Study of hit detection efficiency

Test beams offer the possibility to measure the hit detection efficiencyof DEPFET modules using a tag and probe technique. The approach followed here is to reconstruct tracks in the telescope requiring hits on the six Mimosa26 sensors only (tag). Then, we can probe whether we find a hit on the DEPFET sensor from the same particle close to the predicted impact point on the device under test. A complication of the efficiency measurement is the rather long integration time of the Mimosa26 planes (114µs) compared

u [mm]

Figure 5.12.: Left: Number of telescope tracks with at least5hits in the six Mimosa26 sensors shown in the local u, v coordinates of the tested DEPFET module (B). The size of the trigger area was adjusted to the sensitive area of the tested sensor (black rectangle). The density of primary tracks peaks at the position of the tested sensor while secondary tracks are scattered over the entire acceptance of the telescope. Right: Distribution of the number of telescope tracks per event. Only events with a single track in the entire telescope are used for the efficiency measurement.

to the much shorter integration time of the DEPFET module (4.8µs). Only the first track which triggers the readout is guaranteed to be in the integration time of both the Mimosa26 planes and the DEPFET module. Secondary tracks crossing the telescope within the integration time of the Mimosa26 planes but more than4.8µs after the trigger cannot be detected on the DEPFET plane but will be reconstructed in the telescope. In other words, secondary tracks hitting the sensitive area of the DEPFET module bias the measured hit efficiencydownwards. The approach followed here is to suppress the effect of secondary tracks by vetoing all events with more than one track in the telescope acceptance³.

The efficiency measurements were performed for Module B from a sample of120GeV pion tracks measured at CERN, see Fig. 5.12. The track reconstruction requires five hits on Mimosa26 planes with a trackχ² value below100in the fully aligned telescope. The number of tracks per trigger hitting the small active area of Module B was significantly enhanced by integrating a further trigger plane based on the FE-I4 chip into the TLU as described in Obermann [77]. The main advantage of the new trigger plane is the possibility to trigger on particle tracks traversing an adjustable trigger area which can be positioned around the active sensor area of the device under test. A total number of0.6million tracks was available for measurements of the hit detection efficiency in single track events. A hit on the device under test was matched to the particle track if the track residuals were below100µm in theu andv directions.

Fig. 5.13 shows the two dimensional map of efficiencies for all pixels on the sensor. The statistical errors for a pixel by pixel map are in the order of1 %limited by the rather small number of300tracks

3The optimal solution to this problem would be the installation of another (reference) DEPFET module in order to tag the first track in an event. However, the available DAQ system was limited to operate a single DEPFET module in the test beam.

columns

Figure 5.13.: Measured efficiencyfor all32×64pixels of Module B. The average number of tracks per pixel used for the measurement is300and results in a statistical uncertainty of∼1 %. A total of four rows and parts of one column are masked (white pixels). Apart from masked pixels the measured efficiency is uniform over the sensor area within the statistical errors.

per pixel. Despite the limited statistical accuracy, the two dimensional map demonstrates that only few pixels were masked and shows a uniform hit efficiency over the whole sensor area. A statistically more accurate measurement of the hit efficiency is given Fig. 5.14 where hits in the same pixel column or row are accumulated. The measured hit detection efficiency is better than99.5 % for all working pixel columns and rows. The efficiency drop in the outermost two columns to 99.2 % is still under investigation. The expected hit detection efficiency from the detector simulation is99.7 %and agrees well with the experimental result.

In Fig. 5.15 the detection efficiency for pions under perpendicular incidence is shown as a function of the threshold on the signal in the seed pixel (the pixel with largest signal in the cluster). The minimum value of the seed threshold is given by the smallest usable zero suppression threshold. The zero suppres-sion threshold used here is3LSB or525electrons (Module B). The hit detection efficiency is measured

columns

Figure 5.14.: Measured efficiencyfor Module B averaged over columns (left) and rows (right). Apart from columns (rows) with masked pixels, the efficiency is>99.5 %in the whole sensor area. The efficiency drop to99.2 %in the outermost is still under investigation.

seed threshold [e]

0 2000 4000 6000 8000 10000 12000

efficiency

Figure 5.15.: Measured efficiency for Module B averaged over the whole sensor area for different thresholds on the seed signal (left). The minimum usable seed threshold was3LSB or525 electrons limited by the smallest usable zero suppression threshold. The measured effi-ciency is>99.5 %for a seed threshold of525electrons but falls below99 %for thresholds larger than1200electrons (right).

for seed thresholds from525electrons to roughly12000electrons. The statistical error on the efficiency is<0.1 %for all seed thresholds. The main experimental error is the calibration of the least significant bit in terms of an equivalent number of electrons. The most important observation is the steep decrease of the hit efficiency for thresholds larger than1200electrons. The origin of this effect will be discussed in more detail in the next section.