Readout of the CGEM-IT

In parallel to the validation with the planar GEMs, as soon as the different compo-nents of the CGEM-IT (CGEM detector layers, on-detector and off-detector electronics) were produced and individually validated, integration tests have been carried out in order to assess the functionality of the full system. Figure 4.35 shows the Layer 1 of the CGEM-IT assembled and fully instrumented at the INFN Laboratories of Ferrara, where the CGEM detectors have been developed and characterized.

Figure 4.35: CGEM-IT Layer 1 fully instrumented at Ferrara INFN laboratories. The de-tector is readout by 16 Front-End Boards (32 TIGER) connected to 4 GEMROC (bottom right of the figure) through the LV-DATA Patch Cards (below the CGEM); in the top right corner a small part of the water-cooling system can be spotted.

Compared to the DAQ system used in the beam tests, here, due to the higher number of electronics channels to be managed, the TIGER chips are controlled and readout by the GEMROC modules. Still no GEM Data Collectors are employed, so the GEMROCs are directly readout by the PC running the acquisition through its Ethernet port (UDP communication).

Automatic tools to monitor the system status and configure the TIGER chips have been developed. The delays for the LVDS links TX and RX alignment are automatically set in the optimum point for bug-free data transmission. Custom Python scripts allow to perform threshold scans on every channel and set each threshold to a value specified

by the user. These settings can be then tested in a short acquisition to evaluate the noise occupancy on each channel and eventually fine tune the threshold of screamer channels [65]. For this purpose, a Graphical User Front-end Interface (GUFI, [73]) to select the configuration of the GEMROC modules and the TIGER chips and run and monitor the acquisition has been developed. This software will be later integrated in the BESIII CGEM slow control utility for the experiment run operations.

Figure 4.36 (left) shows the results of a threshold scan performed on the 64 channels of one chip. From this kind of scans the noise level for all the channels of the CGEM-IT Layer 1 has been evaluated. The results are displayed in Figure 4.36 (right). It can be observed the different distribution for X and V strips. The former have all the same length and thus the same capacitance, hence the noise level should be similar for all the strips, while the latter have very different lengths and thus a broader noise distribution.

The longest V strips are similar to the X strips, as this measurement seems to confirm.

Figure 4.36: 64-channel threshold scan (left) and noise distribution measured on the CGEM-IT Layer 1 for X and V strips [73].

During this phase, also the synchronous operation of more than one GEMROC mod-ule was assessed. For this test, an external pulser was employed to generate the reset and trigger signals which were then propagated from the master GEMROC to the three slave GEMROCs.

In order to complete the readout chain validation, interconnection tests between the GEMROC and the GEM-DC have been carried out in February 2019. The setup is shown in Figure 4.37 and includes a FEB controlled by one GEMROC module, coupled to a GEM-DC installed on a VME crate together with a VME-USB Brigde CAEN V1718 board for the communication with the DAQ PC.

These tests proved the correct functionality of the bi-directional communication between the GEMROC module and GEM-DC over optical fiber link at 2 Gb/s. GEMROC

Figure 4.37: GEMROC-GEM DC interconnection test setup.

trigger-matched data packets are correctly transmitted to the GEM-DC and then sent to the PC through the VME. GEM-DC control signals allow to properly read and write the GEMROC configuration registers, while the TIGER configuration is handled through an Ethernet connection. Preliminary tests of the GEM-DC “event building” function have been carried out, some minor revisions to adjust this feature to the CGEM-IT data format are now ongoing.

Instrumentation of the full CGEM-IT detector

The three layers composing the CGEM-IT and all related electronics have been shipped to IHEP (Institute of High Energy Physics, Beijing) in the second half of 2018.

The integration of the full system started in November 2018.

Figure 4.38 shows the IHEP laboratory in which the different CGEM layers are mounted, instrumented and tested. The three layers are firstly tested separately to as-sess their behavior after the shipment. GEM detectors, like most gaseous detectors, need some gas and HV conditioning in order to reach their nominal operation values. In par-allel the connectivity of the detector HV and the on-detector electronics LV and data is validated. After that, they are assembled together for tracking measurements using cosmic rays and sources.

