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A novel pyrolytic-graphite-on-silicon device for neutron monochromatization

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Abstract

In the quest of improving neutron scattering instrumentation a new style of dynamically focusing monochromator is proposed where highly oriented pyrolytic graphite (HOPG) crystals are bonded on bendable single crystal silicon blades. This development followed a study of the elastic limit of HOPG that revealed serious technical difficulties when bending thin blades of this very fragile material to achieve dynamical focusing. The critical breakage thickness was found to be less than 0.1 mm for a radius of curvature of 1.5 m. The novel PG-on-Si (POSI) concept replaces the linear arrays of individually oriented HOPG pieces employed in existing devices. It facilitates the integration of two separate HOPG and Si monochromators into a single device. Mechanical tests, X-ray and neutron diffraction studies of the POSI system confirmed the feasibility of these single and dual monochromator designs that feature high performance and augmented functionality at reduced complexity and cost.

Introduction

The performance of neutron monochromator crystals is determined by several factors. To provide high reflectivity a small capture cross section and a high scattering power density equivalent to a big coherent scattering length and a small crystallographic unit cell are essential. To keep the level of background intensity low, the incoherent, diffuse elastic and inelastic scattering cross sections of the material should be small. Parasitic scattering and contamination from higher orders should be reduced as much as possible. The range of lattice plane orientation, called mosaic spread when replacing the real defect structure of imperfect crystals by the mosaic model, and the amount of strain generated by the elastic deformation of nearly perfect crystals must meet the flux versus resolution optimization of a specific instrument scenario. The probably most problematic requirement concerns the uniformity of the angular and spatial lattice deformations inside the diffracting volume of sufficiently big mosaic crystals.

The crystalline materials that most frequently serve as monochromators for thermal and cold neutrons are highly oriented pyrolytic graphite (HOPG) and bendable stacks of nearly perfect silicon crystals (BPSC), see, e.g., the reviews given in [1], [2]. For thermal and hot neutrons the performances of diamond and beryllium mosaic crystals were shown to be superior to those of the traditional plastically deformed copper and germanium crystals. However, the large-scale production of sufficiently big crystals with an appropriate defect configuration encountered serious problems [2], [3]. Recently, the application of hot pressed bcc-iron has been reported [4]. Stacks of plastically deformed silicon and germanium wafers have been studied too. The deformation was achieved by successive bending and reflattening of thin wafers at temperatures close to the melting point [5], [6]. A still different deformation technique is applied in astrophysics where bent Si crystal stacks create images of stars by focusing high energy gamma rays. Here, a fixed curvature of the lattice planes was generated by the strains produced when machining fine grooves into thin Si wafers [7].

In this paper a recent monochromator design is reported where pyrolytic graphite pieces are attached to perfect silicon crystal supports, a technique designated in the following by the acronym “POSI”. The main motivation of these efforts was the need for high transparency so that the non-diffracted neutrons can be used by other instruments situated downstream of the monochromator. Examples are the monochromator of the strain scanner HETU recently installed at the Mianyang facility in China [8] and the premonochromator of the USANS instrument Kookaburra at ANSTO, Australia [9]. In these applications well characterized HOPG pieces were soldered with indium metal on perfect Si crystals serving as supports with special shapes, namely facetted plates for single focusing and a toroid for double focusing, respectively. In both cases the meridional focusing is static, optimized for a single neutron wavelength band or a restricted spectral range.

To overcome this limitation the following questions arise: how can we expand the POSI technique to become dynamically focusing which implies a variable curvature of both HOPG and Si? Elastic bending of stacks composed of thin silicon blades is a well-established technique, but how thin must HOPG plates be so that they can be bent? Because both materials are complementary in terms of flux and resolution, another question has been addressed: would it be possible to integrate two HOPG and Si monochromators into a single, dynamically focusing device generating a neutron beam sending two wavelength bands to the sample either simultaneously or one after the other? Such a dual POSI monochromator would not only be more compact but also less expensive because only one mechanical subsystem is needed. Regarding applications, for example irreversible time-dependent phenomena such as structural phase transformations could be studied simultaneously under high resolution-low intensity and high flux-low resolution conditions. First, we briefly review the basic properties of existing HOPG and Si monochromators.

