Tests with Synchrotron Radiation

In practice, only a fraction of the photons impinging on the detector are registered because of the entrance window absorption. Furthermore, a part of the charge generated in the detector volume is lost during the transfer to the readout anode.

The quantum detection efficiency (QDE) is a measure for the loss of the detected signal because of these two effects; it is defined as the ratio of the number of electrons detected in the real detector to the number of electrons detected with an ideal detector without absorption and with perfect charge transfer. Therefore, together with the conversion factor from photons to ADC counts (see Sec.5.4.1), the QDE can be used to convert the measured intensity into an absolute photon number. Another efficiency measure frequently used to describe CCD detectors is the detective quantum efficiency (DQE), which gives the degradation of the signal-to-noise ratio of the measured signal due to the detector [96].

The quantum detection efficiency has been measured by Hartmann et al. [29]

for a pn-CCD with entrance window properties similar to those of the STXM

0.6

0.4

0.2

200 500 1000 2000 5000

0.0

Energy [eV]

Efficiency Silicon K−Edge

Oxygen K−Edge

Figure 5.12: Measured quantum detection efficiency of a pn-CCD with similar proper-ties to the pn-CCD employed in the STXM [29]. Points: measured quantum detection efficiency. Line: calculated efficiency fitted to measured points.

pn-CCD (see Fig. 5.12). The QDE is affected mainly by the absorption of the SiO₂ entrance window, so silicon and oxygen K absorption edges are visible. A function taking into account the absorption and charge transfer is fitted to the measured points. It gives an extrapolated quantum detection efficiency at the carbon absorption edge (283 eV) of 52 %.

The effectiveness of the pn-CCD anode reset mechanism via the Reset FET has been tested with the STXM setup at a photon energy of 280 eV. Fig. 5.13shows the measured intensity as a function of time for several three different of the Reset FET gate pulses (see Fig.5.3). Without resetting the anode (RFGA HI =−7 V), the measured intensity decays within 200 ms to about half the initial value after the beamline shutter is opened and overshoots into the negative range when the shutter is closed. This effect is attributable to the charge accumulated at the pn-CCD readout anodes. The anodes are read out differentially with the charge before the row transfer to the anode taken as the reference value. If the charge cannot flow off the anode between subsequent readouts, the reference value is larger than zero, and the intensity measured decreases. If the incident photon intensity drops to zero, a positive reference value is subtracted from zero, so the measured intensity becomes negative.

If the Reset FET is triggered, the decay and the negative overshoot can be reduced significantly, but a long positive trail of approximately 10 % of the inten-sity with open shutter appears. This behavior can be explained by an imperfect charge transport of the pn-CCD. It can possibly be remedied by changing the voltages relevant for the pn-CCD charge transport. After the initial decay, the

80 Chapter 5. The STXM detector

0 1 2 3 4 5

0 200 400 600 800 1000

Intensity [a.u.]

Time [s]

Frame number

-3V -5V No reset: -7V

Figure 5.13: Test of the pn-CCD Reset FET mechanism. The detector is illuminated with 280 eV X-rays and read out continuously. The beamline shutter is opened and closed once. The Reset FET gate voltage oscillates between RFGA LOW voltage, which is set to −7 V, and RFGA HI. Shown are the intensities in ADC-counts per frame without active anode reset (RFGA HI = −7 V) and for anode reset with two different RFGA HI voltages.

intensity with a voltage of RFGA HI =−3 V is more than twice as high than the intensity without anode reset.

Fig.5.14 shows a measurement of the pn-CCD linearity for several RFGA HI voltages. The X-ray intensity is varied by changing the air gap between the OSA and the detector vacuum window. With active anode reset, the progression of the measurements deviates clearly from the theoretical curve at higher air gaps (corresponding to lower intensities), so the detector does not behave linearly. This non-linearity could be explained by an incorrect setting of the analog-to-digital conversion time and could be avoided with a modified sequencer configuration.

The maximum photon rate up to which the detector is linear can be calcu-lated from the saturation visible in Fig. 5.14. With an exposure time of 40 ms, saturation is reached at about 2×10⁻⁵ counts/frame. At an exposure time of t_e= 40 ms, one pixel of the annular diffraction pattern (see Fig.5.4) is illuminated with 1/72 of the intensity of the whole frame. Therefore, saturation occurs at 2800 counts/pixel or 1400 photons/pixel at 430 eV, assuming that the saturation affects only these brightest pixels. With continuous readout, where the maxi-mum intensity per pixel is only 1/1646 of the frame intensity, this corresponds to a saturation photon rate of 4×10⁸photons/s at 430 eV or 6×10⁸photons/s at 283 eV.

1×10³ 2×10³ 5×10

800 1200 1600 2000 2400

[Counts / (frame

Air gap [µm]

-5V-6V No reset: -7V Air absorption

1×10⁴ 2×10⁴

800 1200 1600 2000 2400

[Counts / (frame

Air gap [µm]

-5V-6V No reset: -7V Air absorption

Figure 5.14: Test of the pn-CCD linearity with continuous readout (left) and with an exposure time of te = 40 ms (right) for several RFGA HI voltages. Shown are the intensities in ADC-counts per frame normalized to a beam current of 150 mA as a function of air gap. For comparison, a straight line with a slope calculated from the tabulated air absorption [31] is shown. The detector is illuminated with X-rays of an energy of 430 eV.

Chapter 6

First Measurements with the