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

Abstract

This paper discusses design principles and possible performances of an “in-beam” ultracold neutron (UCN) source for the European Spallation Source (ESS). The key components of the proposed neutron delivery system are nested-mirror optics (NMO), which image the bright neutron emission surface of the large liquid-deuterium moderator, studied within the HighNESS project, onto a remotely located superfluid-helium converter. Bandpass supermirrors, with optional polarization capability, enable the selective transport of those neutrons that are most effective for UCN production, exploiting the single-phonon conversion process that is possible for neutrons having wavelengths within a narrow range centered on 8.9 A ˚. NMO are capable of extracting and refocusing neutrons with small transport losses under the large solid angle available at the ESS Large Beam Port (LBP), allowing the converter to be placed far away from the high-radiation area in the ESS shielding bunker, where the source stays accessible for trouble-shooting while facilitating a low-background environment for nearby UCN experiments. Various configurations of the beam and converter are possible, including a large-volume converter – with or without a magnetic reflector – for a large total UCN production rate, or a beam focused onto a small converter for highest possible UCN density. The source performances estimated by first simulations of a baseline version presented in this paper, including a saturated UCN density on the order of 10 5 cm − 3 , motivate further study and the development of NMO beyond the first prototypes that have been recently investigated experimentally.

Very cold and ultra cold neutron sources for ESS

“…Some notable examples from particle physics that can profit from large and divergent neutron beams include a search for neutron-antineutron oscillations and measurements of the static electric dipole moment of the neutron [1,11,45]. Nested mirror optics [21,66] seem particularly efficient for delivery of such beams and will be discussed for an "in-beam" ultracold neutron source [8,17,44] in a separate contribution to these workshop proceedings [67].…”

“…The brightness of the lower moderator at the NNBAR opening at this wavelength, at 5 MW average power, is about 3.4 × 10 11 n/cm 2 /s/sr/Å. This number serves as key input to calculating the neutron flux incident on an in-beam UCN source, see [67] for more details.…”

“…4), where the moderator can be viewed under the largest possible solid angle (for further details see Ref. [67]).…”

Approaches to high-density storage experiments with in-situ production and detection of ultracold neutrons

“…ANNI will use a high-brilliance moderator, and has been optimized for beam experiments requiring small or moderate divergence [46]. The ESS Large Beam Port (LBP) will use a moderator with high integral flux, and extract a very large beam with large solid angle and divergence that could be delivered into a convertor using focusing optics [64].…”

“…The in-situ densities ρ are estimated by extrapolation from numbers measured at SUN-2 without any corrections applied, while the values ρ 700 = P I × 700 s are intended to give some notion of the scope remaining for future improvements by substantially increasing UCN storage times in the convertor vessel. Note that not all beams have the same usable area, and that the numbers given for ESS/LBP assume the use of focusing optics [64] Beamline The values given for P I and ρ correspond to the phase I configuration, which uses to a CYTOP-on-nickel convertor vessel rather than V c = 233 neV [12].…”