Particle Physics Masterclass 2015

Introduction to ISIS

By Dr Chris Frost


The image below shows some of the physicists who pioneered the research that allowed ISIS to be developed and used.


The Quantum Scale

Neils Bohr and Albert Einstein were not altogether happy with what they were finding out.

Niels Henrik David Bohr (7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922.


“Anyone who is not shocked by quantum theory has not understood it”


“Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the “old one.” I, at any rate, am convinced that He (GOD) does not throw dice.”

Some of the important work was taking place in the UK


Cavendish Laboratory staff and graduate students in 1932. There are 9 Nobel Prizewinners in this picture.

(Fourth row) N.S. Alexander. P. Wright. A.G. Hill. J.L. Pawsey. G. Occhialini. H. Miller.

(Third row) W.E. Duncanson. E.C.Childs. T.G.P. Tarrant. J.M. McDougall. R.C. Evans. E.S S. Shire. E.L.C. White. F.H. Nicoll. R.M. Chaudhri. B.V. Bowden. W.B. Lewis.

(Second row) P.C. Ho. C.B. Mohr. H.W.S. Massey. M.L. Oliphant. E.T.S. Walton. C.E. Wynn-Williams. J.K. Roberts. N. Feather. Miss Davies. Miss Marie Sparshott. J.P. Gott

(Front row) J.A. Ratcliffe. P. Kapitza. J. Chadwick. R. Ladenberg. Prof. Sir J.J. Thomson. Prof. Lor d. Rutherford. Prof. C.T.R. Wilson. F.E. Aston. C.D. Ellis. P.M.S. Blackett. J.D. Cockroft.

Arguably 1932 was the year of greatest achievement at the Cavendish with three discoveries of such importance that they truly deserve the overworked accolade “breakthrough”. For the previous two decades, the standard model of the atom had contained two types of particles: the light, negatively charged electrons and lumps of positively charged matter called protons. Then in the space of a few months, the number of known fundamental particles doubled from two to four; the atoms of the lighter elements, far from being indivisible, were broken open at will; and powerful new machinery for both producing and detecting the particles came into use. These discoveries gave nuclear physics a new impulse. And the four men most closely involved at the Cavendish became Nobel laureates.

One scientist of especial importance as far as ISIS is concerned is James Chadwick who discovered the neutron.


Sir James Chadwick, CH, FRS (20 October 1891 – 24 July 1974) was an English physicist who was awarded the 1935 Nobel Prize in Physics for his discovery of the neutron in 1932. In 1941, he wrote the final draft of the MAUD Report, which inspired the U.S. government to begin serious atomic bomb research efforts. He was the head of the British team that worked on the Manhattan Project during the Second World War. He was knighted in England in 1945 for his achievements in physics.

In 1930 the German physicists Bothe and Becker bombarded the light metal beryllium with alpha particles, and noticed that a very penetrating radiation was emitted. This radiation was non-ionising, and they assumed it was gamma rays.

In 1932 Irène and Frédéric Joliot-Curie investigated this radiation in France. They let the radiation hit a block of paraffin wax, and found it caused the wax to emit protons. They measured the speeds of these protons and found that the gamma rays would have to be incredibly energetic to knock them from the wax.

Chadwick reported the Joliot-Curie’s experiment to Rutherford, who did not believe that gamma rays could account for the protons from the wax. He and Chadwick were convinced that the beryllium was emitting neutrons. Neutrons have nearly the same mass as protons, so should knock protons from a wax block fairly easily.

Chadwick worked day and night to prove the neutron theory, studying the beryllium radiation with an ionisation counter and a cloud chamber. He found that the wax could be replaced with other light substances, even beryllium, and that protons were still produced.


Chadwick’s apparatus consisted of the neutron chamber, in which neutrons (first believed to be protons) are produced, and the ionisation chamber which detects protons.


The air is removed from the neutron chamber through the thin chimney, producing a vacuum in the chamber.