Section snippets

Monochromator crystal properties

HOPG has been employed for more than half a century as a highly efficient monochromator and filter material for thermal and cold neutrons. It is a one-dimensional mosaic crystal featuring strong covalent bonding of the carbon atoms in the hexagonal base plane and weak van-der-Waals forces along the c-axis. The excellent performance of HOPG is due to both its crystallographic properties and the favorable nuclear properties of carbon such as high coherent scattering power and small capture

Purpose

In the present study two avenues were followed to build a POSI monochromator with continuous, variable curvature. The first idea was to intercalate sufficiently thin HOPG pieces between thicker Si blades and to bend the stack. The HOPG crystals would cover the optically active length across the central area of the longer Si blades mounted in a bending device while the remaining gaps towards both ends would be filled with shims. Because the resistance to the bending force scales with the third

Principle

A second way was examined to realize dynamically focusing POSI monochromators by inclining flat HOPG pieces bonded on the rectangular ribs of a thin Si support crystal. When bending the Si crystal the HOPG plates are mutually inclined forming a curved polygonal shape that is closer to the ideal elliptical or circular figure than the linear arrays presently used. Since the surface deformation of the ribs during bending is very small, the force acting on the bond and therefore the risk of

Concept of a dual monochromator

The concept of a dual monochromator is inspired by the complementarity and the compatibility of HOPG and Si monochromators. It seeks to reduce the mechanical parts, the complexity, the space occupied and thus the cost of the existing systems while increasing functionality. The POSI scheme permits to integrate both HOPG and Si crystals into a single unit that focuses the useful neutrons on the sample while transmitting the non-diffracted beam. The dual monochromator consists of a POSI system

Summary and perspectives

In an attempt to achieve dynamical focusing by curved HOPG crystals the bendability of Momentive grade ZYA crystals was studied as a function of the thickness and the radius of curvature. To achieve a minimum radius of 1.5 m without breakage the thickness must be below 0.1 mm, but there is still a non-zero risk of damage after several bending and flattening cycles. The results show that the elasticity limit is sample-dependent. Therefore the method of bending thin HOPG blades is not

Declaration of Competing Interest

The authors declare that there is no competing interest.

Acknowledgments

The neutron experiments were carried out at the CANAM infrastructure of the NPI CAS Řež supported through MŠMT project No. LM2015056. The support by the MEYS project LM2018111 (participation of the Czech Republic in the European Spallation Source) is gratefully acknowledged.

References (23)

  • KirschtP. et al.

    Nucl. Instrum. Methods Phys. Res. A

    (2018)
  • HirakaH. et al.

    Nucl. Instrum. Methods Phys. Res. A

    (2011)
  • VogtT. et al.

    Nucl. Instrum. Methods Phys. Res. A

    (1994)
  • MikulaP. et al.

    Nucl. Instrum. Methods Phys. Res. A

    (2004)
  • MikulaP. et al.

    Phys. B

    (2000)
  • KimuraH. et al.

    Nucl. Instrum. Methods Phys. Res. A

    (2002)
  • FreundA.K. et al.

    Nucl. Instrum. Methods Phys. Res. A

    (2011)
  • SmeeS.A. et al.

    Nucl. Instrum. Methods Phys. Res. A

    (2001)
  • Toft-PetersenR. et al.

    Nucl. Instrum. Methods Phys. Res. A

    (2020)
  • AndersonI.S. et al.

    International Tables for Crystallography, Vol. C

    (2006)
  • FreundA.K.

    J. Appl. Crystallogr.

    (2009)
  • Cited by (0)

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