Figure 4.38: IHEP laboratory where the CGEM-IT is assembled. Layer 1 is mounted on the mechanical system used to insert L1 inside L2 and then inside L3. In this picture also two prototype CGEM layers built during the development of the CGEM-IT are present.

Figure 4.39: Test setups for Layer 2 stand-alone (left) and Layer 1 and 2 assembled to-gether (right).

Figure 4.39 (left) shows the setup for the Layer 2 characterization, while the system incorporating the first two layers is displayed in Figure 4.39 (right). In both cases, the external trigger is provided by two scintillators placed above and below the detector.

In order to simulate the BESIII L1 trigger latency, a 8.6 μs delay is applied to the scin-tillators coincidence signal before it is sent to the GEMROC modules which produce trigger-matched data inside a programmable 1.5 μs trigger time window. The trigger signal is also injected into one unused TIGER channel, set in TDC mode, to check the proper functioning of the GEMROC trigger-match operations and provide a global time reference.

Figures 4.40 and 4.41 show the noise level for the Layer 2 of the CGEM-IT. On one hand, the noise level of the X strips is flat along all the strips, except for some outliers, since they all have the same capacitance. On the other hand, the noise level of the V strips is not flat and follows the strips length profile. In general, V strips show a lower noise since their width is much smaller (130 μm vs 570 μm). The presence of groups of outliers implies that the system still needs to be optimized. The sectors showing this increased noise are currently being investigated from the point of view of the detector itself and its related electronics. Some improvements have already been observed by enhancing the grounding connection between the detector and the Front-End Boards.

Strip X

0 200 400 600 800 1000 1200 1400

noise level [a.u.]

Noise Layer 2 (strip X)

Strip V

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

noise level [a.u.]

Noise Layer 2 (strip V)

Figure 4.40: Layer 2 noise level profile for X (top) and V (bottom) strips.

h6

Noise layer 2 (strip X)

h5

Noise Layer 2 (strip V)

Figure 4.41: Layer 2 noise level distribution for X (left) and V (right) strips.

Cosmic rays data acquisition

At the time of this writing, the assembly of Layer 1 and Layer 2 has been completed and cosmic ray data acquisition is currently ongoing. Some very preliminary results provide a first evaluation of the quality of collected data.

The time information of each fired strip is used to reconstruct the particles track inside the GEMs 5 mm drift gap (refer to 1.2.2). In order to estimate the consistency of this time measurement, the difference between the timestamp of the fired strips and the trigger time reference, for a detector gain of 9000, is plotted in Figure 4.42.

Figure 4.42: Strips time distribution around the trigger time for a gain of 9000.

The resulting distribution is fitted using a “Constant + Gaussian” probability density function, obtaining a FWHM of about 125 ns. Considering a drift velocity, 𝑣_𝑑, of 45

μm/ns, it is possible to extract the width of the drift gap as:

𝑑 = FWHM ⋅ 𝑣_𝑑 = 45 μm/ns ⋅ 125 ns ≈ 5.6 mm (4.6) This value is in good agreement with the expected 5 mm drift gap width.

The same measurement is repeated with a different gain setting. The strips time distribution for a gain of 12000 is depicted in Figure 4.43.

Figure 4.43: Strips time distribution around the trigger time for a gain of 12000.

Here we obtain a very similar result, proving that the drift time does not depend on the detector HV. The strips time distribution is constant for two different gain settings and compatible with the gap width. A more complete and precise measurement will be given by the clusterization algorithm which allows to extract and study the time information of each fired strip comprised in one cluster.

For what concerns cosmic rays tracking, the alignment of the two layers and the development of the software analysis tools for the events reconstruction are currently ongoing. In the next months, also Layer 3 will be included in the system and the inte-gration of the full CGEM-IT detector will be completed.