Measurements of Attenuation Length of scintillating and clear fibers

D. Cassettari, O. Marago University of Pisa, Department of physics

P.za Torricelli, 2 - 56100 Pisa, Italy

R. Brunetti University of Bologna, Department of physics

Via Irnerio, 46 - 40126 Bologna, Italy

J. Valls University of Valencia, Spain

M. Atac, M. Mishina Fermilab- P.0.Box 500 Batavia, Illinois - 60510

August,14 1995

Abstract

The purpose of our work was to measure the attenuation length of scintillating and clear fibers. This is important in order to know the amount of light we expect at the end of the path from the central detector to the readout <levices. The work was carried out at Fermilab. The experimental setup consisted in a dark box in which we put a scintillating fiber and a clear fiber spliced together. The scintillating fiber was illuminated with a UV lamp and the light yield was measured by a photodiode at the end of the clear fiber. Concerning scintillating fibers, we tested Kuraray 3HF, Kuraray vinyl 3HF, and Bicron 3HF, while con­cerning clear fibers we tested Kuraray S-type, Kuraray non-S-type and Bicron S-type. As far as we can say that there is no significant trend of attenuation length versus diameter in Kuraray 3HF scintillating fibers, we can observe, on the ot her hand, an increasing trend in clear fibers, although we need more points to approach a final conclusion. As we expected, we also noticed a difference between single clad and double clad fibers.

1 Motivations

ln the run II, in the CDF detector there will be a new kind of tracker positioned between SVX and the CTC: IFT (Intermediate Fiber Tracking). It consists of 6 axial layers and 6 stereo layers of scintillating multiclad fibers; every layer is composed by a staggered doublet to cover all the solid angle and to avoid spatial gaps between the sensitive cores of the fibers. We will use small diameter fibers because we want low occupancy, high resolution and we want to have the amount of material small in order to keep the multiple Coulomb scattering of particles to a minimum. A scintillating fiber is composed by a polystyrene sensitive core doped with PTP (1 % ) as a primary dye and 3HF (1500 ppm) or vinyl 3HF (1000 ppm) as a secondary dye. When a particle goes through a fiber it releases energy that is absorbed by polystyrene and PTP. As shown in figure 1, these materiais have short Stokes shift and their emission spectrum is just above the absorption spectrum of the 3HF; because of its large Stokes shift

FERMILAB-CONF-95-432-PPD

3HF can minimize the self absorption effect. Finally we have an emission of green light that is transmitted through a clear fiber and then it is detected by VLPC (Visible Light Photon Counter). During its path inside a fiber, the light is attenuated mainly because of two factors: self absorpion inside the core and reflections on the clads. We expect that the first one is more important in scintillating fibers, with respect to clear ones, because the core is doped and so it contains more impurities, while clear fibers core is composed by non-doped polystyrene. On the contrary the second factor should have the sarne weight in both scintillating and clear fibers because the clads are made of the sarne materials and the core's refraction index should not be strongly affected by the dyes. ln IFf the light follows a long way (severals meters) togo out, thus it is important to determine the attenuation length of scintillating and clear fibers, that is the purpose of our work.

1.0

= 0.8 o .... Vl Vl

'§ 0.6 ~

0.4

0.2

0.2

= o 0.4

ã "" ~ 0.6

,.Q

< 0.8

Polystyrene

1.0 Polystyrene

P-TP

550 600 650

Wavelength (nm)

Figure 1: Emission and absorption spectra of the sensitive core.

2 Experimental Setup

A scintillating fiber was spliced to a clear fiber and both of them were put in a dark box. To make the splicing, first of all we polished both ends with a d iamond fly cutting machine, then we spliced them into a plastic ferrule with 5-min Epoxy. The cutting machine was used also to polish the other end of the clear fiber, which was coupled with optical grease to a silicon photodiode 1 .

~-------. --·-~ ..... ...._,

Dark box º"'"''"'l é ! U\t Lamp

Clear fiber Splicing point [

~ Photodiode -

L1 m Sciniilluting

i'iber

- Picoammeter GPIB >-------- PC

Figure 2: Experimental setup.

The scintillating fiber was stretched straight in the box and illuminated by an ultraviolet (UV) lamp 2 Hg(Ar). This UV lamp presents an intense peak of"' 90% of output light in the short wave UV (253.7 nm). Thus the scintillating fiber produces green light that goes through the clear fiber until the photodiode. Eventually the light yield is measured by a picoammeter 3 , which is connected to a PC through a GPIB interface card. The output signal from the picoammeter is sampled by the PC and the final result is an average between 50 measurements.

