Neutrons and Lollipops: What do they have in common?
Dr Silvia Capelli
ISIS Neutron and Muon Source
Rutherford Appleton Laboratory
Harwell Science Campus, Didcot, UK
Crystals can be found anywhere around us: from sugar and salt on our dining tables, to snowflakes and gemstones in nature, up in roofs in solar panels or in electrical cars’ batteries. Determining the relative arrangement of atoms in crystalline materials is one of the key points in understanding their properties and possible applications but also in designing new materials. Neutron crystallography is one of the core techniques in this context.
On Wednesday 21st April I was lucky enough to attend this lecture. The following are my notes (and any mistakes are mine) and I would like to thank Dr Capelli for sharing the images from her lecture.
“Why water boils at 100°C and methane at -161°C, why blood is red and grass is green, why diamond is hard and wax is soft, why glaciers flow and iron gets hard when you hammer it, how muscles contract, how sunlight makes plants grow and how living organisms have been able to evolve into ever more complex forms … the answers to all these problems have come from structural analysis.“ Max F. Perutz, 1996.
Max Ferdinand Perutz OM CH CBE FRS (19 May 1914 – 6 February 2002) was an Austrian-born British molecular biologist, who shared the 1962 Nobel Prize for Chemistry with John Kendrew, for their studies of the structures of haemoglobin and myoglobin. He went on to win the Royal Medal of the Royal Society in 1971 and the Copley Medal in 1979. At Cambridge he founded and chaired (1962–79) The Medical Research Council (MRC) Laboratory of Molecular Biology (LMB), fourteen of whose scientists have won Nobel Prizes. Perutz’s contributions to molecular biology in Cambridge are documented in The History of the University of Cambridge: Volume 4 (1870 to 1990) published by the Cambridge University Press in 1992.
1) Introduction to Crystallography. This is a relatively new discipline involving diffraction. What do we learn in a diffraction experiment?
2) How are diffraction experiments carried out
3) Why are neutrons used in diffraction experiments?
4) Examples of the science and its applications
Looking back …
Wilhelm Conrad Röntgen (27 March 1845 – 10 February 1923) was a German mechanical engineer and physicist, who, on 8 November 1895, produced and detected electromagnetic radiation in a wavelength range known as X-rays or Röntgen rays, an achievement that earned him the first Nobel Prize in Physics in 1901. In honour of his accomplishments, in 2004 the International Union of Pure and Applied Chemistry (IUPAC) named element 111, roentgenium, a radioactive element with multiple unstable isotopes, after him.
Wilhelm Roentgen accidently discovered X-rays. He was investigating the external effects from the various types of vacuum tube equipment — apparatuses from Heinrich Hertz, Johann Hittorf, William Crookes, Nikola Tesla and Philipp von Lenard — when an electrical discharge was passed through them. In early November he repeated an experiment with one of Lenard’s tubes in which a thin aluminium window had been added to permit the cathode rays to exit the tube but a cardboard covering was added to protect the aluminium from damage by the strong electrostatic field that produces the cathode rays. He knew the cardboard covering prevented light from escaping, yet he observed that the invisible cathode rays caused a fluorescent effect on a small cardboard screen painted with barium platinocyanide when it was placed close to the aluminium window. It occurred to Röntgen that the Crookes–Hittorf tube, which had a much thicker glass wall than the Lenard tube, might also cause this fluorescent effect.
In the late afternoon of 8 November 1895, he was determined to test his idea. He carefully constructed a black cardboard covering similar to the one he had used on the Lenard tube. He covered the Crookes–Hittorf tube with the cardboard and attached electrodes to a Ruhmkorff coil to generate an electrostatic charge. Before setting up the barium platinocyanide screen to test his idea, he darkened the room to test the opacity of his cardboard cover. As he passed the Ruhmkorff coil charge through the tube, he determined that the cover was light-tight and turned to prepare the next step of the experiment. It was at this point that he noticed a faint shimmering from a bench a few feet away from the tube. To be sure, he tried several more discharges and saw the same shimmering each time. Striking a match, he discovered the shimmering had come from the location of the barium platinocyanide screen he had been intending to use next.
He speculated that a new kind of ray might be responsible. 8 November was a Friday, so he took advantage of the weekend to repeat his experiments and made his first notes. In the following weeks he ate and slept in his laboratory as he investigated many properties of the new rays he temporarily termed “X-rays”, using the mathematical designation (“X”) for something unknown. The new rays came to bear his name in many languages as “Röntgen rays” (and the associated X-ray radiograms as “Röntgenograms”).
