SuperSUN – a new high-density source for ultracold neutrons – SuperSUN

Ultra-cold neutrons (UCN) play an important role to address key questions of particle physics at the low-energy, high-precision frontier, complementary to experiments done at high-energy particle accelerators. UCN are so slow that they can be trapped in material and magnetic bottles, which makes them very well suited for precise measurements. Experiments with UCN on one hand provide values of observables important for applications ranging from particle physics to cosmology. On the other hand they challenge the Standard Model of particle physics. A particularly notable example is the search for a non-vanishing neutron electric dipole moment (EDM) which investigates violation of the fundamental CP-symmetry and the puzzle why there is so much more matter in the universe than antimatter. A further example is the neutron lifetime which impacts the abundances of light chemical elements in big-bang nucleosynthesis. Experiments on gravitation at short distances in the micrometre range probe scenarios of extra-dimensions that might explain why gravitation is so much weaker than for instance the electrostatic force between two charged bodies. Further experiments address questions of Lorentz invariance, dark matter and quantum-mechanical effects that can be excellently studied in optics with very slow neutrons.

The best present UCN sources deliver densities in the order of only a few tens per cubic centimetre, so that many fundamental physics projects hit counting statistical limits. SuperSUN is proposed as a novel UCN source with the goal to introduce a valuable new tool with strong impact. It will enable experiments involving small-to-medium size vessels for trapping of UCN under much improved counting statistical conditions, respectively, enable previously impossible experiments. UCN production is based on the well established conversion of cold to ultracold neutrons via inelastic scattering in superfluid helium. A novel feature and key component of SuperSUN is a magnetic multipole reflector for a drastic enhancement of the UCN density with respect to an existing prototype superfluid-helium UCN source installed in a cold neutron beam. The reflector repels low field seeking UCN and thus strongly reduces losses due to UCN collisions with the material walls of the converter. This concept will lead to a drastic improvement of previous UCN storage time constants and hence provide a polarised UCN density beyond 1000 per ccm. For the proposed length of 3 m the source volume is about 22 litres. This source will be excellently suited to provide UCN for a magnetic trap of a neutron lifetime experiment and to fill the Ramsey cells of various projects to search for the neutron EDM. UCN will become fully polarised in the magnetic reflector, which is a very welcome feature for such experiments. Modern experiments on short range gravitation, where only a small neutron phase space element is extracted from the source operated in current mode, will also strongly benefit from this development.

The project takes advantage of the experience the applicant has gained in his developments of two superfluid-helium UCN source prototypes that led recently to a world-record UCN density surpassing 120 per ccm. It also takes advantage of the well advanced UCN source infrastructure at the ILL in Grenoble, and synergy with the expertise in coating technology brought in by our collaboration partner from Munich. The project SuperSUN has already been evaluated by a committee of non-ILL experts for neutron instrumentation and obtained strong scientific endorsement. ILL‘s Scientific Council therefore asked the ILL Management to find funding for SuperSUN. Support from ANR would enable its immediate realization. ILL has financed and is committed to provide a beam position, already optimised for this project, as soon as funding will become available.