Alpha particles are produced by the decay of radioactive polonium in the neutron chamber. The alpha-particles hit the nearby beryllium target, producing a release of neurons. The reaction that knocks neutrons (n) from the beryllium target is:



A paraffin wax target is put between the two chambers. Neutrons may hit atoms within the wax which release protons, which are then detected in the ionisation chamber.

Within a month Chadwick had conclusive proof of the existence of the neutron. He published his findings in the journal, Nature, on February 27, 1932.

The alpha-particles from the radioactive source hit the beryllium nuclei and transformed them into carbon nuclei, leaving one free neutron. When this neutron hit the hydrogen nuclei in the wax it could knock a proton free, in the same way that a white snooker ball can transfer all its energy to a red snooker ball.


Ernest Rutherford gave the best description of a neutron as a highly penetrating neutral particle with a mass similar to the proton. We now know it is not a combination of an electron and a proton. Quantum mechanics restricts an electron from getting that close to the proton, and measurements of nuclear ‘spin’ provide experimental proof that the nucleus does not contain electrons.


Ernest Rutherford, 1st Baron Rutherford of Nelson, OM FRS (30 August 1871 – 19 October 1937) was a New Zealand-born British physicist who became known as the father of nuclear physics. Encyclopædia Britannica considers him to be the greatest experimentalist since Michael Faraday (1791–1867).

Chadwick knew the neutron wasn’t formed from an electron and a proton, and explained in his Nobel lecture that it seemed ‘useless to discuss whether the neutron and proton are elementary particles or not’. He knew that a more powerful investigation of the neutron was necessary to decide if it was made up of anything else. We now know that the neutron and the proton are made of even tinier particles called quarks.

To further confuse matters, free neutrons are not stable. If a neutron is outside the nucleus for several minutes it will transform into a proton, an electron, and an extremely light particle called a neutrino. The decay occurs because one of the quarks inside the neutron has transformed into a different quark, producing an additional positive charge in the particle.

Neutrons are very penetrating because they are uncharged. This makes them very useful to nuclear physicists, as they can be fired into the nucleus without being repelled like the proton. A neutron can even be made to stop inside a nucleus, transforming elements into more massive types.

“Positive results in the search for ‘neutrons ‘ would add considerable to the existing knowledge on the subject of the construction of matter, and as such would be of greatest interest to science, but, to humanity in general the ultimate success or otherwise of the experiments that were being carried out in this direction would make no difference”

From an interview with James Chadwick reported in The Times, Monday 29th February 1932.

Richard Feynman stated that

“All things are made of atoms – little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another”


Richard Phillips Feynman, (May 11, 1918 – February 15, 1988) was an American theoretical physicist known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics, and the physics of the superfluidity of supercooled liquid helium, as well as in particle physics (he proposed the parton model).

How big is an atom?

An atom is the smallest unit of matter that defines the chemical elements. Every solid, liquid, gas, and plasma is made up of neutral or ionized atoms. Atoms are very small: the size of atoms is measured in picometres – trillionths (1 x E−12) of a metre.

Below is an illustration of the helium atom, depicting the nucleus (pink) and the electron cloud distribution (black). The nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case. The black bar is one angstrom (1 x E−10 m or 100 pm).


How do we know how atoms are arranged in materials?

In physics, Bragg’s law gives the angles for coherent and incoherent scattering from a crystal lattice. When X-rays are incident on an atom, they make the electronic cloud move as does any electromagnetic wave. The movement of these charges re-radiates waves with the same frequency, blurred slightly due to a variety of effects; this phenomenon is known as Rayleigh scattering (or elastic scattering). The scattered waves can themselves be scattered but this secondary scattering is assumed to be negligible.

A similar process occurs upon scattering neutron waves from the nuclei or by a coherent spin interaction with an unpaired electron. These re-emitted wave fields interfere with each other either constructively or destructively (overlapping waves either add up together to produce stronger peaks or are subtracted from each other to some degree), producing a diffraction pattern on a detector or film. The resulting wave interference pattern is the basis of diffraction analysis. This analysis is called Bragg diffraction.

