FFD/TDR Version 2

01 Dec. 2015 JINR, Dubna

LHEP JINR

Dubna, December 2015

MPD

Fast Forward Detector

Technical Design Report

2

FFD group Project leader: V. I. Yurevich Participants: Joint Institute for Nuclear Research, Dubna

G. N. Agakishiev, G. S. Averichev, D. N. Bogoslovsky, L. G. Efimov, V. Yu. Rogov,

S. V. Sergeev, V. V. Tikhomirov, A. A. Timoshenko, A. N. Zubarev, G. A. Yarigin,

V. I. Yurevich

V.G. Khlopin Radium Institute, St. Petersburg

O. I. Batenkov, A. S. Veschikov

3

Contents

1. Introduction 2. General description of the FFD

2.1. Requirements to the detector 2.2. Concept of the detector 2.3. The FFD sub-systems

3. Detector design

4. FFD module

4.1. Detector module elements and prototypes 4.2. Photodetector 4.3. Lead converter 4.4. Radiator 4.5. Front-end electronics

5. Test measurements and results

5.1. Tests in laboratory 5.2. Beam tests 5.3. Tests in magnetic field

6. FFD performance

6.1. Introduction 6.2. Detection of photons 6.3. Detection of charged particles 6.4. Vertex 6.5. Efficiency of L0 trigger 6.6. Start signal for TOF detector 6.7. Background

7. The FFD electronics

7.1. Concept 7.2. Electronics of sub-detectors

7.2.1. Trigger Logic Unit 7.2.2. Mezzanine cards 7.2.3. TLU prototype and tests

7.3. L0 trigger electronics 7.4. Readout electronics 7.5. High voltage power supply

8. Calibration system 9. Detector control system

9.1. General consideration and requirements 9.2. Concept 9.3. Electronics and software 9.4. Interaction with slow control system of MPD

10. The detector cable system

4

11. Installation in MPD

11.1. General 11.2. Detector assembling 11.3. Installation in MPD setup 11.4. FFD cable lines and racks with electronics

12. Schedule and Cost

References

5

1. Introduction

The Fast Forward Detector (FFD) is important part of Multi-Purpose Detector (MPD)

setup for study of Au + Au collisions with beams of NICA collider in an energy interval 3

≤ NNs ≤ 11 GeV. The main aims of the FFD are (i) fast and effective triggering events of

Au+Au collisions in center of MPD setup and (ii) generation of start pulse for TOF detector.

Fast triggering nucleus – nucleus collisions in interaction point of experimental setup

and precision TOF measurements with picosecond time resolution are important features of

all experiments at RHIC and LHC colliders. For this aims two-arm modular detector with

fast Cherenkov or scintillation counters is used.

In the PHENIX experiment, the Beam-Beam Counter (BBC) is used as the start

detector [1] and it consists of two arrays of Cherenkov quartz counters located very close to

the beam pipe at a distance of 144 cm from IP covering pseudorapidity range of 3.0

< |η| < 3.9. A Hamamatsu R3432 fine mesh dynode PMT is used to detect the Cherenkov

light and this PMT is capable of operation in strong magnetic fields. The fully implemented

and installed BBC of 128 channels showed a single detector time resolution of 52 ± 2 ps at

RHIC for 130 GeV per nucleon Au + Au collisions [2].

The time-zero Cherenkov detectors [3] in the PHOBOS experiment are located at |Z| =

5.3 m from IP. The resolution of each read-out channel was about 60 ps after corrections on

the slewing effect [4] which causes a correlation between the time t recorded for a pulse and

the size of the pulse, or amplitude A.

Two arrays of the ALICE T0 detector [5] each consisting of 12 PMTs are located from

IP at a distance of 70 cm on one side, covering the pseudorapidity range of 2.9 < |η| < 3.3

and at 370 cm on other side, covering the pseudorapidity range of 5 < |η| < 4.5. The 3.0-cm

thick quartz Cherenkov radiators are optically coupled to the fine mesh dynode PMTs FEU-

187, produced by firm Electron, Russia. In test runs with a beam of negative pions and

Kaons, a time resolution of 37 ps was obtained for the detector with 3.0-cm diameter

radiator and better time resolution of 28 ps was obtained with smaller 2.0-cm diameter

radiator [6].

The STAR start detector VPD also consists of two identical arrays with 19 read-out

channels. Each read-out channel consists of a Hamamatsu R5946 fine mesh dynode PMT,

a 1 cm thick scintillator (Eljen EJ-204), and a 6.4-mm Pb converter (~1.13 X0). The primary

photons hit the Pb layer and generate by pair production process some ultra-relativistic

6

electrons which come in the scintillator and initiate a light pulse. The time resolution of

single detector channel of ~150 ps was measured in runs Au + Au at NNs = 39 and 62.4

GeV (2010) and p + p at energy of 590 GeV (2013) [7].

NICA operates at much lower energy of beams than RHIC and LHC. The kinetic energy

and normalized velocity of the NICA heavy ions is shown in Fig. 1-1 as a function of NNs .

Fig. 1-1. The kinetic energy and normalized velocity of the NICA heavy ions as a function

of NNs .

It leads to rather low multiplicity of the secondary particles where the charged particles

are not ultrarelativistic (even the spectator protons). As a result the effective triggering the

collisions and picosecond time resolution of the start signal are two nontrivial problems

which must be solved in MPD project. To solve these problems, a new Cherenkov detector

FFD was proposed. It has two large area modular arrays which detect both the fast charged

particles and the high energy photons from π0-decays.

7

2. General description of the FFD

2.1. Requirements to the detector

Among other detectors of the MPD setup, the FFD plays a key role in fast effective

triggering nucleus – nucleus collisions in center of the setup with approximately 100%-

efficiency for central and semi-central Au + Au collisions identifying z-position of the

collision with an uncertainty less than 5 cm. Implementation of any method of event

selection on centrality is an important additional issue.

The main requirements are

Fast and effective triggering events of Au + Au collisions in center of the MPD setup.

The detector must be able to see each beam crossings with the dead time < 75 ns.

Generation of the start pulse for TOF detector with time resolution t < 50 ps.

The detector range must be out of the interval -2 < η < 2.

The detector must operate in the MPD magnetic field with B = 0.5 T.

The installation and deinstallation of the detector into the TPC inner tube must be easily

carried out.

Besides, there are some additional important tasks where the FFD might be a useful

instrument. It can much help in adjustment of beam-beam collisions in the center of MPD

setup with operative control of the collision rate and interaction point position during a run.

Mechanics and geometry of the FFD must be implemented in structure of the MPD

setup.

2.2. Concept of the detector

An optimal solution to realize the requirements is to use a well-known phenomenon of

multiple production of pions in central and semi-central collisions of relativistic heavy

nuclei. A detection of the photons from neutral pions decays can help to reach the best

timing for the start signal and the vertex resolution. The high efficiency of the fast triggering

Au + Au collisions in center of the MPD setup can be obtained by registration both the

photons and charged particles.

8

The concept of the detector with picosecond time resolution is based on the key idea –

to register a fraction of high-rapidity photons and the most relativistic charged particles in

both directions from IP by modular arrays of Cherenkov detectors with a large active area.

The high-energy photons are registered by their conversion to electrons inside a lead plate

with thickness of 2X0. The application of advanced photodetectors MCP-PMTs, Planacon

type, from Photonis in the detector modules helps to realize (i) the picosecond timing, (ii)

the large modular array with needed granularity and a small dead area, and (iii) good

operation in the MPD magnetic field.

The FFD modular sub-detectors FFDE and FFDW are placed at a distance of 100 cm to

the left and to the right from IP in center of the MPD setup as it is schematically shown in

Fig.2-1. The FFD must trigger the collisions in an interval |z| < 25 cm from the MPD center

along the beam axis. The each sub-detector array consists of 20 identical modules with total

granularity of 80 channels. The FFD covers an angular range 3.0° < |θ| < 10.8° or a

pseudorapidity interval of 2.35 < |η| <3.63. The FFD layout in the MPD setup is shown in

Fig. 2-2.

Fig. 2-1. A scheme of FFD arrays.

