2.7 Classification of X-ray Detectors
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focus X-rays, but only very close to the center of the field of view as it would suffer
from extreme coma.
For X-ray astronomy observations, Wolter–I telescope is an annular sector mirror
composed of parabolic and hyperbolic mirror combination, and its incident aperture
is a thin ring; only X-ray photons that firstly come to the parabolic mirror and then
the hyperbolic one can be focused on the detectors located at focal point. Hyperbolic
mirrors with same focal length and different radius are nested by multiple layers to
increase effective collection area of telescope. Furthermore, the sensitive detectors
are installed on the focal point and the telescope can image for X-ray sources within
a sky area of 1° × 1°. The space resolution of the telescope is usually relied on the
mirrors’ focusing capability and the detectors’ positioning accuracy.
Coded-aperture masks are grids, gratings or other patterns of materials opaque to
the various wavelengths of electromagnetic radiations, which are usually high-energy
radiations such as X-rays and gamma rays. As mentioned earlier in the section, it
is impractical that imaging is made at the X-ray and gamma-ray bands by lenses or
mirrors. Modulation imaging by aperture is thereby often used instead. The pinhole
camera is the most basic form of such a modulation imager, but its disadvantage
is low throughput, as its small aperture allows through little radiation. Only a tiny
fraction of the light passes through the pinhole, which causes the low Signal-toNoise Ratio (SNR). To solve this problem, the mask can contain many holes, in
one of several particular patterns, for example. Multiple masks, at varying distances
from a detector, add flexibility to this tool. Specifically, the modulation collimator
was used to identify the first cosmic X-ray source and thus to launch the new field
of X-ray astronomy in 1965.
Obviously, the coded-aperture mask imaging technique originated from the
pinhole camera. A mask with the pinholes is mounted ahead position-sensitive
detector and incident X-rays pass through the pinholes. And thus, the X-rays within
sky area will be cast upon the detector by the pinholes and an image or shadow of
X-ray intensity is obtained. The imaging resolution depends on the pinhole size and
the position resolution of the detector. The blocking and penetrating radiation of a
coded-aperture mask can be expressed, respectively, as 1 and 0. As X-rays from a
direction come to a coded-aperture mask, the shadow of the mask will be cast upon
the position-sensitive detector. The shadow’s position changes with the incoming
direction. As the X-rays come from various directions, these shadows will be overlapped on the detector. If the autocorrelation value of the mask’s modulation function
is designed as 1, the intensity distribution of the X-ray sources within the sky area
can be obtained by the correlation analysis. In other words, the properties of the original radiation sources can then be mathematically reconstructed from the shadows.
There are three key problems for the coded-aperture mask: how to increase collecting
area (or duty cycle), how to design the coded mask with autocorrelation and how to
develop highly position-sensitive detector. The coded-aperture imaging technology
is particularly used to detect hard X-rays and the higher imaging resolution can
usually be achieved by the technology.
73
focus X-rays, but only very close to the center of the field of view as it would suffer
from extreme coma.
For X-ray astronomy observations, Wolter–I telescope is an annular sector mirror
composed of parabolic and hyperbolic mirror combination, and its incident aperture
is a thin ring; only X-ray photons that firstly come to the parabolic mirror and then
the hyperbolic one can be focused on the detectors located at focal point. Hyperbolic
mirrors with same focal length and different radius are nested by multiple layers to
increase effective collection area of telescope. Furthermore, the sensitive detectors
are installed on the focal point and the telescope can image for X-ray sources within
a sky area of 1° × 1°. The space resolution of the telescope is usually relied on the
mirrors’ focusing capability and the detectors’ positioning accuracy.
Coded-aperture masks are grids, gratings or other patterns of materials opaque to
the various wavelengths of electromagnetic radiations, which are usually high-energy
radiations such as X-rays and gamma rays. As mentioned earlier in the section, it
is impractical that imaging is made at the X-ray and gamma-ray bands by lenses or
mirrors. Modulation imaging by aperture is thereby often used instead. The pinhole
camera is the most basic form of such a modulation imager, but its disadvantage
is low throughput, as its small aperture allows through little radiation. Only a tiny
fraction of the light passes through the pinhole, which causes the low Signal-toNoise Ratio (SNR). To solve this problem, the mask can contain many holes, in
one of several particular patterns, for example. Multiple masks, at varying distances
from a detector, add flexibility to this tool. Specifically, the modulation collimator
was used to identify the first cosmic X-ray source and thus to launch the new field
of X-ray astronomy in 1965.
Obviously, the coded-aperture mask imaging technique originated from the
pinhole camera. A mask with the pinholes is mounted ahead position-sensitive
detector and incident X-rays pass through the pinholes. And thus, the X-rays within
sky area will be cast upon the detector by the pinholes and an image or shadow of
X-ray intensity is obtained. The imaging resolution depends on the pinhole size and
the position resolution of the detector. The blocking and penetrating radiation of a
coded-aperture mask can be expressed, respectively, as 1 and 0. As X-rays from a
direction come to a coded-aperture mask, the shadow of the mask will be cast upon
the position-sensitive detector. The shadow’s position changes with the incoming
direction. As the X-rays come from various directions, these shadows will be overlapped on the detector. If the autocorrelation value of the mask’s modulation function
is designed as 1, the intensity distribution of the X-ray sources within the sky area
can be obtained by the correlation analysis. In other words, the properties of the original radiation sources can then be mathematically reconstructed from the shadows.
There are three key problems for the coded-aperture mask: how to increase collecting
area (or duty cycle), how to design the coded mask with autocorrelation and how to
develop highly position-sensitive detector. The coded-aperture imaging technology
is particularly used to detect hard X-rays and the higher imaging resolution can
usually be achieved by the technology.