At one point while he was investigating the ability of various materials to stop the rays, Röntgen brought a small piece of lead into position while a discharge was occurring. Röntgen thus saw the first radiographic image, his own flickering ghostly skeleton on the barium platinocyanide screen. He later reported that it was at this point that he determined to continue his experiments in secrecy, because he feared for his professional reputation if his observations were in error.
Nearly two weeks after his discovery, he took the very first picture using X-rays of his wife Anna Bertha’s hand. When she saw her skeleton she exclaimed “I have seen my death!”
Above right shows the first medical X-ray by Wilhelm Röntgen of his wife Anna Bertha Ludwig’s hand.
Max Theodor Felix von Laue (9 October 1879 – 24 April 1960) was a German physicist who won the Nobel Prize in Physics in 1914 for his discovery of the diffraction of X-rays by crystals. In addition to his scientific endeavours with contributions in optics, crystallography, quantum theory, superconductivity, and the theory of relativity, he had a number of administrative positions which advanced and guided German scientific research and development during four decades. A strong objector to National Socialism, he was instrumental in re-establishing and organizing German science after World War II.
In 1912 Max Von Laue wondered what would happen if he passed short wavelength waves through a crystal when discussing the propagation of light in crystals with a colleague. He supposed that the much shorter electromagnetic rays, which X-rays were supposed to be, would cause some kind of diffraction or interference phenomena in a medium and that a crystal could provide this medium. This discussion laid the basis for modern X-ray crystallography and a few months later, in April 1912, the first demonstration of X-ray diffraction from a crystal lattice was achieved.
The initial experiment involved passing X-rays through a crystal of copper sulphate, then with some crystals of the mineral, Blende, which produced a pattern of spots.
But why was a pattern of spots produced?
A regular array of spots on a photographic emulsion results from X rays scattered by certain groups of parallel atomic planes within the crystal. When a thin, pencil-like beam of X rays is allowed to impinge on the crystal, those of certain wavelengths will be oriented at just the proper angle to a group of atomic planes so that they will combine in phase to produce intense, regularly spaced spots on a film or plate centred around the central image from the beam, which passes through undeviated. Laue patterns, first detected by Max von Laue, a German physicist, are invaluable for crystal analysis.
The interference patterns supported the interpretation of X-rays as electromagnetic waves. Remarkably, these findings also had an exceptional resonance among crystallographers: those well-defined spots were seen as conclusive evidence that atoms arrange in a space-lattice configuration in crystals. As Alfred Tutton — an English crystallographer — stated in November 1912 “the space-lattice structure of crystals … is now rendered visible to our eyes”
1912-1913: Bragg’s law
“… the spots in Laue’s crystallographs can be shown to be due to partial reflection of the incident beam in sets of parallel planes in the crystal.. “ (W.L. Bragg). The spots indicate the shape of the crystal.
https://en.wikipedia.org/wiki/William_Henry_Bragg (Above left)
Sir William Henry Bragg OM KBE PRS (2 July 1862 – 12 March 1942) was a British physicist, chemist, mathematician and active sportsman who uniquely shared a Nobel Prize with his son Lawrence Bragg – the 1915 Nobel Prize in Physics: “for their services in the analysis of crystal structure by means of X-rays”. The mineral Braggite is named after him and his son. He was knighted in 1920.
https://en.wikipedia.org/wiki/Lawrence_Bragg (Above right)
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 Bragg’s law of X-ray diffraction, which is basic for the determination of crystal structure. He was joint winner (with his father, William Henry Bragg) of the Nobel Prize in Physics in 1915: “For their services in the analysis of crystal structure by means of X-ray”, an important step in the development of X-ray crystallography.
The composition of X-rays was unknown at the time; W H Bragg argued that X-rays are streams of particles whilst others argued that they are waves. Max von Laue directed an X-ray beam at a crystal in front of a photographic plate; alongside of the spot where the beam struck there were additional spots from deflected rays — hence X-rays are waves. In 1912, as a first-year research student at Cambridge, W L Bragg, while strolling by the river, had the insight that crystals made from parallel sheets of atoms would not diffract X-ray beams that struck their surface at most angles because X-rays deflected by collisions with atoms would be out of phase, cancelling one another out. However, when the X-ray beam stuck at an angle at which the distances it passed between atomic sheets in the crystal equalled the X-ray’s wavelength then those deflected would be in phase and produce a spot on a nearby film. From this insight he wrote the simple Bragg equation that relates the wavelength of the X-ray and the distance between atomic sheets in a simple crystal to the angles at which an impinging X-ray beam would be reflected.