L = 100 cm

Au Au

γ

γ

π

π

p

p p

FFD E FFD W

2.35 < |η| < 3.63

θ

25 cm

3.0˚ < |θ| < 10.8˚

9

Fig. 2-2. The FFD layout in the MPD setup.

The pulses from common MCP-PMTs outputs are used for fast triggering the

collisions in a selected z-interval in center of the MPD setup. The collision position is

defined from the time difference between the first pulses from the sub-detectors FFDE and

FFDW.

The pulses of all 160 individual channels of the FFD are fed to readout electronics

together with signals from TOF detector for off-line time-of-flight analysis. For each event

the precise position of IP is given by analysis of TPC tracks. Thus, the first single hit

produced by high-energy photon or the most relativistic charged particle in FFD can be used

as the start pulse for TOF detector. Appearance of a number of such pulses improves the

time resolution of the start signal.

The detector performance was studied by Monte-Carlo simulation with LAQGSM and

GEANT4 codes and the results are described in detail in Chapter 6.

2.3. The FFD sub-systems

A number of various sub-systems are needed for operation of the FFD and realization

of the detector goals in MPD. The main parts of the detector are schematically shown in Fig.

2-3. The FFD system consists of

FFDE

FFDW

10

the two FFD arrays of detector modules,

the electronics and power supplies of the sub-detectors,

the L0 trigger electronics,

the readout electronics,

the laser calibration system,

the detector control system.

The concept, design, prototyping, and production of the sub-systems and their

elements are described in the TRD. The design of FFD modular arrays is discussed in

Chapter 3; the detector module, main elements, and prototyping are described in Chapter 4;

the description of test measurements with the module prototypes and obtained results are

given in Chapter 5; the FFD electronics including the sub-detector electronics, LV and HV

power supplies, the L0 trigger, and readout electronics are described in Chapter 7; the

Chapter 8 is devoted to the laser calibration system; the detector control system is presented

in Chapter 9; the information about the FFD cable system is given in Chapter 10.

Fig. 2-3. A scheme of the FFD sub-systems.

11

3. Detector design

The space for the FFD arrays is limited by the requirements coming from the geometry

of the MPD setup. The FFD inner diameter is 102 mm which is a bit larger of the beam pipe

diameter. The FFD maximum diameter is 400 mm which is defined by the inner diameter of

the TPC and some space needed for a mechanical support and installation of IT detector in

center of the MPD setup (in future). A scheme of front side of the FFD array with 20

modules and 80-channel granularity is shown in Fig. 3-1.

Fig. 3-1. A scheme of the front side of the FFD array with 20 modules.

The Autodesk Inventor Professional is used for 3D design of the FFD.

The outer dimensions of the FFD module are 64 × 64 × 132 mm3 and its mass is 1.5

kg. The module housing is made from aluminum alloy and all other materials are

nonmagnetic, besides the MCP-PMT. Inside side surfaces of aluminum housing have small

detents for positioning MCP-PMT inside module housing. Some light pressure to fix the

quartz radiator on MCP-PMT surface is achieved by a thin layer of soft rubber between the

lead plate and the quartz bars. Special thin housings for the MCP-PMT and quartz bars are

produced from plastic with a 3D printer, it improves the light protection and mechanical

support of these elements inside the module.

The active area and granularity of FFD module are defined by the size and granularity

of the quartz radiator which has total size 56 × 56 mm2 and consists of four identical bars 28

12

× 28 × 15 mm3. The size of the front side of the module is 64 × 64 mm2. Thus, the quartz

radiator covers 77% of the module cross section. The total active area of FFD is 2 × 627

cm2.

Fig. 3-2. Design drawing of FFD module.

The mechanics of FFD cell structure consists of four identical parts, one of them is

shown in Fig. 3-3 and it has 5 cells for FFD modules. It is produced from special plastic

with a 3D printer.

Fig. 3-3. Design drawing of the cells for 5 detector modules (1/4 part of the FFD array).

13

The FFD is mounted around the beam pipe using upper and bottom parts of the FFD

array. The upper part with mechanical support is shown in Fig. 3-4. The design drawing of

FFD array with 20 modules is shown in Fig. 3-5. The difference in the flight paths of

individual channels leads to uncertainty of the arrival time of photons of 15 ps. In the vertex

electronics of L0 trigger the different delays of incoming pulses from FFD modules are

adjusted to equal value by means of special precise delays with 10-ps step.

Fig. 3-4. Design drawing of the upper part of FFD cell structure.

Fig. 3-5. Design drawing of the FFD array with 20 modules.

14

4. FFD module

4.1. Module elements and prototypes

The modules of the FFD arrays are main elements of the detector and they define

characteristics of the FFD (the time resolution, the efficiency of photon and charged particle

detection, the immunity to magnetic field). The module must be compact with cross section

close to the MCP-PMT size to reduce the dead area in FFD array. The module has 2 2 cell

structure which is defined by usage of 2 2 structure of the quartz radiator and FEE

channels.

Since 2010 three different versions of the detector module were designed, simulated,

prototyped, and tested with cosmic rays and beams. In 2014 the pilot set of 12 units were

produced and tested.

The internal content of the third version of FFD module is shown in Fig. 4-1. The main

module elements (quartz radiator, MCP-PMT, and FEE board) are shown separately in

Fig. 4-2.

Fig. 4-1. The third version of FFD module: 1 – Pb plate (converter of high-energy photons), 2 – quartz radiator bars, 3 – MCP-PMT XP85012/A1-Q, 4 – FEE board, 5 – module housing, 6 – HV connector, 7 – SMA outputs of analog signals, 8 – HDMI cable (LVDS signals + LV for FEE).

1 2

3

4

5

6

7

8

a

15

Fig. 4-2. Photo of main elements of FFD module.

The final module design has only some small modifications in comparison with the

prototype shown in Fig. 4-1. These changes concern of mechanical supports of MCP-PMT

and quartz bars inside the module housing, the input of optical fiber on the front side of the

module housing, and decreasing a number of SMA connectors (analog outputs) from 5 to 1

(only the common analog pulse from MCP-PMT will be used for control of the module

operation).

4.2. Photodetector

The appearance of MCP-PMT Planacon [8, 9] provoked a wide development of

advanced detectors with pico-second time resolution for present and future experiments

BaBar [10], ATLAS [11], Belle [12], LHCb [13], and PANDA [14]. This device, shown in

Fig. 4-3, has rectangular form with a photocathode of 53 x 53 mm2 which is sensitive in

visible and ultraviolet region and occupies 81% of the front surface that is very important for

designing fast Cherenkov detectors with dense packing MCP-PMTs into a large-scale

detector array.

The careful experimental tests with picosecond lasers, relativistic beams of single-

charged particles, and in magnetic fields with ramping up from zero to 1.5 T were carried

out by different groups with aim to study the most critical characteristics of the MCP-PMTs

for developing new generation of detectors with picosecond time resolution [12, 14, 15, 16].

These studies showed the following: (i) signals from anode pixels are characterized by fairly

Quartz radiator

XP85012/A1-Q

FEE board

16

flat response with variation factor of 1.5 with rather low cross talk, (ii) the single photon

time resolution does not depends on the magnetic field, (iii) Planacon XP85012 has stable

operation up to the single photon rate ~1MHz/cm2 at gain 106, (iv) the lifetime depends on

the integrated anode charge.

The tests carried out by DIRC group from PANDA experiment at GSI showed that the

lifetime of Planacon depends on the integrated anode charge and the gain of XP85012

decreases only by factor of 0.9 with increasing anode charge up to 100 mC/cm2 (moreover,

experts of Photonis inform that lifetime of Planacon devices has been recently much

increased).

It leads to the conclusion that the FFD modules based on MCP-PMTs XP85012 will

operate without essential change of characteristics during about 10 years of beam time. This

estimation is based on the beam conditions planned for MPD/NICA, results of our

simulation, and low gain regime of the MCP-PMT operation.

Operation of Planacon at low gain demonstrates good time resolution and gives some

advantages. It is better for aging and rate issues and by lowering the gain detector becomes

sensitive only to relativistic charged particles traveling through Cherenkov radiator and it

does not see a background with a few photoelectron deposits from γ-rays. Additional

amplification is provided by fast frond-end electronics (FEE).

