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Neutrons Probing Matter

Introduction
Merkur
Neutron Tomography
Neutron tomographic view of the Roman bronze sculpture „Mercur from Thalwil“, an important exhibit of the Swiss National Museum (Zurich).

To use neutrons for this non-destructive investigation is mandatory due to the high lead content of the alloy, which makes the common X-ray techniques impossible to apply. The data were produced at the NEUTRA station of the Swiss spallation neutron source SINQ (PSI Villigen). http://neutra.web.psi.ch/

Regions showing high-degrees of corrosion are shown in red.

Image: Courtesy of Dr. Eberhard Lehman. .
Chadwick
Sir James Chadwick, 1935 Nobel prize.

Source Gonville & Caius College Web Site, Cambridge.

Almost everybody knows that neutrons are one of the three basic constituents of matter besides electrons and protons. It is however less widely known that neutron beams can be used to probe the structure and the dynamics of matter. Neutrons allow the investigation of a wide range of fundamental properties in various fields from physics and chemistry through materials science to biology, medicine and even environmental sciences. The arrival of SwissNeutronics within the MaNEP network has motivated us to present some basic information on neutrons and also some recent scientific activities of MaNEP scientists at the Paul Scherrer Institut (PSI) where a continuous neutron source is available.

Today's Applications
Neutron Scattering is not only used for fundamental studies in physics but also has numerous other applications. For example, neutrons can probe deep into the materials involved in aircraft engineering to give microscopic insights into the constraints affecting the operational lifetimes of these components. Similarly, neutron tomography allows for interesting investigations (see picture besides). Neutron scattering also improves our understanding of disordered crystalline materials, such as glasses and liquids, which are central to optical communication, chemical and biochemical engineering, food sciences, the pharmaceutical industry and molecular biology.

See most recent results at http://sinq.web.psi.ch/

History of sciences: The Neutron Discoverer
James Chadwick (1891-1974) was an English physicist educated at the Universities of Manchester and Cambridge who also studied under Hans Geiger at the Technische Hochschule in Berlin. From 1923 he worked with Ernest Rutherford at the Cavendish Laboratory in Cambridge, studying the transmutation of elements by bombarding them with alpha particles. They investigated the nature of the atomic nucleus, identifying the proton (the nucleus of the hydrogen atom) as a constituent of the nuclei of other atoms. In 1932, Chadwick made a fundamental discovery in the domain of nuclear science: he proved the existence of neutrons.
Chadwick observed that beryllium, when exposed to bombardment by alpha particles, released an unknown radiation that in turn ejected protons from the nuclei of various substances. Chadwick interpreted this radiation as being composed of particles of mass approximately equal to that of the proton, but without electrical charge: neutrons (see below). This discovery provided a new tool for inducing atomic disintegration since neutrons, being electrically uncharged, can penetrate undeflected into the atomic nucleus. Chadwick received numerous prizes and won the Nobel Prize for Physics in 1935; he was also knighted in 1945.
Adapted from Encyclopædia Britannica and Nobel Lectures, Elsevier.

What is a Neutron
A neutron is an electrically neutral subatomic particle with approximately the mass of a proton which is commonly found in the nuclei of all atoms (except hydrogen).
Neutron Sources
The way to obtain free neutrons is not straightforward since one needs to break the nuclear forces. Neutron beams can be produced from nuclear fission reactors (for example at the Laue Langevin Institute in Grenoble) or may be obtained when a proton beam hits a heavy metal target (for example the spallation neutron source SINQ at PSI). A spallation source has the advantage of delivering an extremely intense neutron beam with low heat production in contrast to traditional reactor sources which suffer from intense heat production in the reactor core.
Neutron Guides
Neutron guides are needed to bring neutrons from the source to the experiment site. Similarly to light mirrors, neutron guides are made of glass with polished surfaces, coated with special neutron reflecting materials such as Ni-Ti supermirrors. They allow intense neutron beams to be transported over long distances with low neutron losses.

See the neutron guide concept of SwissNeutronics.

Why are Neutrons a Versatile Probe
Neutron beams are a type of radiation with properties that make them an extremely versatile and unique analytical probe as explained below.
Seeing Inside Matter
Due to their neutral character, neutrons can penetrate matter with almost no attenuation (only a few elements can attenuate neutron beams) and in a non-destructive way, thereby giving access to its properties. Neutrons interact differently with each component inside matter allowing for high contrast imaging between elements of similar nature. Furthermore neutrons have a wavelength of the order of the distance between atoms. This allows them to "see" matter in the 0.1-100nm range, ena-bling the investigation of crystal structures and microstructures of materials.
Sensing Magnetism
Neutrons have an intrinsic magnetic nature (spin) which leads to a very high sensitivity to the magnetic properties of matter.
Feeling Vibrations
Neutrons interact with moving atoms inside matter, thus changing their speed and direction and allowing us to probe and measure the dynamic properties of matter.

Read the scientific article of the newsletter Nr 7.

Neutron Scattering
The way neutrons diffuse through matter depends on the physical properties of the materials investigated. This allows for investigations inside materials (in contrast with surface sensitive probes); such analyses are commonly referred as neutron scattering techniques. Bertram Brockhouse and Clifford Shull received the 1994 Nobel Prize in Physics for pioneering contributions to their development.
In simple terms, neutron scattering experiments help answer where atoms "are" and what atoms "do".


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