Test and characterization of SiPMs for the upgrade of MEGII high resolution Timing Counter

The MEG II experiment based at PSI (Zuerich, Switzerland) has been committed and is taking data since 2021 to improve sensitivity on the decay μ+ → e+ γ. The pixelated Timing Counter (pTC), consisting of two arrays of 512 5 mm thick scintillator pixel each, read out by 6 3×3 mm2, 50 μm cell, Silicon Photomultipliers (SiPMs) from AdvanSiD, for a total of 6144 SiPM, achieves an overall resolution in the positron impact time of ∼43 ps when exploiting multiple measurements. To additionally improve this resolution, 4×4 mm2, 40 μm cell SiPMs have been selected to substitute a fraction of the old ones (about 1000 of them overall). By means of an automated test system, a first group of them has been characterized (measuring their breakdown voltage and their I-V curves), to match as much as possible SiPMs with the same gains in each pixel, in order to maximize the pixel time resolution. Such automated test system will be presented, together with some preliminary results on single pixel time resolution and the expectations of the average time resolution of the pTC in the coming years.


The MEGII experiment
Within the Standard Model, the flavor of charged leptons is conserved in very good approximation, and therefore charged Lepton Flavor Violation is expected to be unobservable.On the other hand, many physics models beyond the Standard Model predict charged Lepton Flavor Violation within the experimental reach, and therefore processes like μ + →  + γ decay are probes for testing new physics.The MEG experiment, at the Paul Scherrer Institute (PSI, Switzerland), has been searching for the μ + →  + γ, down to a Branching Ratio (BR) 5 • 10 −13 , exploiting the most intense continuous muon beam in the world and innovative detectors.After the conclusion in 2013 of the data taking with the first version of the detector [1], an upgrade (MEGII) with improved performances has been designed [2], built, and now it is operational in the phase of data taking.An improvement of the sensitivity by one order of magnitude is expected.
In figure 1 the layout of MEGII detector is shown.-1 -

MEGII Timing Counter
The main task of the pixelated Timing Counter [3,4] (pTC) in the MEG II experiment is to measure the positron time of arrival with an accuracy of 30 ps.In figure 2, the Timing Counter is shown.The pTC is composed of 512 pixels and each of them consists of a 5 mm thick plastic scintillator tile and of two arrays of six SiPMs, in series connection, attached at both sides as shown in figure 3.  -2 -

JINST 19 C03004 2 The upgrade of MEGII pixelated Timing Counter
The SiPM chosen for the original pTC of the MEG II experiment are the AdvanSiD, model ASD-NUV3S-P.It is a 3×3 mm 2 device, 50 μm cell, surface mount version.To improve time resolution we decided to use the model ASD-NUV4S-P which is a 4×4 mm 2 device, 40 μm cell.The active surface has been therefore increased by 78%, which should guarantee a better time resolution.

Characterization of SiliconPMs
We purchased 100 of ASD-NUV4S-P devices for test.In order to perform quality control and to precisely define the working parameters of the SiPMs during the experiment, each SiPM must be characterized.In particular the I-V curve should be measured in order to extract the breakdown voltage.In fact a good time resolution is obtained when all SiPMs on the same pixel have the same gain G = CV OV , where C is the device capacitance (practically the same for all SiPMs) and V OV is the difference between the bias voltage and the breakdown voltage.With this goal in mind, we developed an automatic test system that can measure sequentially the I-V curve and some parameters of 32 surface mount SiPMs.
The temperature of all SiPMs in the test system can be regulated in the range from 10 • C to 50 • C for all the 32 devices with a resolution of 0.1 • C and with a stability smaller than 0.2 • C over one day.

The SiPM automated test system
As can be seen in figure 4  -3 - The SiPM tray consists of a copper support that can host 32 surface mount SiPMs in appropriate housing (hole).As displayed in figure 5, left, devices are inserted with the window downward.In order to protect the optical face, a plastic black foil is added between SiPM and copper.Once the devices have been inserted, another copper support is mounted on the first one.It contains the sharp spring contacts (see figure 5, right) that touch SiPM pads.Each couple of spring contacts is connected to the electrical switch; therefore each device can be connected one at a time to the Picoammeter/Voltage Source, that sequentially increases the bias voltage and measures the dark current produced by the SiPM.In this way, it is possible to get the "reverse" I-V current curve and, from it, the breakdown voltage at the selected temperature.Once tested a device, the PC switches to the following one and starts again the measurement.Before starting the measurements it is important to set the temperature and wait until the selected value is reached by means of heater resistor and/or Peltier cells.In the same way, applying direct polarization it is possible to measure the quenching resistor.Moreover, by making the I-V curves at various temperature it is possible also to get a "breakdown voltage temperature coefficient".

Results
Using the automatic test system, 100 detectors have been characterized measuring the I-V curve at a fixed temperature of 30 • C: the curves for all the SiPM tested are shown in figure 6.The breakdown voltages were calculated from those I-V curves.By means of these values, some SiliconPMs electronic boards have been produced and mounted on scintillators in order to get some tests pixels.-4 -Those pixels have been characterized in terms of time resolution in order to get a comparison with the previous ones.
In figure 7 it is possible to see the comparison between the time resolution of pixels with old 3×3 mm 2 , 50 μm SiPMs and the one obtained with the new 4×4 mm 2 , 40 μm devices.With the 3×3 mm 2 SiPMs a time resolution for the single pixel of ∼100 ps was achieved, which leads to a pTC time resolution for the determination of the crossing time of the positron of ∼43 ps once the measurements in all pixels crossed (∼9 on average) are combined.Analogously, with the new 4×4 mm 2 SiPMs, where a time resolution for the single pixel of ∼70 ps can be achieved, the crossing time of the positron is expected to be measured with a resolution down to ∼30 ps.In figure 8 it is presented the distribution of the breakdown voltages for all the 100 SiPMs tested.The bimodal distribution is related to SiPMs produced on different batches.-5 -

Conclusions
A sample of 4×4 mm 2 SiPMs from AdvanSiD have been tested by measuring their I-V curves and breakdown voltages with the automated system developed for MEG.The time resolution of a pixel equipped with these SiPMs has been also measured, obtaining a time resolution ∼30% better than the one obtained with 3×3 mm 2 SiPMs, in accordance with the expectations based on the ratio of the active areas.On the basis of these tests, we will procede to a partial upgrading of the pTC pixel installing ∼100 pixels equipped with 4×4 mm 2 SiPMs with the improved time resolution.

Figure 3 .
Figure 3.The single pixel of the pTC: scintillator covered by a diffusive (and light protection) material and SiPM electronic board with SiPM.SiPM are connected in a series configuration.
, the system is based on a PC controlled (USB) Keithley Picoammeter/Voltage Source model 6487, an home made (based on GEFRAN 1200) temperature controller able to drive both heater resistor and Peltier cells, a PC controlled (by means of USB National Instruments Analog-Digital I/O model NI USB-6008), an home made 32ch electrical switch (Device Under Test selector) and finally a 32 SiPMs housing system support.

Figure 5 .
Figure 5. Left: the housing for SiliconPM on the tray (SiPM are mounted with contacts upside); right: the spring contacts for touching SiPMs pads.

Figure 7 .
Figure 7. Time resolution comparison between the old and the new Timing Counter Pixel.The pTC time resolution for positron timing (43 ps) is obtained combining the pixels crossed by the positrons (∼9 on average).

Figure 8 .
Figure 8. Breakdown voltage distribution of ASD-NUV4S-P Advansid SiPM.The bimodal distribution is related to SiPMs produced on different batches.