Study of photodetectors available on a market and results obtained with various

photodetectors in other laboratories [15] led us to the conclusion that MCP-PMT Planacon

XP85012/A1-Q or XP85012/A1-S from Photonis is the best solution for our application and

it covers all requirements to the photodetector of FFD module.

Fig. 4.3. A view of MCP-PMT XP85012/A1 (Photonis).

17

Main characteristics of XP85012/A1-Q (S) are listed below • Planacon size: 59 59 24.5 mm3

• Photocathode of 53 53 mm2 occupies 81% of front surface

• 2-mm quartz input window

• Sensitive in visible and ultraviolet region

• 8 8 multianode topology

• Chevron assembly of two MCPs

• MCP pore size: 25 m

• HVmax : - 2000 V

• Typical gain factor: ~ 105 – 106

• Rise time: 0.6 ns

• Transit time spread, TTS : ~ 37 ps

• Low noise, dark current: 1 – 3 nA (typical)

• High immunity to magnetic field

• Mass: 128 g

The quantum efficiency of XP85012/A1-Q (S) is shown in Fig. 4-4 and the device

drawing is shown in Fig. 4-5.

Fig. 4-4. The quantum efficiency of XP85012/A1-Q

18

Fig. 4-5. The drawing of XP85012/A1. A high immunity to the magnetic field is the crucial factor for detectors operating

because the FFD will function in strong magnetic field of the MPD with B = 0.5 T. A typical

result of gain variance in magnetic field is shown in Fig. 4-6.

Fig. 4-6. Study of gain variance of MCP-PMT XP85012 in magnetic field (the result is given by Burle Co.).

19

4.3. Lead converter

The photons are detected in FFD module via conversion to electrons in a lead plate

placed in front of quartz radiator. It is known that efficiency of photon registration depends

on the converter thickness. The results of simulation of single photon registration by FFD

array is shown in Fig. 4-7 for energy spectrum of photons produced in Au + Au collisions.

The lead thickness was selected from 2 to 20 mm. The efficiency grows up with increasing

lead thickness but above 10 mm the dependence becomes weak. Finally the 10-mm lead

converter was chosen. The lead plates have some small holes needed for optic fiber inputs

and screws to fix the converters on the front panels of module housings.

Fig. 4-7. Efficiency of single photon registration: the left figure – vs. bias on multiplicity of the Cherenkov photons at different thickness of the lead converter, the right figure – vs. thickness at the fixed bias of 100 Cherenkov photons.

4.4. Radiator

The radiator consists of a set of four quartz radiator bars with dimensions 28 28 15

mm. The radiator transverse size is a bit larger and close to the photocathode size of

Planacon MCP-PMT. The 15-mm thickness of quartz bar is a compromise value because

farther increasing gives worse time resolution due to larger time dispersion of Cherenkov

photon arrival on MCP-PMT photocathode [16] and shorter radiator length leads to smaller

number of photoelectrons Npe from the photocathode and as a result the time resolution

degrades.

20

The radiators are produced by firm “Fluoride”, St. Petersburg from the optical quartz

KU-1. All surfaces of the quartz bars are polished and the side surfaces are covered by thin

aluminum layer for reflection of Cherenkov photons.

The silicon oil from Dow Corning Co. with high transmittance for UV photons is used

as optical grease between quartz bars and 2-mm MCP-PMT window made from quartz or

sapphire.

The number of Cherenkov photons produced in a radiator with length L by charged

particle with Z = 1 and β = 1 is calculated by formula

The estimation of Nph for UV region and region of MCP-PMT sensitivity is given in Table

together with the photoelectron number Npe. It is clearly seen that the contribution of UV

region to Cherenkov detector response is ~50%.

The dependencies of Nph and Npe, the quantum efficiency of XP85012/A1-Q QE, and

the optical grease transmission on wavy length of photons are shown in Fig. 4-8.

170<λ<270 170<λ<670

Nph

Npe

979

147

1909

278

Fig. 4-8. The dependencies of Nph and Npe, the quantum efficiency of XP85012/A1-Q QE, and the optical grease transmission on wavy length of photons.

dn

LN ph 22

1)

)(

11(2

2

1

21

4.5. Front-end electronics

In accordance with the FFD design, MCP-PMT XP85012/A1 with 64 anode pads is

transformed into the 2 2 photodetector by merging 16 pads (4 × 4) of each cell into a

single channel. The pulses coming from the anode pads have a small time jitter and they are

used for production of the start signal for TOF detector. Additionally to the anode pad

signals, this device has a common MCP output. The time resolution obtained with this signal

in [13] was slightly worse than the result obtained with anode signal. Our test measurement

confirms this conclusion and these pulses from all counters of FFD arrays are used for fast

finding of pseudo-Vertex and generating L0 trigger signal. Thus, the module front-end

electronics (FEE) has 5 channels of amplifiers and discriminators on the FEE board, four

channels accept signals from the anode pads and one more channel accepts a signal from the

specific MCPs output. A view of the FEE board is shown in Fig. 4-2.

The preliminary tests with 1-ns light pulses from a LED source demonstrated the

necessity of careful design of FEE inputs to provide a minimum time dispersion of signals

from individual anode pads. The compensation of this effect is carried out by

implementation of different length of input traces on the FEE board.

A functional scheme of the FEE channel is shown in Fig. 4-9. The major elements are

the low-noise input amplifier with BFR93A transistor, the pulse shaper minimizing signal to

noise ratio, the RC chain filtering the signal frequency, the fast amplifier with ~ 40 dB gain

(MAR-8 chip of DC – 8GHz bandwidth), and LMH7220 discriminator (1.2 GHz, Low

Distortion Operational Amp). The amplifiers provide analog pulses with ~ 1.3-ns rise time

and the discriminators produce LVDS signals which lengths increase with amplitude of

input pulses. The analog pulses are fed to SMA output connectors and the LVDS pulses

together with LV power lines are fed to the sub-detector electronics unit with a high quality

HDMI cable.

Fig. 4-9. A functional scheme of the FEE channel.

22

In test measurements with the module prototypes the analog and LVDS pulses from

all channels were used. The output signals of the final FFD module are five LVDS pulses

and the MCP analog pulse, the first pulses are fed to the trigger and MPD readout

electronics and the last one is used for the module operation control and it is fed to 5-GS/s

digitizer CAEN mod. x742.

The design and development of FFD module have been reported and published

elsewhere in [17 – 22].

23

5. Test measurements and results

5.1. Tests in laboratory

The detector modules are fabricated in special equipped room for detector production.

Then the prepared modules are tested with LED pulses and cosmic rays in laboratory. The

goal of this stage is study of the detector operation, detector response, signal to noise ratio,

tuning LV and HV regime, and measurements of time resolution. The experimental stand

with tested modules is shown in Fig. 5-1.

Fig. 5-1. The experimental stand with tested FFD modules in the laboratory.

More careful study of the detector response and time resolution was carried out with

high-energy deuteron beam of Nuclotron at JINR. The module prototypes prepared for the

beam tests in 2014 are shown in Fig. 5-2.

24

Fig. 5-2. The FFD modules prepared for tests with the deuteron beam.

5.2. Beam tests

The experimental study of detector module performance was carried out with a 3.5-

GeV deuteron beam of the Nuclotron at LHEP/JINR. The measurements were carried out in

period 2013 – 2014 at a novel external beam channel specially created for tests

measurements with prototypes of sub-detectors of MPD and BM@N. The beam intensity

was varied from 103 to 105 deuterons per 2-s spill. A photo and a scheme of the

experimental setup at the beam line are shown in Fig. 5-3 and Fig.5-4 respectively.

Two pairs of detector modules shown in Fig. 10.3 were installed for the test along the

beam axis. The modules did not contain lead converters. The first pair of modules D1 and

D2 were placed behind the first MWPC, the second pair D3 and D4 – at a distance of 2.7 m

behind the first pair. The prototypes of RPCs of TOF detector were located in the middle of

the experimental setup. The distance between modules in the pairs was ~22 cm.

