Modular LED Relative Gain Monitoring of the GlueX Barrel CalorimterI

T. Bogarta,1

aThomas Jefferson National Accelerator Facility (Jefferson Laboratory), Newport News, Virginia, 23606, USA

Abstract

The GlueX Barrel Calorimeter is one of the main components of the GlueX Detector. GlueX will be investigating gluonic degreesof freedom in Quantum Chromodynamics. The relative gain of the photodetectors for the calorimeter will be monitored usingmodular LED driver systems. The BCAL system consists of a global controller that feeds power, bias voltage and trigger signals to96 local controllers situated at the ends of the 48 BCAL modules, which drive 40 LEDs associated with the 40 light guides at theend of each module. The LEDs are mounted on small boards that will be pulsed using controller boards. 3840 blue LEDs will beused to pulse the BCAL readout cells via their light guides. The system is currently being installed on the detector and the testedperformance is presented herein.

1. Project Background1

In elementary physics forces decrease with the separation be-2

tween interacting objects, which is not the case in regards to3

the permanent confinement of quarks. The Flux-Tube Model4

explains the confinement of quarks through gluonic flux tubes5

as shown in Fig. 1.6

Figure 1: Sketch of the Flux-Tube Model as described by Quantum Chromody-namics.

When excited, these tubes can add degrees of freedom to the7

quark system. These states, where not only quarks but also8

IThis work was carried out as part of the GlueX Project at Jefferson Lab.1Student at Christopher Newport University, Newport News, Virginia,

23606, USA.

gluons effectively participate in the full dynamics, are what the9

GlueX Experiment will ultimately be searching for.10

1.1. Quantum Chromodynamics11

The GlueX Experiment in Hall D at Jefferson Lab intends12

to provide information regarding the nature of the confinement13

in Quantum Chromodynamics by searching for exotic hybrid14

mesons. Quantum Chromodynamics (QCD) provides an under-15

standing of the behavior of quarks and gluons at high energies;16

however, from predictions at low energies, it remains difficult17

to obtain a quantitative observation from QCD into the field of18

hadrons, mesons, and baryons. Gluons are elementary particles19

that operate as exchange particles that are the strong force be-20

tween quarks. As quarks make up hadrons, gluons are the force21

that binds quarks into hadrons.22

1.2. Barrel Calorimeter Overview23

The experiment will begin with physics runs starting in 2015.24

One of the key subsystems is the barrel calorimeter (BCAL).25

The BCAL consists of 48 modules of a scintillating fiber ma-26

trix embedded in lead [4]. It will circle inside of the GlueX’s27

superconducting solenoid and will act as a sampling calorime-28

ter. The BCAL will be readout using large-area arrays of Sili-29

Preprint submitted to Elsevier December 8, 2013

con Photomultipliers (SiPMs). It is important to track the gain30

for the detector. Specifically, the SiPM arrays are a novel tech-31

nology and their performance has to be monitored and studied32

extensively, in term of gain shifts and degradation of resolution.33

A Light Emitting Diode (LED) based system has been de-34

signed to meet these requirements. It has been principally de-35

signed to monitor the relative gain of the photosensors. The ab-36

solute calibration will be accomplished using cosmic rays and37

specific beam-originating interactions.38

The chosen system is modular and compact. The BCAL sys-39

tem consists of control boards that each feed power, bias and40

four trigger signals to 40 LEDs mounted on individual pulser41

boards, which also contain the driver circuits. With this de-42

sign the number of cables is minimized and the use of bulky43

coaxial cables is avoided. The LED-based monitoring system44

achieves some redundancy using a geometry where each LED45

illuminates two photosensors at opposite ends of the BCAL si-46

multaneously.47

2. BCAL Monitoring48

The BCAL consists of 48 optically isolated calorimeter mod-49

ules, each having a trapezoidal cross section and forming a hol-50

low cylinder, respectively, as depicted in Fig. 2. This detector51

is an electromagnetic sampling calorimeter, comprised of layers52

of lead sheets and scintillating fibers.53

It is important that the fibers yield the highest possible54

amount of light, measured by the sensors as photoelectrons,55

as this feature impacts the energy and timing resolutions as56

well as the detection threshold. The light transmitted by the57

fibers is collected by 40 acrylic light guides, glued at each end58

of every module (a total of 80 per module), and funneled to59

large-area. These devices offer gain as well as energy and tim-60

ing resolutions comparable to those of a PMT, are immune to61

magnetic fields, but require simple electronic amplification and62

summing circuits [6]. The 40 SiPMs, their electronics, a tem-63

perature compensation system, a cooling system and connectors64

are all integrated into a single assembly that bolts onto the alu-65

minum base plate of each module. External readout electronics66

mounted in standard crates include low-voltage bias and power67

for the preamplifiers, a flash ADC (FADC), a discriminator and68

a Time to Digital Converter (TDC).69

11.77 cm

8.51 cm

22.46 cm

BCAL end view

BCAL top half cutaway(a) (b)

