A hydrogen leak-tight, transparent cryogenic sample container for ultracold-neutron transmission measurements Stefan Döge, and Jürgen Hingerl Citation: Review of Scientific Instruments 89, 033903 (2018); doi: 10.1063/1.4996296 View online: https://doi.org/10.1063/1.4996296 View Table of Contents: http://aip.scitation.org/toc/rsi/89/3 Published by the American Institute of Physics REVIEW OF SCIENTIFIC INSTRUMENTS 89, 033903 (2018) A hydrogen leak-tight, transparent cryogenic sample container for ultracold-neutron transmission measurements Stefan Doge¨ 1,2,3,a) and Jurgen¨ Hingerl1,2 1Institut Laue–Langevin, 71 Avenue des Martyrs, F-38042 Grenoble Cedex 9, France 2Physik-Department, Technische Universitat¨ Munchen,¨ D-85748 Garching, Germany 3Ecole´ Doctorale de Physique, Universite´ Grenoble Alpes, F-38402 Saint-Martin-d’Heres,` France (Received 15 July 2017; accepted 11 February 2018; published online 6 March 2018) The 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. Published by AIP Publishing. https://doi.org/10.1063/1.4996296 I. INTRODUCTION II. THE NEED FOR AN IMPROVED SAMPLE CONTAINER The scattering of thermal and cold neutrons from cryo- genic liquids and solids is a well-established experimental If one wants to measure the transmission of ultracold neu- technique. Most of the time, sample containers and their trons through a cryogenic liquid or solid, the windows of the neutron beam windows are machined from aluminum alloy sample container need to be as highly polished as possible because of its favorable post-irradiation behavior (short half- (center-line average roughness Ra < 10 Å) in order to minimize life of 2.5 min of the 28Al isotope, which is created by 27Al undesired scattering from surfaces. Machined, rough-surface capturing a neutron), low neutron scattering and absorption aluminum windows are fairly easy to make and have been cross section, and easy workability. Thermal and cold neu- used in UCN transmission experiments before.8–11 As our tests trons have wavelengths of 1–10 Å and are therefore practi- with single-side polished aluminum windows and unpolished cally insensitive to the aluminum’s neutron-optical potential, as well as polished aluminum foils (Ra in the range of a few material inhomogeneities, and surface roughness. µm) have shown, they are not suitable for UCN transmission Slow neutrons—especially ultracold neutrons (UCNs), experiments because of significant UCN scattering from the with a velocity of only a few meters per second and a vacuum–aluminum and aluminum–sample interfaces due to wavelength of several hundred Ångstrom—are,¨ however, surface roughness. sensitive to surface roughness,1 material,2,3 and magnetic In addition, thin aluminum windows in the range of inhomogeneities.4 Besides that, aluminum windows are not 0.15–0.3 mm tend to bulge at a pressure difference of about optically transparent and thus do not allow for an on-line 1 bar. This results in a poorly defined sample thickness, which control of the sample. In UCN applications, the advantage translates directly to a large error in the scattering cross section. of the low neutron-optical potential of aluminum (54 neV) is Atchison et al.8,9 used an initial sample thickness of 10.0 mm. more than offset by the drawbacks that the use of this material However, after bulging of the windows, computer simulations entails. suggested an “effective thickness” of 11.1 mm. Considering In this article, we present an improved sample con- the relation of sample thickness d and the total cross section tainer design which addresses the issues of previous σtot in the transmission equation [Eq. (1), where I0 is the trans- sample containers and permits reliable UCN transmission mitted UCN flux through an empty sample container, I(d) is measurements on cryogenic crystals, such as solid deu- the transmitted UCN flux through a sample of thickness d, and terium (sD2). Measurements of this kind are needed to inter- Nv is the molecular number density of the sample], one imme- pret the performance of operating sD2-based UCN sources diately understands the importance of a well-defined sample worldwide.5–7 thickness, I(d) = e−Nvσtotd. (1) I0 a)Author to whom correspondence should be addressed: Stefan Doege, Besides exhibiting a low surface roughness, sample con- [email protected] tainer windows for UCN transmission experiments should 0034-6748/2018/89(3)/033903/8/$30.00 89, 033903-1 Published by AIP Publishing. 