Two scintillation counters were used for trigger pulse production for each beam

particle passing through the experimental area. The information from MWPCs was used for

track reconstruction.

The LVDS signals from modules D1 – D4 were fed via HDMI cables to a special

board which aim was (i) production and control of low voltages for FEE, and (ii) transport

the LVDS signals to inputs of a VME module TDC32VL [23].

The analog pulses were fed from the modules to two digitizers Evaluation Board

DRS4 V4 designed at PSI [24]. This device is based on a DRS4 chip which is a switched

capacitor array digitizing at sampling speed of 5 GS/s that corresponds to 200-ps binning. It

25

has SMA connectors for four input channels and USB 2.0 port for data readout, control, and

power voltage.

Fig. 5-3. The layout of FFD modules D1 – D4 together with other detectors on beam line of

MPD-test area.

Fig. 5-4. A scheme of the experimental setup for study of detector modules: S1, S2 – the scintillation counters, MWPC1, MWPC2 – the multiwire proportional chambers, D1 – D4 – the tested detector modules, RPCs – the resistive plate chambers of TOF detector.

26

Thus, two different readout methods were applied in the time-of-flight measurements

for estimation of time resolution of the Cherenkov detector. In the first approach, used also

for TOF RPC detectors, the LVDS signals were fed to TDC32VL modules. A length of

LVDS pulses brings information about a pulse height used for time – pulse length correction

in off-line analyses. The second method was digitizing form of the analog pulses. The rise

time of the pulses after FEE corresponds to 6 time bins on the front slope of the pulses. It is

enough for a good interpolation and finding t0 position on the time scale in off-line analyses.

In the measurements with DRS4 digitizer we studied (i) the form of pulses from

analog outputs of detector modules, (ii) the pulse height distribution for beam particles

hitting the back surface of module, (iii) the cross talk response, and (iv) the time resolution

of single detector channel.

Typical responses of the detectors D1 – D4 are shown in Fig. 5-5 for 10 events

induced by 3.5-GeV deuterons. Here the deuteron tracks passed the central area of quartz

radiators. The rise time of pulses is 1.3 ns, the pulse width is 6 ns.

The rotation of detector by 180° leads to decreasing the pulse height by a factor of ~3.

Thus, in a real experiment background particles will mostly give much smaller responses

which can be rejected by a discriminator.

The measurements showed a small cross talk between module channels with an

negligible contribution to the detector responses.

Fig. 5-5. The pulses of FFD modules for 10 events measured with the digitizer Evaluation Board DRS4 V4.

27

A result of the TOF measurements with two pairs of detector modules is shown in

Fig. 5-6. The linear fit of pulse front was used for t0 finding for each detector pulse.

The obtained TOF peaks are well approximated by Gauss distribution with σ ≈ 33.5 ps.

Fig. 5-6. TOF measurements with pair of the modules and Evaluation Board DRS4 V4 (run 2014): D1 – D2 (left) and D3 – D4 (right).

The beam velocity spread was less of a few ps and it gives a negligible contribution.

The time jitter of the DRS4 digitizer increases with delay between pulses fed on different

inputs and our estimation gives σDRS ≈ 14 ps for the test measurements with beam. Taking

into account these values, one can estimate the time resolution of the detector module σt ≈

21.5 ps.

The TOF peak obtained with LVDS pulses fed to TDC32VL is shown in Fig. 5-7.

Here the time resolution of single channel chain is σt = 41 ps that is a bit worse of the result

obtained with the digitizer.

Fig. 5-7. The result of TOF measurements with LVDS pulses fed to TDC32VL.

28

Fig. 5-8. The TOF peaks obtained with pair of modules CD1 and CD2 and digitizer CAEN mod.N6742 in measurement with 3.5-GeV/n deuteron beam.

The TOF measurements 2015 with 3.5-GeV/n deuterons and 5 GS/s digitizer CAEN

mod.N6742 were carried out with pair of modules CD1 and CD2 on beam line of BM@N

setup. The results are shown in Fig. 5-8. The data were obtained for four individual channels

of the modules. The time resolution of single detector channel with electronics is σt = 33 –

36 ps that is worse of the results earlier obtained with Evaluation Board DRS4 V4 but it also

satisfies to the requirement.

The beam tests showed that the developed FFD modules provide the required time

resolution and it depends on method and readout electronics used.

Two versions of radiators were considered at R&D stage: (1) a quartz radiator 53×53

mm2 equal to photocathode area of XP85012, (2) a quartz radiator 59×59 mm2 equal to full

size of XP85012. The detector responses were studied in special measurements with

deuteron beam and two MWPC for finding track position of each beam particle on radiator

of tested module. The measurements were carried out with pair of the modules and readout

electronics TDC32VL. A scheme of 29.5×29.5 mm2 quartz bar layout with a set of 4×4

virtual cells used for detector response analysis is shown in Fig. 5-9. The results of the test

measurements show a degradation of pulse height and TOF distributions in virtual cells

29

placed along a perimeter of XP85012 for the larger radiator as it is shown in Fig. 5-10 and

Fig. 5-11 respectively.

To get better characteristics we decided to use quartz radiator 53×53 mm2 in final

version of FFD modules.

Fig. 5-9. A scheme of 29.5×29.5 mm2 quartz bar layout with a set of 4×4 virtual cells.

Fig. 5-10. Distributions of LVDS pulse length measured with TDC32VL.

XP85012/A1-Q

Quartz bar

29.529.5 mm2

Photocathode

30

Fig. 5-11. TOF measurements with pair of the modules and TDC32VL.

5.3. Tests in magnetic field

Operation of FFD modules in magnetic field with B = 0.5 T was studied with a

prototype of T0 detector (BM@N experiment) [18 – 22] based on the same modules and

placed inside BM@N magnet as it is shown in Fig. 5-12. The measurements were made in

February – March 2015 during BM@N run with a deuteron beam for the modules with

vertical orientation which were reproduced the experimental conditions for the FFD in MPD

setup. To get the same gain we slightly increased (~20 V) the high voltage in comparison

with magnitudes used for MCP-PMTs operation without magnetic field.

The pulse height distributions and time resolution were studied with LED pulses and in

measurements with 3.5-GeV/n carbon beam. The C ions interacted with Cu target 10 mm

diam.×7 mm and produced secondary charged particles and photons. A fraction of the

secondaries came to the T0 modules and produced pulses. A Cherenkov beam detector with

31

11-mm quartz radiator generated start signals for TOF measurements with time resolution

σt = 27 ps.

Fig. 5-12. (a) Detector layout at BM@N beam line: SD – the Cherenkov start detector,

T0 detector with FFD modules, (b) – the view of modular array of T0 detector (February

– March 2015).

The measured pulse height distributions overlap a wide interval as it is expected from

MC simulation. Preliminary estimation of the time resolution of the detector modules

obtained with pulses from common output fed to CAEN digitizer is 67 ps and after

subtraction of a contribution coming from geometry it becomes σt ≈ 57 ps. The analysis is in

progress and we hope to get better magnitude for individual channels of the modules.

Making the measurements we observed an effect of TOF peak shift in magnetic field.

The shifts were different for different orientation of the detector modules in magnetic field

of BM@N magnet. This effect must be taken into account when the time calibration of FFD

channels in magnetic field of MPD will be carried out.

The experimental study of FFD Cherenkov modules have been reported and published

elsewhere in [17 – 22].

T0 detector

a b

Beam SD

32

6. FFD performance

6.1. Introduction

The LAQGSM plus GEANT4 codes were applied for Monte Carlo simulation of Au +

Au collisions to study trigger and time performance of the FFD. Under this, 1-mm

aluminum beam pipe, magnetic field of 0.5 T, lead layer with thickness of 10 mm to convert

photons to electrons, as well as FFD geometry and materials were taken into account. The

collisions occur in the center of MPD setup.

6.2. Detection of photons

An example of energy spectrum of photons produced in Au + Au collisions at NNs =

7 GeV is shown in Fig. 6-1. The spectrum covers a broad energy range and indicates the

most photons have to be within the interval from 100 MeV to a few GeV.