3 Measurements and results

3.1 Scintillating fibers

For the measurements of attenuation length in scintillating fibers, the UV lamp was moved along the fiber and we picked up the signal for each position; then the experimental points were fitted with an exponential and from the fitting we obtained the attenuation length À:

I(x) =lo exp(-x/">..) 1 Graseby Optronics model #221 Silicon Photodiode. 2 0RIEL model 6035 with ORIEL model 6060 DC power supply. 3 KEITHLEY picoammeter.

(1)

where x is the position of the UV lamp and I( x) is the light yield. ln table 1 different types of scintillating fibers we tested are shown, together with clear fibers used in the mea­surement and attenuation lengths obtained from the fitting.

Scintillating Fibers

Type ])iameter(mm) Spliced to Att. Length (m)

5.025 ± 0.003 (*)

Kuraray S-type 4.88 ± 0.02 (*)

3HF (1500 ppm) 0.5 d = 0.8mm 4.85 ± 0.01 (**)

5 m long 4.53 ± 0.03 (**)

4.498 ± 0.002

4.571 ± 0.008

4.015 ± 0.002

Kuraray S-type 4. 723 ± 0.003 (*)

3HF (1500 ppm) 0.835 d = 0.83 mm 4.68 ± 0.01 (*)

3 m long 4.265 ± 0.003 (*)

Kuraray S-type 3.691 ± 0.002

vinyl 3HF (1000 ppm) 0.93 d=lmm 3.815 ± 0.001 (*)

3 m long 4.115 ± 0.001 (*)

Bicron S-type 4.027 ± 0.001

3HF (1500 ppm) 0.83 d = 0.9mm 3.937 ± 0.002

3 m long

Table 1: Attenuation lengths for scintillating fibers. The stars indicate which measurements are made with the sarne set offibers.

ln figure 3 attenuation curves are shown and we can notice that they are well fitted by exponentials. We can also see that different couples of fibers have different amounts of light, that is the correspondent curves are far one from the other. Since each curve has been normalized to the source, different amount of light are mostly dueto different lengths of the clear fibers used in the measurement (see 0.5 mm fibers compared to other types) and to some inevitable differences in the splicing points: in fact we made them by hand and so the amount of light transmitted changes from time to time.

ln figure 4 there is a summary of our results conceming scintillating fibers. For Kuraray 3HF we have two diameters and we can observe that there is no significant trend of the attenuation length versus diameter within our resolution (which is~ 103). Since vinyl 3HF fibers are single clad, they have an attenuation length smaller than Kuraray 3HF. Conceming Bicron fibers, these results suggest that they are single clad too (the attenuation lengths are comparable to those of the vinyl 3HF). Further more, looking at them through a microscope we could not find the second clad and so, although Bicron company sent us these fibers as multiclad, we conclude that they are single clad actually.

Ul -1 :t: 1 o e :J

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Attenuatíon Length ín Scíntíllatíng Fíbers

' ' -. --.. -~ ................... --... ---:·. -. -. -................ .

: :

,.:. .......................................................... 3HF .. d.:::.0.:.B3.5.mm .. .

2 3

*· À .:::.4.~3 m O À= 4l7 m e À= 4~7 m

VINYL 3HF A :\ = 4~ 1 m B À= 3~8 m *À= 3;7 m

···-BICRON·JHF·· ············· · ~À= 4l0 m r_, :\ = 3~9 m

3HF d=o:.s mm O :\ = 4l0 m e À= 4l9 m

.::::*:::~ .. ?. .. 5.Io::n;···· .. :.~ .. X.:::.4i5 .m . . .­

..... /\ ... ~. :::.4.l9.m ... /\ À= 4l9 m

..... ti.Ã = 4t5 rii ........... .

4 5 6 Dístance from splíced poínt (m)

Figure 3: Attenuation curves for scintillatingfibers.

Attenuatíon Length versus Scíntíllatíng Fíber Díameters ~ 6

E ..e +-' oi e Q) _j

e 5.5 o

+-' o :::l e Q)

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : : . . . . : : . .

--. -.... -~................. -----. --:- -. --. -. -.................. ~................... . . -. --~ -. -........................ :. --... --.

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..................................... • ........................... L ........................ .l.. ....................... J.

t

4.5 ....................... t ....................... , ........................... : ........................ \ .......................... ~~ .......... .

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4 .................. ···*····· ..................... i ........................... ] ........................... l ..... 1t ................ L ....................... . '{;:(

• . . * 3HF:1500 ppm kURARAY : · · •

............. 9..=::.9.~.:5Jl1_1!.J.! .. 9..=::.9.'.§.~.?. .. ~.~ ... :........ .... .. j . 3.5 : : ; ---------~---····················---~---·

3 0.4

• VINYL 3HF 1 ood ppm KURP)RAY d=0~93 mm : :

: : ' * 3HF:1500 ppm BICRON d=0~83 mm

0.5 0.6 0.7 0.8 0.9 1

Scíntíllatíng Fíber Díameter (mm)

Figure 4: Attenuation length VS diameter for scintillatingfibers.

