structure of the crystalline alloy. To obtain high sensitivity to such non-periodic structures, NOVA is designed to detect every neutron diffracted off the sample. Further, in the inelastic scattering of neutrons off each atom, neutrons transfer momentum to the atoms inside sample. NOVA can measure the momentum change, called momentum transfer, of each scattered neutron.
Wide momentum transfer range of NOVA
NOVA’s precursor at KEK, KENS-HIT, looked at mechanical alloying in graphite hydrogen storage. The alloy was composed of layers of graphite, to which hydrogen atoms had adsorbed. By measuring the distance between hydrogen atoms and nearby carbon atoms, the team deduced the positions of the hydrogen atoms. “The KENS-HIT scientists found out that about half of the hydrogen atoms were closely bound to carbon atoms, while the other half were trapped between the graphite layers. The hydrogen atoms that were located further from the graphite were the ones which were easily extracted by bringing up the temperature of the alloy,” explains Otomo.
While KENS-HIT provided valuable information, it was only able to observe the small-scale structure of materials. To observe the larger-scale structure of graphite required another spectrometer called KENS-SWAN. “Now, with NOVA, both large- and small-scale structure can be reconstructed from just one shot of neutrons. Further, the resulting images will have more than twice the resolution due to the intense neutron beam,” says Otomo.
Utilizing NOVA's ability to detect neutrons scattered at any angle, the team has just demonstrated the ability to determine the position of hydrogen atoms in a sample of vanadium deutrides. Even though neutrons are more sensitive to hydrogen than X-ray, precise determination of the location of hydrogen atoms is still a challenge. Thus, the analysis of NOVA results requires extra care. A hydrogen nucleus has one neutron and one proton. Because the mass of a hydrogen nucleus is comparable to that of a single neutron, the position of a hydrogen atom can easily be disturbed by an inelastic collision with a neutron. Additionally, molecules in liquid water move around. “The correction method for hydrogen has been tested at NOVA,” says Otomo. “We were able to confirm the utility of the analytical method.”
The bug-shaped vacuum chamber
NOVA’s vacuum chamber is probably one of the most bizarre that has ever existed. As Otomo notes, vacuum chambers are not easy to manufacture, and generally have simple shapes, like a cube. Look at the NOVA. The shape you see is exactly the shape of vacuum chamber NOVA has.
“Originally, we were not sure if it would be possible to build a chamber with such a complex shape,” says Otomo. “Very few company was willing to accept the original design contract.” The team drew the sketches of NOVA vacuum chamber from scratch, and reshaped it a bit to make the design look more reasonable. For example, the original smoothly curving edges became octagonal, because curved vacuum chamber surfaces turned out to be hard to produce.
“Luckily, companies were very understanding of our scientific interests. Our project is also well funded by the Department of the New Energy and Industrial Technology Development Organization (NEDO), and so we were able to pursue an optimal design for reliable performance,” Otomo says. In March 2009, the vacuum chamber was completed by the Kobe Steel Group, just as designed. The design had been in Otomo’s mind since 2001. “It was great to see NOVA materializing, just as we had envisioned.”
Interdisciplinary collaboration at KEK
In recent years, a new trend of interdisciplinary collaboration has flowered at KEK. In large part, this trend is due to the efforts of the recently established Detector Technology Project at KEK. “Prior to this, institutes within KEK were rather closed societies. We at KEK's Institute of Materials Structure Science (IMSS) developed everything by ourselves, including detectors,” says Otomo. “Now, detector physicists at the KEK's Institute of Particle and Nuclear Studies (IPNS) join our team and help us make the most of advanced detector technology.”
“J-PARC produces a very intense neutron beam. The beam needs to be well focused to minimize the noise. The high performance GEM allows us to do this with high-quality monitoring,” says Otomo. “However, GEM is still a new technology, and has not yet been employed in large scale.” Because of the uncertainties surrounding this new technology, the team chose the time-tested and proven helium wire chambers, for detecting the less intense, scattered neutrons.
In one way, Otomo’s role actually extends to all 23 neutron beamlines at J-PARC. Back in 2003 when NOVA was still seeking approval, Otomo wholeheartedly pursued one ambition: to unify all the software used for neutron analyses at J-PARC. This was not an easy mission because “skilled physicists are inclined to write their own code for their own purposes.”
Another important software component, the data acquisition system (DAQ), of the neutron facility has been unified by a separate effort. The DAQ handles everything from the digitization of triggered data, the trigger selection, to storage. DAQ experts from the DTP, in joint effort with Otomo, developed a unified DAQ that can successfully handle the high rate of events at the neutron facility.
The NOVA team is currently analyzing the data acquired during last year's trial run. Otomo said that their first priority was to design and develop a stable system using solid technology to realize hard performance. So far it has gone well.
The team will begin examining various hydrogen storage alloys this fall. “Hydrogen energy is an important, environmentally friendly energy source for the future,” says Otomo. “NOVA has great potential for examining not just hydrogen storage materials, but more generally non-crystalline structures and liquids. Our biological world is filled with them.” Even after the long-awaited discovery of practical hydrogen storage materials, NOVA will continue to provide important advances far into the future.
|copyright (c) 2010, HIGH ENERGY ACCELERATOR RESEARCH ORGANIZATION, KEK
1-1 Oho, Tsukuba, Ibaraki 305-0801 Japan