[Paper Review] Ultra-cold Neutron Production in Anti-ferromagnetic Oxygen Solid

C. Y. Liu, A. R. Young|ArXiv.org|Jun 2, 2004

Atomic and Subatomic Physics Research4 references3 citations

TL;DR

This paper proposes solid oxygen (S-O₂) in its anti-ferromagnetic α-phase as a novel ultra-cold neutron (UCN) source, leveraging neutron-magnon inelastic scattering to produce UCN. The mechanism offers a 10-fold increase in UCN flux over solid deuterium due to reduced upscattering and lower nuclear absorption, with optimal operation at 2 K.

ABSTRACT

Spin waves, or magnons, in the anti-ferromagnetic $α$ phase of solid oxygen provide a novel mechanism for ultra-cold neutron (UCN) production. Magnons dominate the energy exchange mechanisms for cold neutrons and UCN in solid $α$-oxygen, much in the same way as do phonons in solid deuterium superthermal UCN sources. We present calculations of UCN production and upscattering rates in S-O$_2$. The results indicate that S-O$_2$ is potentially a much more efficient UCN source material than solid deuterium.

Motivation & Objective

• To explore solid oxygen (S-O₂) in its α-phase as a superthermal ultra-cold neutron (UCN) source with enhanced performance over existing materials.
• To analyze the role of anti-ferromagnetic magnons in neutron energy downscattering for UCN production.
• To compare UCN production and loss mechanisms in S-O₂ with those in solid deuterium, identifying key advantages.
• To determine the optimal operating temperature for S-O₂ by evaluating temperature-dependent loss processes.
• To assess the feasibility of S-O₂ as a next-generation UCN source based on theoretical modeling of scattering cross sections and loss rates.

Proposed method

• Calculates neutron-magnon and neutron-phonon scattering cross sections in solid oxygen using the magnetic scattering potential and spin wave dispersion relations.
• Applies the scattering law formalism S(κ, ω) to model inelastic neutron-magnon scattering, incorporating the magnon dispersion and density of states.
• Uses the effective scattering length r₀ = 5.4 fm derived from the neutron magnetic moment and electron spin coupling to quantify magnetic scattering strength.
• Evaluates UCN production rate via neutron-magnon inelastic scattering and upscattering rate via magnon absorption, using the Boltzmann factor e⁻⁸K/T to model thermal suppression.
• Compares UCN loss mechanisms: nuclear absorption, magnon upscattering, para-molecule effects, and ozone formation, using theoretical estimates and experimental analogs.
• Estimates the UCN density and flux enhancement in S-O₂ relative to solid deuterium by combining reduced upscattering, lower absorption, and infinite elastic mean free path.

Experimental results

Research questions

• RQ1 Can anti-ferromagnetic magnons in solid oxygen provide an efficient mechanism for ultra-cold neutron production?
• RQ2 How does the UCN production rate in S-O₂ compare quantitatively to that in solid deuterium?
• RQ3 What is the dominant UCN loss mechanism in S-O₂, and how does it compare to losses in solid deuterium?
• RQ4 At what temperature is the UCN source performance in S-O₂ maximized, considering thermal suppression of magnon processes?
• RQ5 To what extent can the UCN flux in S-O₂ exceed that of solid deuterium, given reduced upscattering and absorption?

Key findings

• The neutron-magnon inelastic scattering cross section in S-O₂ is comparable in strength to the neutron-phonon scattering in solid deuterium.
• UCN upscattering via magnon absorption is exponentially suppressed at low temperatures, with a suppression factor of e⁻⁸K/T, leading to significantly reduced loss rates.
• The nuclear absorption cross section of oxygen is smaller than that of deuterium, resulting in a longer UCN lifetime in S-O₂.
• Losses from para-molecule interactions, a major source in solid deuterium, are absent in S-O₂ due to the absence of rotational states in ¹⁶O₂.
• Theoretical calculations predict a UCN density in S-O₂ that is approximately an order of magnitude higher than in solid deuterium under the same incident neutron flux.
• A hundredfold increase in UCN flux over solid deuterium is theoretically feasible by exploiting reduced upscattering, lower absorption, and infinite elastic mean free path, with optimal operation at 2 K.