Data obtained via different methods can provide additional insight into the sys- tem being studied. With this in mind, XEOM1 has been designed to operate using a number of different detectors. Clearly the primary detector should be an imaging device and two different CCD cameras have been used with XEOM1. The array of angle ports around the base of the optical column allow for many other secondary detectors to be mounted for parallel measurement of other sig- nals.
FLI CCD Camera
The first camera purchased for XEOM1 was a Microline ML1109 spectroscopy CCD camera manufactured by Finger Lakes Instrumentation (FLI) LLC, Lima, USA[8]. The basic specifications of the camera as advertised by the manufac- turer are outlined here. As will be seen in Section 4.4, the camera — particularly the sensor — has fallen short of expectations.
One of the most appealing aspects of this camera, in terms of integrating it into the XEOM1 system, is its size. In order to keep XEOM1 as portable as possible, it has been important to use relatively compact components. In the case of the camera, weight is also of critical concern as it must be supported by the rail onto which the optical column is mounted. A 25 mm diameter shutter
Sensor Specifications
Sensor Hamamatsu S10140-1009
Array size 1024×506
Pixel size 12µm
Full well capacity 75 000 e−
Sensor dimensions 24.58×6.072 mm
Electronics Specifications
Digitization speed 500 kHz
Typical system noise 10 e−RMS @ 500 kHz
Typical maximum cooling 65◦C below ambient
Typical dark current <0.04 e−pixel−1s−1 @−40◦C
Linearity >99 %
Table 3.1: FLI Microline ML1109 spectroscopy CCD camera specifications.
with a 40 ms open/close time is integrated into the camera. The compactness of the FLI camera compromises its cooling capability which is provided by a single- stage thermoelectric cooler. However, the inherently low thermal noise level of the back-thinned S10140-1009 CCD sensor (Hamamatsu Photonics K.K., Japan) fitted in the camera more than makes up for the modest cooling level.
Andor CCD Camera
While it was intended that a second CCD camera be purchased to replace the FLI camera, freeing it up to be integrated into the planned spectroscopy arm, it was the difficulties we had with the first camera which eventually prompted the decision. The quantum efficiency curves for the sensors in both the Andor and FLI cameras are given in Figure 3.9. The UV-enhanced coating applied to the e2v sensor provides reasonable efficiency in the UV region, although it reduces the efficiency somewhat for visible wavelengths when compared with the stan- dard broadband coating (∼60 % rather than∼90 %). Further details about the Andor camera and continued comparison with the FLI Microline can be found
in Section 4.4.3.
Sensor Specifications
Sensor e2v CCD42-40 NIMO Back Illuminated
Array size 2048×2048
Pixel size 13.5µm
Full well capacity 100 000 e− (minimum)
Sensor dimensions 27.6×27.6 mm
Electronics Specifications
Digitization speed 5 MHz (maximum)
Typical system noise 31.5 e− RMS @ 5 MHz
Typical maximum cooling 70◦C below ambient
Typical dark current 0.4 e−pixel−1s−1 @−70◦C
Linearity >99 %
Table 3.2: Andor iKon-L large area sensor CCD camera specifications. The readout speed may be adjusted; reducing this lowers the read noise in the final image.
Hamamatsu Photomultiplier Tube
The main detector on ODXAS1, the Hamamatsu H8259-01 PM tube, is now be- ing used as a secondary detector on XEOM1. The flange on the shutter has been fitted with a hollow plug which is designed to engage with the 10° angle ports. Adapters have also been made so that the PM tube can be fitted to the 45° port, and also in place of the CCD camera at the end of the optical column. In the latter configuration, the PM sensor lies in the image plane of the optics. Utilizing the PM tube in this manner allows for total XEOL spectra to be acquired in sit- uations where lateral resolution is not required, while still taking advantage of the greater light collection capability of the optical column. Experiments in this configuration have been carried out in order to compare the total XEOL intensity measured using the FLI CCD camera and the PM tube.
100 80 60 40 20 0 Quantum Efficiency / % 1200 1000 800 600 400 200 Wavelength / nm
Figure 3.9: Quantum efficiency curves for the Hamamatsu S10140-1009 and e2v CCD42-40 NIMO CCD sensors fitted in the FLI and Andor cameras respectively. Data taken from the manufacturer-supplied data sheets.
Avalanche Photodiode
For parallel measurement of fluorescence X-ray absorption spectra an avalanche photodiode (APD) detector has been used (supplied by the XMaS beamline). This consists of a silicon avalanche photodiode chip (PerkinElmer Inc., Hopkin- ton, USA) and integrated preamplifier. The chip active area is 5×5 mm and is
protected by an 80µm thick beryllium window mounted onto a KF-16 vacuum
flange. To minimize the detection of backscattered X-rays the fluorescence de- tector should ideally be positioned close to 90° from the X-ray beam axis. Since the minimum angle between sample surface normal and the beam axis is 30° (imposed by the angle ports), the maximum angle of the APD w.r.t. the beam is 50° (see Section 4.1.1).