HPC detectors operate on a different principle to CCD and CMOS detectors and
function as arrays of independent point detectors [72, 74]. The detector surface
consists of a pixelated substrate made of doped silicon, GaAs or CdTe that absorbs
X-rays and produce electron-hole pairs in proportion to the X-ray photon energy.
The pixels are biased to drive the electrons or holes through a metal interconnect to a
readout chip. (The “hybrid” in the name refers to the way in which the sensor and
readout chip are made separately from different materials and later bonded to create a
working detector.) The readout chip compares the electrical signal to pre-set thresholds and increments a counter if the signal is within the threshold. Compared to CCD
and CMOS detectors, X-ray photons are detected directly with no analogue-todigital conversion, and if the thresholds are calibrated for the X-ray energy, the
detector can effectively reject background noise.
While HPC detectors address the primary deficiencies of CCD and CMOS
detectors, the technology nonetheless has some limitations. As with CMOS detectors, the integrated electronics require large pixels. Amplifying and detecting a signal
blocks (“paralyses”) the detector for a short period of time – typically hundreds of
nanoseconds – during which it cannot count additional photons. Each pixel therefore
has a maximum count rate beyond which it will effectively saturate, which is
typically on the order of 10
5
–10
7 counts per pixel per second. The operating
principle also means that if two photons are captured by a pixel at the same time,
only one will be counted. For completeness, it is worth noting that the digital
counters in the pixels are limited to a maximum by the bit depth, but this is in
practice much higher than for CCD and CMOS detectors and is only likely to be an
issue for long exposures at high flux. For example, the 20-bit counter used on some
PILATUS detectors can count up to 2
20
% 10
6 photons per pixel during a single
exposure, which can be compared to a typical ~10
4
–10
5 photons per pixel for CCD
and CMOS detectors.
A second important feature of HPC detectors is the vastly improved readout
speed. The counters are individually addressable by design and can be read out in
parallel, which makes it possible to read out images in milliseconds, i.e. up to three
orders of magnitude faster than CCD/CMOS detectors. In most detectors, the
readout electronics and counter are connected through logic gates that can be
electronically controlled (gated) by an external trigger signal, allowing them to be
active for periods of time on the order of hundreds of ns, much shorter than the
readout time.
For completeness, we note that the count rate limits of HPC detectors make them
unsuitable for use with XFEL sources. As a result, several XFELs use detectors
based on CCD and CMOS technologies, while others use modified HPC detectors
that integrate the electronic signal over time [75, 76].
Finally, a very recent development to HPC detector technology changes the
fundamental way the data is read out. Rather than counting photons to generate a
2D image, these detectors output a continuous data stream of “events” in which each
detected photon is labelled with a timestamp and the 2D coordinates of the pixel that
detected it [77]. This completely eliminates the readout time, and the stream may
also incorporate external trigger signals for synchronisation with, e.g. a pulsed laser.
230
L. E. Hatcher et al.
function as arrays of independent point detectors [72, 74]. The detector surface
consists of a pixelated substrate made of doped silicon, GaAs or CdTe that absorbs
X-rays and produce electron-hole pairs in proportion to the X-ray photon energy.
The pixels are biased to drive the electrons or holes through a metal interconnect to a
readout chip. (The “hybrid” in the name refers to the way in which the sensor and
readout chip are made separately from different materials and later bonded to create a
working detector.) The readout chip compares the electrical signal to pre-set thresholds and increments a counter if the signal is within the threshold. Compared to CCD
and CMOS detectors, X-ray photons are detected directly with no analogue-todigital conversion, and if the thresholds are calibrated for the X-ray energy, the
detector can effectively reject background noise.
While HPC detectors address the primary deficiencies of CCD and CMOS
detectors, the technology nonetheless has some limitations. As with CMOS detectors, the integrated electronics require large pixels. Amplifying and detecting a signal
blocks (“paralyses”) the detector for a short period of time – typically hundreds of
nanoseconds – during which it cannot count additional photons. Each pixel therefore
has a maximum count rate beyond which it will effectively saturate, which is
typically on the order of 10
5
–10
7 counts per pixel per second. The operating
principle also means that if two photons are captured by a pixel at the same time,
only one will be counted. For completeness, it is worth noting that the digital
counters in the pixels are limited to a maximum by the bit depth, but this is in
practice much higher than for CCD and CMOS detectors and is only likely to be an
issue for long exposures at high flux. For example, the 20-bit counter used on some
PILATUS detectors can count up to 2
20
% 10
6 photons per pixel during a single
exposure, which can be compared to a typical ~10
4
–10
5 photons per pixel for CCD
and CMOS detectors.
A second important feature of HPC detectors is the vastly improved readout
speed. The counters are individually addressable by design and can be read out in
parallel, which makes it possible to read out images in milliseconds, i.e. up to three
orders of magnitude faster than CCD/CMOS detectors. In most detectors, the
readout electronics and counter are connected through logic gates that can be
electronically controlled (gated) by an external trigger signal, allowing them to be
active for periods of time on the order of hundreds of ns, much shorter than the
readout time.
For completeness, we note that the count rate limits of HPC detectors make them
unsuitable for use with XFEL sources. As a result, several XFELs use detectors
based on CCD and CMOS technologies, while others use modified HPC detectors
that integrate the electronic signal over time [75, 76].
Finally, a very recent development to HPC detector technology changes the
fundamental way the data is read out. Rather than counting photons to generate a
2D image, these detectors output a continuous data stream of “events” in which each
detected photon is labelled with a timestamp and the 2D coordinates of the pixel that
detected it [77]. This completely eliminates the readout time, and the stream may
also incorporate external trigger signals for synchronisation with, e.g. a pulsed laser.
230
L. E. Hatcher et al.