NL1041110B1

NL1041110B1 - Method for assembling an imaging x-ray optic.

Info

Publication number: NL1041110B1
Application number: NL1041110A
Authority: NL
Inventors: Marcel Ackermann Dr; Giuseppe Vacanti Dr; Vervest Msc Mark
Original assignee: Cosine Science&Computing B V
Priority date: 2014-12-17
Filing date: 2014-12-17
Publication date: 2016-09-22
Also published as: NL1041110A

Abstract

Method for assembling an imaging x-ray optic utilizing wedged mirror plates in a Wolter-I configuration wherein the difference in wedge between two stacked plate pairs is 2 wedge units. The wedged mirror plates are manufactured by using commercial high-quality silicon wafers which are diced, structured, wedged, coated, bent and stacked. Two of such stacks are then assembled into a confocal configuration to form a mirror module, which is a complete x-ray imaging system. Hundreds of such mirror modules are finally integrated and mounted to form an x-ray optic.

Description

Title: Method for assembling an imaging x-ray optic

Abstract

Description

Field and background of the invention

The present invention relates generally to the field of high energy imaging optics. This includes a dedicated method for assembling a Wolter type x-ray telescope utilizing silicon wafers as mirror plates. More specifically, the present invention provides a new method to focus and image x or gamma rays using a particular configuration of optical modules based on stacked mirror plates .

In a preferred solution, the invention uses the current production method of Silicon Pore Optics (as described for example in "Making the ATHENA optics using Silicon Pore Optics", M. Collon et al) in a configuration that satisfies the Wolter-I telescope configuration (Wolter patent US2759106).

Silicon Pore Optics is a technology developed to enable large area x-ray optics. Silicon Pore Optics (SPO) are nested silicon mirrors in a Wolter type configuration that focus grazing incidence high energy photons by using commercial high-guality silicon wafers which are diced, structured, wedged, coated, bent and stacked. Two of such stacks are then assembled into a confocal configuration to form a mirror module, which is a complete x-ray imaging system. Hundreds of such mirror modules are finally integrated and mounted to form an x-ray optic as described in the Beijersbergen and Bavdaz patent US7321127B2. A wedge must be added to each primary-secondary pair of plates to make the angle at which on axis x-rays meet the primary mirror at each radius comply with the Wolter-I configuration and its approximations(Figure 1). This makes all plates be perfectly positioned to image on-axis rays to the centre of the focal plane.This wedge is introduced by first growing a uniform Si02 oxide layer on the plate, and then etching it away so that a varying plate thickness results .

Future x-ray missions with shorter focal lengths and /or larger apertures, such as for example the ESA x-ray mission ATHENA (collecting area 2m2 at 1 keV with a 5" angular resolution and a wide field of view of 40'x40'), require the manufacturing of steeper wedges and longer plates, leading to thicker wedges. This taxes current technologies to grow Si02 layers on wafers, therefore the physical limitations of the present wedging process have been reached.

It is apparent that for mirror modules for which the current wedging technology is insufficient, a new approach is required in order to meet the scientific requirements. This invention presents a solution which overcomes the limitations described above, mitigating the requirement of thick wedges and defining a new configuration of wedged SPO mirrors .

Patent application number US7881432, describing a method for assembling 2 or more mirror plate stacks, and the publications "Performance of Silicon Pore Optics" (M.Collon et al) and "Silicon Pore Optics for Astrophysical Missions" (G. Vacanti et al) describing the SPO development status are incorporated herein by reference.

Detailed description

The basic geometry of reflections on a single primarysecondary plate pair is known and can be found in detail in "Silicon Pore Optics for Astrophysical Missions" (G. Vacanti et al).

Considering the combination of a primary and secondary surface in the standard Wolter-I approximation (Figure 1), the basic equation describing the incident angle a, where a is the grazing incidence angle of an on-axis photon on the primary mirror, is:

where R is the distance from the mirror surface to the the optical axis, and F is the nominal focal length of the system.

Considering the geometry of a general two-plate system the total deflection undergone by an on-axis x-ray (this is a photon with an incidence angle θ=0 deg, parallel to the optical axis of the imaging system) after two reflections βρ and β6 (once on the primary mirror Mp and once on the secondary mirror Ms) is

In a stack of plates arranged from larger to smaller radius, let δ be the amount by which the angle a of each primary plate must be changed for the system to maintain its optical configuration. We call this amount δ the wedge unit, that is the plate-to-plate change in grazing incident angle a that is required for the system to bring on-axis photons to a common focus.

