2. A brief history of muons
Back in 1910, Jesuit priest Thomas Wulf noticed that electrometers (instruments that can detect radioactive decay) detected higher radiation rates at the top of the Eiffel Tower than at the bottom. A few years later, Victor Hess used a hot air balloon to demonstrate the link between altitude and the response of the electrometer, postulating that this was evidence of extra-terrestrial radiation. Physicist Robert Millikan took this one step further, sending remote controlled electrometers up to the stratosphere to detect what he went on to name “cosmic rays”.
Millikan’s group at the California Institute of Technology (Caltech) went on to discover a new particle responsible for the majority of cosmic ray effects – the muon. High up in the Earth’s atmosphere, cosmic rays collide with particles, creating muons that then rain down on Earth. An area the size of a coin is hit by a muon about once a minute. Since the 1950s, the study of muons has needed more intense sources – which has driven the development of new particle accelerators.
In 1957, Garwin, Lederman and Weinrich published Observations of the Failure of Conservation in which they observed that;
“It seems possible that polarised positive and negative muons will become a powerful tool for exploring magnetic fields in nuclei… atoms and interatomic regions.”
This suggestion proved prophetic. Muon spectroscopy was used to develop the understanding of magnetic materials such as nickel and iron. By implanting positive muons in the material of interest their high sensitivity to small magnetic fields uncovered new and unexpected magnetic features.
The history of muons at ISIS
The ISIS neutron and muon source first offered a user programme for scientists from the UK and internationally as part of a collaboration involving Grenoble, Uppsala, Munich and Parma as well as ISIS and funded by the European Community. The first muon spin rotation test spectrum was recorded on 23 March 1987 in a rudimentary single-detector set-up. A spectrometer comprised of of 32 detectors in two arrays was commissioned soon after; this was the precursor to the present MuSR spectrometer. It was soon hard to satisfy the demand for beamtime, either from the original consortium or from prospective new users. A second European grant allowed the original beamline to be split into three separate experimental areas. In addition to the original instrument, a new spectrometer (EMU) was built, optimised for specific types of measurement and the third area DEVA made available for special development projects.
The early 1990s saw the construction of an entire new muon beamline complex at ISIS. Sponsored by RIKEN and funded by Japan, this became the RIKEN-RAL muon facility, one of the largest UK–Japan science collaborations. Alongside experimental areas for fundamental muon science and muon catalysed fusion, the ARGUS spectrometer was provided for condensed matter and materials science studies. Benefiting from a channel allowing muons to be generated from positrons decaying in flight, the RIKEN-RAL muon facility can produce negative as well as positive muons, with variable momenta enabling studies inside pressure cells. First muons were produced at RIKEN-RAL in 1994.
In 2005, the European muon facility received a major in-house grant, enabling the development of a new high field muon instrument opening up new areas of research at the facility. More recently, upgrades of the beamline and instrument detectors has kept the facility state-of-the-art. In 2018, ownership and operation of the RIKEN-RAL facility passed to ISIS under a new five year agreement. As part of this agreement, the beamlines were refurbished to enable them to continue to operate for many years to come.
How ISIS makes muons
Nature makes muons from high energy particles in space (cosmic rays). But to get the intense muon beams at ISIS, we do something different.
- Fifty times a second, two bunches of protons are accelerated in the synchrotron and then diverted towards target station.
- The proton beam collides with a thin piece of carbon.
- The high-energy protons strike the nuclei of carbon atoms, causing them to emit pions – unstable exotic particles.
- Pions are so unstable that within a few nanoseconds they decay into neutrinos (particles with no electric charge and almost no mass) and muons.
- The muons are then channeled into ISIS’s muon experiment areas.
Muons are exotic relatives of electrons, but with more mass. They are unstable particles that decay in about two millionths of a second to positrons and neutrinos.
Melanin: a potential new interface for bioelectronics
Semiconductors are used in all modern electronics. They’re currently made from inorganic elements or compounds, such as silicon or gallium arsenide, and rely on electrons to carry an electric current. In biological systems, ions carry the current, and so the two types of system do not interact with one another. The existence of organic semiconductors was first demonstrated in the 1970s, with experiments carried out on melanin, better known as a skin pigment.