033903-2 S. Doge¨ and J. Hingerl Rev. Sci. Instrum. 89, 033903 (2018) be made from a material with a low absorption cross sec- III. DESIGN REQUIREMENTS OF THE SAMPLE tion to maximize the neutron flux to the sample and with a CONTAINER AND OPTICAL ACCESS 12,13 low neutron-optical potential to transmit UCNs with as A thick and therefore mechanically strong glass slab or low as possible energy. All materials are practically imper- collar does not present an obstacle in optical applications. By vious to UCNs with a kinetic energy below their respective contrast, in our case, the glass windows need to be as thin as neutron-optical potential. Materials of choice are thus sili- possible to minimize the absorption of UCNs. First, we tried con, transparent vitreous silica and synthetic quartz (SiO2; 0.5 mm thick vitreous silica windows in our sample container, naming convention suggested by Laufer14 to clarify the nam- 15 but they imploded in a vacuum test at about 800 mbar pres- ing variations of quartz ), and sapphire (Al2O3). The latter sure difference. Same-size windows, but with a thickness of three are optically transparent and allow for an observation 1.0 mm, withstood a pressure difference of 1 bar and more and of the condensation and crystal growth processes in the sam- thus became our window material of choice. ple container along the neutron beam axis. Amorphous silica In brief, the design requirements were as follows: has the unique advantage of not producing small-angle scat- tering inside the material.16 In our sample containers, we • highly polished and thin windows (d = 1 mm), the used transparent vitreous silica wafers purchased from Plan windows need to be easily removable; Optik AG, Elsoff, Germany. All of the following reported • optically transparent windows with a low neutron- results were obtained using sample containers with these optical potential and of high purity (no scattering length wafers. density inhomogeneities in the material); The fact that the surfaces of the silica windows have a • vacuum seal needs to be easily demountable and negligible influence on the measured UCN transmission is hydrogen-tight down to 4.5 K; demonstrated by the virtually identical transmissivity through • sample thickness needs to be well defined, the same one d = 1.0 mm window and two d = 0.525 mm windows (see across the entire sample area, and adjustable over a wide Fig.1). If the surfaces had a large impact, four vacuum–silica range within the same sample container; interfaces would transmit substantially less UCNs than two • sample container needs to withstand 2 bar hydrogen such interfaces. The calculated neutron-optical potential12,13,17 pressure against high vacuum with a maximum pres- of our amorphous silica windows based on the volumetric sure of 10 3 mbar, preferably in the range of 10 5 to mass density provided by the manufacturer (ρ = 2.203 g/cm3) 10 7 mbar, in order for the closed-cycle refrigerator to is 90.6 neV at room temperature and the same at cryo- work smoothly and to minimize UCN up-scattering on temperatures due to a volume contraction of less than one per residual gas molecules. mille. The foot of the sample container was designed to be Since the aforementioned suitable materials are com- mounted onto the two-stage cold-head of the cryostat described monly supplied as flat wafers and it is very difficult to make in the work of Doge¨ et al.11 with a number of modifications: them into one-piece structures (like a flat window plus clamp 11 The stainless steel neutron guides facing directly the sam- with screw holes) that would fit into our sample container, ple container were removed. An illumination unit with nine we had to develop a hydrogen-tight seal to join the flat wafers white LEDs was mounted on one side of the sample container and the clamps made from aluminum alloy AlMg3 (AA5754). (see Fig.2). The LED support had a recess for a 50.8 mm FIG. 2. The cryostat is shown with the transparent sample container mounted. From left to right, the components are neutron guide adapter, neutron colli- mator, outer cryostat body, cryo-shield connected to the 1st cooling stage of FIG.