- Email: [email protected]
Spectral attenuation length of scintillating fibers
L.*H __
Nuclear Instruments and Methods in Physics Research A 360 (1995) 24.5-247
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS
!% 53 !!I ELSEVIER
“E~~t~~”
Spectral attenuation length of scintillating fibers Guido Drexlin, Veit Eberhard Xernforschungszentrum
Kurlsru~e, institutor
Kernphysi~
*, Dirk Hunkel, B. Zeitnitz
Uni~Iersi~ of K~rlsr~e, ~nsti~t $ir ~xperimentelie K~rnp~~s~~
76021 Karlsruhe, Germany
Abstract A double spectrometer allows the precise measurement of the spectral attenuation length of scintillating fibers. Exciting the fibers with a Nz-laser at different points and measuring the wavelength dependent light intensity on both ends of the fiber simultaneously, enables a measurement of the attenuation length which is pra~i~~ly independent of systematic uncertainties. The experimental setup can additionally be used for the measurement of the relative light output. Six types of scintillating fibers from four manufacturers (Bicron, Kuraray, Pol.Hi.Tech, and Plastifo) were tested. For different fibers the wavelength dependent attenuation lengths were measured from 0.3 m up to 20 m with an accuracy as good as 1%.
1. Introduction In data sheets from manufacturers and in most measurements cited in literature only the integral attenuation length of scintillating fibers is considered and no account is taken of the spectral composition of the scintillation light. In general the spectrum of scintillation light is altered due to different attenuation processes and therefore depends strongly on the length of the scintillating fiber. The wavelength dependent quantum efficiency of photomultipliers or photodiodes enhances this effect. Therefore the integral attenuation length is only a rough quality criterium. However, for the design and simulation of scintillating fiber detectors the precise knowledge of the spectral attenuation length and light output is essential.
decompose the light and reflect its spectral com~nents with a wavelength dependent angle. Turnable spectrometer arms of 50 cm length, supported by ajustable precision aluminum plates are fixed below the center of each reflection grating. On each end of the spectrometer arms the decomposed scintillation light is picked up by a plastic fiber (Toray-PF, 1 mm) and ligh~i~d to two VALVO XP3462 photom~tipliers respectively. The spectrometer arms cover an angular range from 10” to 90” or, in terms of wavelength, from 390 nm to 900 nm. The spectrometers were calibrated by a dye laser to an accuracy of 0.1 nm. The spectrometer resolution depends on the gratings, the lenses, and the width of the optical slit and was calculated for a wavelength of 500 nm to be A h/h = 0.25%, which is identical to the measured resolution.
2. Double spectrometer 3. Measurement of the spectral attenuation length For the measurement of the spectral attenuation length a double spectrometer was constructed [4]. The scintillating fiber to be tested is clamped on an optical bench and is transversally excited by a N,-laser at any position along the fiber. The pulsed N,-laser emits UV-light of 337 nm with a pulse width of 3 ns (FWHM). On both ends of the fiber an identical spectrometer is built to measure the wavelength dependent light output of the fiber. In front of the fiber ends there are optical slits of 160 p,rn each. Two achromatic quartz lenses (f= 8 cm) focus the emitted scintillation light onto holographic reflection gratings with 1200 lines per mm fl]. The gratings
* Corresponding author. 0168-9002/95/$09.50
Exciting the scintillating fiber by the N,-laser at a distance x from the middle of the fiber of total length 1 produces a spectral intensity Z,,,(x, A) in the left and right spectrometers of: I,( x, A) = I,( A) e-(1/a-x)/A(A), 1,(x, A) =I,(
A) e-(1/2+X)/n(~)~
(1) (2)
Z,, denotes the initial intensity of light at the excitation point, A the wavelength of light, whereas A is the attenuation length. The signals S,, measured at the photomultipliers of both spectrometers at a certain wavelength h are related to the intensity of light I,,, by Sr,&
A) = Gr,Jr,,(~~
A),
Q 1995 Elsevier Science B.V. All rights reserved
SSDI 0168-9002(95)00096-8
V. TRENDS IN CALORIMETRY
G. Drexlin ef al. / Nucl, hstr. and Me& in Plqs. Res. A 360 (1995) 24%247
z44h
where Gr,, contains the gain of the photomultipljers, the spectral efficiency of the photocathode and the spectral transmittance of the plastic fibers. The signals S, and S, can be measured simultaneously and since the scintillation process is isotropic [2,3]. the ratio
Q(x,
4(x, A)
A) = ~l(x
f
Gr
=
ce
_‘,.,>,(*)
1
is independent of the initial excitation intensity I,. A second measurement at a different position x’ eliminates the unknown ratio of gain- and efficiency-factors C,/G, if the gains of the two photomuItipliers show a good linearity over a wide dynamic range. The linearity of the photomultipliers was measured in an additional experiment [4] to be better than 1%. The double ratio of two independent measurements at positions x and x’ thus is: -2x/A(A)
Q(x, A)
G,/G,e
(2(x', A)
G,/G,e-'"'/<""'
-=
in the fiber. The intensity of the laser pulses are precisely monitored by a photodiode for offline correction of the pulse intensity variations. The spectral intensity of the resulting scintillation light is measured for wavelengths from 400 nm to 600 nm with a stepsize of 10 nm. The measurement of each fiber type is repeated on several samples and the results are averaged.