W H Bragg built an apparatus in which a crystal could be rotated to precise angles while measuring the energy of reflections. This enabled father and son to measure the distances between the atomic sheets in a number of simple crystals. They calculated the spacing of the atoms from the weight of the crystal and Avogadro’s constant which enabled them to measure the wavelengths of the X-rays produced by different metallic targets in the X-ray tubes. W H Bragg reported their results at meetings and in a paper, giving credit to “his son” (unnamed) for the equation, but not as a co-author, which gave his son “some heartaches”, which he never overcame.
Bragg diffraction (also referred to as the Bragg formulation of X-ray diffraction) was first proposed by Lawrence Bragg and his father William Henry Bragg in 1913 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.
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.
Bragg diffraction occurs when radiation, with a 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 difference between the path lengths of the two waves is equal to an integer multiple (n) of the wavelength. The path difference between two waves undergoing interference is given by 2dsinθ, where θ is the scattering angle.
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. Atoms become “real” objects that can be seen experimentally!!! A milestone in science allowing atoms to be seen.
What is a crystal? A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are usually identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations. The scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification.
The word crystal derives from the Ancient Greek word κρύσταλλος (krustallos), meaning both “ice” and “rock crystal”, from κρύος (kruos), “icy cold, frost”
From there on…
1913 – W.L. Bragg: structure determination of NaCl (Cubic unit cell), KCl (Cubic unit cell), KBr (Cubic unit cell), KI single crystals (Cubic unit cell), followed by diamond (The crystal structure of diamond is comprised of tetrahedrally coordinated C atoms. The unit cell is fcc, with 2 carbon atoms per lattice point, and 8 atoms per cell), calcite (single hexagonal shape), pyrite (unit cell is composed of a Fe face-centred cubic sub lattice into which the S ions are embedded)…..
1919 – Niggli : first space group tables useful for structure determination Geometrische Kristallographie des Diskontinuums
Paul Niggli (26 June 1888 – 13 January 1953) was a Swiss crystallographer who was a leader in the field of X-ray crystallography.
He originated the idea of a systematic deduction of the patterns in the internal structure of crystals by means of X-ray data. He supplied a complete outline of methods that have since been used to determine these patterns. There are 230 possible different internal patterns for different crystals. Because the patterns describe a three-dimensional arrangement, they are known as space groups. Niggli also developed a notation that described the individual space groups, and co-authored a definitive set of tables describing them.
1932 – J. Chadwick: discovery of neutrons
Sir James Chadwick, CH, FRS (20 October 1891 – 24 July 1974) was a British physicist who was awarded the 1935 Nobel Prize in Physics for his discovery of the neutron in 1932.
A schematic diagram of the experiment used to discover the neutron in 1932. At left, a polonium source was used to irradiate beryllium with alpha particles, which induced an uncharged radiation. When this radiation struck paraffin wax, protons were ejected. The protons were observed using a small ionization chamber. Adapted from Chadwick (1932).
1934 – J.D. Bernal & D. Hodgkin : first X-rays pattern of a single crystal of the protein pepsin
John Desmond Bernal FRS (10 May 1901 – 15 September 1971) was an Irish scientist who pioneered the use of X-ray crystallography in molecular biology (below left).
Dorothy Mary Crowfoot Hodgkin OM FRS HonFRSC (12 May 1910 – 29 July 1994) was a British chemist who developed protein crystallography, for which she won the Nobel Prize in Chemistry in 1964 (above right)
Pepsin is an endopeptidase that breaks down proteins into smaller peptides (that is, a protease). It is produced in the stomach and is one of the main digestive enzymes in the digestive systems of humans and many other animals, where it helps digest the proteins in food. Pepsin has a three dimensional structure, of which one or more polypeptide chains twist and fold, bringing together a small number of amino acids to form the active site, or the location on the enzyme where the substrate binds and the reaction takes place. Pepsin is an aspartic protease, using a catalytic aspartate in its active site
1936 – Halban & Preiswerk: first neutron diffraction experiment
https://en.wikipedia.org/wiki/Hans_von_Halban (below left)
Hans Heinrich von Halban (24 January 1908 – 28 November 1964) was a French physicist
Peter Preiswerk (16 January 1907 – 28 January 1972) was a Swiss physicist and co-founder of CERN (above right).
