Development of the FoCal-E PAD detector and its electronics
for the LHC-ALICE experiment††
Motoi Inaba, Tatsuya Chujo , Masahiro Hirano for the ALICE FoCal collaboration
Tsukuba University of Technology, Amakubo 4-3-15, Tsukuba, Japan.
PurposeIn order to study the initial state of the high-energy heavyion collision, it is proposed to install a forward calorimeter(FoCal) in the LHC-ALICE experiment.
FoCal is going to have an electro-magnetic calorimeter(FoCal-E) to measure the direct photons and a hadroncalorimeter (FoCal-H) to observe the jets, and the FoCal-E
consists of low-granularity silicon-pad (PAD) modules andhigh-granularity silicon-pixel (MAPS) modules.
The first beam test of the FoCal-E, namely a combination ofPAD and MAPS modules, was carried out at CERN PS and SPS
beam lines in 2014.
We developed a trigger signal converting circuit, a trigger
signal processor, a temperature monitor using precision
digital thermo-sensors, independent regulated power circuits
and isolated high-voltage generators for PAD modules.
Low voltage (+4.1V and -4.1V)

Independent regulated power circuits
An isolated high-voltage generator

High voltage(-100V)

Independent regulated power circuits
An isolated high-voltage generator
PCPC

A precision digital thermo-sensor

The APV25 hybrid board
A summing board
Precision digital
thermo-sensors

Independent regulated power circuits
An isolated high-voltage generator

The FoCal-E PAD module
Each FoCal-E PAD module has 4 pairs of a tungsten tile andthe 8 x 8 silicon photodiode array as shown in Fig. 1. The
sizes of a tungsten tile and a photodiode cell are 94.3 x 94.3
mm2 and 11.3 x 11.3 mm2, respectively.
The output signal is derived as the current sum of 4
photodiode cells in the same position by a summing board.
The summing board makes low-gain and high-gain output
signals. Prototypes of the PAD module and the summingboard were designed and fabricated at ORNL.
We used frontend hybrid boards with the APV25 analog
pipeline ASIC, the CERN RD51 SRS-ADC-C-Card and SRS-FECCard to read out signals of the FoCal-E PAD module at thefirst beam test in 2014 as shown in Fig.2. We are going tohave new frontend hybrid boards with the VMM2 mixedsignal ASIC and SRS-D-Card.

Fig. 1 FoCal-E PAD modules at CERN PS beam line.

NEW CIRCUITS

Independent regulated power circuits
An isolated high-voltage generator† University of Tsukuba, Tennodai 1-1-1, Tsukuba, Japan.

A temperature monitor

The APV25 hybrid board
Small scintillation
counters (PMT)

PAD

PAD
MAPS

MAPS

A control signal from DAQ to the APV25 hybrid
and the data from the APV25 hybrid to DAQDAQSRSSRSfor
PAD FEC-Card ADC-C-Card

A NEW CIRCUITA control signal from the temperature monitor to thethermo-sensor and the data from the thermo-sensor
to the temperature monitor

A large scintillation
counter (PMT)BeamPADA summing board

PC

NEW CIRCUITS

A trigger signal converting
circuit and a trigger signal
processor

PC

Trigger data
(10-bit parallel)

A trigger counter
Trigger information to DAQ

DAQ for MAPS

A trigger signal

Discriminatorsand gate generators
High voltage modules(by Universiteit Utrecht)

Triggersignals(NIM modules)

A new temperature monitor
Fig. 3 shows a temperature monitor with high-precision
digital thermo-sensors to manage the temperature-sensitive
photodiodes. The thermo-sensors are controlled through the1-wired serial interface, and it is easy to increase the numberof sensors up to 160. In the remote control mode, it sends
the temperature data to a personal computer through theUSB interface. Figs. 4 and 5 show the exterior of thetemperature monitor and thermo-sensors, respectively.

Fig. 2 The beam test setup and developed circuits.

New exclusive low voltage and high voltage supplies
Since the APV25 hybrid board and a summing board werevery sensitive to the electrical noise on the power lines, wemade the independent regulated power circuits and isolated
high-voltage generators. It could bring the good results toimprove S/N on the frontend electronics for PAD.
An example of
the waveform of
the PAD signal

Improved !
Fig. 7 The PAD signal

Fig. 6 A new trigger signal processor.

Fig. 3 A new temperature monitor.

Fig. 4 The exterior of thetemperature monitor.

Fig. 5 High-precision digital
thermo-sensors.

A new trigger signal processor
First, we developed a trigger signal converting circuitwith the CPLD to merge the PAD data with the MAPS
data because they were working on different DAQs asshown in Fig. 2. The circuit receives the 10-bit parallel
trigger data from a trigger counter through optocouplers
and sends a serial trigger data with the timing andchannel information to DAQ for PAD. The circuit andDAQ for PAD can be connected with each other throughan ordinary HDMI cable.
And then, we developed a new trigger signal processor
with the FPGA as shown in Fig. 6. The new circuit canread out the discriminated signals of three scintillation
counters directly and make a higher-precision serial
trigger data for PAD. It is easy to modify the trigger
configuration through the USB interface. The circuit also
has three output ports for 8-digit 7-segment LEDs.
The new trigger signal processor is going to be used at
the next beam test.

Fig. 8 The pedestal distribution

Fig. 9 σ of the pedestal distribution

Summary
We have developed the trigger signal converting circuit,the new trigger signal processor, the new temperaturemonitor using precision digital thermo-sensors, the new
independent regulated power circuits and isolated highvoltage generators for PAD modules.
The new temperature monitor has got the important data
to manage the temperature-sensitive photodiodes, and the
new regulated power circuits and high-voltage generators
have cut off the electric noise on the power lines successfully.
We are planning to do the next beam test to study theenergy resolution and the dynamic range of the FoCal-E PAD
modules using the new trigger signal processor at CERN PSand SPS beam lines in October, 2015.