and the efficiency with which the primary electron beam produced at the cathode is
converted to X-rays by the anode material. The power is a product of the cathode
emission current and the accelerating voltage. The conversion efficiency is proportional to the accelerating voltage and atomic number Z of the anode material and is
around 1.3, 1.9 and 2.1% for Cu, Mo and Ag tubes operating at 50 kV. The power a
tube can be operated at is limited by the heat load on the anode, which must be
carefully controlled to avoid rapid degradation. The anode is therefore typically
water cooled.
Dramatic technical advances in this area in the last 30 years have led to substantial
improvements in the brilliance of laboratory X-ray sources. Microfocus X-ray
sources, capable of focussing the X-ray beam down to a much smaller spot size
than conventional sources, can enhance the brilliance for smaller crystals by ~20Â
compared to a traditional sealed-tube source. Rotating anode sources, where the
anode is moved to spread the electron beam over a larger section of the material to
reduce the heat load and allow the tube to operate at higher power, can produce
~60–120Â higher brilliance when combined with microfocus optics. Other new
technologies such as liquid metal target and nanofocus sources offer even higher
brilliance, and continuing technological advances in this area make it likely that even
brighter laboratory X-ray sources will be available in the near future [67].
Despite the promising improvements in laboratory X-ray technology, as
discussed in Sect. 4.1, pump-(multi)probe SCXRD measurements are severely
diffraction-limited due to the need for very short X-ray probe times. At shorter target
time resolutions, brighter X-ray sources are thus required to achieve sufficient
signal-to-noise ratio in a reasonable timeframe. As such, most TR-SCXRD studies
have been conducted at third-generation synchrotron sources, where the average
X-ray brilliance is more than a billion times higher than a laboratory X-ray source
[67, 68]. The advent of X-ray free electron laser (XFEL) sources has further
increased the peak brilliance achievable by a further billion- to trillion-fold, enabling
even smaller crystals to be studied at very short timescales.
The required time resolution also dictates the choice of X-ray source. Unlike
laboratory sources, the manner in which synchrotron and XFEL radiation is generated results in a natural temporal structure [69–71]. Synchrotron sources produce
X-rays in bunches with bunch widths in the region of picoseconds and typical
separations between bunches of a few nanoseconds within the main bunch train,
and single bunches can be isolated in the hybrid fill patterns to obtain a couple of
hundred nanosecond separation to the main bunch train. This structure places a
lower limit on the achievable time resolution. At nanosecond time resolution,
experiments can be conducted by selecting single X-ray bunches from the synchrotron beam, while for experiments at microsecond and longer timescales, the synchrotron beam appears continuous, and techniques such as mechanical chopping or
electronically gating the detector can be used to produce probes of the desired width.
Further discussion of fast TR diffraction experiments at synchrotrons can be found in
chapter “Time-Resolved Single-Crystal X-Ray Crystallography”. XFEL facilities
provide access to even shorter timescales, with current sources producing X-ray
pulses as short as a few femtoseconds and developments targeting attosecond time
resolution.
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converted to X-rays by the anode material. The power is a product of the cathode
emission current and the accelerating voltage. The conversion efficiency is proportional to the accelerating voltage and atomic number Z of the anode material and is
around 1.3, 1.9 and 2.1% for Cu, Mo and Ag tubes operating at 50 kV. The power a
tube can be operated at is limited by the heat load on the anode, which must be
carefully controlled to avoid rapid degradation. The anode is therefore typically
water cooled.
Dramatic technical advances in this area in the last 30 years have led to substantial
improvements in the brilliance of laboratory X-ray sources. Microfocus X-ray
sources, capable of focussing the X-ray beam down to a much smaller spot size
than conventional sources, can enhance the brilliance for smaller crystals by ~20Â
compared to a traditional sealed-tube source. Rotating anode sources, where the
anode is moved to spread the electron beam over a larger section of the material to
reduce the heat load and allow the tube to operate at higher power, can produce
~60–120Â higher brilliance when combined with microfocus optics. Other new
technologies such as liquid metal target and nanofocus sources offer even higher
brilliance, and continuing technological advances in this area make it likely that even
brighter laboratory X-ray sources will be available in the near future [67].
Despite the promising improvements in laboratory X-ray technology, as
discussed in Sect. 4.1, pump-(multi)probe SCXRD measurements are severely
diffraction-limited due to the need for very short X-ray probe times. At shorter target
time resolutions, brighter X-ray sources are thus required to achieve sufficient
signal-to-noise ratio in a reasonable timeframe. As such, most TR-SCXRD studies
have been conducted at third-generation synchrotron sources, where the average
X-ray brilliance is more than a billion times higher than a laboratory X-ray source
[67, 68]. The advent of X-ray free electron laser (XFEL) sources has further
increased the peak brilliance achievable by a further billion- to trillion-fold, enabling
even smaller crystals to be studied at very short timescales.
The required time resolution also dictates the choice of X-ray source. Unlike
laboratory sources, the manner in which synchrotron and XFEL radiation is generated results in a natural temporal structure [69–71]. Synchrotron sources produce
X-rays in bunches with bunch widths in the region of picoseconds and typical
separations between bunches of a few nanoseconds within the main bunch train,
and single bunches can be isolated in the hybrid fill patterns to obtain a couple of
hundred nanosecond separation to the main bunch train. This structure places a
lower limit on the achievable time resolution. At nanosecond time resolution,
experiments can be conducted by selecting single X-ray bunches from the synchrotron beam, while for experiments at microsecond and longer timescales, the synchrotron beam appears continuous, and techniques such as mechanical chopping or
electronically gating the detector can be used to produce probes of the desired width.
Further discussion of fast TR diffraction experiments at synchrotrons can be found in
chapter “Time-Resolved Single-Crystal X-Ray Crystallography”. XFEL facilities
provide access to even shorter timescales, with current sources producing X-ray
pulses as short as a few femtoseconds and developments targeting attosecond time
resolution.
228
L. E. Hatcher et al.