arXiv subjects

Christoph Morkel

Publications and source records attributed to Christoph Morkel.

6 recordsLinked to original sources

arXiv2026

Ultracold Neutron Sources: An Essentially Enhanced Optimal Operation Temperature for Ortho-Deuterium-Based Superthermal Converters

There is a longstanding dogma in the physics of ultracold neutrons (UCN): The optimal temperature of ortho-deuterium based UCN-sources must not exceed $5\,\mathrm{K}$. This value has been established by the pioneering work of Golub et al. decades ago, utilizing previously established cross sections for solid ortho-deuterium. But recently, an improved calculation of the crucial 1-phonon up-scattering cross section - using the so-called corrected Incoherent Approximation - leads to a new upper limit for the optimal operation temperature of ortho-deuterium based UCN sources. After a discussion of the relevant cross sections of the $\mathrm{D}_2$-molecule we conclude that the optimal temperature lies between $10\,\mathrm{K}$ and $12\,\mathrm{K}$, a factor 2 higher than estimated earlier. This offers significantly relaxed cryogenic conditions for UCN experiments with solid ortho-deuterium and especially for the operation of ortho-deuterium based ultracold neutron sources.

physics.ins-det↗

arXiv2023

Direct measurement of the scattering cross sections of liquid ortho-deuterium for ultracold neutrons and comparison with model calculations

Liquid deuterium is a fluid between the quantum and classical regimes. It attracts interest from fundamental research for the verification of quantum calculations but also from neutron physics as it is a widely used neutron moderator medium. We have measured the scattering cross sections of liquid ortho-deuterium in the ultracold-neutron range for four different temperatures (19-23 K) and compared them with calculations from a parameter-free analytical calculation model as well as with previous measurements at 19 K by another research group. All three show remarkable agreement, which establishes the validity of the calculation model and proves it is a reliable basis for the derivation of scattering kernels. The deconvolution of our measured transmission data changed the cross section results noticeably only for neutrons faster than 10 m/s. We found the total scattering cross section of liquid deuterium to be inversely proportional to velocity, as is predicted by theory.

nucl-ex↗

arXiv2021

Incoherent approximation for neutron up-scattering cross sections and its corrections for slow neutrons and low crystal temperatures

The incoherent approximation (IA) is often used for calculating the one-phonon inelastic neutron scattering cross section for arbitrary solids. It is valid for thermal neutrons but for slow neutrons it requires a correction, which is significant for isotopes that are strong coherent scatterers. In this article, we present the extension of the Placzek--Van Hove corrections for slow neutrons in the limit of low temperatures using the example of solid \emph{ortho}-deuterium (sD$_2$). Our approach yields realistic one-phonon up-scattering cross sections for sD$_2$ and shows the IA to be a factor of 2 to 5 too high for ultracold neutron (UCN) up-scattering in sD$_2$. Our calculations are compared with previously published Monte Carlo calculations of the one-phonon cross section based on the dynamic structure function $S(q,ω)$ of polycrystalline \emph{ortho}-deuterium and are found to be consistent with them. Furthermore, we provide the means for easily replicable calculations of the one-phonon up-scattering cross sections of solid \emph{ortho}-deuterium for slow neutrons. These should from now on be used in calculations and simulations of UCN scattering in sD$_2$.

nucl-th↗

arXiv2021

Measured velocity spectra and neutron densities of the PF2 ultracold-neutron beam ports at the Institut Laue--Langevin

Ultracold neutrons (UCNs) are a useful tool for fundamental physics experiments. They can be used to probe the lifetime of free neutrons, search for new CP violating processes and exotic interactions beyond the Standard Model, perform Ramsey spectroscopy, and carry out neutron-optical interference experiments. All of these experiments require high neutron count rates for good statistics. For optimal exploitation of experimental beam time, these experiments need to be prepared and, at times, even simulated in advance. To this end, it is crucial to know the velocity-dependent UCN flux at each beam position. Knowing the absolute neutron flux also allows for an absolute calibration of previously gathered data. Using the same time-of-fight experimental setup, we have measured the differential neutron flux of three out of the four UCN beam ports at the PF2 instrument at Institut Laue--Langevin, Grenoble. These beam ports are commonly used for UCN flux experiments and proof-of-principle tests.

physics.ins-det↗

arXiv2021

Scattering of ultracold neutrons from rough surfaces of metal foils

The transparency of metal foils for ultracold neutrons (UCNs) plays an important role in the design of future high-density UCN sources, which will feed a number of fundamental physics experiments. In this work, we describe and discuss the measured transmission of a collimated beam of very slow neutrons (UCNs and very cold neutrons) through foils of Al, Cu, and Zr of various thicknesses at room temperature. Our goal was to separate scattering and absorption in the sample bulk from surface scattering, and to quantify the contribution of the surface. We were able to demonstrate that the surface roughness of these foils caused a significant fraction of UCN scattering. The surface roughness parameter $b$ extracted from UCN measurements was shown to be of the same order of magnitude as the surface parameter determined by atomic-force microscopy. They lie in the order of several hundreds of angstroms. Using the formalism developed here, transmission data from previous neutron-optical experiments were re-analyzed and their surface roughness parameter $b$ was extracted.

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arXiv2015

Scattering cross sections of liquid deuterium for ultracold neutrons: Experimental results and a calculation model

We present scattering cross sections $σ_\text{scatt}$ of ultracold neutrons (UCN) in liquid deuterium at T = 20.6 K, as recently measured by means of a transmission experiment. The indispensable thorough raw data treatment procedure is explained. A calculation model for coherent and incoherent scattering in liquid deuterium in the hydrodynamic limit based on appropriate physical concepts is provided and shown to fit the data well. The applicability of the incoherent approximation for UCN scattering in liquid deuterium was tested and found to deliver acceptable results.

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