Fig. 6-1. Energy spectrum of photons passing through the front surface of FFD array for

Au+Au collisions at NNs = 7 GeV.

The energetic electrons produced in the lead plate by a high-energy photon pass

through the quartz radiator generating a high response in MCP-PMT. The number of

Cherenkov photons produced in the quartz radiator and MCP-PMT window was studied

with MC simulation for different energies of incoming photons 50, 100, 200, and 500 MeV.

The Cherenkov photon distributions are shown in Fig. 6-2.

The first peak at ~1850 Ch.ph. corresponds to single electron escaping lead converter

and passing through quartz radiator. The overlapping interval depends on a photon energy

33

and for 500-MeV photons the maximum number of Cerenkov photons reaches 20 000. Thus,

the dynamical range of pulse height generated by photons in FFD module is equal to ~ 10.

Fig. 6-2. The distributions of Cherenkov photon number in dependence of energy of incoming photons 50, 100, 200, and 500 MeV (results of MC simulation)

The efficiency of photon detection by FFD module is shown in Fig. 6-3 as a function of

photon energy. It increases with energy and reaches of 75% at energies above 400 MeV.

Fg. 6-3. Efficiency of detection of high energy photons by FFD module. The high energy photons are mainly produced in decay of neutral pions generated in

collisions of nucleons-participants. Multiplicity of the pions and photons grows up with

energy and number of participant-nucleons, centrality. Thus, a number of hits produced by

photons in FFD sub-detector increases with beam energy and decreases with the impact

34

parameter as shown in Fig. 6-4 for Au + Au collisions at two energies NNs = 5 and 11 GeV

(threshold is 1000 Cherenkov photons).

Fig. 6-4. The multiplicity of hits produced by photons in a single FFD array for Au + Au

collisions at NNs = 5 (a) and 11 GeV (b) in dependence of the impact parameter.

6.3. Detection of charged particles

The charged particle 2D-distributions, the momentum vs. pseudorapidity, are shown in

Fig. 6-5 for Au + Au collisions at two different energies NNs = 5 (a) and 11 GeV (b)

where the intervals covered by FFD sub-detectors are also shown. At the lower energy the

spectator-protons region corresponds to the FFD acceptance and one may expect a dominate

contribution of these protons to the detector response. But at the highest energy the most

spectators move at smaller angles through the holes in center of the FFD arrays and in this

case they give a smaller contribution to the FFD response. Thus, the FFD response, the

number of hits, produced by the charged particles depends on the beam energy and it is the

a

b

35

sum of two contributions from (1) the particles produced in the collisions of participant-

nucleons and (2) the particles produced in the spectators decay.

The velocity distributions of charged particles coming to the FFD are shown in Fig. 6-

6 for the same energies of the collisions. At the lower energy (Fig. 6-6 a) the spectator-

protons move with velocity 0.925 and their contribution looks like the wide peak around

this value. Here the most relativistic particles are the energetic pions producing the small

shoulder on the right tail of the peak. At the highest energy NNs = 11 GeV the arriving

particles become more relativistic and produce pulses in FFD with a small time uncertainty.

Fig. 6-5. The 2D-plots of the momentum vs. pseudorapidity for charged particles produced in

Au + Au collisions at two different energies NNs = 5 (a) and 11 GeV (b).

FFD FFD a

FFD FFD

b

36

Fig. 6-6. The velocity distributions of charged particles produced in Au + Au collisions at

two different energies NNs = 5 (a) and 11 GeV (b).

The energetic charged particles pass through 7-mm aluminum front panel of the

module housing plus 10-mm lead converter and produce enough Cherenkov photons in the

quartz radiator to be registered in FFD. The most relativistic particles produce of ~1800

Cherenkov photons ad form a peak in pulse height distribution. This peak corresponds to the

single electron peak in Fig. 6-2.

The multiplicity of hits produced by charged particles in a single FFD array is shown

in Fig. 6-7 for Au + Au collisions at NNs = 5 (a) and 11 GeV (b) as afunction of the

impact parameter. The wide peak at the lower energy in a range of the semi-central

collisions corresponds to arrival of the spectator-protons. The monotonic increasing a

number of hits with the centrality is observed at the maximum energy. The both sub-

detectors have some hits induced by the charged particles and it leads to high efficiency of

triggering the collision in a range of the impact parameter b < 12 fm.

a

b

37

Fig. 6-7. The multiplicity of hits produced by charged particles in a single FFD array for Au

+ Au collisions at NNs = 5 (a) and 11 GeV (b) in dependence of the impact parameter.

6.4. Vertex

The pulses from both sub-detectors TE and TW with small time jitter are needed for

vertex determination of the IP position in center of MPD setup with uncertainty less than a

few cm. Note that a time shift of 0.1 ns corresponds to a position error of 3 cm. The quality

of vertex determination by FFD is shown in Fig. 6-8 for Au + Au collisions in center of

MPD at NNs = 5 (a) and 11 GeV (b) (min. bias). In the central and semi-central collisions

the photons detected in both FFD arrays form the narrow peak. The events beyond the peak

correspond to the cases when no photons come to the acceptance of a single or both sub-

detectors and the hit produced by the first charged particle is used for generation of T pulse.

a

b

38

Fig. 6-8. The vertex determination by FFD for Au + Au collisions in center of MPD at

NNs = 5 (a) and 11 GeV (b) (min. bias).

6.5. Efficiency of L0 trigger

The detection of high energy photons and charged particles defines the vertex / L0

trigger efficiency which is a function of beam energy and centrality of Au+Au collisions.

The trigger efficiency for NNs = 5 and 11 GeV is shown in Fig. 6-9 as a function of the

impact parameter. The three different cases are shown where the signals of sub-detectors are

generated in (i) 8 inner modules (the blue symbols), (ii) 12 outer modules (the red symbols),

and (iii) all 20 modules (the black symbols). When all the modules are used the efficiency is

100% for the both energies in a range b < 11 fm. For more peripheral collisions the

efficiency falls down with the impact parameter.

sNN = 5 GeV

3 cm

sNN = 11 GeV

39

Fig. 6-9. The trigger efficiency for the collisions in center of MPD at NNs = 5 and 11 GeV.

6.6. Start signal for TOF detector

The start signal for TOF measurements is defined in off-line analyses of the time and

length of LVDS pulses from FFD written with TDC72VHL. The pulse produced by photon

or charged particle with the highest velocity is considered as the best candidate of the start

signal for TOF detector. The Fig. 6-10 shows the time dispersion of the start signal for Au +

Au collisions in center of MPD at two energies NNs = 5 and 11 GeV as a function of the

impact parameter. The line of 50-ps delay of the pulse appearance is also shown (it

corresponds to the requirement for the time resolution of start signal). At the lower energy a

small time dispersion of the pulses is observed which does not exceed 10 ps up to b = 8 fm.

For more peripheral collisions a fraction of events with the delayed pulses appears and a

number of these events increases with the impact parameter. But in a range b < 10 fm the

fraction of events with pulses delayed more than 50 ps is rather small. At the energy of 11

sNN = 5 GeV

sNN = 11 GeV

40

GeV the situation becomes better and only for very peripheral collisions (b > 12 fm ) some

events with pulse delay above 50 ps are observed.

Fig. 6-10. The time dispersion of the start signal for Au + Au collisions in center of MPD at

two energies NNs = 5 and 11 GeV as a function of the impact parameter.

We note that in the central and semi central collisions with several hits N in FFD

induced by high-energy photons, the time resolution of start signal improves as N1/2.

6.7. Background

Additionally, a background of low energy electrons traveling in magnetic field along

the beam line was studied by means of MC simulation. These electrons traveling in the

magnetic field of MPD hit the radiators of FFD modules and produce small responses which

are mainly have a random distribution over time scale. We found that the intensity of these

background events is rather low and it gives negligible influence on detector response and

time characteristics.

50 ps

sNN = 5 GeV

50 ps

sNN = 11 GeV

41

7. The FFD electronics

7.1. Concept

The FFD is main detector providing the fast identification of nucleus – nucleus

collisions in the center of MPD setup. It generates the L0 trigger signal for the main

electronics and other detectors of MPD. Since a beams crossing occurs every 75 ns, the dead

time of the trigger electronics chain for generation of the L0 trigger pulse must be less than

70 ns.

