Development of UCN sources at PNPI

Abstract

This article reviews the development of various sources for ultracold neutrons (UCNs) at the Petersburg Nuclear Physics Institute (PNPI). For 45 years, PNPI has proposed and manufactured cryogenic devices for neutron conversion to low energies. Based on beryllium, hydrogen and deuterium, they can be operated in the intense radiation fields near the core of a nuclear reactor. A more recently launched UCN source development utilizes superfluid helium (He-II) as conversion medium. Initially proposed and designed for PNPI’s old WWR-M reactor, the project has been reshaped to equip the institute’s PIK reactor with a modern UCN source of this type. The projected UCN density in the closed source chamber is 2.2 × 103 cm−3, which, as calculations of neutron transport show, will provide 200 cm−3 in the chambers of a neutron EDM spectrometer connected to the source by a UCN guide. Experiments at PNPI with a full-scale UCN source model have demonstrated that a heat load of 60 W can be removed from the He-II in the converter at a temperature of 1.37 K. This fact confirms the practical possibility to implement low-temperature converters under “in-pile” conditions with large heat inflows. The review concludes with a presentation of various proposed options for a He-II based UCN source at the European Spallation Source.

HighNESS conceptual design report: Volume I

“…Additionally, various UCN sources are either in the construction phase or have been proposed at prominent facilities like FRM-II in Germany, TRIUMF in Canada, LANL and SNS in the USA. For a comprehensive overview of the global UCN landscape, refer to [180].…”

“…Therefore, it is probably not well suited for a SD 2 source, which requires neutrons with such wavelengths. For He-II on the other hand, for which neutrons with λ = 8.9 Å are most effective for UCN production, the Bi filter is very interesting, as discussed in [180].…”

“…For the in-pile option under study in this section, we followed the proposal by Serebrov and Lyamkin [180], consisting of a 58-liter vessel (60 cm × 30 cm × 32 cm) filled with He-II, surrounded by a reflector made of LD 2 and bismuth shielding. The geometry is depicted in Fig.…”

“…This heat load encompasses contributions from both neutrons and photons on the He-II converter as well as the aluminum shell surrounding the He-II. Additionally, there is a decay heat load resulting from aluminum beta activation, which accounts for about 24 % of the total heat load originating from neutrons and photons [181]. Throughout the optimization process, we maintained the He-II volume constant at the preliminary design value of 57.6 L for comparability.…”

Production of ultracold neutrons in a decelerating trap

“…Also nonequilibrium methods for producing UCNs from thermal and cold neutrons are well developed [17]. Thus, solid-deuterium UCN converters have been implemented, for example, at the Los Alamos National Laboratory [42] and at the Paul Scherrer Institute [4], and projects of UCN converters based on 4 He are at different stages of development [3,28,40,44].…”