A proton-accelerator complex will incorporate one of the world's most intense pulsed neutron sources.
Staff at the new Japan Proton Accelerator Research Complex (J-PARC), in the seaside village of Tokaimura in Ibaraki, like to use familiar analogies to introduce the facility. The 330-metre-long building that houses the linear proton accelerator — the engine behind the complex — is similar in size to Tokyo's train station. And the main 50-gigaelectronvolt synchrotron, with its three flattened sides and a diameter of 500 metres, resembles an onigiri — the rice ball found in every convenience store throughout Japan.
J-PARC will support several fields in which Japan has excelled, and if organizers have their way, Japan will stay at the forefront of them. Starting next month, the facility will accelerate and smash protons into a variety of targets to produce beams of subatomic particles such as neutrons, muons, neutrinos and kaons. It promises to provide its users — including a large proportion of international scientists — with new ways to image molecules, probe neutrino physics and more.
Most accelerators are built either to study nuclear physics or fundamental particle physics, or for materials or medical use. J-PARC is designed to do all of that, says director Shoji Nagamiya, in part because it sprang from two earlier proposals that merged. The Tsukuba-based High Energy Accelerator Research Organization was looking to build a 50-gigaelectronvolt ring for neutrino and kaon projects; the Japan Atomic Research Energy Institute, which was later reorganized as the Japan Atomic Energy Agency, wanted a neutron beam. The idea to share an accelerator, which came about in 1998, made sense, but the union was consumed by red tape. The institutes finally signed off on what would become a ¥152-billion (US$1.6-billion) collaboration in 2001. "J-PARC is like an indispensable, high-quality single crystal finally grown after a tremendous struggle," says Kazuyoshi Yamada, a materials researcher at Tohoku University in Sendai who started preparing samples for it two years ago.
At the complex's materials and life-science facility, research kicks off in earnest next month when seven neutron beamlines start running. These will be used to precisely image biological molecules and inorganic crystals, among other things. For instance, Nobuo Niimura of Ibaraki University will lead a protein-structure project; researchers from Hitachi will analyse lithium-based materials used for plug-in electric cars (see page 436); and yet others will study the behaviour of magnetic fields inside superconductors.
J-PARC's neutron beams will be 1 megawatt — intense enough to dramatically speed up the rate of discovery. Earlier neutron studies, using neutrons produced in nuclear reactors, led to only five or so protein structures being completed in a year, says Niimura. At the new complex, this number will jump to 100. Thomas Mason, director of the Spallation Neutron Source at the Oak Ridge National Laboratory in Tennessee (see 'Neutron sources around the world'), says that the two facilities are "a new threshold in performance". In 2009, J-PARC will also start running experiments with the world's most intense pulsed muon beam, exceeding by 30-fold the Rutherford Appleton Laboratory's facility in Didcot, UK.
Meanwhile, the largest and costliest part of the facility — the 50-gigaelectronvolt synchrotron — will be devoted to studies of neutrinos and kaons. A team of some 400 researchers will race to observe for the first time the transition of one variety of neutrinos, called muon neutrinos, into another called electron neutrinos. The observation could offer clues about the range of neutrino masses and about CP violation, a concept key to understanding why matter dominates over antimatter in the Universe.
Groups in China and France are pursuing a similar goal, but with less-energetic neutrinos generated by nuclear reactors. The Fermi National Accelerator Laboratory in Batavia, Illinois, will have a more powerful neutrino source when its NOvA gets up and running after 2014.
To create the beam of muon neutrinos, J-PARC will send a proton beam at a graphite target. Unlike conventional head-on collisions, J-PARC's proton beam will be adjustable to allow collisions at an angle. The angled collision will lower the average energy, but more of the resulting neutrinos will be in the range likely to oscillate into electron neutrinos on their underground trip to the SuperKamiokande detector, 295 kilometres away.
When J-PARC's big synchrotron isn't doing neutrino physics, it will focus on kaons and hypernuclear experiments. For example, scientists will look for evidence that neutral kaons decay into neutral pions plus a neutrino and an antineutrino.
About half of the $10 million needed for the decay experiment, and $8 million needed for other kaon experiments, still await funding. But this year's Nobel prizes may help to give the field a boost. In 2002, the awarding of the Nobel to Masatoshi Koshiba, then at the University of Tokyo, for neutrino work meant that Japan ploughed another ¥16 billion into J-PARC's neutrino project, accelerating its opening. This year, the Nobel physics prize was shared by the High Energy Accelerator Research Organization's Makoto Kobayashi and Kyoto University's Toshihide Maskawa for their work in explaining the dominance of matter over antimatter in the Universe — and some think that might boost funding for the kaon experiments at J-PARC. "It was," says Nagamiya, "very good timing."
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Scientific Reports (2015)