Cold Neutron Source

Cold Neutron Source (CNS) Diagram showing the incoming and outgoing flows of Helium and Deuterium.

The CNS is essentially a volume of liquid deuterium cooled to 20 degrees above absolute zero (-250°C), located less than 25 cm away from the reactor core. The reactor core produces about 20 Megawatts (MW) of energy when fully operational, so a substantial cooling plant is needed to remove the heat from the CNS to keep it cool and the deuterium liquefied.

The CNS was designed by the Petersburg Nuclear Physics Institute, and it is cooled by a 5-kilowatt cryogenic cooling system built by Air Liquide in France.  This is in turn cooled by heat exchangers and two compressors built in Germany by Kaeser.

 
How is the CNS cooled?

The liquid deuterium in the CNS is not pressurised, and the deuterium system operates as a helium-cooled thermosiphon - a passive heat exchanger based on natural convection which circulates and cools liquid without needing a mechanical pump.

In order to further improve its performance, there is a cavity in the CNS on the side facing neutron guides CG-1 to 3 [link to Neutron Guides page].

Producing neutrons in OPAL

OPAL is essentially a neutron factory, producing neutrons that can then be used by specialised instruments to study the properties and structure of a range of materials at a range of sizes from the sub-atomic to large and complex molecules and structures.  In fact, all nuclear reactors produce neutrons in their core and moderators (any substance used to slow down neutrons in nuclear reactors, such as heavy water) but OPAL is specifically designed for optimal neutron production at 20 MW.

Thermal neutrons

In OPAL's heavy-water moderator (Reflector Vessel), neutrons produced by the reactor core exchange energy with the heavy water molecules, giving the neutrons a characteristic energy of around 300 degrees above absolute zero, or about 25 meV.  This energy is similar to the vibrational energy in materials at room temperature, making these thermal neutrons useful for studying how atoms move within molecules or in solids and liquids.  When these thermal neutrons operate as a wave in a diffractometer (such as ECHIDNA or WOMBAT), they have a wavelength of around 2 Ångstroms or 0.2 nanometres.  This is similar to X-ray wavelengths and to the spacing between atoms in solids or liquids, making OPAL's thermal neutrons most useful in determining the structures of important new materials.

Cold neutrons

The CNS cools the neutrons to 20K, roughly 15 times lower in energy than the thermal neutrons. This reduction in temperature makes cold neutrons that are useful for studying superconductivity, magnetic, and other quantum effects that occur in materials at very low temperatures. In addition, the wavelength is now much longer, at 6 Ångstroms (almost a nanometre) or more, making these cold neutrons useful for the study of larger nanoscale objects including: whole polymer molecules; protein molecules; surfactants (soapy molecules important in technology and in cell membranes); defects in metals; and flux vortices in superconductors.

The old HIFAR reactor did not have a CNS, with the consequence that Australian researchers who wanted to study materials on the nanoscale had to travel overseas to access other facilities with cold neutron sources.

Which instruments use the CNS?

ANSTO presently has two cold-neutron instruments (QUOKKA and PLATYPUS) studying structure on the nanoscale, with two more (KOOKABURRA and BILBY) under construction.

In addition, there are three more cold-neutron instruments (PELICAN, SIKA and EMU) under construction, which are designed to do cold-neutron spectroscopy - measuring changes in the neutrons' energy and looking at the vibration or diffusion of molecules, atoms and magnetic moments - with much finer resolution than instruments using thermal neutrons.  Roughly half of the scientific program at OPAL uses cold neutrons and is completely dependent on the CNS.

How do the neutrons get from the CNS to the instruments?

The neutrons are transported from the CNS to the instruments by long, neutron guides that are rectangular in cross-section. These guides act a bit like optical fibres, relaying a beam of neutrons instead of light. Each guide has a complex, highly polished and multi-layered metal coating inside, called a supermirror, that bounces the neutrons along the guide to the instruments, as well as absorbing stray neutrons, preventing their escape.

At present, OPAL has two cold neutron guides (CG-1 and CG-3) leading into the Neutron Guide Hall on one side of the cold source, with a third large simple beam (CG-4) on the other side of the cold source for the Taiwan-funded SIKA cold-neutron 3-Axis Spectrometer.

However, ANSTO now has funding to install another guide, CG-2, between CG-1 and CG-3 and this will be installed in 2012-2013.

More about the neutron beam guides