1945 – C.W. Bunn: definition of Chemical Crystallography as “X-ray methods used for the identification of solid substances and the determination of atomic configurations”
Charles William Bunn (15 January 1905-13 April 1990) was a British chemist and largely self-taught crystallographer
1953 – Hautpman & Karle: Direct methods and solution of the phase problem (theory and mathematical description fully understood!)
Herbert Aaron Hauptman (February 14, 1917 – October 23, 2011) was an American mathematician and Nobel laureate. He pioneered and developed a mathematical method that has changed the whole field of chemistry and opened a new era in research in determination of molecular structures of crystallized materials.
Jerome Karle (born Jerome Karfunkle; June 18, 1918 – June 6, 2013) was an American physical chemist. Jointly with Herbert A. Hauptman, he was awarded the Nobel Prize in Chemistry in 1985, for the direct analysis of crystal structures using X-ray scattering techniques
1953 – Watson & Crick, Wilkins & Franklin: Structure of DNA!
https://en.wikipedia.org/wiki/Francis_Crick (below left)
Francis Harry Compton Crick OM FRS (8 June 1916 – 28 July 2004) was a British molecular biologist, biophysicist, and neuroscientist, most noted for being a co-discoverer of the structure of the DNA molecule in 1953 with James Watson, work which was based partly on fundamental studies done by Rosalind Franklin and Maurice Wilkins. Together with Watson and Wilkins, he was jointly awarded the 1962 Nobel Prize in Physiology or Medicine “for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material”.
https://en.wikipedia.org/wiki/James_Watson (above right)
James Dewey Watson (born April 6, 1928) is an American molecular biologist, geneticist and zoologist, best known as one of the co-discoverers of the structure of DNA in 1953 with Francis Crick and Rosalind Franklin. Watson, Crick, and Maurice Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine “for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material”.
https://en.wikipedia.org/wiki/Rosalind_Franklin (below left)
Rosalind Elsie Franklin (25 July 1920 – 16 April 1958) was an English chemist and X-ray crystallographer who made contributions to the understanding of the molecular structures of DNA (deoxyribonucleic acid), RNA (ribonucleic acid), viruses, coal, and graphite. Although her works on coal and viruses were appreciated in her lifetime, her contributions to the discovery of the structure of DNA were largely recognised posthumously.
https://en.wikipedia.org/wiki/Maurice_Wilkins (above right)
Maurice Hugh Frederick Wilkins CBE FRS (15 December 1916 – 5 October 2004) was a New Zealand-born British physicist and molecular biologist, and Nobel laureate whose research contributed to the scientific understanding of phosphorescence, isotope separation, optical microscopy and X-ray diffraction, and to the development of radar. He is best known for his work at King’s College London on the structure of DNA.
A primary technique for structural analysis of biological molecule is X-ray crystallography. The wavelength of X-rays is about the same as the space between the atoms in crystalline matter. Deflected X-rays can give an image pattern on a photographic plate, whose angles when analysed mathematically can lead to the details of each atoms arrange with respect to the other atoms.
To solve the structure of DNA four ideas had to come together. That the phosphate backbone was on the outside, bases on the inside, that the molecule was a double helix, that the strands were antiparallel and that it had a specific base pairing.
Franklin found out that by bundling super thin strands of DNA and zapping them with a super fine x-ray beam there were two forms of hydration — the A form (easy to photograph) that is dry and the B form (hard to photograph) that is wet. Her B form photographs showed a fuzzy cross which meant a helix. Since the water would be attracted to the phosphates in the backbone, and the DNA was easily hydrated and dehydrated, she guessed that the backbone was on the outside and the bases were on the inside. The first part of the problem was solved.
Photo 51 X-ray diffraction of wet DNA showing the B form double helix taken by Rosalind Franklin and Raymond Gosling on Friday, 2nd May, 1952 by long exposure started the previous day. Being a perfectionist Franklin would not release any data until she had more information on the A form which was giving the most data.
Not releasing the information on the B form proved to be Franklin’s downfall, for she got bogged down with calculations and obsessed in trying to determine whether the A form was helical. Gosling got so frustrated trying to visualise the geometry of the arcs that oranges were used to simulate the spatial relations of the several curves. Finally on Friday the 18th July, 1952 Rosalind took up her fountain pen and, in capital letters, wrote, on a 3 x 6 inch card with a hand inked black border, a death notice for the DNA helix (Crystalline) which she and Gosling signed; referring only to the A form “crystalline” DNA. Franklin continued to waste most of the winter of 1952 with work on the A form. She did meet with Crick who tried to offer advice, but, since his character was typically patronising, she rejected an opportunity to collaborate. An opportunity missed, for they would most certainly have solved the puzzle in months.
