Abstract
We propose a new method for production of ultracold neutrons (UCNs) in superfluid helium. The principal idea consists in installing a helium UCN source into an external beam of thermal or cold neutrons and in surrounding this source with a solid methane moderator/reflector cooled down to ~4 K. The moderator plays the role of an external source of cold neutrons needed to produce UCNs. The flux of accumulated neutrons could exceed the flux of incident neutrons due to their numerous reflections from methane; also the source size could be significantly larger than the incident beam diameter. We provide preliminary calculations of cooling of neutrons. These calculations show that such a source being installed at an intense source of thermal or cold neutrons like the ILL or PIK reactor or the ESS spallation source could provide the UCN density 105 cm−3, the production rate 107 UCN/s−1. Main advantages of such an UCN source include its low radiative and thermal load, relatively low cost, and convenient accessibility for any maintenance. We have carried out an experiment on cooling of thermal neutrons in a methane cavity. The data confirm the results of our calculations of the spectrum and flux of neutrons in the methane cavity.
Highlights
Further progress in the field of studies with ultracold neutrons (UCNs) [1, 2], as a tool for nuclear and particle physics [3,4,5], is often limited by available UCN densities
Before calculating parameters of the proposed UCN source, we estimate the flux of thermal neutrons, which could be achieved in an external neutron guide of existing neutron sources
Our calculations showed that the installation of a helium UCN source in an external beam of thermal neutrons at the ILL reactor in Grenoble, at the PIK reactor in Gatchina, or at the ESS spallation source in Lund would allow achieving the UCN density of ∼105 UCN⋅cm−3
Summary
Further progress in the field of studies with ultracold neutrons (UCNs) [1, 2], as a tool for nuclear and particle physics [3,4,5], is often limited by available UCN densities. The total energy of exited phonons could be found within a much broader energy range, and UCNs could be produced in a multiphonon process from a much broader energy range of incident neutrons Both processes would give comparable contributions to the UCN production provided that the initial cold neutron spectrum is broad. It was shown in the cited work that the produced UCNs could be stored in superfluid helium for a long time if the helium temperature is ∼1 K or lower. Advantages of helium sources consist of the transparency of the physical processes involved, the relatively low cost, and the possibility to install them outside of a reactor/spallation source zone
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