Bragg diffraction (also referred to as the Bragg formulation of X-ray diffraction) was first proposed by William Lawrence Bragg and William Henry Bragg in 1931 in response to their discovery that crystalline solids produced surprising patterns of reflected X-rays (in contrast to that of, say, a liquid). They found that these crystals, at certain specific wavelengths and incident angles, produced intense peaks of reflected radiation. The concept of Bragg diffraction applies equally to neutron diffraction and electron diffraction processes. Both neutron and X-ray wavelengths are comparable with inter-atomic distances (~150 pm) and thus are an excellent probe for this length scale.

William Lawrence Bragg explained this result by modelling the crystal as a set of discrete parallel planes separated by a constant parameter d. It was proposed that the incident X-ray radiation would produce a Bragg peak if their reflections off the various planes interfered constructively. The interference is constructive when the phase shift is a multiple of 2π; this condition can be expressed by Bragg’s law and was first presented by Sir William Lawrence Bragg on 11 November 1912 to the Cambridge Philosophical Society. Although simple, Bragg’s law confirmed the existence of real particles at the atomic scale, as well as providing a powerful new tool for studying crystals in the form of X-ray and neutron diffraction. William Lawrence Bragg and his father, Sir William Henry Bragg, were awarded the Nobel Prize in physics in 1915 for their work in determining crystal structures beginning with NaCl, ZnS, and diamond. They are the only father-son team to jointly win. William Lawrence Bragg was 25 years old, making him then, the youngest physics Nobel laureate.

Bragg diffraction occurs when radiation, with wavelength comparable to atomic spacings, is scattered in a specular fashion by the atoms of a crystalline system, and undergoes constructive interference. For a crystalline solid, the waves are scattered from lattice planes separated by the interplanar distance d. When the scattered waves interfere constructively, they remain in phase since the path length of each wave is equal to an integer multiple of the wavelength. The path difference between two waves undergoing interference is given by 2dsinθ, where θ is the scattering angle (see figure on the right). The effect of the constructive or destructive interference intensifies because of the cumulative effect of reflection in successive crystallographic planes of the crystalline lattice (as described by Miller notation). This leads to Bragg’s law, which describes the condition on θ for the constructive interference to be at its strongest:



where n is a positive integer and λ is the wavelength of incident wave. Note that moving particles, including electrons, protons and neutrons, have an associated wavelength called de Broglie wavelength. A diffraction pattern is obtained by measuring the intensity of scattered waves as a function of scattering angle. Very strong intensities known as Bragg peaks are obtained in the diffraction pattern at the points where the scattering angles satisfy Bragg condition.

(Below left)

Sir William Lawrence Bragg CH OBE MC FRS (31 March 1890 – 1 July 1971) was an Australian-born British physicist and X-ray crystallographer, discoverer (1912) of the Bragg law of X-ray diffraction, which is basic for the determination of crystal structure.


(Above right)

Sir William Henry Bragg OM, KBE, PRS (2 July 1862 – 12 March 1942) was a British physicist, chemist and mathematician. He was joint winner (with his son, William Lawrence Bragg) of the Nobel Prize in Physics in 1915.

Coherent neutron diffraction (Bragg scattering by crystal lattice planes) was first demonstrated in 1936 by Mitchel & Powers and Halban & Preiswerk as an exercise in wave mechanics

The possibility of using the scattering of neutrons as a probe of materials developed with the availability of copious quantities of slow neutrons from reactors after 1945. Fermi’s group used Bragg scattering to measure nuclear cross-sections

So what do neutrons do?

Fired into samples, they can tell us where atoms are and how they are moving deep inside materials.

This atom’s-eye view helps us to explain why substances have the properties they do – for example, how they conduct electricity, why they have particular magnetic properties, or how tough they are – and so can help us make new substances that are useful in everyday applications.

When neutrons interact with the atoms in a solid or liquid, they scatter off the nuclei in a characteristic manner that depends on the atomic positions. The angles at which the neutrons emerge from the sample tell us the distances between the atoms. Any changes in the neutron energies as they pass through the samples also reveal the motions of atoms and molecules.