Wilkins had been given a clear copy of Franklin’s B form photograph by Raymond Gosling in a corridor a few days early, showed it to Watson without Franklin’s consent or knowledge.
Max Perutz, head of the Medical Research Council Unit housed at the Cavendish Laboratory in Cambridge, where Crick was a research student, received a government report that contained the data presented by Franklin at her departmental seminar. The report was not confidential, but it was private. Perutz passed the report on to Crick without asking Randall or Franklin’s permission.
Watson and Crick now had all Franklin’s data which showed that DNA was a multiple helix. Crick, who had worked on proteins, soon realised that Franklin’s data implied an antiparallel double helix. On the 10th of February Franklin began working on the B form again. According to Franklin’s note book she had had the antiparallel idea for the A form, but had not applied it to the B form, had she done so she would have solved part three of the puzzle earlier.
Francis Crick shows James Watson the model of DNA that they started building on Wednesday, 4th March and finished in the evening of Saturday, 7th March, 1953 in their room number 103 of the Austin Wing at the Cavendish Laboratories, Cambridge, using a slide-rule.
Franklin produced a draft paper, with Gosling, dated the 17th of March, 1953, which outlined that the molecule was a double helix, had specific base pairing and the antiparallel A form, which had not been applied to the B form. She did not realise that Watson and Crick were racing to publish first, which they did on the 18th of March, 1953, so beating her because she had not published.
The Watson and Crick paper entitled “A Structure for Deoxyribose Nucleic Acid” written on the 2nd of April, 1953 and published in “Nature” on the 25th April, 1953 cited no authorities or historical record. It opened with the sentence “We wish to suggest a structure for the salt of deoxyribose nucleic acid (DNA). This structure has novel features which are of considerable biological interest.”. It contains no experimental proofs. It contains only hypotheses. No acknowledgement is made to Franklin and Wilkins at King’s College, London beyond the following statement. “We have also been stimulated by a knowledge of the general nature of the unpublished results and ideas of Dr. M.H.F. Wilkins, Dr. R.E. Franklin, and their co-workers at King’s College London.”.
In 1962 Watson, Crick and Wilkins received the Nobel Prize in Physiology or Medicine. They proposed that the DNA molecule takes the shape of a double helix, an elegantly simple structure that resembles a gently twisted ladder. The rails of the ladder are made of alternating units of phosphate and the sugar deoxyribose; the rungs are each composed of a pair of nitrogen-containing nucleotides. In their Nobel lectures they cite 98 references, none are Franklin’s. Only Wilkins included her in his acknowledgements. Franklin died in 1958. The Nobel Prize is only awarded to living persons.
A new discipline: Crystallography
… moving forward …
What can be learnt in a diffraction experiment?
Inter and intra-molecular interactions (Van der Waals, H-Bonding…)
Molecular motion in the solid state (complementary information in respect to “spectroscopic” techniques)
Phase transitions, Polymorphism
Arrangement /interconnections in layered/hybrid structures
Short range ordering of atoms or molecules (diffuse scattering)
–> Interactions, reactions paths, transformations
–> Understanding/designing molecular properties
Materials have different forms …
single crystals, powders, poly-crystalline materials
… and need different techniques …
Protein crystal uses diamond light source
Big crystal need orientating – uses neutron diffraction
… and produce different patterns!
But first a bit of history …
Thomas Young FRS (13 June 1773 – 10 May 1829) was a British polymath and physician. In his own judgment, of his many achievements the most important was to establish the wave theory of light. To do so, he had to overcome the century-old view, expressed in the venerable Isaac Newton’s Opticks, that light is a particle. Nevertheless, in the early-19th century Young put forth a number of theoretical reasons supporting the wave theory of light, and he developed two enduring demonstrations to support this viewpoint. With the ripple tank he demonstrated the idea of interference in the context of water waves. With the Young’s interference experiment, or double-slit experiment, he demonstrated interference in the context of light as a wave.
Diffraction results from the interference of an infinite number of waves emitted by a continuous distribution of source points. What happens when an electromagnetic wave interacts with a crystal.
However the wave theory of light could not explain the photoelectric effect.
The photoelectric effect refers to the emission, or ejection, of electrons from the surface of, generally, a metal in response to incident light.