(c) (d)

single moduleend

65 cm

25 cm

48 m

odules

390 cm

180 c

m 30-cm targetbeamline

BCAL

11 o126o

390 cm

65 cmBCAL

Figure 2: Sketch of Barrel Calorimeter readout. (a) A three-dimensional ren-dering of the BCAL; (b) top-half cutaway (partial side view) of a BCAL moduleshowing its polar angle coverage and location with respect to the GlueX LH2target; (c) end view of the BCAL depicting all 48 azimuthal modules and (d) anend view of a single module showing the readout segmentation: four rings (in-ner to outer) comprised of sums of 1, 2, 3, and 4 readout rows, respectively,and four azimuthal slices (columns), resulting in a total of 16 summed readoutzones. More details can be found in the text.

The light guides, at the end of each module, are arranged70

in four columns and ten rows as shown in Fig. 2d. The light71

guides in each of the ten rows are slightly different, growing ra-72

dially outward in the detector. The light is funneled by different73

amounts to the SiPM arrays, all of the same size, located at the74

output. The signals of the sensors are summed together within75

each column. The SiPMs of the innermost row are readout indi-76

vidually, the second and third rows are summed, the next three77

after those are summed and the final four are also summed to-78

gether as shown in Fig. 3. This results in a 1:2:3:4 summing79

scheme for each column and 16 FADCs connected to each end80

of a module. TDCs are connected to the first three sums, 1:2:3,81

2

for a total of 12 channels per side. The power is distributed to82

ten SiPMs at a time along rows and different summing groups,83

which allows individual SiPMs to be powered and readout dur-84

ing special runs. SiPMs are sorted according to their breakdown85

voltages first, and their bias voltage is further individualized by86

setting trim resistors.87

Figure 3: Left: Bias Map - SiPMs of the same color are those powered bya common power supply. Right: Charge summing map - SiPMs of the samecolor and shade are those which are summed and sent to one read-out channel.

2.1. BCAL LED Monitoring Design88

The monitoring system is designed to track the stability of89

the SiPM output signals about once per day between data runs.90

To minimize cabling, simplify construction and reduce size and91

cost while maintaining a degree of flexibility in its pulsing op-92

tions, a modular LED driver system has been designed in three93

levels. At the top level, a global controller feeds power, bias94

voltage and four trigger signals to 96 local controllers situated95

at the ends of the 48 BCAL modules, one controller per module96

per side. At the next level, each local controller (or BCAL con-97

troller) pulses the 40 LEDs associated with the 40 light guides98

at the end of a module as four independent trigger groups. Each99

group corresponds to one of the four columns of ten light guides100

shown in Fig. 2d. LEDs in the same group are fired simultane-101

ously by one of the four trigger signals, illuminating an entire102

column of light guides. At the third level, a miniature LED103

pulser circuit board is attached directly onto each light guide,104

directing light towards the opposite end of the module. LED105

Figure 4: Configuration of the BCAL LED monitoring system. The 335 mmbespoke flat flexible cable caters for the slightly varying pitch between lightguides.

Figure 5: Left panel: One end of a prototype BCAL module, showing two ofits four columns of light guides. The LED pulsers are mounted on the side ofthe light guides attached to a common flex cable. The free end of the flex cableconnects to the control board on the left (not shown here). Right panel: BCALLED pulser board connected to a flat flexible cable. The LED (not shown) issoldered on the bottom layer, right under the 3-pin MOSFET transistor in themiddle of the board’s bottom edge. The light beam points downwards and isparallel to the board plane.

