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 conversion of cold to ultracold neutrons via inelastic scattering in superfluid helium. While the conversion process is well established, the benefits of helium based superthermal sources needs to be experimentally demonstrated. This investigation started with SUN and SUN2 which are considered as the two prototypes of SuperSUN. A novel feature and key component of SuperSUN is a magnetic multipole reflector for a drastic enhancement of the UCN density. 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 inside SuperSUN.

The source will first be commissioned as an intermediate stage without the magnetic reflector, called phase I before pursuing with the full performance in phase II. In this final stage, SuperSUN aims for a density of fully polarized UCNs beyond 1000 per cm3 inside the source which would correspond to a density of about 40 UCN per cm3 in the experimental cells of PanEDM, SuperSUN first user. If the commissioning confirms these numbers, SuperSUN will be the UCN source with the highest density in the world and the first one to produce polarized UCN.

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

The coldneutron beam is represented by a green arrow labeled CN.

a:  the 100-L liquid helium bath
b: the needle valve
c: the 1-K pot
d: the 4He superleak
e: the 3He pumping column
f: the 3He impedance

g: the 3He/4He heat exchanger
h: the UCN box
i: the conversion volume at 0.6 K
j: the two beryllium windows
k: the UCN extraction system
l: the UCN valve
m: the superconducting magnet in a separate liquid helium bath: for SuperSUN phase II this replaces part of the phase I 4-K screen
n: the 4-K cryostat.