Electronics

weak signals from the external noise and crosstalk from the pulses driving the LED. The temperature sensor and LED use normal metallic wire cables. Figure 4.9 shows the gathered APD holders with the cables and a complete block module.

The next upper module holds 2×2 block modules with the intermediate board. This is the basic unit for slow control because the intermediate board distributes the slow control signals from the back-end system to each crystal module. Figure 4.10 shows the module with all cables.

(a)APD holder boards and frame with cabling. (b)Complete block module

Figure 4.9:Block module. The brown cables are for the APDs, the blue and white cables are for the LEDs, the red and white cables are for the temperature sensor, and the green cables are for grounding each board.

4.2.6 Feedthrough

The feedthrough mediates the electric signals from the vacuum in the detector solenoid to the outside.

The APD signals are so weak that a preamplifier is necessary; however, it is not preferred to place electronics that generate heat in the vacuum. We carefully designed the feedthrough to minimize external noise and crosstalk contamination.

We attempted to make the feedthrough with the PCB to deal with multiple signal lines. The feedthroughs are made of sturdy metal to maintain the high vacuum pressure; however, it is not trivial to pass many cables. The feedthroughs are also a noise source because of the increase in the number of electrical contacts. Using PCBs as the feedthrough can enable dense wirings and help mount any electronics on the board. Although the PCB cannot shut a high-pressure difference and tolerate its stress, we require only a vacuum pressure ofO(1) Pa at the lowest. Figure 4.11 shows a feedthrough prototype attached to the latest ECAL prototype.

4.3 Electronics

4.3.1 Signal Line

The preamplifier shown in Figure 4.12 is designed for the ECAL to enhance the weak charge currents from 16 APDs to magnitudes that can be read by the readout electronics. Further, the preamplifier comprises two types of components: charge sensitive and transimpedance components. The charge sensitive component is used to amplify an input charge from an APD with a gain of 0.15 V/pC. The

4.3. Electronics

Figure 4.10: 2×2 block modules with the inter-mediate board, to which the slow-control cables from the crystal modules are connected.

Figure 4.11: Feedthrough board made of PCB installed in the ECAL prototype. The pream-plifiers and slow-control transmission boards are mounted.

transimpedance part converts it into a couple of differential waveforms. These differential waveforms can prevent themselves from external noises⁷. The entire circuit does not include an explicit shaping circuit to maintain the original time structure of the LYSO light emission for the pile-up separation.

The trigger line includes an adder circuits that sum the waveforms every four channels of the signal line. Further, it includes a shaping circuit with a time constant of about 100 nsec at present, whereas it may change depending on the studies in the future.

EROS is the ECAL version of ROESTI, and it was introduced in Section 3.3. Figure 4.13 shows a picture of the entire EROS. Although it has the same components as that in ROESTI, the only difference is the mezzanine board; instead of ASD, it is connected to the main board. It converts the differential waveform from the preamplifier into a single-end waveform.

Figure 4.12: Preamplifier board handling 16 APD channels from the right connector

Figure 4.13: Waveform digitizer board for the ECAL, EROS. The mez-zanine board on the left side converts the input differential waveforms into single-end waveforms.

7A pair of differential waveforms has the same shapev(t) with opposite polarity asv_±(t)=±v(t). Any noisen(t) can contaminate both waveforms simultaneously with the same phase:v±(t)→v^′_±(t)=v±+n(t). This noise can be eliminated by considering the differencev^′₊(t)−v^′₋(t)∝v(t).

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4.3. Electronics

4.3.2 Trigger Line

Currently, the Phase-II trigger system has two candidate electronics: Pre-trigger and COTTRI (COMET trigger). The former is specific to the ECAL; however, the latter is shared with the Phase-I trigger system. Each systems comprises front-end or mezzanine boards and motherboards. The front-end board processes the trigger line waveforms and compiles them for the motherboard. The motherboard holds several mezzanine boards, collects their data, and generates trigger signals for FC7 if they ex-ceed some trigger criteria. If FC7 makes trigger decisions after receiving the trigger signals, they are distributed to every EROS to digitize and record waveforms. Figure 4.14 displays the pictures of the Pre-trigger and COTTRI electronics. The Pre-trigger and COTTRI electronics contain 8 bit flash ADC chips operating at a 100 MHz sampling frequency. These ADC chips can monitor the in-put waveforms continuously, separately from EROS, and therefore, they can generate trigger signals instantly.

(a)Pre-trigger mezzanine board (b)Pre-trigger motherboard (c)COTTRI front-end board

Figure 4.14:Phase-II trigger electronics.

4.3.3 Slow Control

The ECAL system employs several slow-control modules to maintain the performance. First, the HV for the APDs must be controlled and recorded. Their optimum values vary depending on, for example, temperature and radiation damage. Hence, APD gains should be maintained by adjusting the HV. We developed the prototypes of the controller and monitor separately. However, the controller prototype remains under development and is not remotely controllable. Second, an electronics board is dedicated to monitoring the temperature and driving the LED. Third, slow-control transmission boards are installed on the feedthrough to carry all slow control signals.

The prototypes of the HV controller device and monitor board are displayed in Figure 4.15. Both handle 32 channels. The controller contains a series of capacitors and resistances to stabilize the HV. The individual channel values are adjustable. Furthermore, the controller supplies other voltages reduced by a constant factor to a few volts for the monitor board. The monitor board contains ADC chips to read them, and it communicates with the DAQ machine to transmit them.

Figure 4.16 shows the prototype board that measures the temperature and operates the LEDs. For LED operation, a NIM (nuclear instrumentation module)-standard input from outside generates the LED driving pulse. The board uses adjusters to change the shape of the driving pulse so that the LED

4.3. Electronics light-emission time structure becomes similar to that of the LYSO scintillation. The driving-pulse amplitude is set considerably higher than that of the usual LED activation threshold of about 0.7 V because the threshold is sensitive to temperature. The ND filter moderates the consequent light that is too bright for the APD. However, such a robust driving signal introduces a considerable amount of crosstalk into the nearby APD. Therefore, we use differential signaling for the LED driving pulse. A differential pulse pair looks±0 V in total from the APD, and hence, it can reduce crosstalk.

Finally, the slow-control transmission board was designed for HVs and slow-control signals to access the vacuum region. It also has a simple HV filtering circuit.

(a)Controller device (b)Monitor board

Figure 4.15:Prototype of the (a) controller and (b) monitor devices for the APD HV. Output HVs are adjusted individually with volumes.

Figure 4.16: Prototype of the LED controller and temperature monitor board. The left connector links to the slow-control transmission board.

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5

Performance Evaluation of the StrECAL Prototype

Understanding the response and performance of StrECAL for the signal electrons is the most funda-mental milestone to study the Phase-II sensitivity. At the beginning of 2017, we completed a StrECAL prototype and performed a test-beam experiment to evaluate its performance on March 2017 at ELPH in Tohoku University, Japan. All estimated performances successfully satisfied all requirements.