Energy contained within the incident light is absorbed by electrons within the metal, giving the electrons sufficient energy to be ‘knocked’ out of, that is, emitted from, the surface of the metal.
Using the classical Maxwell wave theory of light, the more intense the incident light the greater the energy with which the electrons should be ejected from the metal. That is, the average energy carried by an ejected (photoelectric) electron should increase with the intensity of the incident light.
In fact this didn’t happen. Rather the energies of the emitted electrons were found to be independent of the intensity of the incident radiation.
Einstein (1905) proposed that the incident light consisted of individual quanta, called photons that interacted with the electrons in the metal like discrete particles, rather than as continuous waves. For a given frequency, or ‘colour,’ of the incident radiation, each photon carried the energy E = hf, where h is Planck’s constant and f is the frequency. Increasing the intensity of the light corresponded, in Einstein’s model, to increasing the number of incident photons per unit time (flux), while the energy of each photon remained the same (as long as the frequency of the radiation was held constant).
Clearly, in Einstein’s model, increasing the intensity of the incident radiation would cause greater numbers of electrons to be ejected, but each electron would carry the same average energy because each incident photon carried the same energy. [This assumes that the dominant process consists of individual photons being absorbed by and resulting in the ejection of a single electron.] Likewise, in Einstein’s model, increasing the frequency f, rather than the intensity, of the incident radiation would increase the average energy of the emitted electrons.
Both of these predictions were confirmed experimentally. Moreover, the rate of increase of the energy of the ejected electrons with increasing frequency, which can be measured, enables one to determine the value of Planck’s constant h.
The photoelectric effect is perhaps the most direct and convincing evidence of the existence of photons and the ‘corpuscular’ nature of light and electromagnetic radiation. That is, it provides undeniable evidence of the quantization of the electromagnetic field and the limitations of the classical field equations of Maxwell.
In 1924 Louis De Broglie proposed that if light can contain particles, which are concentrations of energy incorporated into the wave, then perhaps all particles, like the electron, can be transported as a wave.
This theory set the basis of wave mechanics. It was supported by Einstein, confirmed by the electron diffraction experiments of G P Thomson and Davisson and Germer, and generalized by the work of Schrödinger.
The Davisson–Germer experiment was a 1923-7 experiment by Clinton Davisson and Lester Germer at Western Electric (later Bell Labs), in which electrons, scattered by the surface of a crystal of nickel metal, displayed a diffraction pattern. This confirmed the hypothesis, advanced by Louis de Broglie in 1924, of wave-particle duality, and was an experimental milestone in the creation of quantum mechanics.
So neutrons, being particles, should display wave-like behaviour under certain circumstances, and therefore be diffracted.
So what do we know about neutrons:
In 1911 Earnest Rutherford proposed the nuclear atom
In 1913 Niels Bohr identified the fact that charges are separate in the atom
In 1920 the term neutron was first used
In 1932 James Chadwick discovered the neutron
In 1934 Fermi investigated the interaction of neutrons with matter
In 1936 Halban & Preiswerk carried out the first neutron diffraction experiment
In 1937 Enrico Fermi fled to the USA
In 1942 the first human-made self-sustaining nuclear chain reaction was initiated in Chicago Pile-1, during an experiment led by Enrico Fermi.
In 1944 Shull, Wollan and associates developed neutron scattering techniques utilizing reactor neutrons that occurred at the Oak Ridge National Laboratory.
In 1946 the triple-axis spectrometry method was first developed by Bertram Brockhouse at the National Research Experimental NRX reactor at the Chalk River Laboratories in Canada.
X-rays (Photons) or Neutrons (particles)?
The above information explains why neutron radiography—which uses neutrons to image objects—is very good at visualizing lighter elements and liquids.
Neutron imaging is the process of making an image with neutrons. The resultant image is based on the neutron attenuation properties of the imaged object. Some light elements/materials e.g. water strongly absorb neutrons while many commonly used metals allow neutrons to pass through them.
Neutrons can identify small atoms. Isotopes of the same element will behave differently to neutron imaging because they contain different numbers of neutrons.
How to produce neutrons?
In nuclear fission the nucleus of an atom breaks up into two lighter nuclei. The process may take place spontaneously in some cases or may be induced by the excitation of the nucleus with a variety of particles (e.g., neutrons, protons, deuterons, or alpha particles) or with electromagnetic radiation in the form of gamma rays. In the fission process, a large quantity of energy is released, radioactive products are formed, and several neutrons are emitted. These neutrons can induce fission in a nearby nucleus of fissionable material and release more neutrons that can repeat the sequence, causing a chain reaction in which a large number of nuclei undergo fission and an enormous amount of energy is released.