pulser boards in groups of ten are attached to a common bus106

cable connected to the controller board. The configuration of a107

BCAL controller with 40 LED pulser boards instrumenting one108

end of one module is shown in Fig. 4.109

BCAL controllers receive the four in-coming trigger signals110

and relay each one to one column of ten LEDs using a bespoke111

flat flexible cable to carry the common trigger along with the112

bias and supply voltages, as illustrated in the left panel of Fig. 5.113

The regeneration of trigger signals at each level obviates the114

need to send fast-rising pulses over long cables, improving ro-115

bustness without compromising the emitted pulse characteris-116

tics. The LED monitoring system is independent of the SiPM117

bias distribution system, thus allowing independent monitoring118

of individual SiPMs through each readout sum. In summary,119

the BCAL system employs a total of 3840 (48 × 2 × 40) LED120

3

pulser boards, 384 flat flexible cables, and 96 (48 × 2) con-121

trollers. Details of the BCAL monitoring system can be found122

in Ref. [10].123

Major considerations in the design of the LED light pulser124

were to minimize its physical size, due to the very limited space125

available between light guides as is evident from Fig. 5. Also to126

the very low power dissipation, to minimize temperature vari-127

ations that deteriorate the performance stability of the LEDs,128

achieve nanosecond performance with a wide variety of LED129

types and wavelengths, control the emitted pulse intensity, and130

to minimize any electromagnetic interference that might affect131

the performance of SiPMs. On those grounds, and mainly the132

pulser designs based on monostable multivibrators, avalanche133

transistors or complementary regenerative switches were ex-134

cluded, and we opted for the simple, robust, inexpensive, and135

low power circuit design, measuring 12×13.5 mm2 with almost136

half of its area taken up by the flex cable connector (Fig. 5). The137

use of a connector was necessary to allow the removal and sort-138

ing of pulsers into groups of 10 with similar light intensity for139

each column of light guides.140

The pulser dissipates ≈200µW at 1 kHz, increased with fre-141

quency by ≈70 µW/kHz. The MOSFET allows a maximum142

LED bias voltage of 60 V, although practical bias voltages are143

normally lower than 15 V. The bias determines the intensity of144

the LED light flash. The small loop area of the capacitor dis-145

charge current, along with a ground plane covering the entire146

bottom side of the board, ensure that there is practically no de-147

tectable radiated or near-field interference affecting the SiPMs.148

Conducted interference is minimized by keeping the SiPM elec-149

tronics separated upstream at the level of global controller from150

those of the LED monitoring system.151

The LEDs2 come in a standard surface mount package emit-152

ting blue light at 471 nm, which is at the peak of the fiber emis-153

2Model SMS1105BWC, Bivar, Inc., CA 92618, USA (www.bivar.com)

sion spectrum [8]. Measurements were also carried out using154

green LEDs at 574 nm — where the SiPM quantum efficiency155

drops by about 25% from its peak value — in order to inves-156

tigate whether it conferred any advantage over the blue one,157

which it did not. Each LED emits light parallel to the board158

plane through the near light guide and toward the opposite side159

of the BCAL module. Light enters the fibers and is transmit-160

ted through the module to the sensor at the far end and is also161

reflected from the near end of the module back into the near162

sensor. The design goal was to have about the same light inten-163

sity on both ends of a module to facilitate gain matching. Due to164

attenuation through the module [7], the light intensity at the far165

end must be ≈3 times that at the near end in order to produce166

the same signal at both ends. This goal was achieved during167

tests on a small (58 cm long) prototype calorimeter [9], but the168

ratio ranged from 1-4 from tests on a production module, as ex-169

plained in Section 2.3. A final consideration was redundancy,170

namely171

To evaluate the capability for gain matching, blue, domed172

LEDs were positioned on two light guides glued on either end173

of a 58 cm prototype calorimeter, inside a 3 × 3 mm2 “groove”174

or a 4 mm diameter and 3 mm depth “well”, machined at 6 cm175

and 5 cm, respectively, from the large-area end of each light176

guide. The opening angle of light from the LED was 40◦ and177

the LED beam was aimed towards the calorimeter. Light was178

detected with signal amplitudes of 2.5 V and 7.15 V (ratio of179

2.86) for the near and far signals from the groove, respectively,180

and 0.83 V, 2.65 V (ratio 3.2) from the well. The measurements181

were repeated with a green LED (570 nm), and yielded 1.5 V182

and 3.6 V (ratio of 2.4). All ratios proved the design require-183

ment of equalizing approximately the LED signals on the near184

and far ends of each module.185

The placement of the LEDs on opposing light guides was car-186

ried out in a manner that allowed approximately equal amount187

4

Figure 6: Hole on the side of a light guide, where the LED is to be mounted.Dimensional details are provided in the text.

