A Super Ultra Cold Neutron source

SuperSUN - a Super Ultracold Neutron Source

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.

The production of UCN by SuperSUN 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 source will therefore drastically increase the storage time of the UCNs in the trap.

While the best UCN sources so far provide densities of the order of a few tens of neutrons per cubic centimetre, SuperSUN will provide fully polarized UCNs with a density beyond 1000 per cm3. 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 first instrument that will make use of the UCNS from SuperSUN is panEDM, a search for the EDM (electric dipole moment) of the neutron.

Instrument layout

  1.  isolation vacuum vessel
  2. coldtraps and heat-exchangers for incoming helium gas thermally anchored to the first stage of the cryocooler at about 45 K
  3. regenerator heat-exchangers
  4. second stage of the cryocooler with heat-exchangers
  5. cold needle valve of the 4He circuit
  6. 1-Kelvin 4He evaporation pot
  7. cold needle valve of the 3He circuit
  8. thermal baffles
  9. outer heat shield attached to the first stage of the cryocooler
  10. inner heat shield attached to the second stage of the cryocooler
  11. superleak filter
  12. 3He evaporation pot
  13. copper heat-exchanger
  14. "U"-shaped connection pipe
  15. aluminum converter vessel filled with superfluid 4He
  16. UCN storage/production volume
  17. vertical stainless steel UCN guide
  18. UCN shutter valve thermally anchored to the inner heat shield
  19. thin aluminum cold neutron beam window
  20. stainless steel UCN guide at room temperature.

From F. M. Piegsa et al. (2014) Phys. Rev. C, 90, 015501. DOI:10.1103/PhysRevC.90.015501