The electronics of FFD consists of two sub-detector branch modules called Trigger

Logic Unit (TLU) and a Vertex/L0 trigger Unit (VL0U), a block diagram is shown in Fig. 7-1.

The signals from the branch modules are sent to the VL0U module which generates

L0 trigger based on the FFD signals and signals from some other detectors.

Fig. 7-1. Block diagram of the FFD electronics.

Each TLU is used to perform following jobs:

To generate preprocessed trigger signal based on signals from FEE of the

detector modules,

To get data from individual channels and to deliver the signals to the Vertex /

L0 trigger processor and to the DAQ,

42

To generate and to deliver the LV power to the FEE,

To provide monitoring of TLU,

To control the FEE power supplies.

All these modules are controlled by the FFD detector control system (DCS).

7.2. Electronics of sub-detectors

7.2.1. Trigger Logic Unit The main electronics of FFD sub-detectors called trigger logic unit (TLU) is designed

as a single unit. This device has a modular structure – a motherboard and a set of mezzanine

cards.

The mezzanine cards are:

LV power supply and Fan-out board (PFB) and

Input-output boards (IOB) including

o TTL to NIM converter,

o TTL to TTL 50 Ohm converter,

o Discriminator board.

Each branch of FEE (FFDE and FFDW) has its own TLU board. The TLU handles 20

PFB boards and 8 IOB. The heart of TLU is an Altera Cyclone V GX FPGA used for trigger

signal preprocessing and for individual channels data processing.

The TLU accumulates counts from FEE channels, results of preprocessing etc. The

monitoring data readout is done via RS-link or Ethernet line.

A block diagram of the TLU is shown in Fig. 7-2.

Fan-out signals from PFB are grouped according to their destination. One group of

individual channels goes directly to FPGA for trigger preprocessing, the other group goes to

Molex 76105-0585 connector for DAQ.

The FEE common signals are sent to 4 groups:

DAQ data with Molex 76105-0585 connector,

for debugging and monitoring (connector SMA or similar),

directly to FPGA

to a delay line stage.

In addition to preprocessing of trigger signals the TLU also provides suppression of

noise signals. We use that fact that the detector response signal with small amplitude

produces short output pulse and a large amplitude signal corresponds to the long output

43

signal. Therefore there is a possibility to perform small signal (being a noise) discrimination

by short signal suppression. A diagram of the discriminator is shown in Fig. 7-3.

Fig.7-2. Block scheme of the TLU

Figure 7-3. Block diagram of input stage for common signals.

44

The discrimination level is defined by Programmable Delay#1 implemented inside the

TLU FPGA.

The external Programmable Delay #2 with delay range from 23 to 32 ns allows to

align all FEE common signals in time to provide required trigger timing accuracy.

7.2.2. Mezzanine cards Power Supply and Fan-out Board The Power Supply and Fan-out Board (PFB) provides three independent voltages to

supply detector FEE and fans-out signals from FEE. Each power channel is monitored and

controlled with high precision and could be switched On or Off independently. The PFB

uses MCU STM32F103R4T6B.

The PFB board technical specifications are as follows:

The Negative voltage channel provides current up to 100 mA at -7.3V.

The positive channels provide current up to 150 mA in a voltage range from 4.0V to

8.0V and could be adjusted with ~1mV step.

The channel output voltages and currents are read back by 12-bit ADC.

The FEE individual channels are fan-out 1:2 with Micrel chip SY58608U with jitter

<1 ps. One of these signals is used for preprocessing in FPGA and the second is sent

to DAQ electronics.

The FEE common channels are fan-out 1:4 with Micrel chip SY89832U with jitter

<1ps. One signal is sent directly to FPGA for preprocessing, one goes to DAQ, one

is sent to delay line and then back to FPGA and the last one is used for monitoring

by a scope.

The communication between PFB and the FFD DCS is done via RS serial link.

One FFD branch (TLU) will process signals from 20 FEE modules and will provide

power for them. We combine power supplies and fan-outs on one PCB to provide simplicity

of installation and assembling and improve a reliability of PFB. The PFB block diagram is

shown in Fig. 7-4.

45

Fig. 7-4. Block diagram of the PFB.

The HDMI standard connector has been chosen as a low cost high quality standard

industrial solution for signal and power distribution. The PCI-E standard edge connector is

used for signals delivery from PFB to TLU and to feed power for PFB.

The first prototype of LV power supply have been assembled and successively tested

during BM@N 2015 run. The view of power supply board is shown in Fig. 7-5.

Fig. 7-5. A view of power supply board.

The TLU motherboard also handles IOB that provide input-output connections with

other electronics.

46

Discriminator board

For input signals of different type we use discriminator boards. The Discriminator

board (DIB) – consist of 4 input channels having fast discriminators with 1.5 GHz

equivalent input rise time bandwidth and 700ps propagation delay. The threshold could be

set in range from – 2V to +3V with step around 5 mV. A view of the discriminator board is

shown in Fig. 7-6.

Fig. 7-6. A view of the discriminator board.

TTL-NIM converter board For output connections we use TTL to TTL 50 Ohm converters and TTL to NIM

converters.

The TTL-NIM converter Board (TNB) is used to convert the trigger processor output

TTL signals to NIM signals which could be sent to external devices. The board contains four

converters TTL to NIM. A view of the TTL-NIM convertor board is shown in Fig. 7-7.

TTL-50 Ohm TTL converter Board

A TTL-50 Ohm TTL converter Board (TTB) is used to convert trigger processor

output TTL signals to 50 Ohm TTL signals which could be sent to external devices. The

board contains four converters TTL to 50 Ohm TTL and it is shown in Fig. 7-8. PFB and all

IOB were tested at BM@N 2015 run. All declared parameters have been reached.

47

Fig.7-7. A view of the TTL-NIM convertor board.

Fig.7-8. A view of the TTL-TTL 50 Ohm converter.

7.2.3. TLU prototype and tests

The FFD TLU is a next version of a BM@N T0U which is considered as a prototype

of the sub-detector electronics and it has been tested during the BM@N run in 2015. The

T0U provides the transmission of LVDS signals to readout electronics (TDC), the

production of L0 trigger based on programmable trigger logic (FPGA), and LV power for

the FEE of FFD modules. Twelve FFD modules were used as the beam and T0-detector

counters in the experiment with Nuclotron beams of 3.5-GeV deuterons and carbon ions in

2015. The beam test of the T0U was passed successfully and all declared parameters were

reached.

48

The 3D design and view of T0U are shown in Fig. 7-9 and Fig. 7-10.

Fig.7-9. The 3D design of the T0U module.

Fig. 7-10. A view of T0U module (prototype of TLU): 1 – the output connector for DAQ for TOF start; 2 – the input connector for FEE; 3 – the power supply board; 4 – the output connectors for Vertex/L0 trigger board.

HDMI

LV LVDS

FFD module To readout electronics (TDC32VL)

24 FFD individ.channels

24 FFD individ.channels

12 FFD pulses of comm.outputs + 12 channels (in store) LVDS

LVDS

LVDS

Other detectors, accelerator pulse

SMA inputs

L0 Trigger RS232 / Ethernet

USB2.0 Control and connection with global slow control system (TANGO)

49

7.3. L0 trigger electronics

The vertex/L0 trigger unit (VL0U) uses preprocessed data coming from both TLUs

(TLUE, TLUR ). The VL0U also could process up to 24 signals from other detectors and

devices.

The VL0U has a modular structure – it has a motherboard and 4 different type

mezzanine boards. The motherboard performs the following jobs:

L0 trigger generation using signals from TLUE and TLUW.

Vertex processing.

Start DAQ signal generation based on input signals.

Control and monitoring of the PiLas picosecond laser calibration system.

Monitoring and control of mezzanine cards including:

o Discriminator cards (DIB),

o TTL – NIM converter cards (TNB),

o 50 Ohm TTL out board (TTB),

o Ethernet interface card (ETB),

o Time to digital converter board (TDCB)

Accumulation of the trigger monitoring information. This information will be sent to

the control and Vertex / L0 unit server running at DCS PCs via optical link, RS,

Ethernet or USB 2.0.