of light from each LED to arrive at the near and far SiPMs of188

its readout cell. This was done to simplify gain matching be-189

tween the two photosensors as well as for reasons of redun-190

dancy, namely to ensure that even if one LED ceased function-191

ing the other one would still illuminate both ends. The LEDs192

are glued into pockets drilled into the sides of the light guides.193

2.2. LED Monitoring Test on Calorimeter Modules194

Tests of the LED monitoring system were carried out on a195

small prototype module as well as on a production module. The196

former tests were done employing an earlier version of the read-197

out electronics that suffered from increased noise. The noise198

was due to the proximity of the ground traces on the boards and199

this was rectified for subsequent tests and the production units.200

Otherwise the system functioned well in both cases, although201

the extracted near/far ratio differed, achieving values near one202

(after correcting for the attenuation length difference between203

the prototype and the full length) but values 1-4 for the produc-204

tion module. Nevertheless, this ratio spread does not pose any205

difficulties in the main objective of gain monitoring, as the rel-206

ative stability of each ratio will be checked rather than its abso-207

lute value. Possible reasons for the ratio difference and details208

of the tests are presented below.209

The shorter calorimeter prototype, termed ”mini BCAL”,210

was fully instrumented with SiPMs and their electronics and211

was used for the first comprehensive tests of a complete LED212

system (80 LED pulsers and their two controllers) at the Ex-213

perimental Staging Building (ESB) at Jefferson Lab [11]. Its214

ends are referred to as upstream and downstream, following the215

convention relative to beam direction in Hall D.216

The SiPMs used for these tests were selected based on bin-217

ning their operational voltage for the four bias distribution lines218

per end of the module. In this manner, the common bias sup-219

plied on each line results in approximately the same gain for220

each SiPM. The same principle will be applied for sorting all221

3840 units for the full BCAL.222

During these tests the prototype upstream and downstream223

wedge boards had identical layout. However, this resulted in224

needing to use different bias lines to power the SiPMs on op-225

posite ends of the same readout cell. As this is both confusing226

to the experimenters and results in doubling the cross talk to227

adjacent cells, the production upstream and downstream boards228

will have a mirrored configuration.229

Primarily for reasons of cost, the SiPM signals for the GlueX230

experiment are summed in the following manner: the first, inner231

(to the incoming particles during the experiment) row of SiPMs232

are read individually in each column, the next two rows are233

summed, the next three after those are summed as are the last234

four rows. This is referred to as the 1:2:3:4 summing scheme for235

each column, resulting in 16 readout channels and is sketched236

in Fig. 2. All of these are directed to FADCs (16 channels per237

end per module), whereas only the 1:2:3:4 readouts are directed238

to TDCs (12 channels per end per module).239

The ESB setup included a fine-control bias supply as well240

as a chiller to keep the SiPMs at the constant temperature of241

16◦C. The operating bias was set at an overbias of +1.2 V, i.e.242

in the range from 74-75 V; the SiPM bias is referred to as high243

voltage (HV) to distinguish it from the low voltage applied to244

its electronics amplifier (<8 V).245

Preliminary testing of the HV channels were conducted by246

turning them on one at a time and observing the signals on an247

oscilloscope. During the initial tests, the downstream LEDs248

5

101

102

103

104

Mea

n Si

gnal

(AD

C ch

anne

ls)

8765LED BIas (V)

Upstream Downstream

Figure 7: ADC values of the LED light are plotted as a function of appliedLED bias. The light was detected by the SiPMs in Channels 1 and 2, respec-tively, located at opposite ends of a calorimeter readout cell in one of the middlecolumns.

were flashed first, facing the opposite side of the calorime-249

ter. Upstream-to-downstream ratios of each of the ten SiPMs250

for each column were calculated by the pulse amplitude cap-251

tured on an oscilloscope. In addition, columns adjacent to the252

flashed SiPM channels were investigated for cross talk to es-253

tablish whether the cross talk is an optical or electronic effect.254

The pedestals were determined by evaluating the first fifteen255

samples of the FADC. These values were then subtracted from256

the signal ADC values to obtain the peak values of the signals.257

As expected, a strong correlation in ADC values between cor-258

responding upstream-to-downstream SiPMs was present where259

again the downstream LED was flashing and its light is visible260

between both downstream (near) and upstream (far) SiPM. The261

LED bias was varied from 4.0 to 7.5 V. The signal was minimal262

below 5.0 V, and above this value the resulting SiPM signal263

ranged from 1000-8000 ADC channels as graphed in Fig. 8.264

Based on these results, a bias of 6.4 V (middle of the range)265

was chosen for the remainder of the tests.266

The stability of the LEDs was evaluated, particularly since267

its bias range of operation was narrower than initially expected.268

Most data runs lasted for a few minutes but some continued for269

a few hours. No change in the SiPM output was observed for270

such short periods. Finally, a run lasting 42 hours was found271

580

570

560

550

Mea

n Pe

ak V

alue

(AD

C ch

anne

ls)

