Superfluid-Helium Converter for Accumulation and Extraction of Ultracold Neutrons

Abstract

We report the first successful extraction of accumulated ultracold neutrons (UCN) from a converter of superfluid helium, in which they were produced by downscattering neutrons of a cold beam from the Munich research reactor. Windowless UCN extraction is performed in vertical direction through a mechanical cold valve. This prototype of a versatile UCN source is comprised of a novel cryostat designed to keep the source portable and to allow for rapid cooldown. We measured time constants for UCN storage and extraction into a detector at room temperature, with the converter held at various temperatures between 0.7 and 1.3 K. The UCN production rate inferred from the count rate of extracted UCN is close to the theoretical expectation.

Comparison of ultracold neutron sources for fundamental physics measurements

“…Note that, in contrast to the definition of "buildup mode" measurements in earlier experiments [66,80], the neutron beam stayed continuously on the converter vessel. The beam was switched off only for longer breaks in the measurement, e.g., for modifying the setup from vertical to horizontal extraction.…”

“…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.…”

Experimental study of ultracold neutron production in pressurized superfluid helium

“…UCNs were extracted by an inverted U-shaped stainless-steel UCN guide to a 3 He UCN detector. For further details, see [18,20,33].…”

“…The setup for the He-II UCN production target and UCN extraction was similar to the one described in Refs. [18,20]: The target The CN beam passed through the velocity selector (a), a secondary CN guide (b), which could be closed off with a fast shutter (c), and a beam monitor with circular orifices (d) before it entered the cryostat through a series of thin aluminum windows (e). The He-II target was kept in the Ni-coated stainless-steel pressure container (g); CN entrance and exit sides from aluminum were covered with nickel half-spheres (q).…”

“…Current projects to increase the density of UCNs for physics experiments employ neutron converters of superfluid helium (He-II) [15][16][17][18][19][20] and solid deuterium [21][22][23][24][25][26]. They exploit the concept of a superthermal source [27], where the converter temperature can be much higher than the UCN "temperature."…”