3.2 Clear fibers

To measure the attenuation length in clear fibers, the UV lamp was kept at 1.1 m from the splicing. After each measure we cut and polish the clear fiber (typically we cut 25 cm of clear fiber); in other words we cut, polish, measure and so on. The reason of this particular distance, 1.1 m, is that in IFf the middle point, that is the point just above the vertex, is 1.1 m far from the splicing. It is important to test this point because here the particle has the smallest path inside the fiber and so the fiber produces the smallest amount of light. We tested also another point of the scintillating fiber, which was 2.2 m far from the splicing, to check if there was an effect dueto self absorption inside the scintillating fiber (this is because the attenuation length of the clear fiber depends strongly on frequency). This effect is well described by figure 5, that shows how 3HF spec­trum changes with length of the scintillating fiber. Anyway we found no difference in attenuation length within our resolution, probably thanks to the large Stokes shift.

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__ at 20 cm from Detector

_______ at 300 cm

475 500 525 550 575 600 625 650 675

Wavelength(nm)

Figure 5: Spectrum.

ln table 2 there are various types of fiber we tested, with attenuation lengths obtained in the fitting.

Clear Fibers

Type Diameter (mm) Att. Length (m)

0.5 6.39 ± 0.02 (#) 0.5 5.91±0.03 (*) 0.6 6.90 ± 0.01 (*) 0.8 7.73±0.01 (*) 0.8 7.20 ± 0.01 (*)

Kuraray 0.83 7.04 ± 0.04 (#) S-type 0.85 7.31±0.01 (*)

0.9 7.33 ± 0.03 (*) 1 7.98 ± 0.03 (*) 1 6.826 ± 0.007 (+)

1 7.404 ± 0.005 (+)

1 7.93 ± 0.01 (#)

0.83 9.55 ± 0.07 (*) Kuraray 0.835 8.79 ± 0.06 (*)

non S-type 0.85 9.74 ± 0.02 (*) 0.9 10.67 ± 0.02 (*)

0.965 9.76 ± 0.02 (*)

Bicron 0.75 5.402 ± 0.006 (*) S-type 0.835 6.05 ± 0.01 (*)

0.96 5.53 ± 0.01 (*)

Table 2: Attenuation lengths for clear fibers 4 .

We can notice that there is no important change if we use a different scintillating fiber (for instance see 1 mm S-type Kuraray). ln figures 6, 7 and 8 there are the attenuation curves, that are well fitted by exponentials. Finally figure 9 summarizes our results about clear fibers: for S-type we can notice an increasing trend of attenuation length versus diameter, while we cannot conclude anything about non S-type and Bicron because we miss some points. Non S-type fibers have the best attenuation length, but on the other hand they are more fragile than S-type fibers because they are less ftexible.

4The symbols in the table indicate which type of scintillating fiber was used in the measure: (*): spliced to d = 0.5 mm 3HF scintillating fiber (#): spliced to d = 0.835 mm 3HF scintillating fiber (+): spliced to d = 0.93 mm VINYL 3HF scintillating fiber.

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Attenuotíon Length ín S-type Clear Fíbers Kuroroy

. ' ' '

_ __ . _ _______ . : _ ., _____ Ã- .d::::.Ó.83.m~,.À::::7.Ó4.m _ . ________ ..:, _______________ ;, _______________ .6 ... d::::.i .. mm,.>.;::;:.7.93 .. m. _________ _

___ .l. ______________ L ____________ L _______________ .o ... d::::.i .. mm,.1.\::;:.7.4 .. rr.L ____ .. ____ _ ' '

: : : j j * d=i mm ~=6.8 m . ···-~-- - --······· .. ···········~·······-·······-:···············-~---············· ------·········-~---······--1 .... :-···············~············· .

