7. Irradiation Experiment 109
7.8. Radioactive Source Measurements
7.8. Radioactive Source Measurements
To study the performance of the overall system, measurements with radioactive sources were done as the last measurement before the X-ray tube was activated again. These measurements allow to check the working point of the DEPFETs by looking at the MPV of the cluster charge distributions. In addition it it possible to determine the signal-to-noise ratio. Table7.4 lists the radioactive sources used during the irradiation campaign together with their radiation type and energies. For these measurements the sensor was
Isotope Radiation Type Energy [keV]
90Sr β 546 (max.)
109Cd γ 22.16
Table 7.4.: Radioactive Sources used during the irradiation.
not removed from the X-ray machine cabinet. The radioactive sources were placed a couple of centimetres above the sensor in a central position. For the data taking no external trigger was used. Instead the sensor was continuously read out at a frequency of 10 kHz. A hitmap of a measurement with a 90Sr source can be seen in fig. 7.21 One
Figure 7.21.: Hitmap of the PXD after a 90Sr measurement at the beginning of the irradiation. The beam-spot of the source is clearly visible and the sensor as well as the deactivated ASIC pair. The two yellow lines correspond to two damaged (electrical) gates of the sensor.
of the visible features are the two segments along the x-axis with different pixel pitches.
At the bottom side of the figure one can also identify the positions of the switchers and additional capacities that seem to at least partly shield the DEPFET matrix from the radiation. The third DCD-DHP was not read out during all measurements because a high noise was seen that made a stable readout impossible due to a too high occupancy3. Using the same clustering strategy as described in section 6.4, the recorded data was processed and analysed.
Figure 7.22 shows two cluster charge distributions recorded with the sensor at the beginning and at the end of the irradiation. Both distributions show the expected
3Each DHP can only handle an occupancy of≈3%
Figure 7.22.: Normalised cluster charge distributions before and after the irradiation campaign. For both measurements a90Sr source was used. Reprinted with permission from [87].
Landau distribution but are slightly shifted relative to each other because of a different working point setting.
7.8. Radioactive Source Measurements 7.8.1. Energy Calibration and DEPFET Working Point Evolution
Using two different readioactive sources, it is possible to calculate the total gain of the sensor (DEPFET amplification + DCD amplification). To do this, the energy of the source has to be known and the sensor’s response to it. For aγ source the energy mea-sured by the sensor has a gaussian shape with an MPV corresponding to the energy of the photons. For β radiation with a continuous spectrum, this is more difficult. The energy deposition in Si detectors by charged particles can be described by a Landau distribution. The MPV of the Landau depends on the energy of the particle and the thickness of the material. To correctly model the response of the sensor, a Geant4 sim-ulation was created modelling a simplified version of the sensor (75µm Si block) and electrons coming from a particle gun with energies according to the90Sr spectrum. The
90Sr spectrum was obtained via BetaShape [100] software, which is based on a theoretical model described in [101]. The expected shape of the spectrum measured by the sensor can be seen in fig.7.23. From this simulation an MPV value of 19.4keV was extracted.
With this translation of the MPV values measured by the sensor in ADU to energies in keV, the total gain of the sensor was calculated by performing a linear fit. This analysis was done for every source measurement made during the irradiation. Figure7.24 shows an example of a fit to the data at the end of the irradiation. In order to see a possible gradient of the gain on the sensor, the analysis was done for different regions on the sensor as well. A map of the sensor before the irradiation can be seen in fig.7.25. There is a visible gradient along the rows of the sensor, pixels in the switcher regions furthest away from the DCDs have the highest gain. This tendency was seen on other sensors as well during laboratory measurements.
As described in section 7.4, the target for the threshold shift compensation was the source current measured by the LMU power supply. This was meant to guarantee a constant working point of the DEPFETs. Figure7.26however shows that the total gain of the sensor was not constant during the irradiation. The average gain (over the whole sensor) was ≈ 1.4ADU/keV at the beginning of the irradiation and ≈ 1.65ADU/keV at the end. This behaviour indicates that the threshold shift was over-compensated by the readjustment of the gate voltage. It is important to remember that the gain of a single DEPFET cell is not affected by the irradiation, only the working point is shifted.
Through the LMU power supply only the mean source currentI˜D is known, which is the mean over the drain-source currents of all gates.
I˜D ∝ 1
Keeping I˜D constant does not necessarily guarantee that the mean gg ∝ VG−Vthr is constant as well. This is especially the case when the threshold shifts Vthr,i of the gates vary, which is true due to the inhomogeneity of the irradiation.
Because only a global adjustment of the gate voltage was performed to keepI˜D
con-stant, the working points of the DEPFETs across the sensor were shifted relative to each other. This can be seen clearly when looking at the evolution of the gain, shown in fig. 7.27. The figure shows the gains of the six switcher regions across the sensor.
The regions at the border of the sensor, where less dose was received, show a larger gain increase than the central region.
At Belle II no such strong inhomogeneity in received dose is expected. Therefore, using the mean source current to keep the working point constant should be more reliable than in this measurement campaign.
Figure 7.23.: Deposited energy of a90Sr beta source in 75µm Si, simulated with Geant4.
7.8. Radioactive Source Measurements
Figure 7.24.: Energy calibration at the end of the irradiation at≈210kGy.
Figure 7.25.: System gain across the sensor at the start of the irradiation. The sensor is divided into ASIC/Switcher regions. The DCD-DHP pairs are located on the left side and the Switchers on the bottom side of the figure. No data of the third DCD-DHP pair was taken during the radioactive source measurements.
Figure 7.26.: Evolution of the energy calibration during the irradiation.
Figure 7.27.: Relative gain as a function of time.
7.8. Radioactive Source Measurements 7.8.2. Signal-to-Noise Ratio
One of the most crucial observables by which the performance of the PXD is quantified is the signal-to-noise ratio (SNR), which can be estimated by using radioactive sources.
The calculation of the SNR was done in the same manner as for the phase 2 PXD data, described in section6.4.3. Figure7.28shows two examples of these SNR distributions at the beginning of the irradiation. As the noise across the sensor was rather homogeneous and stable over time, the form of the distribution is very similar to the cluster charge distribution.
As the SNR depends on the working point of the sensor, the same segmented analysis which was done for the sensor gain was repeated for the SNR as well. A map of the SNR across the sensor at the beginning of the irradiation can be seen in fig.7.29.
The evolution of the SNR, which can be seen in Figure7.30, is in turn very similar to the evolution of the system gain, discussed in section7.8.1.
(a)90Sr (b)109Cd
Figure 7.28.: Signal to noise histograms for90Sr (left) and109Cd (right) with fitted Lan-dau and Gaussian distribution, respectively.
Figure 7.29.: SNR across the sensor at the beginning of the irradiation. The data was recorded with a 90Sr source.
Figure 7.30.: Evolution of the signal-to-noise ratio during the irradiation. Reprinted with permission from [87].