=
e”“‘-.r,/,I(A)
The only parameters left are the measured signals S,.rt the positions of the excitation x and x’ and the attenuation length -4. The attenuation length can be derived therefore from:
5. Results Six types of scintillating fibers from four different manufacturers (Bicron, Pol.Hi.Tech, Kuraray and Plastifo) were tested with the described setup. Fig. la shows the attenuation length as a function of wavelength of two versions of the BCF-12 fiber from Bicron. Both versions consist basically of the same materials, but in one, denoted by BCF-12 + , high purity materials and a more careful production process were used. The form of the distributions is characteristic for all fibers tested here, with a minimum at 440 nm and an increase towards greater wavelengths. The minimum can be explained by self-absorption on the dyes in the fiber, as pure polystyrol, which is the basic material, does not show
S,( x. A)&( x’, A) A( A) = 2(x’ -x)/In
S,( x, A)&( x’, A) ’
All unknown parameters like the initial excitation intensity, Z,, the gain and efficiency factors cancel out. Moreover, the measurements can be repeated at several other excitation points for consistency tests. A further advantage is the fact, that only the attenuation mechanisms between the two excitation points contribute to the measurement, whereas attenuation processes between the excitation points and the fiber ends cancel out. The attenuation length is measured on approximately 2 m long samples for wavelengths from 400 nm to 600 nm with a stepsize of 10 nm. The fiber is excited at five points at distances of 0 cm, 530 cm and _t 60 cm from the middle of the fiber by 200 laser shots at each position. The signals from the left and right end photomultipl~ers are fed into a Q-ADC of a CAhJAC data acquisition system, which also controls the measurement. In advance of every laser shot the pedestal of the ADC is measured. All measured data are stored on magnetic tape for offline evaluation.
4. Measurement of the spectral light output For the measurement of the spectral light output, the fiber is excited by the laser at a distance of 10 cm from one fiber end and the spectral light output is measured by the spectrometer on the near side. This is to minimize effects on the light spectrum due to attenuation processes
400
450
SO0
550
Wavelength [n?l Fig. 1. (a) Attenuation length of Bicron fibers BCF-12 + (circles) and BCF-12 (squares) as a function of wavelength. (b) Light output of BCF- 12 + as a fun~~on of wavelength (solid line). The scaIe is ~ormaIized to the maximum yield at 470 nm. The dashed lines show the calculated light output for I m, 5 m and 10 m fiber length.
G. Drexlin et al. /Nucl. Instr. and Meth. in Phys. Res. A 360 (1995) 245-247
-
4
Om
i .,: 400
450
Fig. 2. (a) Attenuation length of function of wavelength. (b) Light as a function of wavelength (solid the maximum yield of BCF-12+ show the calculated light output length.
500
550 Wavelength
[:I?]
