Research
Overview|Colloidal Diffusion|Capillary Wave Dynamics|Polymeric Sponge Phases|CCD X-Ray Detectors
Peter Falus, Simon Mochrie, Matt Borthwick
Fast CCD
cameras for x-ray photon correlation spectroscopy and time-resolved
x-ray scattering and imaging.
X-ray sources have shown
tremendous advances over the last several decades, with even more brilliant
sources anticipated in the near future (LINK TO ...). Nevertheless, as increasingly
challenging experiments are attempted and the demand for synchrotron beam time
grows, in order to collect the most meaningful data most efficiently, it is
essential to optimize the beam line optics,
and the x-ray detection scheme. However, in contrast to the intensive effort
to increase source brilliance and improve beam line optics, the development
of x-ray detectors has often seemed a relatively neglected area, except in the
field of macromolecular crystallography, where
large-area devices, costing hundreds of thousands or millions of dollars, have
become prevalent
We have implemented a new, inexpensive, fast, charge-coupled device (CCD)-based,
x-ray area detector -- the SMD1M60
-- in the context of a research effort to carry out x-ray photon correlation
spectroscopy (XPCS) experiments.
PCS experiments with x-rays are much more challenging than those using lasers. This is, first, a partially coherent x-ray beam, which contains far fewer photons than a laser beam. Second the x-ray scattering cross-section is far smaller than for light scattering. The upshot is that the crucial aspect of an XPCS experiment is usually the signal-to-noise ratio (SNR). Thus, the source should be as brilliant as possible. The beamline optics should preserve the source brilliance. The sample must be studied in a manner that minimizes as far as possible x-ray sample damage. Beamline and synchrotron stability is essential to achieve clean results. Because XPCS is SNR-starved enough beamtime must be allocated. Finally, the best possible detector should be used.
A detector optimized for XPCS should collect as many of the photons as it can,
over as wide a solid angular range as possible. At the same time, its spatial
resolution should be sufficiently fine to partially
resolve the speckle that results when a sample is illuminated by partially coherent
radiation. It should have a small dead time and high quantum efficiency. Crucially,
it should have a time resolution that is commensurate with the dynamics of the
subject systems of interest -- the faster the available time resolution, the
more systems that there are that are potentially accessible to study.
Until now, XPCS experiments
have employed CCD cameras with full-frame data rates of only about 0.3~Hz. As
a result, the time needed for CCD readout has been a serious limitation in the
development and application of XPCS. In many instances, a time delay between
successive images
of 3~s is too long to capture an interesting time evolution of the speckle pattern.
This problem can be overcome using so-called kinetics mode, where only a portion
of the CCD is illuminated and data from
successive images, closely separated in time, are stored in the unilluminated
portion of the CCD for later readout. However, because the full-frame data rate
remains 0.3~Hz and only a small portion
of the CCD is used to collect data, the SNR in this case is much poorer than
would be possible if the full-frame readout time was comparable with or less
than the required time step. Thus, at the outset of this project, in order to
carry forward our scientific research program, we were confronted by an urgent
need for a faster XPCS detector, that would acquire data much more efficiently
than earlier detectors, and that was also inexpensive. Rather than designing
and assembling our own detector ``from scratch'', we instead embarked upon a
modular approach, using commercial components and following industry standards,
to create a photon-counting x-ray area detector system.
The key feature of the SMD1M60 as a detector for XPCS experiments is that it
permits us to continuously acquire images, consisting of individual photon events,
at full-frame data rates of up to 60 Hz and frame data rates of up to 500 Hz.
Thus, it is straightforward to acquire data with a time resolution of as little
as 2~ms, and data from a considerably larger solid angle can be collected if
a
time resolution of 17~ms is acceptable. The much greater data rate possible
with the SMD1M60 compared to earlier generation cameras permits a many-fold
increase in the XPCS SNR in cases where sub-second time steps are called for.
It is not the readout time {\em per se} that
is important. Indeed, several groups have reported optical PCS setups based
on
video cameras taking images at 30~Hz \cite{wong:93,cipelletti:99,others}. However,
in these cases the overall data rates were many times smaller than with the
SMD1M60. Instead, what leads to the overall superior data rate possible with
the SMD1M60 is the combination of the frame readout time, the number of pixels
per frame, and the number of bits per pixel. In addition, the SMD1M60 is based
on an inexpensive, commercially-available CCD camera. It is also lightweight
and conveniently transportable to the synchrotron.
Beyond XPCS, because of the superior data rates possible, we expect that the
SMD1M60 will be valuable in time-resolved x-ray scattering measurements of all
sorts, including imaging applications.
In addition, we have found it capable of collecting superior small angle x-ray
scattering (SAXS) data. The SMD1M60 may also prove valuable in x-ray imaging
applications.