Tour of ISIS
Scott Lawrie, who is involved in research using ISIS, kindly, took us around. The reason for the microphone was because the noise made it. This is what he had to say about his job at ISIS:
I work for the UK’s Science and Technology Facilities Council (STFC) on the ISIS facility near Oxford. This is a large international tool which uses subatomic particles called neutrons and muons to study the micro-structure of materials. These particles can look deep inside solid structures to see flaws, in much the same way that one might have an x-ray at hospital to non-invasively tell if your bone is broken. However more importantly, ISIS can actually tell you what makes bone work at the atomic level. Everything in the world is made of the same soup of atoms, but the particular arrangement of atoms in the cells of your bones makes bone strong, light, flexible and self-healing. ISIS is a tool for anyone to use to try and understand the properties of any new or exotic material they are interested in. Be it oil, glass, superconducting magnets, antibiotics, turbine blades, liquid crystals, moon rock, carbon nanotubes, even spider webs! Any and all materials used – or with the potential to use – to make our everyday lives and the UK economy better can be studied at ISIS.
The neutrons and muons crated at ISIS are very rare. There aren’t many places in the world able to make enough of them to use in a working facility. To make a lot of them requires a large, complicated machine like ISIS. ISIS is made of two main components: a particle accelerator and a target. Particles leaving the accelerator at near light-speed smash into the target. In much the same way as a bullet from a gun hitting a wall sends concrete shrapnel everywhere, our particles hitting the target send neutrons flying in all directions. Suffice to say it is a lot more complicated than that: over 10,000 components in the accelerator and target need to work together to make the neutrons and muons and send them to the experimental stations to use in material studies.
I work at the very start of the accelerator on a device called an ‘ion source’. As the name implies, it is a source of ions: specifically negative hydrogen ions. A hydrogen atom consists of a proton with an electron orbiting round it. We put hydrogen into our source and apply high voltages, strong magnetic fields, high temperatures and a dash of caesium (which, as you may remember from chemistry class, is highly reactive to water…!) to force a second electron onto a hydrogen atom to make it a negatively charged ion. This all takes place in a plasma – what the Sun is made from – so it requires a lot of equipment and expertise to get right. In fact there is an element of gut feeling and hand waving about the complex processes that happen in the plasma, which leads to us jokingly calling the process ‘ion sourcery’! I am currently in the middle of a Ph.D. trying the unravel some of the magic and make the ISIS ion source a more efficient piece of kit.
Other things I work on include designing large electromagnets, placing and project managing large value orders, performing computer simulations, building electronic circuits, computer programming, testing equipment and occasionally crawling around under the machinery connecting things together and fault finding. It’s a very varied job with challenging project specifications and I think it’s the best career an experimental physicist could get into. I feel lucky every day that I work for STFC and ISIS and look forward to going to work each morning.
The above picture shows a working model of ISIS
ISIS is a pulsed neutron and muon source. It uses the techniques of muon spectroscopy and neutron scattering to probe the structure and dynamics of condensed matter on a microscopic scale ranging from the subatomic to the macromolecular.
Neutrons are the uncharged part of the nucleus of an atom and muons are elementary particles similar to the electron, but with a much greater mass (105.7 MeV/c^2). At ISIS the neutrons are created by accelerating ‘bunches’ of protons in a synchrotron, then colliding these with a heavy tungsten metal target, under a constant cooling load to dissipate the heat from the 160 kW proton beam. The impacts cause neutrons to spall off the tungsten atoms, and the neutrons are channelled through guides, or beamlines, to around 20 instruments, each individually optimised for the study of different types of interactions between the neutron beam and matter. The target station and most of the instruments are set in a large hall. Neutrons are a dangerous form of radiation, so the target and beamlines are heavily shielded with concrete.
ISIS produces muons by colliding a fraction of the proton beam with a graphite target, producing pions which decay rapidly into muons, delivered in a spin-polarised beam to sample stations.
The statistical accumulation of deflected neutrons at different positions beyond the sample can be used to find the structure of a material, and the loss or gain of energy by neutrons can reveal the dynamic behaviour of parts of a sample, for example diffusive processes in solids.
Hundreds of experiments are performed annually at ISIS by visiting researchers from around the world, in diverse science areas including physics, chemistry, materials engineering, earth sciences, biology and archaeology.
How does ISIS work?
To make a neutron beam, you need to first have some particles travelling REALLY fast.
To make this happen, ISIS has two particle accelerators. There is a linear accelerator called the Linac and a circular accelerator, called the synchrotron.
Particles are first accelerated in the linear accelerator and separated into bunches like sausages. By the end of the accelerator, the particles are travelling at 37% of the speed of light!
They are then passed into the synchrotron, where they are accelerated even more. By the time they have finished 10,000 laps of the synchrotron they are travelling at around 84% of the speed of light!
The particles then travel towards a tungsten target, no bigger than a packet of biscuits that is situated in the experimental hall. When the particles smash into the target, neutrons are dislodged from the atoms. These neutrons are then detected by the neutron instruments.