A block diagram of the VL0U is shown in Fig. 7-11.

Figure 7-11. A Block diagram of the VLOU.

50

Preprocessed common signal from each TLU passes programmable delay controlled

by DCS via FPGA. It has a programmable delay chip IC SY89295 or IC854S296I-33 (the

same chip is used in TLU) which provides quite stable adjustable delay in a range from 3.2

to 14.8 ns with 10 ps step and a small time jitter.

The vertex processor is shown in Fig. 7-12. The length of cable coming from the

TLUW (right branch) is as short as possible and its signal arrives to VL0U before the signal

of TLUE (left branch). The right branch pulse for the whole acceptable interaction area

signals is used as the start signal for vertex coordinate processing and it generates the

"Vertex gate signal". The arrival of the left branch signal during the "Vertex gate signal"

means that the interaction takes place inside an acceptable range in the center of MPD setup.

The geometrical boundaries of the acceptable interaction area are selected and tuned by the

adjustable delays of the first branch signal and gate length.

Figure 7-12. A Block diagram of the vertex processor.

7.4. Readout electronics The LVDS signals from FFD sub-detector, 80 individual channels and 20 common

channels, plus signal from the laser reference detector are fed to two units of TDC72VHL

(25 ps multi-hit time stamping TDC developed and produced in Laboratory of high energy

physics, JINR) by five short cables Molex P/N 11102512xx with connectors Molex 76105-

0585. The same readout electronics and cables are used for TOF detector.

51

The 5-GS/s 32-inputs digitizers CAEN mod. x742 (2 units per sub-detector) are used

for detector operation control and calibration purpose with common analog and LVDS

pulses from the FFD modules.

Multi-hit TDCs are applied for information about past / future history of interactions in

MPD during the active time of TPC and other detectors. For this purpose the pulses of the

L0 trigger, TE, TW, and some other fast detectors (ZDC) are used as input signals.

7.5. High voltage power supply

To provide HV power to the FFD detectors we plan to use HV power supplies

produced by the HVSys company, Dubna. The power supply consists of a crate with 12 HV

units which provide output voltage up to 3 KV with current up to 5 mA. Communication

with FFD DCS is performed by RS232 serial link or USB/RS bridge.

Since we have 20 PMTs at each FFD branch we will have two HV crates at each MPD

side.

The HV system control follows the FFD DCS concept. The HV system will have one

low-level server controlling four power supply crates with their own Com-ports (hardware

ports or virtual for USB or Ethernet connection) running four independent threads. This

server will have GUIs for FFD experts who will be able to set proper working voltages,

current limits etc. The crate control and setup GUI are shown in Fig. 7-13.

Fig. 7-13. A view of the HV crate control and setup GUIs panel.

52

The central DCS will be allowed only to switch ON/OFF all channel simultaneously

and to select and download existing predefined HV configuration from the local repository.

The actual values of voltages, currents etc. will be published to central DCS by Tango-

server retrieving data from low-level server.

53

8. Calibration system

A special method for precision time calibration of FFD channels monitoring the

detector operation is required. For this purpose we developed a system based on PiLas laser

with 20-ps pulse width and 405-nm wave length. The laser and the optical system were

produced and delivered by Advanced Laser Diode Systems (Germany). Main parts of the

system are

the PiLas control unit,

the box with laser head and optical system,

the quartz fiber bundles,

the reference detector.

The optical system was specially designed for our application. By means of this

system the laser beam is input into quartz fibers of the common bundle with a low loss of

the beam power and good uniformity over the fibers. The optical fiber bundles were

manufactured by BIOLITEC Co. The multi-mode optical fibers WF100/140/300N were

used in the bundles. The test carried out with 15-m fiber and laser showed that input pulse

with 45-ps FWHM becomes 90-ps after passing the fiber.

The photodetector MCP-PMT PP0365G from Photonis is used as the reference

detector. The characteristics of this photodetector are listed below

MCP double , chevron, with 6-μm pore size Quartz window Photocathode diam.: 17.5 mm Rise time: 200 ps Sensitivity in UV range, QE: ≈ 25–30 % Typical gain: 7×105

A view of these elements (besides the optical fiber bundles) is shown in Fig. 8-1.

A scheme of the calibration system is shown in Fig. 8-2. The laser beam is focused on

the fused end of optical fiber bundle which is split into two 7-m bundles for the FFD sub-

detectors and 10 fibers for the reference detector and tests. The first bundles connect via SC-

SC patch panels with the next 7-m fiber bundles passing through the MPD facility to the

FFD modules.

54

Fig. 8-1. A view of the PiLas control unit, the box with laser head and optical system, and

the photodetector MCP-PMT PP0365G.

Fig. 8-2. A scheme of the laser beam transmission to the FFD modules.

PiLas Control Unit Laser head with Optical system

Reference MCP-PMT

55

9. Detector control system

9.1. General consideration and requirements

The detector control system (DCS) provides the control and monitoring of following

sub-systems:

the HV power supplies for MPC-PMTs,

the LV power supplies for frond-end electronics,

the control of FFD and L0 trigger logic operation,

the laser calibration,

the local DAQ for calibration and monitoring of the FFD.

9.2. Concept

Each subsystem has its own low level server handling communication between a

control PC and a subsystem. The server also provides GUIs for subsystem state presentation

and for the expert level control and monitoring of subsystem.

The relevant server information is published by each server in a shared memory as a

text having XML structure. The information update is followed by triggering of "Windows

named event".

This information is taken by a Tango DCS server and passed to the DCS tree of whole

detector. The general interest information archiving is provided by Tango system. Servers

also could provide archiving of private information to local files at the DCS PC.

Some fraction of the information could be published by a private web-server running

at the DCS PC.

The FFD experts could have full control of all systems and MPD shifters should have

limited control on subsystems. Shifters will be able to switch On/Off channels and select

and download predefined configurations of subsystem parameters.

Configurations files are generated by experts using server GUIs. These files are stored

as local files and could be only read-out by Tango server for publishing and archiving.

The DCS servers publish all relevant information for the Tango server and receive limited

set of commands from the Tango server.

The general FFD DCS scheme is shown in Fig. 9-1.

56

Fig. 9-1. The diagram of FFD DCS.

9.3. Electronics and software

The DCS PC runs Windows as operation system. The servers are written using Delphi

and MS VS development platforms.

The prototype of a FFD DCS web-server has been developed and tested. To provide

data presentation in a real-time mode we use AJAX technology. We expect that our single

server will be able to provide connection to tens of http clients.

HV system HV server

Tango server

Config.library

HV system

HV system LV server

Config.library

LV system

TLU&VL0U electronics

Trigger server

Config.library

Laser

Reference PMT

Local readout system

Calibration server Splitter

Data processing

Calib.data archive

to M

PD

DC

S

WEB server

Ser

ial

link

or

Eth

erne

t

Pub

lish

ed i

nfor

mat

ion

Laser "Start"

FE

E

57

To provide a stable work of servers the "Watch Dog" windows service program has

been developed. This service checks presence of all servers in a PC memory and if a server

process is missing then the service restarts the server.

The electronics of DCS has been described above.

As the main communication protocol we will use serial link (RS232, RS422 or RS485)

or Ethernet-based TCP/IP protocol. As the TCP/IP node hardware we plan to use WizNet

W5300 chip which has been tested at a prototype board. We got a communication speed up

to 3.5 Mbytes/s at a point-to-point connection.

As the base microcontroller in self-made boards we use microcontrollers of STM32F

and STM8L families from ST Microelectronics.

9.4. Interaction with slow control system of MPD

The FFD DCS is a self-consistent system and it will not have any internal partitioning.

It could be partitioned only at FFD/MPD interface.

The states of FFD FSM will follow standard MPD scheme. FFD DCS will publish

some fraction of internal information to MPD DCS and it will receive a limited set of

commands from MPD DCS including commands to switch On or Off power supply

channels and commands to download predefined named configuration to a subsystem. Used

channels of FE, LV or HV should be defined inside the configuration file.