2) Spallation process
Single event reaction Pulsed flow of neutrons 30 neutrons/proton hit
Spallation is a process in which fragments of material (spall) are ejected from a body due to impact or stress.
Nuclear spallation occurs naturally in Earth’s atmosphere owing to the impacts of cosmic rays, and also on the surfaces of bodies in space such as meteorites and the Moon.
Nuclear spallation is one of the processes by which a particle accelerator may be used to produce a beam of neutrons. A particle beam consisting of protons at around 1 GeV are shot into a target consisting of mercury, tantalum, lead or another heavy metal. The target nucleii are excited and upon deexcitation, 20 to 30 neutrons are expelled per nucleus. Although this is a far more expensive way of producing neutron beams than by a chain reaction of nuclear fission in a nuclear reactor, it has the advantage that the beam can be pulsed with relative ease. Furthermore the energetic cost of one spallation neutron is six times lower than that of a neutron gained via nuclear fission. In contrast to nuclear fission, the spallation neutrons cannot trigger further spallation or fission processes to produce further neutrons. Therefore, there is no chain-reaction, which makes the process non-critical. The concept of nuclear spallation was first coined by Nobelist Glenn T. Seaborg in his doctoral thesis on the inelastic scattering of neutrons in 1937
Neutron beams with multi-MeV energies can be produced using laser based acceleration mechanisms in a two stage process whereby ions are accelerated and then converted into neutrons through nuclear reactions, as shown in the following diagram. Fast neutron beams have applications in neutron radiography imaging and fast neutron therapy. Compared to conventional accelerators, laser based techniques offer the advantage of reduced charged particle acceleration distances, from the meter scale to the millimetre scale due to the high electrical fields supported by plasmas, and high instantaneous neutron production rates, due to the short pulse duration of the laser and the corresponding short acceleration time.
How to select the neutron energy
1) For a monochromatic source use a single crystal monochromator. The crystals could be germanium, silicon, copper of carbon.
2) For a polychromatic source you need to use a moderator such as water, methane or helium. A neutron moderator is a medium that reduces the speed of fast neutrons.
How to detect neutrons
Neutrons have no charge so you need to see how they react with nuclei.
1) Capture: This method of detection relies on the absorption of the neutron by an atom with the simultaneous emission of a γ-ray photon, often referred to as an (n,γ) reaction. Since the absorbing material must absorb neutrons and be capable of existing in gaseous form, then the choice of materials is very limited. The most common is 3He gas.
2) Scintillation process: the neutron is detected in a smaller region of physical space compared to a gas detector due to the higher density of absorbing material. The material can be zinc sulphide or 6Lithium (and other materials too).
Harwell Science Campus
Diamond light source
Diamond Light Source (“Diamond”) is the UK’s national synchrotron science facility located at the Harwell Science and Innovation Campus in Oxfordshire. Its purpose is to produce intense beams of light whose special characteristics are useful in many areas of scientific research. In particular it can be used to investigate the structure and properties of a wide range of materials from proteins (to provide information for designing new and better drugs), and engineering components (such as a fan blade from an aero-engine) to conservation of archaeological artefacts (for example Henry VIII’s flagship the Mary Rose.
Trajectories of electrons can be “bent” by magnetic fields
Interaction between magnetic field and electrons produce x-rays that are let out tangentially into the beamlines
ISIS Neutron and Muon Source is a pulsed neutron and muon source. It is situated at the Rutherford Appleton Laboratory of the Science and Technology Facilities Council, on the Harwell Science and Innovation Campus in Oxfordshire, United Kingdom. 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.
Protons hit a metal target (spallation process) at such high velocity that the metal nuclei break up, letting neutrons escape.
What do we learn?
Inter and intra molecular interactions;
Molecular motion in the solid state;
Phase transitions, polymorph arrangements etc;
Establish models and motions of molecules;
Establish the structure of molecules such as sucrose;
This analysis provides the first precise molecular parameters for sucrose. All hydrogen atoms are included. Carbon-carbon distances are 1.51 to 1.53 Å; carbon-oxygen 1.40 to 1.44 Å; carbon-hydrogen 1.08 to 1.11 Å; oxygen-hydrogen 0.94 to 0.99 Å. The furanose ring conformation differs from that in sucrose sodium bromide dihydrate. Hydrogen bonds (two of them intramolecular) utilize every hydroxyl group except one.
