A Neutron Microscope Using a Nested Wolter-I Condenser and a Bank of Diffractive-Refractive Achromatic Objectives

TL;DR

This work proposes a neutron microscope concept that combines a nested Wolter-I condenser with a bank of achromatic neutron optics (CRLs and FZPs) to overcome the inherent divergence and chromatic limitations of neutron imaging. The authors use ray tracing and prototype feasibility tests to show that a high-flux condenser can deliver up to ~100× flux density gains, while CRL/FZP objective banks, including achromats/apochromats, can achieve 2–10 μm resolution over a 4×4 mm$^2$ field of view for monochromatic beams and broadened bandwidth via chromatic-correcting designs. They outline two implementation pathways—CRL-based and FZP-based banks—each with trade-offs in numerical aperture, field of view, and manufacturability, and discuss strategies for broadband operation, detector deployment, and advanced data-analysis methods for cone-beam tomography. The proposed approach enables simultaneous acquisition of hundreds of projections, potentially delivering X-ray micro-CT-like spatial/angular resolution in neutron tomography for large samples and complex sample environments, with significant impact on materials science, energy storage, and polarized neutron imaging.

Abstract

We propose a nested Wolter-I mirror design for a neutron condenser, which is based on established X-ray telescope technology. We demonstrate through simulations that it can increase the flux density at the ESS imaging instrument ODIN by up to two orders of magnitude. Experimental measurements of reflectivity and figure errors on a prototype mirror element confirm the technical feasibility of the approach. Then, we discuss design strategies for an imaging objective to fully exploit the condenser specifications while achieving spatial resolutions comparable to those of X-ray micro-CT instruments. Analytically, we show that for monochromatic beams suitable solutions exist employing arrays of hundreds of identical objectives, realized either as compound refractive lenses (CRLs) or Fresnel zone plates (FZPs). To mitigate the inherent chromatic aberration of these optics, each individual objective could be replaced by an achromatic FZP/CRL combination. Key optical properties of the resulting microscope are estimated. This novel full-field microscopy concept for highly divergent, polychromatic neutron beams has the potential to improve temporal and spatial resolution for large samples and sample environments and to enable the simultaneous acquisition of hundreds of projections in neutron tomography.

Figures (8)

• Figure 1: Wolter optic based condenser design. Line rendering of the proposed focusing optic with 20 shells of conical symmetry.
• Figure 2: Results of McStas simulations of the Wolter-I type condenser for use at the ODIN instrument. a) Spectrum of ODIN with (purple) and without (green) the Frame Rate Multiplication, FRM, chopper activated. b) The divergence of the beam just upstream of the condenser. c) spatial distribution of the beam just downstream of the condenser. d) 2D-divergence distribution, e) and f) horizontal and vertical divergence as function of position along the beam, x, g) spatial distribution, all of d)-g) measured at the focal point. i) Gain factor of neutron flux as function of distance from the nominal focal point. The first peak in i) corresponds to the focal point, the second peak stems from a geometrical artifact caused by a single reflection on the Wolter-I optic. The resulting artificial beam spot at 0.6 m is correspondingly diffuse, see h).
• Figure 3: Measured neutron reflectometry of curved NiC/Ti supermirror-coated glass at three azimuthal positions, $\Psi$.
• Figure 4: Illustration of the concept of a bank of objectives, where each objective is a compound refractive lens. The optical axes (dashed lines) coincides in the sample plane. Adapted from Leemreize2019.
• Figure 5: Manufacturing specifications for a single FZP objective in terms of outermost zone width $\Delta r$ as function of magnification for three materials. The corresponding numerical aperture is shown. Moreover the required aspect rations of the outermost zone for three materials for maximum efficiency is shown. The example relates to a fixed diameter $D = 4$ mm, wavelength $\lambda =$4 Å and an experimental laboratory allowing $L = 8$ m.