KR_SLAC_Jan03 1

Review of R&D carried out for MINOS active detector

……………………………..Keith Ruddick, University of Minnesota

• With particular emphasis on liquid scintillator• …………………………………but also relevant for solid• Possible liquid scintillators• Light collection; WLS fibers• Containment; importance of high reflectivity• Light yields• Aging studies• Mechanical integrity• NIM A463 (2001) 194

KR_SLAC_Jan03 2

The concept

• Liquid scintillator contained in cells with reflecting walls

• Scintillation light eventually hits wavelength-shifting fiber

• Absorption and re-emission in WLS fiber – trapped light

• Fiber-optic to external photodetector

WLS fiber

Particle trajectory

 

KR_SLAC_Jan03 3

Many liquid scintillators available

-all have max 425 nm; decay2 ns;light yields fall 20-25% with oxygenation

Scintillator types Rel. light yield (nominal)

Description

BC517L, EJ321-L 1.00 Mineral-oil based/pseudocumeneFlashpoint 215F

BC517H, EJ… 1.25 Same, higher fluor concentrationFlashpoint 180F

Scintisafe, Ecoscint,…

1.25 Phenylxylolethane based (PXE)Flashpoint >300FNo oxygenation effect??Biodegradable/non-toxic

RS 100 (Chooz) 1.25 Isopropylbiphelyl-basedMore expensive??

 

KR_SLAC_Jan03 4

Scintillator emission spectra

0

0.2

0.4

0.6

0.8

1

1.2

375 425 475 525 575Wavelength (nm)

Rel

. I

nte

nsi

ty

MINOS scintillator

Ecoscint-A

Ecoscint-O

Scintisafe

BC517

UV lightFiber-optic cable to spectrometer

KR_SLAC_Jan03 5

Various liquid scintillator light yields relative to MINOS solid scintillator

Cs137 gammas in liquid scintillators

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500 600

Channel number

Series1

Series2

Series3

Series4

Series5

Series6

Series7

MINOS scintillatorBC517L (Bicron)Ecoscint-O (Nat'l Diagnostics)Scintisafe 30% (Fisher Chem)Ultimagold (Packard)EJ-399-04 (Eljen Technology)EJ-321-L (Eljen Technology)

137Cs gammas

Compton edge at 0.48 MeV

PMT

KR_SLAC_Jan03 6

Wavelength-shifting (WLS) fiber

• Multi-clad fibers from Kuraray (Bicron/St. Gobain much less reproducible)

• Have measured emission spectra, light attenuation, chemical interaction with scintillators, etc., etc.

• Decay time 7.5 ns, v=0.58c• Light capture fraction=

 

Outer-cladding – polyfluor n=1.41

Inner-cladding – acrylic n=1.50

Polystyrene core with Y-11 fluor n=1.59

%7.559.12

41.159.1

2

inner

outerinner

n

nn

5.7%

KR_SLAC_Jan03 7

WLS fibers: attenuation properties

• Attenuation curve has two components (apparently)

• Sum of 2 exponentials gives quite good fit

• Short component with att1 m

• Long component with att10 m

• Attenuation due to self absorption - independent of fiber radius

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7 8 9 10

Distance along fiber(m)

Relat

ive lig

ht yie

ld

KR_SLAC_Jan03 8

Light yield vs fiber position in cell (1 mm diam fiber in 3 3 cm cell)

1 3 5 7 9

11 13 15 17 19 21 23 25 27 29

S1

S7

S13

S19

S25

0

0.2

0.4

0.6

0.8

1

1.2

1-1.2

0.8-1

0.6-0.8

0.4-0.6

0.2-0.4

0-0.2

KR_SLAC_Jan03 9

Self-absorption in the WLS fiber

• We have measured spectra coming from fiber end as function of position of excitation

• Source of “double-exponential” attenuation is rapid self-absorption of 500 nm emission (1 m att. length) followed by slower absorption (10 m) at greater wavelengths, characteristic of polystyrene clear fiber

KR_SLAC_Jan03 10

WLS fiber attenuation lengths vs wavelength

0.01

0.1

1

10

100

1000

10000

350 370 390 410 430 450 470 490 510 530 550

w avelength (nm)

atte

n le

ngth

(cm

)

Manufacturer’s data (Bicron)

Calculated from spectral data vs distance

KR_SLAC_Jan03 11

Light collection from end of fiber

Light intensity from fiber end

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80 100

Angle (degrees)

Rel

ativ

e in

ten

sity

sinmax=(ncore2-nclad

2)

= 47o lens focusing?

