When comparing the half logarithmic plots for all three detectors the most prominent difference is that the PILATUS3 seems to detect reflections over a wider range of intensities. However, this increase in the number of reflections comes at a price. Low intensity reflections (reflections with less than 100 counts) show variations of up to 50 %. This fact may be the consequence of the underlying working principles of the detectors. The design of the sensor features rather large pixel borders which are insensitive to X-ray photons (cf. Figure 80). All photons impinging on the border are not counted. The influence of one photon not counted
Figure 89: Half logarithmic plot of the variation of the peak intensity vs. the mean intensity for the BRUKERAPEXII detector at an exposure time of 60 s.
is of course bigger for low intensity reflections (< 100 counts). Something similar applies to the PHOTON100 detector. Due to the large insensitive area some photons may not be detected. This effect is however minimised by using focusing lenses (Figure 78). Besides this it also has to be noted that the PILATUS3 practically does not have any noise. Therefore, reflections can be detected which will vanish in the noise of the APEXII and the PHOTON100. Considering just the range of intensity which is present for all three detectors, all of them show variations below 10 %.
The higher sensitivity of the DECTRISPILATUS3 may not only be advantageous for home lab sources. On the one hand a higher number of reflections can be detected with a lower exposure time but on the other hand many of these reflections might have high standard uncertainties.
timing shutter
During data collections at the beamline 15-ID-B of the Advanced Photon Source (Argonne National Labs, Chicago, USA) several problems have been noticed. As stated by Jakob Hey, it is difficult to pinpoint one source of error.
Besides the fact that the used APEXII CCD detector is not suitable for such an intense beam, our attention was drawn to the timing shutter as a potential source of error. Due to the very intense beam and the APEXII CCD detector, using exposure times in the range of 0.3 seconds is nothing unusual.
Jakob Hey already mentioned the connection between the synchronisation of the goniometer stepper motors and the timing shutter[35]. He found out that the parameters OPENDELAY and CLOSEDELAY defined in the configuration file of the Bruker D8 Firmware are crucial to a precise measurement at such low exposure times.
Besides this source of error, the actual time the shutter is open could be subject to error at these exposure times. Due to the fact that the goniometer as well as the timing shutter used at BL 15-ID-B are stock Bruker products, a simple investigation could be carried out at one of our home diffractometers. To precisely measure the exposure time, a circuit using a LED and a phototransistor was designed in cooperation with the electronics workshop (cf. Figure 90). The phototransistor was placed at the one end of the Timing shutter while the LED was placed on the other. To tightly fit the parts to the diffractometer and shield the detecting circuit from stray light, special casings for the phototransistor and the LED were machined. By opening the timing shutter the phototransistor gets illuminated and a signal can be detected. The signals were recorded by a Vellemann 4-channel USB-Datalogger. The frequency of the data
Figure 90: Circuit diagram of the detecting electronics. R1 = 10 kΩ, R2 = 47 kΩ, R3 = 1 kΩ, R4 = 100 kΩ, T1 = BPY62, T2 = BC557.
logger was 100 Hz, making it possible to examine shutter fluctuations of up to 0.01 s. At an exposure time of 0.3 s this translates to 3.33% error. Measurements were done at 1, 0.5 and 0.3 s of shutter opening time.
7.1 Results
Table 23: Overview of the requested and mean shutter opening time and the standard deviation.
Requested exposure time [s] Mean shutter opening time [s] Standard deviation [s]
0.3 0.291 0.004
0.5 0.492 0.005
calculated from a number of 1000 measurements. The results of this calculation are summarised in Table 23. It can be seen that the mean shutter opening time is about 0.01 s shorter for all requested exposure times. The standard deviations are in a range from 0.004 to 0.006 s. Within the boundaries of the experimental setup this indicates that there is a small error introduced by the mechanical shutter.
7.2 Conclusion
In conclusion, a simple device for measuring the shutter opening time could be built. With the help of the USB data logger the exposure times could be measured with a sampling frequency of 100 Hz or 0.01 s. By using a large number of measurements, a rough guess of the accuracy of the mechanical shutter could be obtained. The actual mean shutter opening times vary statistically by about 0.01 s from the requested exposure times. For further testing, it would be advantageous to use data loggers with higher sampling rates to more accurately determine the error introduced by the mechanical shutter. For the lowest exposure time tested, the deviation of 0.01 s equals to an error of 3.33 %. However, this extremely low exposure time is not used very often but with the further development of more and more powerful X-ray sources they might be necessary to avoid overloading the detector. The most efficient way to eliminate all errors introduced by mechanical shutters is simply removing them. Recent detectors are capable of shutterless operation. However, it needs to be proven that shutterless data acquisition is suitable for measurements used in charge density investigations, as these do need extremely high data quality.
In July 2016 it was possible to obtain 24 hours of beam time at the beam line BL02B at the SPring-8 synchrotron facility in Japan in cooperation with the
workgroup of Bo
studies. BL02B1 is equipped with two switchable detectors, a four circle goniometer and an open flow liquid helium cooling (Figure 92). For fast investigation of the crystal quality and routine structure determination, a RIGAKU
MERCURY2 CCD detector can be used. For the measurement of high resolution charge density data sets, the also present custom
RIGAKU cylindrical image plate detector is used.
The image plate does cover a 2θ range from -60 ° to 145 °, which makes it possible to obtain high resolution raw images in one shot. The superior dynamic range of the image plate is suitable to cope with the high intensity low angle reflections as well as weak reflections at high angles at the same time. However, these advantages come at a price: the readout time.
Due to the readout process, the dead time between two images is approximately seven
minutes. Therefore, measurements take more time than at comparable synchrotron sources or beam lines (e.g. Advanced Photon Source, Argonne National Labs,
Figure 92: Setup at beam line BL02B1 at the Spring-8 synchrotron radiation facility, Japan.
Figure 93: Schlenck line installed at BL02B1, Spring-8, Japan.
Chicago, USA). In addition, the laboratory at BL02B1 was upgraded to simplify handling of air sensitive samples (Figure 93).