Looking at a diagram of ISIS above, you can see that it starts with an ion source which involves hydrogen gas and caesium. The process produces negative hydrogen ions, with two electrons orbiting each proton. These ions are passed into a linear accelerator where, incredibly, they are accelerated to about 35 per cent of the speed of light over a distance of about 50 metres. From here, stripped of their electrons, by being passed through thin aluminium foil, to become protons, they are injected into the circular synchrotron. Its 163-metre circumference is composed of 10 sections, each consisting of an electric field to accelerate bunches of protons and magnetic fields to steer and collimate the pulsed beam. The strength of the bending magnets has to be increased in synchronicity with the increasing speed of the protons, to keep them in the same tight trajectory. After 10 000 trips around the synchrotron, the protons are travelling at 84 per cent of the speed of light. They can then be extracted using three fast kicker magnets in which the current rises from zero to 6500 amps in less than 100 nanoseconds.
Passing through a thin graphite target, two to three per cent of the proton beam produces muons (heavy electrons) for use in muon spin spectrometry. The remaining protons strike a tantalum-clad tungsten spallation target, where each proton can eject about 20 neutrons. The target is water-cooled to prevent the 160 kW proton beam melting it. The neutrons are then slowed down or moderated to increase their de Broglie wavelength before they are channelled through guides, or beamlines, to about 20 instruments, individually optimised for the study of different types of matter. A separate beamline takes protons to the ISIS second target station. Optimised for low energy neutrons, this provides greater capacity at ISIS and opens up new areas of research.
Spallation is a process in which fragments of material (spall) are ejected from a body due to impact or stress.
Louis-Victor-Pierre-Raymond, 7th duc de Broglie, (15 August 1892 – 19 March 1987) was a French physicist who made ground breaking contributions to quantum theory.
In his 1924 PhD thesis he postulated the wave nature of electrons and suggested that all matter has wave properties. This concept is known as wave-particle duality or the de Broglie hypothesis. He won the Nobel Prize for Physics in 1929. The de Broglie wavelength is the wavelength of the wave behaviour of a matter particle. Believe it or not you have a de Broglie wavelength.
The images above show parts of target 2
WISH is a high-resolution long-wavelength magnetic diffractometer for Target 2. Designed primarily for powder diffraction at long d-spacing in magnetic and large unit cell systems, with the option of enabling single-crystal and polarised beam experiments. The choppers chop the beam.
The above picture shows inside the WISH instrument
The above picture shows part of a vacuum pump.
The apparatus requires very low temperatures provided by liquid nitrogen and liquid helium.
Offspec is an advanced reflectometer giving access to nanometre length scales parallel and perpendicular to interfaces. It uses the technique of neutron spin-echo to encode the path that neutrons take through the instrument.
Measurements of specular reflectivity give information about structure perpendicular to a surface interface, but an increasing number of important science and technology issues in the study of thin films, multilayers and interfaces concern structure in the plane of the interface.
Zoom is under construction. It will be a flexible, high count rate small-angle scattering instrument for advanced materials, magnetism, environment science, pharmacy and healthcare to study length scales 2‑2000 nm. For the first time at ISIS, it will offer polarised small angle neutron scattering and will use novel focusing devices and high-resolution detectors to reach smaller Q, to complement the Sans2d instrument, without building a very long beam line. Zoom will start commissioning experiments in February 2015.
Chipir is under construction. It will be an instrument for rapid testing of effects of high energy neutrons and it will be one of the first dedicated facilities outside of the US to look at how silicon microchips respond to cosmic neutron radiation.
Nimrod is a near and intermediate range order diffractometer designed to provide continuous access to length scales ranging from the interatomic (<1 Å) through to the mesoscopic (>300 Å).
Quadrupole electromagnets, abbreviated Q-magnets, consist of groups of four magnets laid out so that in the planar multipole expansion of the field the dipole terms cancel and where the lowest significant terms in the field equations are quadrupole. Quadrupole magnets are useful as they create a magnetic field whose magnitude grows rapidly with the radial distance from its longitudinal axis. This is used in particle beam focusing.
Neutrons can be fired into a sample and penetrate deep enough to reveal the microscopic structure of solids and liquids. The diffraction of the neutrons can tell us where atoms are. This technique has been used to assess how pipes used in the offshore oil and gas industry are affected by the installation process.
SAN2D, SANDALS, LOQ and NIMROD use small angle scattering to investigate structures of between 1 and 100 nanometres such as polymers of biological molecules. In a recent study SANS2D was used to look at the interaction between a nanocarrier and a drug in order to optimise delivery to the target in the body.
CRISP, OFFSPEC, INTER, POLFREF and SURF allow scientists to measure the structure of thin films. The technique, called neutron reflectometry, involves shining a beam of neutrons onto an extremely flat surface. The reflectivity profile created can tell us in detail about the structure of the surface. This technique has been used to study bacterial membranes in E. Coli and their interaction with antibiotics.
LET, OSIRIS, VESUVIO, TOSCA, MAPS, IRIS, MARI and MERLIN use neutron spectroscopy to explore the hopping motions of atoms, the rotational modes of molecules, and the magnetic and vibrational motions of atoms. For example an experiment on TOSCA has revealed the dynamics of the antibiotic penicillin.
EMU, MuSR, HiFi and Argus use muons as a complementary probe to neutrons and can be used to study condensed matter and molecular systems. They are used in a diverse range of scientific experiments from probing magnetism, superconductivity and charge transport to investigate hydrogen behaviour in semiconductors. Muons exist for only a fraction of a second before decaying; however this is long enough for the muons to get an “idea” of what the atoms are doing.