KR_SLAC_Jan03 12

Containment material

• For plastics, the crucial property is the glass transition temperature Tg above which creep may occur:

Polymer Tg(oC)

polyethylene -80

polypropylene

-17

PVC 80 - 90

polystyrene 90

acrylicpolycarbonate

105167

Tg

PVC is cheapest

KR_SLAC_Jan03 13

PVC reflectivity

• Many samples were obtained from a Color House (Korlin Concentrates, Stratford, Ont)

• Reflectivity measured using spectrophotometer (Lynn Miller, Indiana)

• Best is 0.965 at 425 nm - has >10% TiO2

• Testing done with 8 m long extrusions from LB Plastics

• 8-cell wide extrusions “off-the-shelf” - 2.4 cm square cells

Reflectivity vs wavelength

0.50.55

0.60.65

0.70.75

0.80.85

0.90.95

1

350 400 450 500 550

Wavelength (nm)

Ref

lect

ivit

y

Alliance PVC

LB Plastics PVC

15% TiO2 sample

KR_SLAC_Jan03 14

Diffuse light scattering at TiO2 reflector

Reflected light

Incident light

Intensity cos (Lambert’s law for diffuse reflection)

 

Scattered light intensity

0

0.2

0.4

0.6

0.8

1

1.2

0 30 60 90

Angle (degrees)

Rel

ativ

e in

ten

sity

Measurements with MINOS reflector

Few % (roughly) specular reflection

KR_SLAC_Jan03 15

Light yield vs fiber diameter

• Assume square cells

• Prob of hitting fiber

• Light yield

• w is cell width, d is fiber diam, a = (1 – average

reflectivity)• Normalized to MINOS geometry

(1.2 mm fiber) for comparison• Light yield independent of

cell size• Approximately proportional

to fiber diameter

daw1

1

a

d

daw

w

1

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.25 0.5 0.75 1 1.25 1.5 1.75

Fiber diameter (mm)

Ligh

t yie

ld re

lativ

e to

MIN

OS

2 x 2 cm cells

3 x 3

4 x 4

5 x 5

KR_SLAC_Jan03 16

Light yield using cosmic rays

• Photoelectron spectrum from cosmic ray muons at 7.5 m

• BC517L in “off-the-shelf” PVC extrusion i.e., TiO2 not optimized1.0 mm fiber

• 1.0 mm fiber; no mirror• Measured using HPD (qe

= 12%)• 3.2 pe average 27

photons

KR_SLAC_Jan03 17

Chemical interaction between components?

• Aging studies use d(effect)/dt exp(-/kT)

• Measured possible changes in mechanical properties of PVC with BC517L at elevated temperatures (none)

• No effects (< 5%) of BC517L on multiclad fibers at 50C over 8 months (also no effects on exposed polystyrene core for >2 years - ECAL

• No change in BC517 light yield at elevated temperatures over period of 8 months with PVC: scintillator ratio 3 times nominal

• All PXE scintillators dissolve fibers – more interaction correlates with more light yield

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80 100

Time (days)

Rel

ativ

e li

gh

t yi

eld

Black:35C

Red: 50C

KR_SLAC_Jan03 18

Scaling stresses and strains

PStress in web =

wt

wP

Deflection w

ww Et

Pwll

w

lw

tw

Max. stress in wall = 2

2

2t

Pw

Max. deflection = 3

4

.32 t

w

E

P

wt

P

(500 psi)

(.003 cm)

(1600 psi)

(.01 cm)

• Numbers in parentheses are for the “off-the shelf” PVC extrusions at a pressure of 20 psi ( 20 m at 45o); w=2.5 cm, tw=1 mm, t=2 mm

• For PVC, E=3.5 to 8105 psi; tensile strength = 7300 psi

KR_SLAC_Jan03 19

Conclusion

• A robust liquid scintillator detector can be made using a mineral-oil based scintillator contained in PVC extrusions

• R&D already done • Such a detector is simple to install – relatively

light weight (scintillator added after installation), no critical tolerances, no fragile components, no complex factory, no maintenance

• MINOS experience for calibration, etc.• Readout scheme: APDs, II/CCD,…..?• Could start building it tomorrow