Pol.Hi.Tech fiber 0046-100 as a output of Pol.Hi.Tech 0046-100 line). The scale is normalized to at 470 nm. The dashed lines for 1 m, 5 m and 10 m fiber
The error bars denote the statistical and sytematical errors, which are only on the order of a few cm in the blue region, where in the green-red region due to the poorer photostatistics the errors approaches the order of 1 m. Above 440 nm the BCF-12 -t- reaches on average a 25% better attenuation length than the ordinary BCF-12 fiber. In this spectral region the attenuation is mostly due to reflection losses and scattering and depends therefore mainly on the purity of the materials and the quality of the surface between core and cladding. Fig. lb shows as a solid line the light output of the BCF-12 + fiber over the wavelength, where the intensity is normalized to the value at 470 nm. The light output curves of all other tested fibers are also normalized with respect to the light output of the BCF-12 + at 470 nm to obtain comparable absolute units. The light output is maximal from 420 nm to 460 nm and decreases then almost exponentially. The minimum of the attenuation length is very near to the maximum of the light output, so that the spectral component with the highest intensity is the most this effect.
247
attenuated. As the length of the fiber increases, the maximum light output is shifted towards greater wavelengths. This is demonstrated in Fig. lb by the dashed lines which denote the calculated light output for 1 m, 5 m and a 10 m long fibers with respect to the spectral attenuation length. This effect has to be considered when selecting the photomultipliers for a fiber detector, favouring the use of greenextended photocathodes. The light output curve for the ordinary BCF-12 is essentially the same but approximatly 20% lower. The fiber Kuraray SCSF81 shows essentially the same behavior as the Bicron BCF-12 + . The spectral attenuation length of a Pol.Hi.Tech 0046100 fiber is shown in Fig. 2a. The attenuation length below 500 nm is much lower than the Bicron BCF-12 + or the Kuraray SCSF81, but the light output curve of the Pol.Hi.Tech 0046-100 fiber in Fig. 2b is on average 25% higher than the BCF-12 + and shows its maximum at 460 nm. However, recently Pol.Hi.Tech has reported a much improved attenuation length of this fiber [S]. We also tested the Plastifo S 101 D scintillating fiber, the only fiber without an optical cladding. Due to high reflection losses caused by the missing cladding we found a maximal spectral attenuation length which is less than 3 m, where all other tested fibers easily achieved values up to IO-20 m.
6. Conclusions With the described experimental setup it is possible for the first time to measure the spectral attenuation length, the spectral light output and homogeneity [4] of scintillating fibers and hence to derive quality criteria of fibers independent from both the length of the fiber and characteristics of the light detector. Moreover the data allow detailed investigations of the different attenuation processes in the fibers. The experimental setup itself is relatively simple and the test of a fiber can be performed in a short time so that it could also be used for the control of the fiber quality during the production line.
References [I] PrZsions [2] [3] [4] f5.j
Beugungsgitter, Technische Daten, Carl Zeiss Oberkochen (1992). J.B. Birks, The Theory and Practice of Scintillation Counting (Pergamon, Oxford, UK, 1964). J.B. Birks, Photophysics of Aromatic Molecules (Wiley, London, 1970). D. Hunkel, Diplom Arbeit, Universit%t Kartsruhe (1994). V. Eberhard, Pol.Hi.Tech s.r.1. S.P. Turanese Km. 44,400 i-67061 Carsoii (AQ), private communication.
V. TRENDS IN ~~RIME~Y
Nuclear Instruments and Methods in Physics Research A 360 (1995) 24.5-247
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS
!% 53 !!I ELSEVIER
“E~~t~~”
Spectral attenuation length of scintillating fibers Guido Drexlin, Veit Eberhard Xernforschungszentrum
Kurlsru~e, institutor
Kernphysi~
*, Dirk Hunkel, B. Zeitnitz
Uni~Iersi~ of K~rlsr~e, ~nsti~t $ir ~xperimentelie K~rnp~~s~~
76021 Karlsruhe, Germany
Abstract A double spectrometer allows the precise measurement of the spectral attenuation length of scintillating fibers. Exciting the fibers with a Nz-laser at different points and measuring the wavelength dependent light intensity on both ends of the fiber simultaneously, enables a measurement of the attenuation length which is pra~i~~ly independent of systematic uncertainties. The experimental setup can additionally be used for the measurement of the relative light output. Six types of scintillating fibers from four manufacturers (Bicron, Kuraray, Pol.Hi.Tech, and Plastifo) were tested. For different fibers the wavelength dependent attenuation lengths were measured from 0.3 m up to 20 m with an accuracy as good as 1%.
