A hydrogen leak-tight, transparent cryogenic sample container for ultracold-neutron transmission measurements

International audienceThe improvement of the number of extractable ultracold neutrons (UCNs) from converters based on solid deuterium (sD2) crystals requires a good understanding of the UCN transport and how the crystal’s morphology influences its transparency to the UCNs. Measurements of the UCN transmission through cryogenic liquids and solids of interest, such as hydrogen (H2) and deuterium (D2), require sample containers with thin, highly polished and optically transparent windows and a well defined sample thickness. One of the most difficult sealing problems is that of light gases like hydrogen and helium at low temperatures against high vacuum. Here we report on the design of a sample container with two 1 mm thin amorphous silica windows cold-welded to aluminum clamps using indium wire gaskets, in order to form a simple, reusable, and hydrogen-tight cryogenic seal. The container meets the above-mentioned requirements and withstands up to 2 bar hydrogen gas pressure against isolation vacuum in the range of 10−5 to 10−7 mbar at temperatures down to 4.5 K. Additionally, photographs of the crystallization process are shown and discussed

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

International audienceUltracold 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

Ultracold neutron storage in a bottle coated with the fluoropolymer CYTOP

The fluoropolymer CYTOP was investigated in order to evaluate its suitability as a coating material for ultracold neutron (UCN) storage vessels. Using neutron reflectometry on CYTOP-coated silicon wafers, its neutron optical potential was measured to be 115.2(2) neV. UCN storage measurements were carried out in a 3.8 l CYTOP-coated aluminum bottle, in which the storage time constant was found to increase from 311(9) s at room temperature to 564(7) s slightly above 10 K. By combining experimental storage data with simulations of the UCN source, the neutron loss factor of CYTOP is estimated to decrease from 1.1(1)$$\times 10^{-4}$$ to 2.7(2)$$\times 10^{-5}$$ at these temperatures, respectively. These results are of particular importance to the next-generation superthermal UCN source SuperSUN, currently under construction at the Institut Laue-Langevin, for which CYTOP is a possible top-surface coating in the UCN production volume