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X-ray and gamma-ray optics

X-ray and gamma-ray optics

Overview

Imaging Optics for electromagnetic radiation above 0.05 keV (X-Ray and Gamma-rays) offer brand-new astrophysical insights. The radiation of interest ranges from the highly red-shifted X-rays emitted by primeval black holes to characteristic supernova Ia Gamma-ray emission lines providing insight in the processes of explosive nucleosynthesis or the deceleration of the cosmic expansion.

Two instrument types offer the possibility to efficiently focus high-energy radiation:

Grazing angle mirrors make use of small angle reflection of X-ray and low-energy gamma-rays. In order to overcome heavy and bulky solid metal reflectors, ongoing technology research is investigating lighter but nonetheless stiff mirror structures, leading to the design of pore optics. Silicon pore optics ("high precision optics, HPO") are currently developed for astrophysics missions on satellites in earth orbit, the lighter but less precise glass pore optics ("micro-pore optics, MPO") are under investigation for missions into the solar system. Additionally, single- and multi-layer coatings are developed to enhance the reflectivity.

Crystal diffraction lenses make use of Laue diffraction in order to focus high-energy gamma-rays. 

Grazing angle mirrors

Next generation X-ray telescopes need new focussing optics technology.

The XEUS optical specifications, given the XMM-Newton design, would require 16 tons of nickel, excluding the support structures! The NXO mirrors need to be made within a mass constraint of 2 tons, which requires a factor 8 reduction in mass in addition to the improvement in the optical qualities. Due to the large mass of the optics the XEUS mission in its original configuration required multiple launches and assembly at the ISS. The technology development is leading to a much lighter spacecraft which can be launched on a single Ariane 5 and be self deployed and no longer requires to be in orbit with the ISS.

All previous mirror technologies lie along a similar trend of increasing mass density with increasing resolving power. Future Missions like Constellation-X and XEUS need a ground-breaking development to overcome this dependency. The mass reduction for the same collecting area will allow more sensitive observations.

Chandra uses highly polished monolithic glass. XMM-Newton uses replicated nickel shells. SCI-AT embarked on the development of new technologies like high performance silicon pores and glass microchannel plates.

Chandra
0.5"
18500 kg/m²
Aeff @1 keV
XMM-Newton
14"
2300 kg/m²
Aeff @1 keV
Si-HPO
5"
200 kg/m²
Aeff @1 keV
Glass-MPO
30"
25 kg/m²
Aeff @1 keV

Optics Developments must not only achieve the science requirements but also be feasible to manufacture in a cost effective manner. In this respect the integration of optics must be simple and the ability to mass produce the optics with a high reliability are important factors. The silicon pore optics lends itself to this scaling advantage.

Micropore Optics

X-ray optics for astrophysics missions requires extremely large collecting areas (>10 m²) in combination with good angular resolution (<5"). The existing technologies of focussing optics use polished glass, electroformed nickel or foils and would lead to excessively heavy and expensive optics, and/or they are not able to produce the required large area. We have developed an entirely novel technology by using pore structures that allow very thin mirrors in the required stiff structure.

In the past three different generic optics technologies have been employed conventionally:

  • Monolithic shells of glass that are figured and highly polished for high angular resolution, but with relatively large mass penalty. They are rigid and relatively easy to support.
  • Thin foils that are fabricated from pliable substrates (plastic or metal) and bent into a conical approximation of a Wolter geometry. These offer the lowest mass but are relatively poor in angular response, especially because distortions arise at their mounting points.
  • Replication of thin shells (usually nickel) from a highly polished mandrel that offers a compromise in terms of mass versus resolution trade-off. These shells are inherently stiffer than foils and can be mounted at one end through fixation onto an accurately machined spider.

The minimisation of the telescope mass and volume becomes of critical importance for the next generation astronomical X-ray telescopes (e.g. NXO) due to their challenging requirements in collecting area and resolution. Reducing the mass of a grazing incidence X-ray optic can be achieved by

  1. using a material that has a lower density and
  2. by reducing the thickness of the mirrors.
Very thin mirrors will in general have more distortions which result in lower imaging resolution. This can be prevented by producing the reflecting surfaces in a tightly interconnected structure, so that the mirrors span only small distances. This is achieved in pore optics.

