First timing measurement with TCT

Today I was able to finally perform the very first timing measurement with our TCT at UZH. In this post I will describe the setup and the results.

Setup description

Below there is a block diagram of the setup:

The pulsed infrared laser^[1]1064 nm, part of our Particulars TCT. is coupled to an optic fiber. This optic fiber then reaches a 50 %/50 % splitter. One of the outputs of this splitter is sent to a 20 meters long optic fiber and then to a mixer, the other output is directly fed in the mixer. As a result we obtain two “identical” pulses with a time difference of 100 ns^[2]For more details on this laser splitting system please see this post.. After this, the laser is shined onto the LGAD. The readout is done using an amplifier^[3]4 GHz, 36 dB, in-house design. and an oscilloscope^[4]Lecroy WaveRunner 9254M, 4 GHz, 40 GS/s..

The device I used for this measurement was

• LGAD AIDA-2020 V1 RUN 11478 W5-DA11 (alias Speedy Gonzalez).

This LGAD was characterized previously using the beta setup and it showed a time resolution of 27 ps^[5]More details on this characterization in this post..

Calibration of laser intensity

In order to perform a realistic timing measurement, the laser intensity has to be properly selected. The reason for this is that the response of the device is different if the number of electron-hole pairs produced changes. The LGADs are intended to be used with minimum ionizing particles (MIP) which are charged particles with \(\beta \gamma \gtrsim 3\) where \(\beta\) and \(\gamma\) are the Lorentz factors. Photons are by no means MIP particles since they have no electric charge. Thus, with the laser we can only mimic the scenario of a MIP particle, but it will never be exactly the same^[6]For more details on MIP particles vs pulsed infrared laser see this post and this post..

To perform the laser intensity calibration, I first exposed the LGAD to beta radiation from an Sr-90 source. After this, the device was exposed to the pulsed infrared laser without changing anything else but the type of radiation. So basically I recorded data in the following two conditions:

Note that everything is the same with the exception of the radiation used to excite the LGAD. Using the data from both configurations I calculated the collected charge and obtained these results^[7]This plot was taken from this post where I compared the response of silicon devices when exposed to MIP radiation and to a laser.:

The distribution of the collected charge with the beta source is shown in black. It follows something similar to a Landau distribution. The other three distributions were obtained with different laser intensities. The laser intensity is controlled by a parameter called “laser pulse width” (LPW), it ranges from 0 % to 100 %, and high values decrease the intensity of the pulse (yes, it is not intuitive). These distributions have a Gaussian shape (please zoom into the plot). From this plot we see that the pink and blue distributions, with LPW of 66.2 % and 66.3 % respectively, provide the best approximation to the distribution with the beta source. Thus, for the timing measurement I decided to use 66.2 %.

Results

I measured the time resolution in two ways:

1. A quick “on the fly” measurement using the oscilloscope.
2. A full offline analysis like we do for the beta setup.

Quick measurement using the oscilloscope

Having set a laser intensity corresponding to LPW = 66.2 %, I proceeded with the timing measurement. I configured the oscilloscope in order to capture the two pulses with 100 ns time separation at each trigger. This is shown below:

For a quick test I set up a timing measurement in the oscilloscope (shown also in the previous image). This was configured to measure the time difference between the rising edge of each pulse, as seen. The time resolution is given by the fluctuations of this time. If we assume that the fluctuations in the time it takes the light to travel through the 20 m optic fiber are negligible then all the fluctuations in the measured time difference are due to the LGAD itself^[8]Here I am also assuming that the fluctuations due to the electronics (amplifier and oscilloscope) are negligible.. Assuming also that the two laser pulses are identical^[9]This is not strictly true since the second pulse is attenuated after having traveled 20 m extra in optic fiber. The attenuation with respect to the first pulse is around 20 %. then the fluctuations due to the LGAD are simply

Using \(\sigma_\text{measured} = 40.6 \text{ ps} \) (see previous image) the time resolution of this LGAD is \(\text{time resolution} = \sigma_\text{LGAD} = 28.7 \text{ ps}\).

This is really close to the \(27 \text{ ps}\) obtained previously with the beta setup^[10]See this post.. The advantage of the measurement with the laser is that it takes much less time (if the intensity does not has to be calibrated for each new device).

Full offline analysis

I also performed a “full offline analysis” of this data. For this I just used the “auto save” function of the oscilloscope to record each waveform. Then I performed the typical procedure we use when we measure two detectors in the beta setup. I looked at the distribution of the time difference for different values of the “constant fraction discriminator” and observed the fluctuations. I obtained this:

The minimum value is for \(k_\text{CFD} = 0.9\) for both the first and the second pulse^[11]This is consistent with our previous characterization of this LGAD using the beta source.. The time resolution at this point is \(27.9 \text{ ps}\) as shown in the previous plot. The time difference at this point is distributed like this:

This result is, again, in very good agreement with our previous characterization of this LGAD using the beta setup.

Conclusions

The first timing measurement of an LGAD device using a TCT (in our lab at UZH) was done. The measurement was done on a device which was previously characterized with the beta source in order to compare both results. The agreement between the two methods is quite good; we obtained a time resolution of \(27.3 \text{ ps}\) using the beta setup and today a time resolution of \(27.9 \text{ ps}\) using the TCT setup. The advantage of the TCT is that it is much faster in doing this measurement as compared to the beta setup (10 minutes vs 3-4 hours), and also that it does not require two identical devices: It can be used to measure on a single device. This result opens the door to perform spacial-timing measurements in 4D pixel detectors.