Since 1990, scientists from the RIKEN Nishina Center for Accelerator-Based Science in Wako have paid regular visits to the Rutherford Appleton Laboratory (RAL) in Oxfordshire, UK. The researchers have adapted part of the unique synchrotron facilities at RAL to create the world’s strongest source of muons: The RIKEN-RAL Muon Facility. The muons, which are classified in the same group as electrons but have 207 times more mass, are used to examine the structure of materials and to explore fusion energy.
The Nishina Center has a distinguished history regarding muons—it is named after Yoshio Nishina, who discovered muons in cosmic rays. Scientists from the Nishina Center quickly spotted a unique opportunity when RAL built a new proton synchrotron, called ISIS, in 1985.
According to RIKEN researcher Katsuhiko Ishida, “ISIS was chosen because it is the only place in the world suitable for producing a pulsed muon beam.” His colleague Isao Watanabe adds: “We began building in 1990 and observed our first muon beam in 1994. Today there are four permanent RIKEN researchers and some postdocs based at RAL, who are joined several times a year by visiting scientists from Japan, like myself.”
Several steps lead to the production of muons for the RIKEN-RAL facility. Firstly, a beam of protons is accelerated to high energies in the circular ISIS synchrotron. The proton beam then passes into the experimental hall, where most of it generates neutrons for other experiments, but about 4% is used to make muons. A graphite target is inserted into the proton beam, producing lots of particles called pions that quickly decay into muons.
The experimental hall at ISIS is an impressive building the size of an aircraft hangar, with the huge proton beamline passing through the center. The RIKEN-RAL Muon Facility consists of four experimental ports encased in concrete and foam on one side of the beamline. On the other side is a UK group also studying muons—an arrangement which, according to Watanabe, fosters some healthy competition. However, the RIKEN team has the advantage of being able to generate both types of muons, positive and negative.
Ishida’s research group uses the negative muons to investigate nuclear fusion, the ‘Promised Land’ for clean energy generation. The biggest advantage of this ‘muon-catalyzed fusion’ is that it could supply fusion energy without the need for extreme conditions such as high temperature and pressure.
How can muons make this happen? The central concept is that each negative muon has exactly the same charge as an electron, so it can knock an electron out of an atom and take its place in orbit around the nucleus. These new ‘muonic’ atoms are much smaller in diameter than normal atoms.
“Muons can catalyze the fusion of two heavy versions of the hydrogen atom, deuterium and tritium, because the heavier muons orbit much closer to the nuclei than electrons,” explains Ishida. “This means the nuclei can move closer together in order to fuse. Thereafter, the muon can move on to catalyze more fusion.”
In theory, this chain reaction could go on thousands of times. However, in practice each muon only generates around 120 fused pairs of deuterium and tritium nuclei before they are lost by sticking to alpha particles. One of the RIKEN team’s biggest achievements was to precisely measure this so-called alpha-sticking effect.
The researchers hope to improve their experimental conditions so that fewer muons are lost by alpha-sticking. An even bigger challenge is to find a more energy-efficient way of producing muons in the first place.
Meanwhile, Watanabe and co-workers use positive muons to investigate the electromagnetic properties of substances. This is possible because positive muons have an intrinsic property called spin, which responds to nearby magnetic fields. When the positive muon decays it emits a positron which contains information on the magnetic field in the sample, and can be easily detected.
This technique, called Muon Spin Rotation, Relaxation and Resonance, or μSR, can measure magnetic field fluctuations on a temporal scale in between those detected by neutron scattering and nuclear magnetic resonance (NMR). This means it can probe systems that have never been studied before.
“I collaborate with more than 50 groups in Japan and five or six in other countries, including semiconductor physicists, chemists studying metal complexes and biologists studying the electronic structure of proteins,” says Watanabe. “We have definitely proven the versatility of μSR for exploring materials.”
Watanabe has been particularly successful in using μSR to explain the mechanisms in so called high-Tc oxides, which enter a superconducting state at relatively high temperatures.
“Metal superconductors, such as those used in levitating trains, need to be cooled with liquid helium, which costs about 2,000 yen per liter!” says Watanabe. “But high-Tc oxides could be put into a superconducting state using only liquid nitrogen, which is much cheaper.”
The RIKEN scientists obviously enjoy coming to RAL, and they hope that their collaboration will continue long into the future.
“Here at Rutherford, the working environment is excellent. The only problem is jet lag,” says Watanabe. “I’d like to develop a computerized system that could be remotely controlled from Japan. This could benefit other Asian scientists that would like to use the facility but cannot afford to travel to Europe. That is my dream.”
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Focus on scientist - Pavel Bakule
Muons are unstable particles with an average lifetime of 2.2 microseconds—a long time in the world of particle decay. This means that when muons are used to probe materials, they penetrate deep into the sample before decaying to provide information on their surroundings.
But how can scientists investigate the surface layer of a sample, or study very thin films? Pavel Bakule, born in Prague, Czech Republic, is a postdoctoral researcher trying to solve this problem by producing very slow muon beams that decay just a few nanometers inside a sample.
Bakule trained as a laser physicist, and during his Doctor of Philosophy at Oxford University he worked on precision spectroscopy of muonium—an atom composed of an electron orbiting a positive muon. His experience with muonium is crucial to slowing muons down.
“The basic idea is quite simple—the problems are more technical,” says Bakule. “We have a heated tungsten foil in which we physically stop the muons. The muons bind to electrons in the tungsten, and slow muonium atoms are emitted from the other side of the film.”
At this stage, the researchers use a laser to knock the electron off the muonium atoms, leaving a slow-moving beam of muons. This can be difficult, as Bakule explains.
“We can only generate a handful of muonium atoms, say about 200 in a space the size of an orange. Trying to hit them with a laser is very difficult, so we need a high intensity pulsed laser.”
The slow muons can provide information on useful materials for nanotechnology, or the emerging field of spintronics. Bakule hopes that the RIKEN team can soon investigate substances under various conditions.
“In future we could synchronize the implantation of muons with other excitations of the sample, for example irradiation with a laser or radiofrequency pulse.”
