An ultracold neutron facility at PSI

Manfred Daum
2000 AIP Conference Proceedings  
A new type of ultra-cold neutron source (SUNS, Spallation Ultra-cold Neutron Source) based on the spallation process is under construction at PSI. A detailed description of the source parameters can be found in Ref. [1]. The essential elements of SUNS are a pulsed proton beam with highest intensity (I p ≥ 2 mA) and a low duty cycle (∼ 1 %), a heavy-element spallation target, and a large moderator and converter system consisting of about 4 m 3 of heavy water at room temperature and 30 dm 3 of
more » ... id deuterium (SD 2 ) at a low temperature (∼ 6 K) for the production of ultra-cold neutrons (UCN). Operating the UCN source in a pulsed mode allows maintaining the SD 2 at low temperatures despite of the large power deposition during a beam pulse of a few seconds. Spallation neutrons are thermalized in the D 2 O, further cooled in the SD 2 and finally, some of them are downscattered into the ultra-cold neutron range (T kin ≤ 250 neV). The pulse duration is long enough to about reach equilibrium between the produced and re-absorbed UCNs in the storage volume (∼2m 3 ). The storage trap is separated from the SD 2 vessel by a reflective shutter, in order to prevent reabsorption of the UCNs in the cold moderator. The proton beam is turned off, and the UCNs can be transferred from the storage volume to an experimental apparatus. The filling of the source storage volume is repeated after about 800 s, i.e. as soon as the UCN density in the storage volume has dropped significantly. The source layout, pulse duration, and storage volume are optimized for a dedicated EDM spectrometer of about 0.2 m 3 volume. Monte Carlo calculations [1] show that at this source, an average UCN density of ∼ 3·10 3 UCN/cm 3 can be delivered to the experiments. This is about two orders of magnitude more than in the present experiment at the reactor in the Institute Laue-Langevin, ILL, Grenoble. This average UCN density corresponds to a pulsed proton beam current I p = 2 mA (600 MeV) with a pulse duration of 8 seconds (1 % duty cycle) and a lead spallation target. In the passed year, many milestones originally required for project approval were reached or test experiments started: 1) Cross check of UCN production calculations and source optimization [3]. 2) Test of the magnetic shielding and stability for an EDM sensitivity of 10 −27 e cm or better [4, 5] . 3) Demonstration of UCN storage times τ ≥ 600 s[6]. 4) Para-to-ortho conversion in deuterium [7, 8] . 5) Measurement of the UCN transmission through SD 2 and Zirconium[9, 10, 11]. 6) Invetsigations of a spallation target operation with tin/lead alloys [12] . 7) Detailed studies of systematic uncertainties and false ef-
doi:10.1063/1.1345387 fatcat:ldhegi6elvb55bbdoyxqsdshla