Newsletter 1/2014. Super-polished copper a new substrate material. Fabrication of neutron collimators launched

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1 Polarizing supermirror m = 5.5 Next generation of metallic guides welded assembly Fabrication of neutron collimators launched Super-polished copper a new substrate material Partnership Next generation of metallic guides welded assembly In 2013 SwissNeutronics produced a new generation of neutron guides using welded aluminum substrates. The Institute Laue-Langevin (ILL) refurbished the H5 guide system that feds several new instruments in the guide hall. SwissNeutronics was awarded the contract for the fabrication of the metallic neutron guide for the in-pile section which is shown in Fig. 1.1 and 1.2 It has a common cross-section at the upstream end and branches into three guides at the downstream end. Both sections are rigidly connected. The common crosssection has dimensions 120 mm (h) 170 mm (w). The individual guides have a height of 120 mm and widths of 60 mm, 67.7 mm and 40 mm. The spatial constraints posed a challenge for the design and the manufacturing of the guide. Therefore SwissNeutronics decided to weld this section while still employing the ruler/trottoir design in order to achieve highest geometrical precision. During the welding process, the temperature of the interior surface was monitored to avoid damage to the supermirror coating (Fig. 1.3). The results demonstrate that the maximum temperature was only about 26 C, which is far away from being critical to the supermirror. Fig. 1.1: Modell of the H5 in-pile guide made from aluminum substrates. Fig. 1.2: Picture of the guide assembly. View from the downstream end. The guide is cladded with borated aluminium plates to reduce the steaming of thermal neutrons. page 1

2 Fig. 1.3: Thermal image of the internal surfaces of the guide during welding. The quality assurance tests demonstrate a performance of the metallic guide that is comparable to guides manufactured from borkron glass N-BK7, i.e. the waviness is in average well below rad (Fig. 1.4), confirming the excellent flatness of the individual aluminum plates. Moreover, an average reflectivity R ave = 0.89 was achieved (Fig. 1.5). Fig. 1.4: The waviness of the assembled guide is well below 10-4 rad for all sides of the common section. The waviness values given in the legend are given in units of 10-4 rad. Fig. 1.5: Compilation of the reflectivity at the critical edge of the supermirror coatings m = 3. The reflectivity of every substrate was measured. page 2

3 SwissNeutronics launches fabrication of neutron collimators In order to define the angular divergence of neutron beams, SwissNeutronics developed recently two kinds of collimators: i) classical foil collimators and ii) solid state devices (Fig. 2.1). The absorption of neutrons by the Mylar foils with absorbing coatings was measured with neutrons ( = 5 Å) (Fig. 2.2). From the refinement of the transmission a 1/10 absorption length of 2.6 m is deduced corresponding to an absorption factor of for thermal neutrons ( = 1.8 Å) at an incident angle of 1. The divergence profile of the foil collimator was also measured and compared with the results of a solid state collimator. The measured divergence is in excellent agreement with the nominal v lu s of 10 d 8 for th foil and solid state device, respectively (Fig. 2.3). Moreover, the desired triangular shape is approximated well and an excellent transmission is achieved. Fig. 2.1: Neutron collimators by SwissNeutronics. Shown are a classical foil collimator (left) and a solid state device (right). A neutron beam of 40 mm 150 mm is collimated here. In conclusion, SwissNeutronics provides state-of-the-art collimators with the following options and characteristics: Soller & radial collimators colli tio s s ll s 3 (1 r d) high transmission foils made from Mylar, Kapton, metal, etc. solid state collimators using Si wafer: t wafer 0.1 typical length: mm Fig. 2.2: Transmission of a Mylar foil with absorbing coating as a function of the angle of incidence of the neutron beam ( = 5 Å). Fig. 2.3: Transmission of foil and solid state collimators for neutron beams. The symbols and lines represent experimental data and a fit to a triangular divergence profile, respectively. page 3

4 Super-polished copper a new substrate material for neutron supermirror Various laboratories (FRM-II, ILL, NIST) already profit from neutron guides made from supermirror coated aluminum substrates for their in-pile sections. Recently, Phil Bentley et al. (European Spallation Source ESS) have demonstrated by means of Monte-Carlo simulations (MCNP5) that Cu reduces the flux of fast neutrons emitted from the tungsten target of the ESS much more efficiently than steel. Thus it is desirable to use Cu as substrate (Fig. 3.1) for the neutron guides close to the moderator within the target station. For this approach, SwissNeutronics has developed a superpolishing process to make Cu suitable as a substrate for supermirror coatings. The reflectivity of supermirror on Cu shows (Fig. 3.2) a similar reflectivity as obtained for typical substrates for neutron guides such as borkron glass and aluminum. These new developments put SwissNeutronics into the position to provide robust Cu-guides for the biological shielding without requiring a gap between the neutron reflecting substrate and the surrounding bulk shielding thus eliminating streaming of high energy neutrons. In addition, the cooling of the substrates is facilitated as Cu is an excellent thermal conductor. Clearly, ESS and other spallation sources will strongly profit from supermirror coated Cu-guides from SwissNeutronics as the costs for shielding can be significantly reduced. Fig. 3.1: Super-polished copper substrate coated with supermirror m = 3. Fig. 3.2: Reflectivity of a supermirror m = 3 on a super-polished copper substrate. page 4

5 Fe/Si polarizing supermirror m = 5.5 with excellent performance Fe/Si polarizing supermirrors are widely used for polarizing devices, e.g. cavities, single or S-shaped benders, and radial wide angle analyzers. For compact devices and polarization of a wide wavelength band, supermirror with large m-values are inevitable. SwissNeutronics is continuously developing Fe/Si polarizing supermirror, recently focusing on m- values larger than 4.5. As a result of this campaign an excellent performance of m = 5.5 supermirror was obtained. Fig. 4.1 shows the measurements of the spin dependent reflectivity and the resulting polarization. The spin-up reflectivity is analyzed using the classical formula for supermirror from the software package McStas. The refined parameter values are: R = 0.67 and m = Due to the Fig. 4.1: Spin dependent reflectivity (R = 67%) and polarization (P > 99%) of a Fe/Si supermirror m = 5.5. almost perfect suppression of the reflection of spin-down neutrons the polarization is better than In conclusion, polarizing supermirror with such an excellent performance offers great options for a variety of applications, including wide angle polarization analysis. 3D-Metrology partnership with Hexagon Metrology SA For the alignment of neutron guides SwissNeutronics employs the most modern instrumentation for 3Dmetrology available, i.e. laser tracker and 3D measurement arm. It enables for a highly accurate alignment while offering also a large flexibility to work in complex areas, e.g. narrow space or remote when radiation fields limit the access. An example for the application is given in our Newsletter from December During the last six years, SwissNeutronics has established a unique expertise for the application of modern 3D-metrology, which is now also offered to industry, where complex components are surveyed against their nominal geometry or have to be aligned for best performance (Fig. 5.1). Hexagon Metrology is the world-wide leading company in the field of 3Dmetrology. They offer a comprehensive range of products and services for all applications of metrology for various industries, e.g. automotive, aerospace, energy, and for medical engineering. In December 2013, the companies Hexagon Metrology and SwissNeutronics established a partnership to provide an extended customer service in the field of mobile 3D-metrology. More information can be found on our webpage: News January Fig. 5.1: Application of a laser tracker and a 3D measurement arm for survey and alignment. page 5