Light Collection Efficiency and Uniformity of Light Guides for the sPHENIXElectromagnetic Calorimeter

S. Stoll, D. Cacace, J. Huang, Z. Parsons, Z.Shi, T. Shimek, C. Woody

sPHENIX EMCalorimeterProposed for the sPHENIX Experiment at RHIC, Brookhaven Nat. Lab. Upton, NY. USA

2

7.5 cm

readout

26 cm total radial space

~14 cm absorber

(h=0)

Light Guides 24,576 total

1 Sector = 96 Blocks

= 384 towers

= 384 light guides

EMCAL Design Specs:

• Coverage: ± 1.1 in h, 2p in f

• Segmentation: Δη x Δφ ≈ 0.025 x 0.025

• Readout channels: 96x256 = 24576 (towers)

• Energy Resolution: sE/E < 15%/√E

• Provide an e/h separation > 100:1

• Approximately projective

• Compact (in order to fit inside Babar solenoid)

• Works inside a 1.5T magnetic field

1 block = 4 towersPhysics Contributions:

• Jet measurements (EM component)

• Photon measurements

• Y measurement

Spacial constraints

• The tungsten powder scintillating fiber calorimeter (SPACAL)

technology was developed by Oleg Tsai. We started with his basic

machined trapezoidal light guide design, and then tuned and optimized

it to fit our projective calorimeter design.

• We used TracePro Ray tracing software to simulate light collection in

various geometries, then produced samples of some of the designs that

gave good results.

• Lab measurements were made using a pulsed LED and a single fiber

scanned across the light guide surface to validate the ray tracing model.

• Some of the designs were produced and installed on detector

prototypes, and then evaluated in beam tests.

• We also explored other options such as diffuse reflecting cavities

• Machining and polishing ~25,000 pieces is cost prohibitive, so we

explored other manufacturing options and designs, such as injection

molding, and a “quad” light guide assembly that would combine 4 tower

light guides into a single manufactured unit. Our detector absorber

blocks are manufactured in 4-tower blocks, so a 4 tower light guide

design is attractive.

3

Overview

Light Guide Design Constraints• Needs to fit in radial space ~ 25mm

• 24,576 towers - Each tower area has ~24mm x 25mm trapezoidal shaped readout face. (22 variants)

• Readout sensors - Hamamatsu S12572-015P MPPCs each have 3mm x 3mm active area. Four SiPMs per tower, 36mm2

total active area. Optically coupled to the lightguide (Momentive RTV615 Refractive Index: 1.406)

• The SiPM pcb is mechanically attached to the LG by a screw in the center of each tower, requiring a tapped #2-56 hole

• The readout surface of each tower is tungsten powder-epoxy composite, with 667 scintillating fibers embedded in it. The fibers are 0.47mm diam. and are spaced 1mm center-center. They emit in the 420-450nm range.

• The lightguide will be epoxied to the surface (BC600 Optical Epoxy).

• So, we need to read out a 506 mm2 area surface with 36 mm2 of sensors.

• The light guide also needs to be as efficient as possible, and also uniform across the entire area.

• For the calorimeter overall, we also need to minimize the effect of gaps between towers, between blocks, and between sectors.

4

Block-block boundaries

Fiber ends in

tungsten powder-epoxy

compositeAdd sipm

photo

SiPM dimensions – 4 per tower

TracePro Raytracing simulation

5

• To simulate the light collection behavior, CAD models of light

guides were evaluated with ray tracing simulations.

• A single fiber, modeled after those in the absorber blocks, was

used to input light into the modeled light guides. The single fiber

was scanned across the input face of the light guide model in

discrete steps.

• The light was then “collected” on the readout surface. The

efficiency is the ratio of rays incident on the sipm array (or pmt) /

rays entering the light guide.

Irradiance map showing distribution of

incident rays on the SiPM array

Ray tracing diagram shows incoming and reflected

light rays

SiPM array

planeInput fiber

• Fiber Diameter 0.47mm

• Core Material – Polystyrene with index: 1.597

• Cladding Material – PMMA with index: 1.494

• 420nm Light Source

• 158mm Length

• there is an absorbing coating, on the outer surface of the fiber cladding to simulate the effect of epoxy in the block to absorb the cladding light.

• Where there is a reflector, this is referring to a perfect mirror placed on the outer surface of the far end of the fiber, core and cladding.