Some neutron discoveries
Dihydrogen bond in a Frustrated Lewis Pair (FLP) system
Frustrated Lewis pair: Lewis acid – base pair that are unable to form a classic donor-acceptor pair due to steric hindrance between the components.
First structural characterisation of a dihydrogen bond for the product of a hydrogen cleavage by a FLP
Charge, orbital and spin ordering in Pr(Sr,Ca)Mn2O7
– Two orbital order transitions at TCo1 ≈ 405K and TCo2 ≈ 300K
– Antiferromagnetic order transition TN ≈ 150K
Diffuse scattering and the mechanism of the phase transition in tri-glycine sulphate
Using neutron and X-ray single crystal diffuse scattering this study shows that hydrogen bond mediated interactions between polarising glycine molecules cause local one-dimensional polarised domains to develop, oriented parallel to the b axis.
Ferroelectricity due to H-bonding interactions
Rather than dipolar interactions form 1D
NH3+ chains that eventually order in 3D
Thermal diffuse scattering in CaTiSiO5 at RT
J. Hudspeth et al., J. Mater. Sci. 48, 6605 (2013).
Single-crystal diffuse scattering data have been collected at room temperature on synthetic titanite using both neutrons and high-energy x-rays. A simple ball-and-springs model reproduces the observed diffuse scattering well, confirming its origin to be primarily due to thermal motion of the atoms.
Comparison of observed and calculated neutron diffuse scattering using DFT. The upper half in each plot are the data, lower half the pattern. Colour scales of the computed images were adjusted to match the experimental data. Bragg peaks are not included in the theoretical patterns.
Ab-initio phonons aid in interpreting thermal diffuse scattering
Et al. and H. Dabkowska, J. Phys. Cond. Mat. 25, 315402 (2013)
DFT: Density functional theory
Inelastic effects in diffraction patterns: NaCl
Direct phonon excitation in a neutron time-of-flight single-crystal Laue diffraction experiment has been observed in a single crystal of NaCl. At room
temperature both phonon emission and excitation leave characteristic features in the diffuse scattering and these are well reproduced using ab initio phonons from density functional theory (DFT). A measurement at 20 K illustrates the effect of thermal population of the phonons, leaving the features corresponding to phonon excitation and strongly suppressing the phonon annihilation. A recipe is given to compute these effects combining DFT results with the geometry of the neutron experiment.
M. J. Gutmann, G. Graziano, S. Mukhopadhyay, K. Refson, and M. v. Zimmermmann, J. Appl. Cryst. 48, 1122 (2015).
Neutron data can show that some materials are not disordered below certain temperatures and can show hydrogen atoms moving about.
Neutrons can study magnetism in order to produce certain materials such as iron-manganese Prussian blue.
Crystalline, Mixed-Valence Manganese Analogue of Prussian Blue: Magnetic, Spectroscopic, X-ray and Neutron Diffraction Studies
Neutron data can indicate, charge, orbital, spin and ordering
2H2 + O2 –> 2H2O
Use organic hydrides. Neutron diffraction data indicates the best structure and which temperature to fill with hydrogen
Structures and Dynamics of the Mixed Dihydrogen/Hydride Complexes [Ru(PCP)(H)(H2)n] (n = 1, 2) and [Ru(PNP)(H)2(H2)]
Chidambaram Gunanathan, Silvia C. Capelli, Ulli Englert, Markus Hölscher and Walter Leitner
Neutron diffraction data from D19@ILL at 18 K (+100 and 200 K)
Extracting physical information from diffraction data
From neutron diffraction data at 18, 100, 200 K you can derive dynamical (and chemically important) features!
Conducting organic polymers
For applications in solar panels, transistors or chemical sensors the key to efficiency is the mobility of charge carriers!
Preserving environment: The giant crystals of Naiça (Mexico)
Giant gypsum crystals, which took 300,000 years to grow. Why do they turn brown?
Possible future science…
New Hybrid materials: new ways of combining Magnetic, optical and electronic properties
Sibille R., Mesbah A., Mazet T., Malaman B., Capelli S., Francois M., J. Sol. State Chem., (2012), 186, 134-141.
New look at macromolecules
Transthyretin: a protein related to neurodgenerative disease
M. Haupt et al, IUCrJ, 1, 429-438 (2014)
Oh, you may be wondering what connects neutrons with lollipops. Well lollipops contain atoms, and atoms contain neutrons.