UCN Source at the TRIGA Mainz
An essential figure of merit which governs the strength of this superthermal UCN source is the thermal neutron flux at the position of the sD2 converter. At the very end of the radial beam port D the measured flux during the 250 MW pulse with a width of ~ 30 ms reaches 2x1015 n/cm2/s during the reactor pulse, which results in a fluence of 6x1013 n/cm2 per pulse. This number is similar to the fluence reached at high flux reactors (~ 1014 n/cm2) for sD2 exposure times of about 1 s. For UCN storage volumes of 20 l filling times larger than 1 s will not increase the UCN number densities due to back diffusion and inevitable UCN losses in the transfer line. Thus, the pulsed UCN source at TRIGA Mainz is highly competitive to superthermal UCN sources presently under construction at large scale facilities with some obvious advantages concerning routine operation and reliability: The average thermal heat load on the sD2 converter and the hydrogen premoderator is negligible.
The newly installed UCN source at beam port D (Fig.1 and Fig.2) currently delivers UCN densities of 10/cm3 in typical storage volumes of 10 to 20 litres , . This number will be further increased by an order of magnitude via the installation of transport tubes coated with 58Ni and the use of solid hydrogen or methane as premoderator.
Fig.1: Schematic drawing of the UCN source at beam port D with beamdump and 900 deflection of the neutron beam upward towards the experimental platform
Fig.2: Photograph of experimental area at beam port D showing an UCN storage chamber (V = 10 L) about 1.5m above the exit of the UCN source
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