40302010Time (hours)

Figure 8: The stability of an upstream LED at the low bias of 6.4 V is graphedover a period of over 42 hours of continuous running. For more details thereader is directed to the text.

to yield a small day-night variation of less than ±3 counts out272

of 565 (±0.5%), most likely due to the temperature variation in273

the ESB. The variation is depicted in Fig. 9.274

Optical cross talk was observed in adjacent columns and275

quantified relative to the flashed ones. The cross talk is visi-276

ble to the naked eye when the LED trigger runs at 1 MHz and277

the LED bias at 10 V, otherwise at lower values it is detectable278

only by an oscilloscope. Even there, none was detectable be-279

low LED bias values of 5.5 V but above this as much as 10%280

appeared in some channels. Specifically, on the near side, there281

was a few percent of cross talk on either side of the cell of the282

firing LED. On the far side the optical cross talk was as large283

as 10%, but produced signals only in the cell adjacent to the284

LED and was confined to the row of fibers that bordered the285

adjacent light guides. The cross talk is peculiar to the geometry286

of the fibers and light guides as illuminated by the LEDs, and,287

therefore, should not be taken as representative of the response288

to particles, which needs to be studied for more representative289

configurations. In other words, the optical cross talk is not an290

effect of the LED monitoring system.291

Measurements were carried out subsequently on a full-scale,292

production module [12]. A snapshot of an LED event is pre-293

sented in Fig. 10. The extracted far/near ratios ranged from one294

6

Figure 9: Typical LED pulses in a readout cell as recorded by its SiPMs, withthe far end (upstream) shown on the left and the near one (downstream) on theright, relative to the placement of the LEDs. The near plot shows a shoulderthat is most likely related to an optical reflection off one of the optical boundarybetween the light guides and the calorimeter’s fibers. Such shoulders were notalways present and in any case do not affect the operation of the monitoringsystem.

to four in all cases examined. There are several factors that295

contribute to this difference (e.g. fiber light output, light guide296

gluing) but the driving factor is most likely the precise place-297

ment and angle of the LEDs in the pockets. However, only the298

relative ratios need to be monitored during the experiment and299

thus the system will function as intended, perhaps by employ-300

ing an online software correction to place all ratios to one, for301

ease of examination by experimenters.302

These tests demonstrated that the signals resulting from303

lower LED bias show clearer signal response and that the LEDs304

can maintain a steady light output voltage of less than 0.5%305

over nearly two days of continuous operation, something well306

beyond any projected running during the experiment.307

3. Summary and Conclusions308

The relative gain of the photodetectors for the GlueX Bar-309

rel Calorimeter (BCAL) will be monitored by employing LED-310

based systems. The LED system is modular with a design311

based on controller boards, each controlling a multitude of LED312

pulser boards, comprising of single-LED boards. The key ob-313

jective is to monitor the performance of the BCAL SiPMs.314

For the entire BCAL monitoring system, 3840 LEDs are315

mounted each on its own single-LED pulser board, glued to316

the side of a light guide and coupled in groups of ten using317

flexible cables. The system was verified to not interfere elec-318

tromagnetically with the SiPMs. The light output of all of the319

LEDs was measured and the results were used to bin the LEDs320

into groups of ten, so as to ensure similar performance across321

a given section of the calorimeter. A full system of 80 LEDs322

and two controller boards were first deployed on a prototype323

module and then on a production module and was examined for324

optimum bias operation, light output and optical cross talk, re-325

vealing no issues of concern. These systems have been installed326

on all BCAL modules.327

4. Acknowledgments328

This work was supported by Jefferson Science Associates,329

LLC, who operates Jefferson Lab under U.S. DOE Contract No.330

DE-AC05-06OR23177, DOE Office of Nuclear Physics grant331

DE-FG02-05ER41374 at Indiana University and NSERC grant332

SAPJ-326516 at the University of Regina. I would like to thank333

David Lawrence for all of his patience and for all his to the334

side explanations when confusion set in; Elton Smith for his335

detailed explanations and allowing me to utilize his previous336

works. I also must give my thanks to the Jefferson Lab for337

allowing me the wonderful opportunity to enjoy my time with338

the facility, assisting the above physicists with their work and339

allowing me access to the buildings I conducted my research in,340

the Continuous Electron Beam Accelerator Facility (CEBAF)341

as well as the Experimental Science Building (ESB).342

Figure Captions343

Fig. 1. To be added later. With Figures?344

Fig. 2 ...345

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