The History behind ISIS

Reporting on the opening of the neutron lab, The Engineer wrote that ‘the equipment — a pulsed source for neutron physics experiments — is intended to provide information about the detailed behaviour of neutrons of known velocities when they meet the materials used in the construction of reactors.’


The ISIS pulsed neutron and muon source is one of the principal research centres operated by the STFC (Science and Technology Facilities Council) at the RAL (Rutherford Appleton Laboratory)


ISIS acts as an extremely sensitive microscope. Using ISIS instruments, scientists can work out where atoms are and measure the forces between them. Neutrons possess a wave-particle duality and have a de Broglie wavelength that allows them to be diffracted by matter in much the same way that x-rays are diffracted by a lattice in x-ray crystallography. But while x-rays interact with the electron cloud, neutrons are uncharged, so they can penetrate deep into matter and interact directly with the nuclei without experiencing electrostatic repulsion.

ISIS is an accelerator driven neutron source (spallation)

A subcritical reactor is a nuclear fission reactor concept that produces fission without achieving criticality. Instead of a sustaining chain reaction, a subcritical reactor uses additional neutrons from an outside source. The neutron source can be a nuclear fusion machine or a neutron source producing neutrons through spallation of heavy nuclei by charged particles such as protons accelerated by a particle accelerator.

Such a device with a reactor coupled to an accelerator is called an Accelerator-driven system (ADS).

A high-intensity proton accelerator with an energy of about 1 GeV, directed towards a spallation target or spallation neutron source. The source located in the heart of the reactor core contains liquid metal which is impacted by the beam, thus releasing neutrons.

For each proton intersecting the spallation target, an average of 20 neutrons is released.




Neutrons can be released by firing bursts of accelerated protons at a heavy metal target, knocking out, or spallating, neutrons from the target’s nuclei. ISIS has significantly developed this spallation neutron approach, which has been extremely successful. It is being taken up elsewhere in the world and will certainly be the basis of any future international neutron facility.

As well as producing neutrons, the proton pulses are also directed at a target designed to produce muons, which are another useful probe of the properties of materials.

The basic requirement for producing neutrons and muons is a powerful accelerator system to generate an energetic proton beam. ISIS has several accelerator stages, culminating in the circular synchrotron accelerator 50 metres across. This produces protons travelling at 84 per cent of the speed of light.

ISIS has the world’s leading expertise and instrumentation in the application of neutrons to condensed matter science


Isis has:

About 2000 users per year;

About 450 publications per year;

About 800 experiments per year;

90” of UK users have high ranking departments

30 Neutrons and Muon instruments involved in spectroscopy, molecular spectroscopy, engineering, diffraction, investigating large scale structures and disordered materials

ISIS staff;

Do 131 invited talks;

Have 65 external committee memberships;

Have 37 scientific or technical meetings organised;

Have 21 visiting appointments at universities;

Have 30 external grants;

Have 50 PhD students supervised

Uses of the properties of Neutrons

Neutron diffraction can be used to establish the structure of low atomic number materials like proteins and surfactants much more easily with lower flux than at a synchrotron radiation source. This is because some low atomic number materials have a higher cross section for neutron interaction than higher atomic weight materials.


One major advantage of neutron diffraction over X-ray diffraction is that the latter is rather insensitive to the presence of hydrogen (H) in a structure, whereas the nuclei 1H and 2H (i.e. Deuterium, D) are strong scatterers for neutrons. The greater scattering power of protons and deuterons means that the position of hydrogen in a crystal and its thermal motions can be determined with greater precision by neutron diffraction. The structures of metal hydride complexes have been assessed by neutron diffraction.


The image on the left shows that X-rays cannot pass through dense materials such as bone and metals however the image on the right shows that the neutrons could pass through the metal container containing the rose, which shows up because the neutrons interact with the hydrogen (and carbon) in the rose. When a fast neutron collides with a light nucleus, it loses a large fraction of its energy. By measuring the rate at which slow neutrons return to the probe after reflecting off of hydrogen nuclei, a neutron probe may determine the water content in soil.

You can use neutrons to investigate nuclear interaction changes without changing the chemistry.


