SuperSUN instrument ready for use
The ILL’s new instrument SuperSUN has just been successfully commissioned. It produces high densities of ultracold neutrons using a “superthermal” production process in superfluid helium. This concept delivers large numbers of neutrons to users for long-duration storage experiments, enabling a next generation of precision measurements.
Ultracold neutrons, or “UCN”, have the amazing property that they can be stored in closed volumes. This feature, due to very low kinetic energies of only ~10-7 eV, opens unique possibilities for particle physics, where the neutron itself is considered as a sample for study. Long-duration storage enables precision measurements of basic properties, such as the neutron’s decay lifetime and permanent electric dipole moment, that provide crucial information about the early universe and its evolution through today. However, UCN can be produced only with very low efficiencies and experimental precision is ultimately limited by the total number detected. Obtaining large numbers of UCN requires both minimizing loss and an extremely high flux of cold neutrons – as available from the ILL.
SuperSUN is designed to provide UCN for storage experiments with long durations of several minutes, seeking to minimize neutron loss at every step . This places it in the category of “high density” UCN sources, in contrast to “high-flux” sources such as the ILL’s PF2 that have served the user community with strong UCN beams for many years. In SuperSUN, cold neutrons become UCN by scattering inelastically in isotopically pure superfluid 4He at temperatures below 0.6 K. Essentially their entire energy and momentum are lost within single scattering events .
During initial operation in 2023, SuperSUN detected 3.8 × 106 UCNs from a single filling cycle of the 14-liter source volume, at a reactor power of 48.6 MW (see Figures 2 and 3). Losses from the closed converter can be well described by two exponentials, attributing a long time constant of 427 s to 57% of the neutrons, and 155 s to the remaining 43%. The observed integral output was accurately predicted based on results from the prototype source SUN-2 , while accounting for differences in cold neutron delivery and the UCN storage volume.
SuperSUN relies on an innovative cold neutron delivery system, including a tapered octagonal guide  and a 3 m long ultrathin circular supermirror guide within the helium UCN converter. For UCN storage it exploits, as a surface coating over the supermirror, the fluoropolymer CYTOP which in dedicated studies has shown exceptional UCN storage properties .
As a user instrument SuperSUN must deliver UCN reliably, and for this depends on an advanced cryogenic system. To name only a few of the challenges that had to be addressed, the system needs to purify the 4He conversion medium by removing the strongly neutron-absorbing isotope 3He, provide sufficient cooling power to maintain the long superfluid converter at stable temperatures below 0.6 K, and be well aligned to the incident beam within cryostats and heavy radiation shields.
SuperSUN was developed in-house with the ILL’s cryogenics service, and builds on the prototype SUN-2 and its predecessor . The source also relies on substantial input and contributions from external partners, including Technische Universität München, Universität Heidelberg, and the University of Illinois Urbana-Champaign. This project has been supported within ILL's Endurance programme. A strong financial funding contribution from the Agence Nationale de la Recherche (ANR) is gratefully acknowledged.
Ongoing work focuses on optimizing the efficiency of UCN transfer to external experiments, in particular the flagship experiment PanEDM . Magnetically enhanced storage will upgrade the source in phase II , providing pre-polarized UCN with even higher densities and longer storage times.
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