Then for plate N in the stack:

and the total deflection for plate N is

This is the classical Wolter-I configuration, and we call this the 1/3 wedge configuration. Actually, any combination that satisfies

brings the on-axis rays to the same focus. This means that there is no need to add 1 wedge unit on the primary plate and 3 units on the secondary plate (Figure 2), but that any combination that has a difference of 2 wedge units between the primary and the secondary mirror will be equivalent for on axis x-rays. The technique can be applied also to the conical and other approximations of Wolter-I type optics. The combination Mp = 1 , Ms = 3 is the correct one from the point of view of minimizing the on-axis aberrations (that is, photons moving parallel the axis of the optic are on axis for each plate). Other combinations are equivalent to rotating each plate slightly with respect to the ideal position: each pair of plates is progressively more off- axis, but the combination of each pair of plates still focuses to the same point in the focal plane.

The maximum thickness of the wedge required is given by the combination of the wedge angle and the plate length. For some combinations of focal length and radii, and in particular for the inner radius mirror modules of the ATHENA X-ray mission, the maximum thickness of the wedge on the secondary plate for the standard 1/3 wedge configuration exceeds the current manufacturable limit by a factor of 3 (Figure 5) . Current wedge geometry with an indication of the length and the thickness of the silicon wafer is shown in Figure 5 a) and b) . The base (1) length and thickness is the same for primary and secondary mirrors but the primary mirror (a) requires a wedge (3) of 318 nm and the secondary mirror (b) a wedge (3) of 955 nm above the socket layer (2) . In Figure 5 c) and d) the foreseen wedge geometry for a longer plate and/or a steeper wedged plate is shown. In this case the the primary mirror (c) requires a wedge (3) of 887 nm and the secondary mirror (d) a wedge of 2665 nm. So, it is evident the at the required oxide thickness for the inner radius of a 1/3 Wolter-I configuration surpasses the presently available oxide thickness by a factor 3.

Because of the afore mentioned total wedge thickness manufacturing limitation, it is worth considering how the wedge thickness might be divided up between the two surfaces (primary and secondary) in a manner that preserves the total angular correction of each pair of plates, while reducing the wedge angle on the secondary plate.

In the present invention it is shown that changing the wedge distribution is equivalent to introducing a progressive rotation into the plates of the stack.

The known combination 1/3 rotates the plates with 1 and 3 wedge units in the same direction (Figure 2). In the second option, the combination 0/2 rotates only the secondary plate (Figure 3). In particular the combination (-1/1) is also possible: the plates are rotated by 1 wedge unit in opposite direction at each reflection (Figure 4).

The present invention provides a method of making plates that bring significant advantages in manufacturability.

In a preferred embodiment, the solution (-1,1) allows using already available silicon wafers as wedged plates: not only is the 1 and -1 wedge already manufacturable with the present process but in addition all plates (primary and secondary) will be physically the same leading to a simplified production process. The rotation angles involved are sufficiently small that the eventual optical performance of the optics is not affected.

The present invention provides new stacking configurations for a x-ray imaging optic using the current plate manufacturing process. The 1/3 configuration has the best imaging performance and the -1/1 is the simplest to manufacture .

Another preferred solution utilises the 0/2 configuration. Also with a 0/2 wedging strategy each plate accumulates one wedge unit of off-axis angle. For example, in the case of 0/2 configuration the off-axis angle is about 3". After 35 plates this amounts to 1.8', not enough to have any significant impact on the angular resolution of the entire system.

Claims

A method for building an X-ray or gamma radiation imaging optic, the aforementioned optic utilizing a plurality of wedge-shaped mirrored silicon plates stacked wherein: a) wedge-shaped stacked silicon mirror plates in a double-plate mirror are called optical configuration Wolter-1 or one of its approximations b) stacked mirror plate pairs have a primary and secondary wedge-shaped mirror plate c) each primary wedge-shaped mirror plate has a wedge unit δ which is the required angle to change the angle α of each primary mirror plate so that photons incident in parallel to the optical axis converge to the same focus. d) each secondary wedge-shaped mirror plate has a wedge that differs from the wedge of the primary mirror plate at two wedge units.

Method for building an X-ray or gamma radiation imaging optic according to claim 1, wherein the wedge for the primary mirror plate is 1 wedge unit and the wedge for the secondary mirror plate is 3 wedge units.

The method for building an X-ray or gamma radiation imaging optic according to claim 1 wherein the wedge for the primary mirror plate is 1 wedge unit and the wedge for the secondary mirror plate is 1 wedge unit.

The method for building an X-ray or gamma radiation imaging optic according to claim 1 wherein the wedge for the primary mirror plate is 0 wedge units and the wedge for the secondary mirror plate is 2 wedge units.

Method for building an X-ray or gamma radiation imaging optic according to any of the preceding claims wherein the wedge-shaped mirror plates are made of silicon wafers.