A next-generation inverse-geometry spallation-driven ultracold neutron source

Abstract

The concept of a next-generation spallation-driven ultracold neutron (UCN) source capable of delivering an integrated flux of ∼ 10 9 UCN s −1 is presented. A novel "inverse geometry" design is used with 40 liters of superfluid 4 He (He-II) as converter cooled with state-of-the-art sub-cooled cryogenic technology to ∼ 1.6 K. Our source design is optimized for a 100 W maximum thermal heat load constraint on the He-II and its vessel. In this paper, we first explore a modified Lujan-Center Mark-3 target for UCN production as a benchmark. We then present the baseline concept of our inverse geometry source design that gives a total UCN production rate in the converter of P UCN = 2.4 × 10 8 s −1 . In our inverse geometry, the spallation target is wrapped symmetrically around the cryogenic UCN converter to permit raster scanning the proton beam over a relatively large volume of tungsten spallation target to reduce the demand on the cooling requirements, which makes it reasonable to assume that water edge-cooling only is sufficient. Our design is refined in several steps to reach P UCN = 2.1 × 10 9 s −1 under our other restriction of 1 MW maximum available proton beam power. We then study effects of the He-II scattering kernel used as well as reductions in P UCN due to pressurization to reach a estimate of P UCN = 1.8 × 10 9 s −1 . Finally, we provide an estimate for the UCN extraction efficiency to show that the total extracted UCN rate out of the converter can be as large as R ex ≈ 6 × 10 8 s −1 for the He-II at 1.6 K out of a 18 cm diameter guide. The UCN extraction loss is dominated by upscattering in the He-II so that if the He-II can be cooled further to 1.4 K, R ex ≈ 8 × 10 8 s −1 can be attained. These extracted UCN rates are around an order of magnitude higher than the strongest proposed sources so far, and is around three orders of magnitude stronger than existing sources.He-II offers much higher thermal conductivities and a UCN upscattering time constant given by τ up ≈ (100 s K 7 )/T 7 (see Sec. VI), resulting in UCN loss times 3 s for a He-II bath at a temperature T = 1.6 K. At these temperatures, the total scattering mean-free-path for 8.9 Å (1 meV) neutrons, the primary UCN producing CN component, is ≈ 18 m. 25 This permits the design of a simple, large volume source where the converter material also serves as the coolant.There are two "in-pile" He-II sources currently under construction. One is at TRIUMF, Canada, where the source is coupled to a 500 MeV proton beam with 20 kW incident beam power. [26][27][28][29] The next generation version of this source will have an expected R ex ∼ 10 6 s −1 and UCN density deliverable to an experimental volume of ∼ 6, 000 cm −3 (both polarized).

Electric dipole moments and the search for new physics

“…Improving the nEDM sensitivity beyond 10 −28 e • cm would require a large-scale apparatus with built-in magnetometry and a high-intensity neutron source. A high-current spallation target, coupled to super-cooled helium, is technically feasible to significantly enhance the UCN yields beyond the capabilities of current UCN sources and increase the density of UCN by several orders of magnitude to reach several thousands per cubic centimeters [142]. To keep pushing the nEDM sensitivity, a new neutron facility (both UCN and cold neutrons) in the US is needed.…”

“…There are many new UCN sources around the world currently operational, being developed, or proposed [134][135][136][137][138][139][140][141][142][143] based on the principles of the superthermal process. Superfluid liquid helium (LHe) and solid deuterium (SD 2 ) have been successfully applied as UCN converters.…”

In-beam superfluid-helium ultracold neutron source for the ESS

“…This is the standard location for neutron cooling devices, in which the large cold-neutron flux leads to a high UCN production rate. Unfortunately, the large heat load on the converter from neutrons and gamma radiation makes it unrealistic to maintain temperatures below 1 K near a high-power primary neutron source, even when a powerful cryogenic plant is available [22,39]. Therefore, in this configuration, the converter is directly connected to an extraction guide, from which UCN stream continuously, delivering a high steady flux of UCN either to an experiment operated in a flow-through mode or to several storage experiments in a time-shared mode [43].…”

Ultracold neutron storage and transport at the PSI UCN source

“…The most prominent example is the search for a permanent electric dipole moment of the neutron (nEDM) [2,3,4,5,6,7,8,9,10]. The best achieved sensitivity for such experiments is limited by counting statistics, and efforts are made worldwide to develop new UCN sources to provide higher intensities [11,12,13]. Besides a high source yield, a highly optimized neutron transport to beamports and experiments is a key ingredient to the performance of UCN sources.…”