Production of ultra-cold neutrons in solid α-oxygen

Abstract

Our recent neutron scattering measurements of phonons and magnons in solid α-oxygen have led us to a new understanding of the production mechanism of ultra-cold-neutrons (UCN) in this super-thermal converter. The UCN production in solid α-oxygen is dominated by the excitation of phonons. The contribution of magnons to UCN production becomes only slightly important above E >10 meV and at E ∼4 meV. Solid α-oxygen is in comparison to solid deuterium less efficient in the down-scattering of thermal or cold neutrons into the UCN energy regime.PACS numbers: 28.20.Cz, 63.20.kk Ultra-cold-neutrons (UCN) are slow enough (∼ 300 neV) to be confined [1] in traps, which can be formed by materials with a high Fermi potential or by a magnetic field (60 neV/T). They can be kept for several minutes in the confinement, and thus be investigated with high precision. UCN are elementary particles that are extremely well suited for low-energy physics experiments. These experiments are investigating fundamental problems unsolved within the framework of the Standard Model [2]. One major experiment is the search for a non-zero electric dipole moment of the neutron [3] (current upper limit 2.9 10 −26 e· cm). Another unique experiment is the precise determination of the lifetime [4] of the free neutron. This value has an important impact on the theory of weak interaction [5].

Investigating solid $α-^{15}$N$_{2}$ as a new source of ultra-cold neutrons

“…The absolute UCN production cross-section is determined by normalizing the scattered intensity to the total molecular cross-section of the sample. The cross-section peaks strongly near 6 meV, and the optimal incident cold neutron temperature is 40 K. The measured cross-section is found to be somewhat lower than that of D 2 and O 2 (see [34]). However, we observe an up-scattering mean free path substantially longer than that of D 2 .…”

“…(3) Upon escaping the nitrogen volume, UCN are boosted by the solid 15 N 2 Fermi potential of ∼ 69 neV. We choose E f = 181 neV, which corresponds to a trappable energy of 250 neV, commensurate with UCN production cross-section calculations in [34].…”

“…The dynamical structure factor is measured using the IN4 TOF spectrometer at the Institut Laue Langevin (ILL) using incident neutrons of wavelength λ = 2.2 Å [32]. The sample environment, cryostat, and sample control system are the same as that used for previous studies of oxygen and deuterium as UCN converters [15,34]. This provides a means of direct comparison between the different converters.…”

“…This provides a means of direct comparison between the different converters. Gaseous 15 N 2 (99.4% isotopic purity) is introduced into the 2 mm annular cell through a needle valve, and condensed using the ILL's orange heliumexchange cryostat (see [15,34] for details). The detector array is calibrated using an isotropically scattering vanadium sample.…”

Cooling neutrons using non-dispersive magnetic excitations

“…At temperatures k B T ≪ E * the excited state is strongly depleted, leading to a suppression of neutron up-scattering back to higher energy. Converter materials investigated so far are superfluid helium [21], solid deuterium [21,22], solid α-oxygen [23,24] and solid 15 N [25]. All cases employ phonons (with a contribution of magnons for α-oxygen), with E * in the order of one to several meV (e.g., 1 meV for superfluid 4 He).…”