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>Top >Feature Story >this page | last update:
09/12/15
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| New milestone at MUSE The Muon Science Establishment, MUSE, at the Japan Accelerator Research Complex (J-PARC) recently produced the world’s highest intensity pulsed muon beam. Read on to learn the team’s hard work, and their plans for the future. |
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 The muon is an elementary particle that is much like an electron, but is 200 times heavier. Because of the larger mass, it is less likely to lose energy from radiation when interacting with matter, and is less affected by electromagnetic fields. Thus, muons can penetrate deeper into matter than electrons would. Muons are uniquely useful because their magnetic spin can map the local magnetic environment inside matter. Alongside the light and the neutron beam, the muon beam is one of three complementary tools used in material science. Muon beams are useful for probing properties such as magnetism and superconductivity (read about it in superconductivity and physicist’s MUSE).
The scope of applications of muon science goes beyond just material science. The wide range of its applications include fundamental sciences such as particle physics, condensed matter physics, and chemistry; as well as applied sciences such as muon catalyzed fusion, biophysics, non-destructive analysis, and beam technology.
Currently, muon-catalyzed fusion is an unpractical power source, as the process consumes more energy than it produces. To fix this problem, scientists need to find a way to produce muons more efficiently. To increase the number of cycles muons can undergo before decaying, it turned out that it is also crucial to deal with the muon loss by the fusion product called alpha particle. The studies found that intense pulsed muon beam can unveil the muon loss process.
MUSE’s challenge The muon beamline at J-PARC extracts protons from a rapid cycle synchrotron (3GeV) ring, and hurls them at a graphite target. The resulting collision produces large numbers of pions. These pions then decay into muons in one of two ways. First, lower energy pions stopped at the graphite target decay inside the target. These muons are called surface muons and have energies around 4 MeV. Second, the higher energy pions that emerge from the target decay while travelling through a six-meter superconducting magnet that extends to muon experimental areas. These muons are called decay muons, and have energies of 5 MeV to 50 MeV.
Sealing with pillow The beam of protons can easily be scattered by just a smattering of air molecules. This contaminates the environment with the resulting radiation. The accelerator scientists, therefore, need to make sure their beam pipe can seal tightly, and maintain a high vacuum. For the beam ducts of the primary proton beamline and the extraction beamline, the team uses a vacuum sealing technology called a pillow seal. The pillow seal was originally developed at the Paul Scherrer Institute (PSI) in Switzerland. The ingenious feature of this vacuum sealing mechanism is the thin foil bag inserted between the flanges that connect vacuum ducts. The foil is filled with compressed air to shut the tiny openings between flanges.
To decrease the leak rate, Miyake, KEK mechanical engineer Nobuhiko Sato, and his collaborators from PSI came up with several sophisticated ideas. The pillow seal has two sets of pillow rings, inner and outer, on each side. Between the inner and outer pillow rings on the supporting flanges, they cut a miniscule groove. The groove is connected to a vacuum pump that vacuums the tiny volume of the groove, so that the pillow can seal the ducts even more tightly. Moreover, the team has found that the smoothness of the flange surface affects the performance. “The design was a success,” says Miyake. “With polished surface of flange, we have found that the leak rate can be brought as low as 10-9 Pascal–m3/s.”
The successful pillow seal design has also been applied to other experiments at J-PARC. For example, the underground neutrino experiment at J-PARC, T2K, has applied the new pillow seal design to their vacuum chambers at the target station. Guiding and shielding The proton beamline runs through multiple dipole and quadrupole magnets to skew and focus the beam. To manage this intense beam of protons, the magnets need be aligned with a 0.1 millimeters precision. However, the real difficulty is that no one is allowed in the facility for maintenance by hand, because of the severe radiation conditions that can fry living things instantly. To deal with this problem, the team designed remote-control mechanisms. However, to manipulate beamline components with the needed high precision, the team developed guide shields. These are huge blocks of iron with rails on them to guide magnets and other beamline components to exactly right positions when they are lowered by crane. The thick iron blocks shield the surroundings from the radiation coming from the beamline, and the guides allow operators to remotely mount and dismount 60-ton magnets with 0.1-millimeter precision.
the target The heart of the muon beamline is the graphite target that produces pions from the intense proton beam. The 3 GeV proton beam will ultimately have a power of one megawatt. This beam will deposit 4 kilowatts of heat at the graphite target, which translates to 1,500 degrees Celsius. Without any cooling, however, the target would reach above 2,000 degrees Celsius. To deal with the heat, the team fabricated an edge-cooled graphite target. A copper frame goes around the 2 centimeters-thick graphite target, and it contains a water pipe that spirals around inside the copper frame three times. This structure keeps the frame under 150 degrees Celsius.
Because of the intense radiation, the current target would not last longer than six months under the expected 1-megawatt proton beam. The team just performed the target exchange operation test in September using J-PARC’s radiation shielding containment box, which is known as a hot cell. The hot cell allows MUSE team members to safely handle radioactive materials using robotic manipulators. J-PARC is the first accelerator facility in Japan that requires a hot cell.
The MUSE team performs target exchange operation in the hot cell developed by J-PARC neutron group. To remotely control the target mounted on the 2-ton stainless steel shield to 0.5-millimeter precision requires a careful design around the target.
Rotating target One goal of the MUSE team has been to increase the lifetime of the target so that target exchange would be less frequent or altogether unnecessary. The team has been working on an alternative target called the rotating target. With the current fixed target, the proton beam hits only one spot. By rotating the target, the effects of the beam are spread over larger areas. This reduces the heat deposit and irradiation effects dramatically, thereby increasing the lifetime of the target.
“The bearing must endure extreme conditions such as high temperature, high radiation, and the use in vacuum,” says Makimura. “The rotating target will decrease the cost of target exchange, the amount of exposure to radiation, and the amount of machine downtime.” Ultra-slow muon source MUSE is a small group of just a dozen physicists and engineers. Every member of the team worked hard to pull off the successful construction of the world’s most intense pulsed muon source in just a few years. “Even though it was planned, it feels good that we achieved the world’s highest intensity at 100 kilowatts,” says Prof. Ryosuke Kadono of KEK, the head of KEK-MSL laboratory and the leader of the KEK-Muon Spin Rotation / Relaxation / Resonance (muSR) group. “In terms of science, however, this is just the start.” The team is now preparing for the various science projects that will be made possible once the target 1-megawatt proton beam intensity is achieved.
“The super omega is a dream machine that will bring unprecedented time resolution, beam size, and yield,” says Miyake. “With the startup of MUSE, we have already taken the first step towards the science of the ultra-slow muons,” says Kadono. “It is up to Japan whether it should continue its strong tradition of pulsed muon beams.” As muon experts around the world would admit, the actual installment of the super omega channel would bring important breakthroughs in muon science. |
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| copyright (c) 2009, HIGH ENERGY ACCELERATOR RESEARCH ORGANIZATION, KEK 1-1 Oho, Tsukuba, Ibaraki 305-0801 Japan |
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