Stern-Gerlach Experiment

The Stern–Gerlach experiment is an important experiment in quantum mechanics on the deflection of particles. This experiment is often used to illustrate basic principles of quantum mechanics.


It involves sending a beam of particles through an inhomogeneous magnetic field and observing their deflection. The results show that particles possess an intrinsic angular momentum that is closely analogous to the angular momentum of a classically spinning object, but that takes only certain quantized values. Another important result is that only one component of a particle’s spin can be measured at one time, meaning that the measurement of the spin along the z-axis destroys information about a particle’s spin along the x and y axis.

The experiment is normally conducted using electrically neutral particles, such as the neutron, or atoms. This avoids the large deflection to the orbit of a charged particle moving through a magnetic field and allows spin-dependent effects to dominate.

The experiment showed that neutrons had a spin if a 1/2.

Neutron scattering experiments

Neutron scattering, the scattering of free neutrons by matter, can refer to either the physical process or the experimental technique which uses this process for the investigation of materials. Neutron scattering as a physical process is of primordial importance in nuclear engineering. Neutron scattering as an experimental technique is used in crystallography, physics, physical chemistry, biophysics, and materials research. It is practised at research reactors and spallation neutron sources that provide neutron radiation of sufficient intensity. Neutron diffraction (elastic scattering) is used for determining structures; Inelastic neutron scattering is used for the study of atomic vibrations and other excitations.

• In a scattering experiment, a beam of radiation is incident on a sample

• The distribution of radiation scattered from the sample is measured

• This is determined by the interaction potential of the radiation and the sample

• The radiation must be coherent (either spatially or temporally or both)


Neutrons can be pictured as matter waves. They can be used in scattering experiments as they have no charge (penetrating), they are weakly interacting, they have a strong magnetic interaction (due to their spin = ½) and they are strongly scattered by light nuclei.

Thermal neutrons have a wavelength of 0.18nm which is comparable with interatomic spacing, and a speed of 2200 m/s

Neutron located “somewhere in Δx”


An intense neutron beam is directed onto the sample of interest; this beam can be viewed as a stream of free particles travelling in the same direction and with the same speed. The free neutrons in the beam interact with the bound nuclei of the atoms in the sample, scattering the beam. These scattered neutrons are recorded by a position-sensitive detector. The resulting data – the intensity of the neutrons scattered by different areas of the sample – is used in mathematical models to determine the shape, size and charge of the scattering objects in the sample.


The incident ki and scattered ks wave vectors are shown, along with the resultant scattering vector q, which is in the plane of the detector. Image courtesy of ILL

Applications today



The image above right (1D spin wave) shows the results of neutrons investigating CuGeO3 using a combination of inelastic neutron scattering measurements it demonstrates that lattice distortions play an important role in determining the magnetic ground state.

The image above left shows the results of neutrons investigating “magnetic monopoles” in spin ice. The bottom row shows predicted neutron scattering data and above is the real data gathered at the ILL, using experimental apparatus that was recently improved by UK funding.

A spin ice is a substance that does not have a single minimal-energy state. It has “spin” degrees of freedom, i.e., it is a magnet, with frustrated interactions that prevent it from completely freezing.

A magnetic monopole is a hypothetical elementary particle in particle physics that is an isolated magnet with only one magnetic pole (a north pole without a south pole or vice versa). They only exist within a special type of material called `spin ice’.


Neutrons can be used to investigate different time- and length-scales

The scattering process can be either elastic (no exchange of energy between neutrons and sample) or inelastic (exchange of energy with the sample): in the first case we speak about diffraction, whereas in the second case we speak about spectroscopy.


An international group of scientists studied these small pea plants with neutrons to watch them during photosynthesis.

Neutrons are being used to study the dynamics of chemical reactions at interfaces for chemical and biochemical engineering, food sciences, drug synthesis and molecular biology.

Neutrons can probe deep into solid objects such as turbine blades, gas pipelines and welds to give a unique microscopic insight into the strains and stresses that affect the operational lifetimes of these crucial engineering components.