· · · <~ d=Q.9 mmj ::\=6.5 jm ' ' ---- ------:----------------, .. _______ --- --.- -Ci~9-:tfn:1nir~-~7. im

__________ _., ___ d:;:;.Q .. 6.mm;_À;=..6._9_jm _____ .. __ d=Ó.9 mmj ::\=6.5 \m

_______ Ç_ .d::::Ó.8.mml.À;=.7.2im __ _ -t( d=1 mm, \=7.98 m

d=Ó.9 mm) À=7.3 )m --i---'----------------~--------------- --0----d=~-.5-mmJ--À-=ôA~m- .. -··--

. .à : () d=0.85 mm, À=7.3 m .:; d=Q.5 mm; À=5.9 :m

' . : ' o : ' ' ' : : '

...... º ...... c?. ............... ~ ................ : ... ~ ........... L .............. ; ................ : ................ ~ .. -- ........... ) ................. : .............. . . ' ' ' ' ' ' ' ' o ' '

: : : o : : -· -· ----------~ -----· · · · · · · · -· · ~ ---· · · · · · · · · · · --:--· --· ----· -· · · · r · · -· · -· · ---· --· 1 · ------- - -----t • - - - - - - - - - - • - - - ·r ----------------:- ----------------~- --------· -----' ' ~ '

"":·:_: .. :_:.i.::::"""":"::::""""-":::_:_,._:_::::::::c"":-:::·:::c::"-"::::·r:_: .. ~::::·:T:: .. ::_:_:.,_:r:::::: .. -.. -: .. :·:·:_:::_::"" : : --------~.:_' _______ '--_' _______ ._~.: __ , .. (.' ------ '.·- - - --- - -- ·-----••••••••••••••• i - • - - - .•••••••••• ;. ••••••••••••••• -:- •••••••••..•.•• -:- ••••••••••••••• ~- ••••••••••••••• ;...... - - . . . . ,,;. . . . . . . . . . . . . ' . . ' ' ' ' ' . ' '

2 3 4 5 6 7 8 9 10 Dístance from splíced poínt (m)

Figure 6: Attenuation curves for clear fibers.

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Attenuatíon Length ín non S-type Clear Fíbers Kuraray

o

. . _________ .: ....................... ; ....................... ; ....................... ; ..................... . . . .

. . . . ' . ' . . . ' ' . ···········--·--·-·······················-············································-·······--·············---··············--·-·········· ······-··············· . . ' ' . . ; . ; ; T . . . . . .

. . . -----. --....... -: .. -... -....... --... -. -. ~ ... -...... --. -..... -... -: . . . . . . . . . . . . . . .. --. -.. ~. -..................... ~ ......... -..... -. -..... ~ ... --........ -.... -.. . ' . . . ' ' ' . . . ' ' . . ' . ' ' . . ' . ' . . . ' . ' ' . . ' . ' . ...................... d=0.$5 mm i ············--····1--···--·----·--------··r·--·--·--------··--··r--·--··--·------·--·

T . : : . : ................................. ~··=··9)4·±-0-;02--i·--· .................. 1 ...................... : ....................... ; .................... ..

. . . ' . . . '

d=0#$3 mm ~ . : ~ .................. .. • .. i\--;;--9 ~55"±'0':07 1 .................... ~ ....................... ~ ..................... -~· ................ ..

d=0.$35 mm : ··········· ..... ·--·i\--=--8/79±{L06·-i--·· .............. . ................................................................. .

d=0.9 mm À i\ = 1 b.67±0.02\

.............................. d=0-;9-e-5--mm· ... ~ .... • i\ = 9)6±0.02 .

2 3 4 5 6 7 Dístance from splíced poínt (m)

Figure 7: Attenuation curves for clear fibers.

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o

Attenuation Length in Clear Fibers of BICRON

--------·--------·········-··-· --------·····················-:---------···--------············:-- ---························· ----

. . . . --a-=0~·9·5 .. n;-n;·-- ... ~ --- ...................... ,. ..... ... ....... ... , ..................... .

• x = 5.5296±0;0064 ····-·························-:---····-----····················}···--··--·--·-···- ............ ; ................................ :---··--························ ... .

~=0.835 mm ! ! x = __ 9):>4.~.iQ_._Q0.9-......................... ;._ ............................................................ -···

 d=0.75 mm

.............. _,, ______ ----·~--=:: .. ?.'.1.Q~ .. :t: ()_._qq_~---- ..................... : ............................... : .................................. .

2 3 4 5 Dístance from splíced poínt (m)

Figure 8: Attenuation curves for clear fibers.

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Attenuatíon Length Versus Clear Fíber Díameter

: : ' .

Kuraray S-type: . '

Kuraray non p-type

Bícron S-type: •

' ' ' ' ' ' . ' ......... ·~ .... --................. , ....... -............ .

• • • •

8 ........ . . ... i .......... .

* * t *

7 ............. .:.*·· ........... . . .. .

* * *

6 ...... -.... -~ ...

* ···*

OA 0.5 0.6 0.7 0.8 0.9 1 1.1

Clear Fíber Díameter (mm)

Figure 9: Attenuation length VS diameter for clear fibers.