The document analyzes data from beam tests of silicon diodes for the High Granularity Calorimeter (HGCal) at CERN. It characterizes the time rise and time over threshold of diodes with different thicknesses and irradiation levels. The analysis shows that time rise and time over threshold decrease with increasing irradiation. For a given irradiation level and thickness, n-type diodes have higher values than p-type diodes. Comparing unirradiated diodes, both metrics increase with thickness. The trends are important for understanding diode performance under the harsh conditions expected at the HL-LHC.

1. Pulse Shape Characterization of Silicon Diodes for HGCal with data
from Beam Test at CERN
Malinda de Silva
CERN Summer Student Report 2016
Supervised by Arabella Martelli

2. 2
Abstract
The High Luminosity phase of the LHC (starting operation in 2025) will provide unprecedented
instantaneous and integrated luminosity, with 25 ns bunch crossing intervals and up to 140 pileup
events. A challenge is to provide excellent physics performance in such a harsh environment to
fully exploit the HL-LHC potentialities and explore new physics frontiers. In this context, the High
Granularity Calorimeter is the detector designed to provide electromagnetic and hadronic energy
coverage and reconstruction in the forward direction of the upgraded CMS. In April 2016 and June
2016, a set of 36 diodes were tested in order to understand various characteristics of its
performance, in order to use them in the upgraded HG Calorimeter. Here, the silicon diodes were
mounted onto a test bench at CERN’s beam test area and exposed to electron showers. Data
received from these diodes were acquired and analysed separately. The objective of this report is
to show the variation of Time Rise, Time Over Threshold with various parameters such as Signal
over Noise Ratio, Irradiation level and Thickness. Time Rise is defined as the time it takes by the
signal to increase its intensity from 10% its peak intensity to 90% its peak intensity value. The
Time Over Threshold here is defined as the time the signal spends over 50% of its peak intensity
value. Given below are the irradiation levels of diodes used in this test.
Methodology for the Analysis
The data used for the analysis was obtained from the results of experiments conducted in April and
June for all diodes (or Pads). Pad 1 and Pad 2 represents the non-irradiated diodes while the rest
of the 4 diodes are irradiated to fluences in the order between 1014
to 1016
as shown in the table
above. The Estimator under study (Time Rise or Time over Threshold) was binned as a function
of various levels of Signal over Noise and fitted with a Gaussians Distribution for each bin to
extract the mean. Summary plots with the obtained trends for Time Rise and Time over Threshold
are shown in Figure 1, 2, 3 and 4 for n Type and p Type diodes. By such plots various conclusions
on the estimator characteristics can be derived.
Type Active Thickness (microns) Fluences/ Irradiation
P-on-N 140 0 0 6.25 x 1015
6.25 x 1015
1.0 x 1016
1.6 x 1016
P-on-N 220 0 0 1.5 x 1015
2.5 x 1015
2.5 x 1015
4 x 1015
P-on-N 290 0 0 4 x 1014
6 x 1014
6 x 1014
9 x 1014
N-on-P 140 0 0 6.25 x 1015
6.25 x 1015
1.0 x 1016
1.6 x 1016
N-on-P 220 0 0 1.5 x 1015
2.5 x 1015
2.5 x 1015
4 x 1015
N-on-P 290 0 0 4 x 1014
6 x 1014
6 x 1014
9 x 1014

3. 3
It should be noted that data for Pad 2 show bad quality due to a problem in the readout cable and
connection. Results for Pad 2 are thus unavailable.
Results
(a) (b)
(c)
Figure 1: Summary Plot of Time Rise vs Signal Over Noise Ratio for n Type diodes of different Thicknesses.
Pad 1 is un-irradiated while Pads 3,4,5,6 are irradiated. (a) for 120 um thickness, (b) for 200 um thickness,
(c) for 300 um thickness
(a) (b)