Fiber Parameters

for simulation

Far

End

Near

End

Fiber

Cladding – Pink

Core – Blue

Light Source inside

fiber.

Light

Guide

3

With Coating and With a Reflector

Polar Angle (Degrees)

0 5 10 15 20 25 30 35 40

Co

un

t

2.5

7.5

12.5

17.5

22.5

0.0

5.0

10.0

15.0

20.0

25.0

6

Distribution of Emitted light from fiber:

Measured data:

Uniformity scan of machined

trapezoidal light guide. SiPM and

PMT readout and TracePro

simulation data.

7

“Stubby” 25mm trapezoidal Light Guide

Edge/center: 0.94 Edge/center: 0.87

25mm

machined

Acrylic LG50mm

machined

Acrylic LG

8

Comparison of Trapezoidal light guides – 50mm and 25mm on pmt

The longer, 50mm light guides had a flatter more uniform response. The shorter 25mm light guides

Had more “roll-off” near the edges. This was also consistent with the Testbeam data.

position (mm)

-30 -20 -10 0 10 20 30

am

plit

ud

e (

mV

)

0

10

20

30

40

50

60

rough surface

painted white

polished

polished

response uniformity of light guides with different surface treatments

white rough polished

white rough polished

polished

Effects of surface treatment – measured data.

9

Light guides with painted or diffuse surfaces (non-specular) show a

20-25% lower efficiency, and less uniform response, with diminished

response near the edges.

Would making the surfaces diffuse reflectors improve the uniformity?

10

Measured Data: Effect of tapped

mounting hole on LG uniformity.

The tapped hole has no apparent effect on the

uniformity. The hole is not deep enough to significantly

“shadow” the SiPMs

Tapped hole and screw

A tapped hole is required to attach the SiPM pcb.

Does it affect the light collection?

11

Winston Cone –

hybrid designs

Unlike a true Winston cone, our

application has approximately square

input and output surfaces

Y Axis (mm)

-15 -12 -9 -6 -3 0 3 6 9 12 15

Eff

iecie

ncy

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Trapezoidal

WCLG 1

WCLG 2

WCLG 3

WCLG 4

WCLG 5

WCLG 6

Read out with PMT Read out with 4 SiPMs

Y Axis (mm)

-12.5 -7.5 -2.5 2.5 7.5 12.5-15.0 -10.0 -5.0 0.0 5.0 10.0 15.0

Eff

icie

ncy

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Trapezoidal

WCLG1

WCLG 2

WCLG 3

WCLG 5

WCLG 6

In simulation, these

light guides were more

uniform when read out

with a PMT (full

coverage), but were

less uniform when

read out with 4 SiPMs

(21% coverage).

WCLGx = Winston Cone Light Guide

Winston cone – hybrid design – cast acrylic

Measured with pmt (full coverage)

Measured data –

with 4 SiPMs (summed output)(Geometric efficiency: SiPM active area / end

area = 36mm2 / 174mm2 = 0.21)

12

For both the measurement

and simulations, with full

coverage of the readout

surface using a pmt, the

response is very uniform…

But with 4 SiPMs , the

response is significantly

less uniform.

Simulation with pmt and SiPMs

pmt

4 SiPMs

13

Measured Uniformity of Quad, injection molded

acrylic light guide on pmt

Measured Uniformity of block + quad light

guide on pmt

…combined response is flatter

“collared” fiber block - read out directly

with pmt (no light guide)

14

0

50

100

150

200

250

0

10

20

30

40

50

-10

0

10

20

30

40

ampl

itude

X position (mm)

Y p

osi

tion (m

m)

apr03 uniformity map of UI block w/fiber "collar" on 7cm pmt. RTVcoupled. LED fiber @ 5mm

X profile at Y= 30mm

X position (mm)

0 10 20 30 40 50 60

am

plit

ude

0

50

100

150

200

250

Y profile at X=15mm

Y position (mm)

-10 0 10 20 30 40

am

plit

ude

0

50

100

150

200

250

• “Collaring brings” the fibers in from the block edges at the

readout end. This improves the light collection near the

block edges

• It also allows for a single light guide shape to be used for

all 25k towers.