Shifters will have a possibility to switch off tripped channels and to suppress error

messages from blocked channels to continue data taking in limited configuration.

58

10. Detector cable system

The FFD sub-detector cable system consists of

High quality HDMI cables with length of 8 m and diameter of 8 mm for transmission of

LVDS pulses and LV power of FEE. The number of these cables is 20 units.

50-ohm coaxial cables with length of 8 m and diameter of 3 mm for transmission of

common analog pulses from detector modules. The number of these cables is 20 units.

HV cables with length of 15 m and diameter of 4 mm for transmission of HV power to

MCP-PMTs of the modules. The number of these cables is 20 units.

The optical fiber bundle with length of 7 m and diameter of 6 mm for transmission of

laser pulse to the modules.

Total cross section of these cables is ~20 cm2. The cables are laid out in a plane behind

TPC to minimize material budget on a path of particles. We use one of 28 outlets in the

magnet for output of the cables.

59

11. Installation in MPD

11.1. General The integration of the FFD into the MPD structure is carried out after the installation

of other barrel detectors (ECAL and TOF), TPC and the beam pipe. Each FFD array consists

of four equal parts with 5 modules each as it is shown in Fig. 3-3 – Fig. 3-5 (Chapter 3).

Such design allows detector mounting around the installed beam pipe. A special moving

platform is needed for the installation of the FFD sub-detectors.

11.2. Detector assembling

The first stages of sub-detector assembling are fulfilled on a special stand which

plays a role of mechanical support and used for further installation of the mounted modular

array into the inner tube of TPC. Here the stages are

1. Mounting of the first lower cell structure on two aluminum rails with two aluminum

frames for cable support as it is shown in Fig. 11-1.

2. Mounting of the second upper cell structure and fixing both parts together.

3. Installation of the detector modules into the cells

4. Connecting the cables to the modules and fixing the cables in the cable support.

Fig. 11-1. The first lower cell structure on two aluminum rails with two aluminum frames for cable support.

60

Next stage is the installation of detector into the MPD. For this purpose the FFD rails are

moved along two special aluminum profiles which are passed through the TPC inner tube

and fixed on the main aluminum frame of the MPD which also supports the TPC. These

profiles are used for mechanical support of the MPD inner detectors. The distance of moving

into the TPC is 70 cm and it is equal to the length of the detector rails. The FFD sub-detector

assembly is shown in Fig. 11-2 (the cables are not shown).

Fig. 11-2. The FFD sub-detector assembly.

11.3. Installation in MPD setup

The installation of the FFD sub-detectors in MPD setup is carried out on a special

platform after installation of the main MPD detectors (TOF, ECAL, TPC) and the beam

pipe. It and has the following stages:

1. Putting the cables along the aluminum profile of TPC outer wheel,

2. Mounting the detector around the beam pipe,

3. Cabling and connecting to FFD modules,

4. Moving the sub-detector array into the TPC inner tube to its position.

The deinstallation is fulfilled in the opposite way.

A drawing of the FFD sub-detectors inside the MPD setup is shown in Fig. 11-3 and a

view of the setup design – in Fig. 11-4. A more detail view of the FFD sub-detector installed

inside the TPC inner tube is shown in Fig. 11-5.

61

Fig. 11-3. A drawing of the FFD sub-detectors inside the MPD setup.

Fig. 11-4. A view of the MPD setup design with the FFD sub-detectors.

FFDE

FFDW

TPC

62

Fig. 11-5. A view of the FFD sub-detector installed inside the TPC inner tube.

63

11.4. FFD cable lines and racks with electronics

The each sub-detector cable output needs 13 13 cm2 hole in a single 50 13 cm2

pass of the MPD as it is shown in Fig. 11-6.

Fig. 11-6. The cable outputs of the FFD sub-detectors.

The racks of the sub-detector electronics are shown in Fig. 11-7. The protected access

to the FFD electronics and cables is needed, only members of the FFD-Trigger group must

be able to operate with this equipment.

Fig. 11-7. The racks of the sub-detector electronics.

64

12. Schedule and Cost The timetable of main tasks on FFD design, R&D, production, tests, and installation is given below. It takes a period till spring 2018. After that the FFD commissioning will begin. The total cost of the Fast Forward Detector is ~1 300 000. 0 $ where 500 000.0 $ is the cost of MCP-PMTs from Photonis.

Task 2011 2012 2013 2014 2015 2016 2017 2018

FFD modules

MCP-PMTs purchase

Front-end electronics

Module mechanics

Beam tests

HV & LV system

Experimental rooms

FFD electronics

L0 trigger electronics

Cable system

Laser calibration system

Detector control system

FFD sub-detector mechanics

Readout electronics

Platform for FFD installation

Installation of FFD

Commissioning

Design, R&D

Installation & Commissioning

Production

Tests

65

References

1. Ikematsu K. et al. Nucl. Instr. Meth. A. 1998. V.411. P.238.

2. Allen M. et al. Nucl. Instr. Meth. A. 2003. V.499. P.549.

3. Back B. B. et al. Nucl. Instr. Meth. A. 2003. V.499. P.603.

4. Bengtson B., Moszynski M. Nucl. Instr. Meth. 1970. V.81. P.109.

5. Bondila M. et al. IEEE Transactions on Nucl. Sci., 2005. V.52. P.1705.

6. ALICE collaboration. The T0 Detector. ALICE TDR 011, CERN-LHCC-2004-025, Sep

(2004).

7. Llope W. J. et al. The STAR vertex position detector. arXiv:1403.6855v1 [physics.ins-det]

26 Mar 2014.

8. www.photonis.com.

9. . Schyns E. Workshop on Large Area Photo-Detectors, Electronics for Particle Physics

and Medical Imaging, Clermont Ferrand, Jan. 28, 2010.

10. Va’vra J. et al. Report SLAC-PUB-12803, Sep. 2007.

11. Albrow M. G. et al. Quartz Cherenkov counters for fast timing: QUARTIC,

arXiv:1207.7248v1 [physics.ins-det] 31 Jul 2012.

12. Korpar S. et al. Nucl. Instr. Meth. A. 2009. V.595. P.169.

13. Garcia L. C. DT Detectors Physics Meeting, CERN, 14 June 2011.

14. Lehmann A. RICH 2010, Cassis, France, May 2010.

15. Va’vra J. et al. RICH 2010, Cassis, France, May 2010; Report SLAC-PUB-14279, Oct.

2010.

16. Britting A. et al. Workshop on fast Cherenkov detectors, Giessen, Apr. 2011.

2011 JINST 6 C10001.

17. Yurevich V. I. et al. Fast forward detector for MPD/NICA project: concept, simulation,

and prototyping. Phys. Part. Nucl. Lett. 2013. V.10 (3). P.258 – 268.

18. Yurevich V. I. Development and study of picosecond start and trigger detector for high-

energy heavy ion experiments. 7th International Conference on New Developments In

Photodetection. 30 June - 4 July 2014, Tours. Nucl. Instr. Meth. A. (2015).

19. Yurevich V. I. Modular Detector with Picosecond Time Resolution. The Technology

and Instrumentation in Particle Physics 2014 (TIPP 2014) conference. 2-6 June 2014,

Amsterdam.

20. Yurevich V. I., Batenkov O. I. et al. Beam test of Cherenkov detector modules with

picosecond time resolution for start and L0 trigger detectors of MPD and BM@N

experiments. Physics of Particles and Nuclei Letters V.12, 2015, p.778.

66

21. Yurevich V. I. Development and study of picosecond start and trigger detector for high-

energy heavy ion experiments. Nucl. Instrum.Methods Phys. Res. A787, 2015, p.308.

22. Yurevich V. I. and Batenkov O. I. Picosecond Cherenkov detectors for high-energy heavy

ion experiments at LHEP/JINR, 13th Pisa Meeting on Advanced Detectors, La Biodola,

Isola d'Elba (Italy) May 24 - 30, 2015.

23. Internet site: afi.jinr.ru/TDC32VL

24. www.psi.ch/drs/evaluation-board.