4. 4
(c)
Figure 2: Summary Plot of Time Rise vs Signal Over Noise Ratio for p Type diodes of different Thicknesses.
Pad 1 is un-irradiated while Pads 3,4,5,6 are irradiated. (a) for 120 um thickness, (b) for 200 um thickness,
(c) for 300 um thickness
(a) (b)
(c)
Figure 3: Summary Plot of Time Over Threshold Vs Signal Over Noise Ratio for n Type diodes of different
Thicknesses. Pad 1 is un-irradiated while Pads 3,4,5,6 are irradiated. (a) for 120 um thickness, (b) for 200
um thickness, (c) for 300 um thickness

5. 5
(b) (b)
(c)
Figure 4: Summary Plot of Time Over Threshold Vs Signal Over Noise Ratio for p Type diodes of different
Thicknesses. Pad 1 is un-irradiated while Pads 3,4,5,6 are irradiated. (a) for 120 um thickness, (b) for 200
um thickness, (c) for 300 um thickness
Discussion
As seen in Figures 1 and 2, the main observation which could be given is that the trend of the Time
Rise is flat above 30% the Signal over Noise. However, for low SNR ratios, the trend deviates
from the expected results. This is mainly due high noise seen in bin 1 (lowest SNR) and few
subsequent bins of all diodes and hence deviating from Gaussian Distribution.
However as seen in Figure 3 and Figure 4, the trend is completely flat for Time over Threshold as
a function of Signal over Noise.
Another observation seen is the fact that when the fluency was increasing, the Time Rise as well
as Time over Threshold values reduced. This may be due to higher damage of the depletion region
of the silicon diodes with a reduction of the effective depletion region.

6. 6
With rising fluence, the number of bins also reduced. This means that higher SNR values were not
obtained when the irradiation is high. It was also observed that p Type diodes had a lower mean
value than n Type diodes. It is consistent with the measurements of the depletion region of the p
Type diodes which were slightly thinner than the n Type diodes.
For un-irradiated diodes, the Time Rise and the Time Over Threshold increases as a function of
thickness. However due to different irradiation levels, direct comparisons cannot be made for the
rest of the diodes. Thus the estimators were studied at as a function of irradiation for different
thicknesses.
Results
Figure 5: Time Rise vs Irradiation Graph. Line represents Pad 1 value while Pad 3,4,5,6 values proceeds
in increasing irradiation order. Each colour represents a specific thickness of a specific type (n type or p
type)

7. 7
Figure 6: Time Over Threshold Vs Irradiation Graph. Line represents Pad 1 value while Pad 3,4,5,6 values
proceeds in increasing irradiation order. Each colour represents a specific thickness of a specific type (n
type or p type)
Discussion
In Figure 5 and Figure 6, Time Rise and Time over Threshold are plotted as a function of irradiation
respectively. The values plotted for the y axis corresponds to the mean values of Time Rise and
Time over Threshold for SNR greater than 30% where the trends are flat and stable.
As observed, all p Type diodes have lower Time Rise and Time Over Threshold than the n Type
counterpart. So the Time Rise and Time Over Threshold decreases with the increase in irradiation
on a given set of diodes of same thickness.
Conclusions and Summary
In this project a set of Silicon Diodes were tested in order to provide a characterization of Time
Rise and Time over Threshold for 3 different thicknesses at different levels of increasing irradiation.
From Figure 1 and 2, conclusions on Time Rise was made with respect to SNR for various
thicknesses. Time Rise trend is overall flat over 30% Signal over Noise Ratio. Similarly, it was
observed that with the increase in fluence, the Time Rise reduces.
Figure 3 and 4 showed that Time over Threshold also has a flat trend as a function of signal over
noise ratios. Also, the Time over Threshold increases with the increase in fluence.
In order to compare results, these were plotted with respect to fluence, in Figure 5 and 6.

8. 8
It was observed that for a given radiation level and thickness, all n Type pads have higher Time
Rise and Time over Threshold than p Type ones. When the radiation level increases however the
Time Rise as well as the Time Over Threshold of diodes with same thickness decreases.
Comparing only pads un-irradiated, it can be observed that both Time Rise and Time Over
Threshold values increase with the thickness.