UNIVERSIT . . AT BONN Physikalisches Institut - Prof. Dr. Norbert ...

6.2. Temporal response 73
amplitude changes.
Furthermore, a distinctive rectangular pattern is visible in the integrator image. This
is the result of the stitching of subsequent measurements because during irradiation the
decrease in signal amplitude varies from pixel to pixel. Tiling the individual measurements
to form the large 9 x 6 view then creates the repetitive pattern. Hence, the first two images
illustrate the need for a good long-term stability of the sensor signals.
The effect of a slow response to a varying input signal is demonstrated in the third image
(see Fig. 6.7(c)). This image shows the fluctuations in the current signals on a short
(1 ms) timescale using the dead time free readout of CIX. At each x-y stage position
10 individual snap-shots of that particular image section were taken and the standard
deviation of these 10 measurements was calculated. Although the long-term decrease in
current amplitude is lost in this measurement, distortions at the edges of the cogwheel are
visible. This is because during the imaging procedure the middle section of the cogwheel
was scanned going from left to right. Like this the remainders of high current signals from
outside the object are superimposed over the low signal section of the cogwheel. Similarly,
when moving out of the brass absorber the current signals only slowly return to the high
amplitudes.
6.2.1 Long-term response
The afore mentioned long-term change in the detector response of CdTe and CdZnTe was
studied in detail for different bias and flux conditions. In these measurements the sensors
were irradiated with a constant beam intensity for five minutes. Following this irradiation
period, the sensor signals were recorded for another five minutes without irradiation. The
irradiation interval of five minutes was chosen as it reflects the typical scan durations
when imaging a larger object with CIX 0.2. In order to evaluate the ASIC’s performance
under such measurement conditions, identical measurements were performed on a 300 µm
thick Si sensor. In this context they will serve as a reference since Si is not limited by
comparable temporal instabilities. The focus of this analysis is placed on the integrator
because of its inherent sensitivity to slow changes in the signal amplitude. In contrast to
this, the CIX counter is only influenced indirectly via a preamplifier baseline shift.
Integrator long-term response - Pulse shapes and mathematical description
Fig. 6.8 shows the time dependence of the average pixel current for a Si, a CdZnTe and
a CdTe sensor at a tube current of 30 µA at 6.5 cm focal spot to detector distance and
at high bias settings. As expected, the Si sensors exhibit no significant temporal changes
during or after an irradiation period. This is not true for the CdTe and CdZnTe detectors
where a significant time-dependence of the signals is observed. Note that the curves in
Fig. 6.8 are representative examples, which were confirmed on other samples of the three
sensor materials.
Starting with the CdZnTe detectors, the current under X-ray irradiation IOn can be described
by a combination of an exponential function with a time constant τOn, specifying
the initial amplitude increase, and a linear function, which parameterizes the subsequent
rise. The observed linear runaway of the detector currents under irradiation has recently
been confirmed in a separate study on CdZnTe samples with high µhτh products at high
bias [53]. At the end of the 5 min irradiation period, the current IOff quickly drops down
to a few percent of the current under irradiation and then slowly returns back to the baseline
following an exponential tangent with the time constant τ long
Off . Even on a timescale
of one hour no current undershoot after tube shut-off is observed. The mathematical de-