Two sets of pores placed back to back reflect the X-rays onto the detector in the focal point (FP).

In a pore optic one wall of each pore is used as the reflecting surface, and the side walls provide extreme stiffness to the structure. The shape of the entirety of reflecting surfaces does not necessarily have to follow exactly the parabolic and hyperbolic surfaces of the Wolter-I design. If the reflecting surfaces are short compared to the focal length, the effect on the imaging resolution of conical instead of parabolic or hyperbolic surfaces ('conical approximation') can be made sufficiently small.

The profile of the pore needs not follow a circle, provided that the width of the pore is smaller than the required size of the focal spot.

Because the walls in the pore structure can be very thin the reflecting surfaces can be stacked very densely, effectively leading to small pores and short optics. This enables the production of high-resolution optics even with flat reflecting surfaces of the pores themselves. The complete system, however, still focuses the rays, since the reflection surfaces are concentric and the inclination of the surfaces rise with the radius. 

SEM image of etched silicon square pore material.

Layered Synthetic Microstructures

A concentrating optic for X-ray and gamma-ray radiation can increase the collecting area beyond the size of the detector, thereby improving the sensitivity of the measurement. Multilayer coating techniques offer the possibility to build even X-ray and gamma-ray mirrors for a concentrating optic.

Reflection of X-ray and gamma-ray radiation off a medium can be very efficient, albeit at very small grazing angles. The models for reflection off a medium, based on a complex index of refraction from the oscillator strength and the Fresnel laws of refraction and reflection, seem to work up to a few hundred kev. As in the X-ray regime, the index of refraction is less than 1 in combination with absorption, resulting in total external reflection.

Single-layer Reflection

Reflection of a uniform medium, called single-layer reflection in contrast to multi-layer reflection, can be achieved by metallic coating (e.g. Ir, Au, Pt, or W). A simple metallic coating has the advantage of a fairly high reflectivity in a very large energy band, from the visible up to the "critical energy", being the energy at which the grazing angle equals the critical angle. At smaller incident angles (< 0.1 deg) and lower energies (> 10 keV) the reflectivity is close to 1 up to the critical energy, assuming that the surface roughness can be sufficiently reduced. A single-layer telescope therefore can be used as a broadband instrument, up to the maximum energy determined by the grazing angles in the optic.

Multi-layer Reflection

The activity includes an assessment of existing measurements and theory for reflectivity performance of layered synthetic microstructures, in the energy range 2 - 70keV. For the XEUS mission a baseline optics design will be adopted, and appropriate coatings proposed for different radial ranges of the mirror locations. The design shall optimise the effective area by increasing reflectivity at lower energies. Test samples will be produced on ESA CFI silicon plates. Methods for patterning the coatings to allow silicon-to-silicon bonding along 150 micron uncoated strips will be determined. Environmental qualification, especially at low temperatures of operation will be demonstrated.

Crystal diffraction lenses

Laue diffraction crystals are required to establish efficient Gamma-ray focussing in the range 100 keV to 1 MeV.

The requirements for energy ranges and effective collecting area are examined. Detailed design and simulation activity to develop an optimized configuration are performed to meet the stated requirements. Different design options for the Laue crystal concept are explored, including Ge, Cu and other crystals with improved mosaic properties gradient crystals, such as Si-Ge, Si stacks, gradient generation with thermal control, and gradient generation by as-built stacking; curved crystal implementation via the stacking of large scale Si wafers or other materials. As well as examining optimal growth techniques, the activity addresses likely requirements for precision crystal cutting, for quality control and crystal selection that may also be a factor in eventual optimized crystal implementation. Eventually, crystal fabrication are implemented and performance measured. Issues of mounting the crystals onto a prototype optical bench must be addressed, including the mounting and bonding of the crystals on a lens frame, precision crystal cutting and means to correct crystal wedge errors.

Applicability to: Gamma-ray Imager

Technology Readiness Target: 2010

Last Update: 1 September 2019
27-Sep-2021 09:32 UT

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