• These blocks will be beam tested in a new prototype (Feb

2018)

Simulation:Locus of points where rays hit the narrow end of the light guide for different fiber positions (indicted by yellow dot)

Square boxes

indicate locations of

SiPMs 15

Measured data:

Uniformity scan of trapezoidal

light guide – 3/4 SiPMs masked

Light distribution –

dependence on location of input. Four SiPMs with Air Gap Flux Efficiency (X Axis)

Position (mm)

-12.5 -7.5 -2.5 2.5 7.5 12.5-15.0 -10.0 -5.0 0.0 5.0 10.0 15.0

Eff

icie

ncy

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21 All SiPMs

SiPM 1

SiPM 2

SiPM 3

SiPM 4

16

50mm

machined

lightguides

25mm inj.

molded

lightguides

50mm

25mm

Test beam results A prototype was tested in 8 GeV electron beam at FermiLab Test Beam Facility (FTBF) /T1044 Jan/2017 This 8x8 tower prototype was instrumented with 50mm machined light guides on the top half and injection molded 25mm light guides on the lower half.

50mm LGs have

more uniform

response, less

significant loss

between towers

25mm LGs have

peaked response,

drops off near

edges, with more

loss between towers

Test beam results A prototype was tested in 8 GeV electron beam at FermiLab Test Beam Facility (FTBF) /T1044 Jan/2017

17

0 Degree -

Normal incidence

10 Degree

rotation

Rotating the detector 10deg

relative to the incident

beam smooths out the

block boundary effects and

reduces “channeling”

This tilt angle was

incorporated into the

calorimeter design in eta

and phi

50mm machined LGs

Inter-block

boundary

Intra-block

tower

boundary

Projection

18

Scintillation

Emission

peakScintillation

Emission

peak

Based on

measurements

and simulations of

the experimental

hall, the sPHENIX

EMCal is expected

to receive

~ 10 krad per run

year.

So cumulative

effects of radiation

should be minimal

- 2-3% over

detector lifetime

Acrylic samples

irradiated with 60Co gamma rays

at BNL SSGRIF

facility.

Conclusions

19

• The short radial space constrains us to a short, “stubby” trapezoidal light guide design. The

longer 50mm light guides were more uniform, but radial space constraints preclude their use.

• Although the Winston cone hybrids had a slightly higher efficiency when their full surface was

read out, they had a less uniform response with the 4 sipm readout.

• Attempts to make the LG surface more uniform by making it a diffuse reflector rather than a

specular reflector made the response less uniform and less efficient.

• We have attempted to mitigate the effect of block boundaries by “collaring” the fibers at the

ends of the absorber blocks to move them in from the light guide edges. This design change will

be evaluated in an upcoming beam test.

• We will also use a single tower, 25mm injection molded light guide with a 1mm “step” in the next

prototype. Simulations suggest that the “step” improves the light collection near the edges of the

light guide.

• Given the proximity of the scintillation emission to the transmission edge in acrylic, it would be

prudent to use UVT acrylic.

References

20

Tsai, O. et al. Results of R&D on a new construction technique for W/ScFi Calorimeters.

J. Phys Conf Ser, 404:012023, 2012.

Tsai, O. et al. Development of a forward Calorimeter system for the STAR experiment.

J. Phys Conf Ser, 587(1):012053, 2015.

Aidala, C. et al, Design and Beam Test Results for the sPHENIX Electromagnetic

and Hadronic Calorimeter Prototypes. arxiv.org/abs/1704.01461

sPHENIX - 2017 Testbeam Analysis Note – Osborne, Huang

Simon, Hertzog, Jones, Rhodes, Yairi. Modeling and scanning lightguides for Pb/SCIFI calorimeters

Nucl Inst & Meth in Phy Res A 335 (1993) 86-101.

TracePro Lambda Research Corp, Littleton, MA USA www.lambdares.com

Momentive Performance Materials Inc. Waterford, NY www.momentive.com (RTV615)

Hamamatsu Corp. Bridgewater, NJ. www.Hamamatsu.com

Other sPHENIX talks/posters at IEEE NSS 2017:

• N-38-1 Test Beam Results and Status of the sPHENIX Calorimeter System – M.E. Connors• N-38-2 Design of a Compact TPC for the sPHENIX Experiment – K. Dehmelt• N-38-3 Design and Performance of the Readout Electronics for the sPHENIX Calorimeters – E. Mannel• N-35-6 The Readout of the sPHENIX Tracking System – M. Purschke• N-03-049 Design Studies for a TPC Readout Plane – B. Azmoun