New source of super-chilled neutrons provides tools for understanding fundamental physics concepts

Ultra-cold neutrons move at very low speeds -- a few metres a second. An average runner could overtake them without trouble. More important from an experimental point of view, they are slow enough that one can store them in special neutron traps for several hundreds of seconds. This enables scientists to perform measurements with high-precision, notably by using resonance techniques. Depending on the experimental circumstances, the neutron can exhibit a variety of aspects. While some experiments may be well described considering the neutron as a simple massive neutral ball, others demonstrate the neutron's wave features following the laws of quantum mechanics. Its internal structure of stupefying complexity made up of quarks and gluons, gives rise to a plethora of static properties and modes of decay determined by the interplay of fundamental forces. Because they can probe a wealth of different phenomena, ultra-cold neutrons have become a precious tool for those working in fundamental physics, particle physics and cosmology.

The very first ultra-cold neutrons were produced near Moscow and Munich at the end of the sixties, and since 1985 ILL's PF2 source has been leading the world in the provision of UCN to experimentalists. PF2 uses a technique conceived by Albert Steyerl and his colleagues in Munich, based on the mechanical slowing down of (already very slow) neutrons exiting a cold liquid deuterium source. The neutrons are reflected by the blades of a turbine, thereby losing most of their remaining energy. They can then be 'bottled' in a trap, typically with a density of 10 UCN per cubic centimetre. The very high reliability of the ILL source has helped international research teams to obtain answers to important questions about the formation of matter after the big bang, the fundamental symmetries underlying particle interactions, or the nature of that mysterious dark matter which we know exists but which we still fail to understand.

Suggestions have been made over many years now for promising new techniques of UCN production, in the hope of arriving at an even more powerful source. The "next generation" source now announced in Physical Review Letters builds on a physical mechanism proposed by Bob Golub and Mike Pendlebury in …1975. Cold neutrons are projected at 450 metres a second into a helium-4 superfluid kept at a temperature of less than 1 Kelvin, i.e., less than one degree above absolute zero. When the neutrons enter the tank of helium they produce elementary vibrations in the liquid and lose most of their kinetic energy; they become ultra-cold.

Although the potential of this mechanism had already been proven in previous experiments, attempts made in the eighties to create a universal ultra-cold neutron source capable of competing with PF2 failed, due to problems in extracting the UCN from the superfluid. Zimmer and his team have now solved this problem in simple and efficient manner. They use a cold valve on a short chimney to extract the UCN accumulated in the source, where they behave like a low-energy gas. This avoids the ultra-cold neutrons having to cross a "window" to exit the source; extraction losses are therefore kept very low. Using this method they were able to achieve a density of around 55 UCN per cubic centimetre, and they also believe there's good potential for improving on this figure. The new apparatus has been built at the ILL as part of a programme to develop new techniques for producing and handling UCN, the ultimate aim being to perform ever more precise measurements in fundamental physics. In fact the GRANIT collaboration is now adapting the source to feed their new spectrometer to be used to test Newton's law of gravity at micrometre scale and thus to improve our understanding of the gravitational force at small distances.

We should note, however, that although the new source is now producing a higher density of ultra-cold neutrons, its constant flux is still well below that of ILL's older source, PF2, which still produces the strongest flux of UCN in the world. PF2 therefore continues to play its prominent role for the scientific community, particularly in experiments involving large experimental volumes.