Experimental study of ultracold neutron production in pressurized superfluid helium

Schmidt-Wellenburg, P.; Bossy, J.; Farhi, Emmanuel; Fertl, M.; Leung, K.; Rahli, A.; Söldner, T.; Zimmer, O.

doi:10.1103/physrevc.92.024004

Cited by 19 publications

(21 citation statements)

References 46 publications

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“…The operating principle of the He-based source is detailed in [68,79] and in publications on the predecessor installation SUN-1, which employs similar concepts [54,65,67,80]. There, UCN densities of at least 55 UCNs/cm 3 in the accumulation vessel were reported.…”

Section: Measurements At the Superfluid He Source Sun-2 At The Inmentioning

confidence: 99%

Comparison of ultracold neutron sources for fundamental physics measurements

et al. 2017

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Ultracold neutrons (UCNs) are key for precision studies of fundamental parameters of the neutron and in searches for new CP violating processes or exotic interactions beyond the Standard Model of particle physics. The most prominent example is the search for a permanent electric dipole moment of the neutron (nEDM). We have performed an experimental comparison of the leading UCN sources currently operating. We have used a 'standard' UCN storage bottle with a volume of 32 liters, comparable in size to nEDM experiments, which allows us to compare the UCN density available at a given beam port.

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Section: Measurements At the Superfluid He Source Sun-2 At The Inmentioning

confidence: 99%

Comparison of ultracold neutron sources for fundamental physics measurements

et al. 2017

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“…We also note that experimental studies of UCN production in pressurized He-II from SVP to 20 bars have been performed. 58 From these results, if we pressurize the He-II converter to ∼ 1 bar, which should be sufficient for suppressing bubble formation that can cause UCN loss, we expect a reduction in P UCN by ∼ 5%. Including this pressurization loss and the effect of the incorrect He-II scattering kernel, P UCN = 1.8 × 10 9 UCN s −1 is expected from our optimized source design.…”

Section: E Effects Of Incorrect He-ii Scattering In Mcnpmentioning

confidence: 78%

“…The up-scattering rate does not increase significantly for such a low pressure. 58 The pressurization moves the state of He-II far from the liquid/vapor line in the phase diagram such that the He-II needs to be locally heated to > 4.2 K before a bubble can be formed. Therefore, we can neglect UCN upscattering caused by helium gas bubbles.…”

Section: B Ucn Up-scattering Loss With He-ii Temperaturementioning

confidence: 99%

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

Leung

Muhrer

Hügle

et al. 2019

Journal of Applied Physics

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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).

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“…Multi-phonon scattering Figure 1: Cross sections for production of ultracold neutrons with energies up to 233.5 neV in superfluid 4 He, calculated from [27] (solid line) and [28] (dashed line).…”

Section: Single-phonon Scatteringmentioning

confidence: 99%

Optimizing neutron moderators for a spallation-driven ultracold-neutron source at TRIUMF

Schreyer

Davis

Kawasaki

et al. 2020

Nuclear Instruments and Methods in Physics Research Section A:

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We report on our efforts to optimize the geometry of neutron moderators and converters for the TRIUMF UltraCold Advanced Neutron (TUCAN) source using MCNP simulations. It will use an existing spallation neutron source driven by a 19.3 kW proton beam delivered by TRIUMF's 520 MeV cyclotron. Spallation neutrons will be moderated in heavy water at room temperature and in liquid deuterium at 20 K, and then superthermally converted to ultracold neutrons in superfluid, isotopically purified 4 He. The helium will be cooled by a 3 He fridge through a 3 He-4 He heat exchanger.The optimization took into account a range of engineering and safety requirements and guided the detailed design of the source. The predicted ultracold-neutron density delivered to a typical experiment is maximized for a production volume of 27 L, achieving a production rate of 1.4 · 10 7 s −1 to 1.6 · 10 7 s −1 with a heat load of 8.1 W. At that heat load, the fridge can cool the superfluid helium to 1.1 K, resulting in a storage lifetime for ultracold neutrons in the source of about 30 s. The most critical performance parameters are the choice of cold moderator and the volume, thickness, and material of the vessel containing the superfluid helium.The source is scheduled to be installed in 2021 and will enable the TUCAN collaboration to measure the electric dipole moment of the neutron with a sensitivity of 10 −27 e cm.

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Experimental study of ultracold neutron production in pressurized superfluid helium

Cited by 19 publications

References 46 publications

Comparison of ultracold neutron sources for fundamental physics measurements

Comparison of ultracold neutron sources for fundamental physics measurements

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

Optimizing neutron moderators for a spallation-driven ultracold-neutron source at TRIUMF

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