4.4 X-Ray Detectors
Current X-ray detectors used for crystallography are based on one of three technologies: charge-coupled device (CCD), complementary metal-oxide-semiconductor
(CMOS) or hybrid photon-counting (HPC) systems.
CCDs are based on early digital imaging technology. Early detectors used a layer
of scintillator material (commonly referred to as a phosphor) to convert incident
X-ray photons into visible photons, which were then directed through fibre optics to
an array of photodiodes to convert the photons to electron/hole pairs. More modern
detectors use materials such as silicon, CdTe and GaAs that absorb X-rays and
produce electron-hole pairs directly (termed “direct detection”). The electrons are
captured in potential wells and accumulated. When the image is read out, the charges
in each well are transferred sequentially to an amplifier, and the signals are digitised
and processed to measure the intensity of X-ray photons received at each pixel and
form a monochrome intensity image.
While well established, CCDs have some notable downsides [72]. Firstly, the
readout process is slow, taking on the order of a second to read out an image, and the
X-rays must be shuttered while this happens. Secondly, thermally generated electrons are also captured in the charge wells, producing a high level of background
noise referred to as a dark current, and mitigating this typically requires actively
cooling the detector. The transfer of charges between wells to the amplifier during
readout can also lead to signal loss and introduce noise. Finally, the charge wells
have a finite capacity that can be exceeded during long exposure times or when
measuring high-intensity reflections. This both saturates the intensity of the pixel
and causes excess electrons to spill over into adjacent wells (an effect termed
“blooming”), leading to distorted peak shapes. The significant background noise
makes it difficult to reliably measure weak reflections, while saturation/blooming
limits the dynamic range and makes it difficult to measure images that contain both
very strong and very weak reflections.
CMOS detectors again evolved from digital photography and improve upon
CCD detectors by attaching addressable amplification and readout logic to each
pixel. By digitising the signal at source and removing the need to transfer charges
between wells during readout, the signal is higher, and CMOS detectors can run at
higher working temperatures with less noise than CCDs. The per-pixel logic also
makes it possible to correct for issues including detector non-linearity, dark current
and detector inhomogeneity at the level of individual pixels [73]. The pixels are
separated with guards that prevent blooming. Although the readout time for a
complete image remains similar to CCDs, addressing individual pixels allows pixels
to be read out in lines to implement a so-called rolling shutter which, if images are
carefully reconstructed after capturing, can considerably reduce the collection time.
On the other hand, saturation remains an issue, limiting the dynamic range, and
incorporating electronics into each pixel requires them to be physically larger than
on CCD detectors.
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
229
Current X-ray detectors used for crystallography are based on one of three technologies: charge-coupled device (CCD), complementary metal-oxide-semiconductor
(CMOS) or hybrid photon-counting (HPC) systems.
CCDs are based on early digital imaging technology. Early detectors used a layer
of scintillator material (commonly referred to as a phosphor) to convert incident
X-ray photons into visible photons, which were then directed through fibre optics to
an array of photodiodes to convert the photons to electron/hole pairs. More modern
detectors use materials such as silicon, CdTe and GaAs that absorb X-rays and
produce electron-hole pairs directly (termed “direct detection”). The electrons are
captured in potential wells and accumulated. When the image is read out, the charges
in each well are transferred sequentially to an amplifier, and the signals are digitised
and processed to measure the intensity of X-ray photons received at each pixel and
form a monochrome intensity image.
While well established, CCDs have some notable downsides [72]. Firstly, the
readout process is slow, taking on the order of a second to read out an image, and the
X-rays must be shuttered while this happens. Secondly, thermally generated electrons are also captured in the charge wells, producing a high level of background
noise referred to as a dark current, and mitigating this typically requires actively
cooling the detector. The transfer of charges between wells to the amplifier during
readout can also lead to signal loss and introduce noise. Finally, the charge wells
have a finite capacity that can be exceeded during long exposure times or when
measuring high-intensity reflections. This both saturates the intensity of the pixel
and causes excess electrons to spill over into adjacent wells (an effect termed
“blooming”), leading to distorted peak shapes. The significant background noise
makes it difficult to reliably measure weak reflections, while saturation/blooming
limits the dynamic range and makes it difficult to measure images that contain both
very strong and very weak reflections.
CMOS detectors again evolved from digital photography and improve upon
CCD detectors by attaching addressable amplification and readout logic to each
pixel. By digitising the signal at source and removing the need to transfer charges
between wells during readout, the signal is higher, and CMOS detectors can run at
higher working temperatures with less noise than CCDs. The per-pixel logic also
makes it possible to correct for issues including detector non-linearity, dark current
and detector inhomogeneity at the level of individual pixels [73]. The pixels are
separated with guards that prevent blooming. Although the readout time for a
complete image remains similar to CCDs, addressing individual pixels allows pixels
to be read out in lines to implement a so-called rolling shutter which, if images are
carefully reconstructed after capturing, can considerably reduce the collection time.
On the other hand, saturation remains an issue, limiting the dynamic range, and
incorporating electronics into each pixel requires them to be physically larger than
on CCD detectors.
Watching Photochemistry Happen: Recent Developments in Dynamic Single-Crystal. . .
229