A team of researchers from the University of Queensland used muon spin spectroscopy to investigate the properties of melanin, and found that both electrons and ions play a role in its conductivity. This research suggests exciting potential for bio-electronic applications, such as medical sensors and tissue stimulation treatments. Organic semiconductors may also provide us with cheaper, greener electronics.
Probing electron transfer with muon spin relaxation
Ferritin is an iron storage protein, produced by almost all living organisms, including bacteria, plants and animals. In the human body it helps to prevent iron levels becoming too high, or too low. The structural and magnetic properties of ferritin have been well studied, and there is interest in the possibility that it could be used in a bio-compatible nano-battery. Muon spin spectroscopy allows investigation of the electron-transfer processes in macromolecules, such as ferritin, at the microscopic level.
Dr Telling and Professor Sue Kilcoyne, from the University of Huddersfield, used Longitudinal Field Muon Spin Relaxation at ISIS to investigate electron-transfer processes in ferritin, apoferritin and their pharmaceutical equivalents. Their fundamental research has since been referenced in a later μSR investigation into differences between ferritin in brains from healthy individuals and Alzheimer’s patients.
"μSR has an advantage over other techniques such as electron spin resonance (ESR) spectroscopy, because it excites molecules and then acts as a probe of the dynamical properties of the excitation."
Dr Mark Telling, ISIS Neutron and Muon Source
9. Cultural heritage
Probing the past with negative muons
Negative muons can be used to non-destructively probe the composition of archaeological artefacts, and the technique can also be used to study engineering samples, bio-systems, and battery materials.
Techniques that determine the elemental composition of samples are often damaging to the sample, and hence unsuitable for use on historical objects. Negative muons can offer non-destructive multi-elemental analyses, based on the measurement of characteristic muonic X-rays emitted after the muon has been captured by the nuclei inside the sample. The idea of using negative muons for chemical analysis was first suggested over 50 years ago, but could not be put into practice until a suitably intense muon source could be built.
Initial tests revealed that the technique is sensitive to all elements, and that depth-dependent studies are possible.
"This work will help develop better analytical protocols which will meet the increasingly stringent demands of non-destructive analysis, without the accompanying drawbacks of surface sensitivity, which can give misleading results."
Prof Mark Pollard, University of Oxford
11. Fundamental research
Muonium sheds light on semiconductor structure
Muonium spectroscopy offers a much-needed solution to the difficulties of detecting isolated or monatomic hydrogen defects in many semiconductors and dielectrics. It was muonium spectroscopy that provided the first atomistic picture of interstitial hydrogen in Group-IV elements. But details of the solid-state chemistry and spin dynamics of muons and protons in the Group-VI semiconductors have been slower to emerge. Researchers have used high-field muon spin spectroscopy to show how positive muons create paramagnetic centres in sulphur. The aim of the research was to provide a model for the solid-state chemistry of interstitial hydrogen in sulphur, and to solve one of the longest standing puzzles in muon spectroscopy – the surprisingly strong depolarisation of muons mimicking ion-implanted protons in this non-magnetic material.
"The paramagnetic centres mimic the neutral reaction product of ion-implanted protons or interstitial atomic hydrogen, which was hitherto unknown in this element. Supercell density-functional calculations were used to confirm the structure for orthorhombic octasulphur."
Dr James Lord, ISIS Neutron and Muon Source
Good chemistry between magnetism and superconductivity
It is challenging to make materials that are both magnetic and superconducting. The usual method is to deposit alternating layers of compounds which are magnetic or superconducting, but the success of this approach is limited by distortions when the two compounds have different sizes at the atomic level. A team of researchers from Spain and the UK have developed a new method in which the layers are formed in solution and brought together by electrostatic attraction. Muon spectroscopy was the perfect tool for probing the coupling of the new materials at a microscopic level, allowing the team to determine the volume of the sample that becomes magnetically ordered and the strength of the superconducting state.
"Rather than build up the material atom by atom, molecule by molecule, nano sheets with different functions are self-assembled in this new technique."
Prof Stephen Blundell, University of Oxford