Neutron studies of nano-particles, low-dimensional systems and magnetism impact upon next generation computer and IT technology, data storage, sensors and superconducting materials.

Neutron spectroscopy measures the atomic and magnetic motions of atoms.


Above left shows the surface of a catalyst, above centre shows a high temperature superconductor structure and above right shows thin-film magnetism studied by neutron reflectometry.


The “flower” (Courtesy: Steve King/ISIS) taken from raw data collected by a neutron camera at ISIS.


“Glass” (Courtesy: Neville Greaves/Aberystwyth University) shows the atomic structure of glass as inferred from data collected in neutron experiments.

Touring ISIS


Sarangan, Abbas, Nicholas, Sulaxan and Sujeethan just before the tour started. As it is quite noisy we all wore headphones and our guide spoke to us through a microphone.


High-intensity chemical interfaces reflectometer offering a unique facility for the study of a range of air/liquid, liquid/liquid, air/solid, and liquid/solid interfaces.

In neutron reflectivity experiments, a narrow beam of neutrons is bounced off a surface. Like beams of light bouncing off a mirror, neutron beams bounce off surfaces at the same angle as they arrive, and are collected by neutron detectors. Inter enables the use of smaller samples and expands the time scales that are observable for dynamic studies, encouraging the investigation of systems more closely aligned with those found in nature and industry.

Example applications

Gene delivery: Finding ways to get medicines to where they are needed in the body

Atmospheric chemistry: Pollutants and cloud formation

Ionic Liquids: ‘Green’ solvents for industry


The students weren’t in any danger



Polref is a polarised neutron reflectometer designed for the study of the magnetic ordering in and between the layers and surfaces of thin film materials.

Through precise control of the neutron spin, unique information on the size and direction of the magnetism as a function of depth can be obtained, allowing very complicated structures to be studied layer by layer.

Example applications


Magnetic nanostructures

Complex biological layered structures

Fundamental magnetism and superconductivity



Time-of-flight Small-Angle Neutron Scattering instrument

Sans2d can be used to examine size, shape, internal structure and spatial arrangement in nanomaterials, ‘soft matter’, and colloidal systems, including those of biological origin, on length scales of between* 0.25-300 nm. SANS does not locate individual atoms but rather looks at the larger structures they form. This gives important insights into many everyday materials and biological systems.

* for comparison, a human hair is about 100,000nm in diameter!

Example applications

Nanoscience: Nanoparticles and liposomes, green chemistry, liquid crystals.

Complex mixtures: Detergent molecules used in commercial products.

Materials: Plastics, composites, metal all rocks, magnetic materials.

Biological macromolecules: Nucleic acids and proteins.




ISIS TS2 instrument WISH sample environment and detector array, exposed for renovation.

Wish is a long-wavelength diffractometer primarily designed 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.

Example applications

Magnetism in covalent systems and under extreme conditions

Large-unit-cell structures

Biological samples




Multi-purpose instrument for SANS, diffraction and spectroscopy utilising the larmor precession of polarised neutrons. Larmor will provide a suite of techniques not currently possible at ISIS and will also expand the range of spatial and temporal length scales to new areas.


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.


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).

Example applications

Complex and confined liquids

Functional and composite materials

Phase behaviour and nucleation





Cold neutron multi-chopper spectrometer for the study of dynamics in condensed matter to understand the microscopic origin of material properties

Example applications

Quantum fluids


Polymers and bio-molecular materials





This instrument is under construction

A neutron imaging and diffraction instrument for materials science, materials processing and engineering

IMAT (Imaging and Materials Science & Engineering) is a neutron imaging and diffraction instrument for materials science, materials processing and engineering. The special features of the instrument will be energy-selective neutron imaging and the combination of neutron imaging and neutron diffraction. It is expected that IMAT will start operating in September 2015.




Nicholas on the far left listening to the guide talking about HIFI

The new high-field muon instrument at ISIS, called HiFi, provides applied longitudinal fields up to 5 T.



Above left is a picture of a working model of ISIS. Above right is a picture showing Mr Clark and Nicholas investigating the model.

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