Read more: 鈥�Instant Expert: Neutron science鈥�
Since James Chadwick鈥檚 discovery, great strides have been made in developing intense sources of neutrons and instruments to detect them. As a result, we can now use neutrons to peer deep inside all manner of materials. They are particularly good at identifying the position of light atoms such as hydrogen, oxygen, carbon and nitrogen in samples. That鈥檚 because neutrons interact with the nucleus, rather than the cloud of electrons around it, and interact with very different strengths with the nucleus of each element at the lighter end of the periodic table. These insights are benefiting areas as varied as healthcare, food science, the environment and engineering.
From monopoles to wings
Every magnet has a north and a south pole. Even if you split a magnet in half, you simply end up with two dipolar magnets. But in the 1930s, British physicist Paul Dirac showed that theoretically a north or south pole could exist on its own.
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Physicists have searched for magnetic 鈥渕onopoles鈥� everywhere, from the debris of high-energy particle collisions to rocks from outer space. A breakthrough came in 2009, when two independent groups found evidence for the phenomenon by firing neutrons at artificial materials called spin ices.
The magnetisation of the particles in a spin ice can have many different arrangements, all of the same energy. If an array is heated up a little, adjacent pairs of particles may have enough energy to flip, creating a north and south pole. What鈥檚 more, each one may flip with its neighbour at no cost in energy, and if this process continues with further neighbours, the poles drift apart. Using neutrons, the teams were able to map out the spin ice鈥檚 magnetic structure and identify small disturbances that indicated the presence of separated defects, analogous to monopoles.
One day, monopoles could be used as a form of computer memory far more compact than anything available today.
Neutrons are also being used to study high-temperature superconductors (see Instant Expert 17, 5 November 2011). These strange materials can maintain a current without any voltage. Although they must be cooled to -200聽掳C in order to function, this temperature is well above that needed for conventional superconductors, which operate at close to absolute zero, or -273聽掳C.
We still don鈥檛 know how high-temperature superconductors work, though. In recent years, researchers have used neutrons to investigate tiny spontaneous loops of current and alternating patterns of spin found within their atomic layers. Both of these phenomena are thought to play a role in allowing electrons to pair up and move around unimpeded in a superconductive state.
If we can unlock the secrets of high-temperature superconductors we might be able to make revolutionary materials that conduct electricity without resistance at room temperature. This could lead to innovations such as power cables that never lose energy and exquisitely sensitive body scanners.
Neutrons are also used to solve problems with existing structures that you don鈥檛 want to fail, such as aircraft wings, railway tracks and turbine blades. Their material properties and performance are largely determined by their nanoscale structure, which is much too small to be examined with ordinary light microscopy. Neutrons, with their shorter wavelengths, provide a new kind of microscope to understand how stress affects these materials and how their properties can be improved for everyday use.
Shaken or stirred?
Many everyday objects, including tools, clothes, food and health products are made of soft materials whose properties arise from their highly complex structures. For more than 30 years, neutron scattering has been the most powerful tool for studying these structures. This is because they are made of long chains of hydrocarbons that contain the light elements that neutrons are so good at distinguishing between.
Take soaps and detergents, for example. Despite their ubiquity, we would still like to make them more useful and reduce their environmental impact. Earlier this year, a team at the Laue-Langevin Institute (ILL) in Grenoble, France, and the University of Bristol, UK, used neutrons to find out whether they could make soap magnetic by incorporating iron into the hydrocarbon chains. Such a soap could be manipulated in a magnetic field, improving water treatment and environmental clean-up.
Other familiar fluids, such as face creams, shampoos and sauces, flow in unusual ways due to the behaviour of their long, chain-like molecules. One phenomenon, called shear-thinning, causes a liquid to become much more runny when it is stirred or shaken. It鈥檚 a process familiar to anyone who has tried to get tomato ketchup out of a glass bottle, only to end up covering the entire plate.
To try to find out why this happens, in 2005 scientists fired a beam of neutrons at various liquids as they applied forces to them. The neutrons revealed the molecular orientation, while a device called a rheometer measured the liquid flow. The established a clear relationship between viscosity and the orientation of the chains, a finding that could help industry predict and tailor how products will leave the bottle or pool in your hand.
Getting biological
Proteins, viruses and cell membranes are naturally rich in light elements and so neutrons are ideal for analysing them. Biologists work alongside neutron scientists to decipher these biological structures and how they carry out their functions. One of the key techniques they use is called deuteration. Some or all of the hydrogen atoms in a sample are replaced with deuterium, a heavy isotope of hydrogen that has a neutron in addition to its single proton. Neutron scattering is so sensitive to light elements that it can tell the two isotopes apart. The sample is then contrasted with undeuterated versions to pick out the location of hydrogen atoms prevalent in biochemical reactions.
One area to benefit is the process of introducing foreign DNA into host cells. It is necessary in gene therapy and in the genetic modification of crops. Many potential agents for the process have been tested using neutron scattering, including the viruses used to inject DNA into cells.
Neutrons have also provided great insight into the transport of cholesterol within cell walls. Cholesterol surrounds every cell, helping carry signals around the body and assisting in the production of hormones. Maintaining the correct levels of cholesterol by redistributing it between and within the cells is vitally important, with abnormalities linked to Alzheimer鈥檚 disease and various cardiovascular disorders. In recent years, neutrons have illuminated these processes, revealing how cells achieve the right equilibrium and what causes the system to break down. This gives vital insight for potential treatments.
When it comes to medicine, there鈥檚 even more to neutrons. Radiopharmaceuticals are one of the best ways to diagnose and treat certain tumours. They deliver a radioactive isotope to cancer cells and kill it with a dose of radiation. But the radiopharmaceuticals being used today are the ones that are most readily available, rather than the ones with the best properties, and this can cause unnecessary damage to surrounding healthy tissue.
Research reactors are now being used to produce new radioisotopes, such as lutetium-177 and terbium-161. Last year, a team from ILL, the Technical University of Munich in Germany and the Paul Scherrer Institute in Villigen, Switzerland, demonstrated a for producing terbium isotopes in large quantities by irradiating samples of gadolinium with neutrons. Gadolinium-160 absorbs a neutron to produce a heavier isotope that undergoes beta decay and transforms into terbium-161. This type of technique is capable of being scaled up to treat hundreds of patients a week.
鈥淣uclear reactors are being used to produce new radioisotopes to diagnose and treat cancer鈥�
Terbium-161 offers a number of useful properties. It emits just the right level of gamma radiation to help trace the radioisotope鈥檚 movement around the body and it emits low-energy electrons, which are able to destroy cancerous cells without damaging too much surrounding tissue. It also has a half-life of about one week, which is long enough for it to be sent to hospitals and short enough not to pose a long-term nuclear waste problem.