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First High Yield Production of Ultracold Neutrons at the TRIGA Reactor

Following a series of promising pilot experiments, the TRIGA reactor at Mainz is now able to provide high yields of ultracold neutrons (UCN). The recent success has resulted from a long-term collaboration of the TU München and Universität Mainz. The joint initiative will have a strong impact on the building of a UCN source in the Forschungsreaktor München II (FRM II), one of the major infrastructure projects of the Excellence Cluster Universe.

UCN are essential to study the specific properties of neutrons, particularly their lifetime and their electrical dipole moment. This kind of research helps scientists in the Excellence Cluster Universe to gain a better understanding of the events taking place in the early Universe. The understanding of matter over antimatter asymmetry in the universe is closely linked to a time reversal symmetry violation. One very promising place to look for it is the electric dipole moment of the neutron which already now provides the most stringent limits. Another measurement is linked to the primordial nucleosynthesis (the first creation of elements in the universe), the rate of which is governed by the lifetime of the neutron which is very poorly known and will be re-measured in Munich.

The TRIGA reactor uses solid frozen Deuterium (sD2) to produce UCN. The free neutrons released in the reactor are cooled down to a temperature of about 0,5 - 3 Millikelvin reducing their velocity to the speed of a walking person. Their energy thereby decreases to values smaller than 200 neV. This superthermal method proves to be far more effective than other production processes like the turbine at ILL. The yields achieved at the pulsed TRIGA reactor lie in the range of 200.000 UCN per pulse detected.

As neutrons are usually bound in atomic nuclei and don’t move around as single particles, scientists have to find ways to get hold of them. The generation of slow ultracold samples provides a method to immobilize the particles. Thus the trapped neutrons can be stored in bottles and magnetic fields over a long period of time, allowing scientists to perform a wide range of experiments.

This news relates to the publication “First production of ultracold neutrons with a solid deuterium source at the pulsed reactor TRIGA Mainz” in The European Physical Journal A 34, 119-127 (2007)


by B. Wankerl

Technische Universitaet Muenchen
Exzellenzcluster Universe

Boltzmannstr. 2
D-85748 Garching

Tel. + 49 89 35831 - 7100
Fax + 49 89 3299 - 4002