1. Introduction In data sheets from manufacturers and in most measurements cited in literature only the integral attenuation length of scintillating fibers is considered and no account is taken of the spectral composition of the scintillation light. In general the spectrum of scintillation light is altered due to different attenuation processes and therefore depends strongly on the length of the scintillating fiber. The wavelength dependent quantum efficiency of photomultipliers or photodiodes enhances this effect. Therefore the integral attenuation length is only a rough quality criterium. However, for the design and simulation of scintillating fiber detectors the precise knowledge of the spectral attenuation length and light output is essential.
decompose the light and reflect its spectral com~nents with a wavelength dependent angle. Turnable spectrometer arms of 50 cm length, supported by ajustable precision aluminum plates are fixed below the center of each reflection grating. On each end of the spectrometer arms the decomposed scintillation light is picked up by a plastic fiber (Toray-PF, 1 mm) and ligh~i~d to two VALVO XP3462 photom~tipliers respectively. The spectrometer arms cover an angular range from 10” to 90” or, in terms of wavelength, from 390 nm to 900 nm. The spectrometers were calibrated by a dye laser to an accuracy of 0.1 nm. The spectrometer resolution depends on the gratings, the lenses, and the width of the optical slit and was calculated for a wavelength of 500 nm to be A h/h = 0.25%, which is identical to the measured resolution.
2. Double spectrometer 3. Measurement of the spectral attenuation length For the measurement of the spectral attenuation length a double spectrometer was constructed [4]. The scintillating fiber to be tested is clamped on an optical bench and is transversally excited by a N,-laser at any position along the fiber. The pulsed N,-laser emits UV-light of 337 nm with a pulse width of 3 ns (FWHM). On both ends of the fiber an identical spectrometer is built to measure the wavelength dependent light output of the fiber. In front of the fiber ends there are optical slits of 160 p,rn each. Two achromatic quartz lenses (f= 8 cm) focus the emitted scintillation light onto holographic reflection gratings with 1200 lines per mm fl]. The gratings
* Corresponding author. 0168-9002/95/$09.50
Exciting the scintillating fiber by the N,-laser at a distance x from the middle of the fiber of total length 1 produces a spectral intensity Z,,,(x, A) in the left and right spectrometers of: I,( x, A) = I,( A) e-(1/a-x)/A(A), 1,(x, A) =I,(
A) e-(1/2+X)/n(~)~
(1) (2)
Z,, denotes the initial intensity of light at the excitation point, A the wavelength of light, whereas A is the attenuation length. The signals S,, measured at the photomultipliers of both spectrometers at a certain wavelength h are related to the intensity of light I,,, by Sr,&
A) = Gr,Jr,,(~~
A),
Q 1995 Elsevier Science B.V. All rights reserved
SSDI 0168-9002(95)00096-8
V. TRENDS IN CALORIMETRY
G. Drexlin ef al. / Nucl, hstr. and Me& in Plqs. Res. A 360 (1995) 24%247
z44h
where Gr,, contains the gain of the photomultipljers, the spectral efficiency of the photocathode and the spectral transmittance of the plastic fibers. The signals S, and S, can be measured simultaneously and since the scintillation process is isotropic [2,3]. the ratio
Q(x,
4(x, A)
A) = ~l(x
f
Gr
=
ce
_‘,.,>,(*)
1
is independent of the initial excitation intensity I,. A second measurement at a different position x’ eliminates the unknown ratio of gain- and efficiency-factors C,/G, if the gains of the two photomuItipliers show a good linearity over a wide dynamic range. The linearity of the photomultipliers was measured in an additional experiment [4] to be better than 1%. The double ratio of two independent measurements at positions x and x’ thus is: -2x/A(A)
Q(x, A)
G,/G,e
(2(x', A)
G,/G,e-'"'/<""'
-=
in the fiber. The intensity of the laser pulses are precisely monitored by a photodiode for offline correction of the pulse intensity variations. The spectral intensity of the resulting scintillation light is measured for wavelengths from 400 nm to 600 nm with a stepsize of 10 nm. The measurement of each fiber type is repeated on several samples and the results are averaged.
=
e”“‘-.r,/,I(A)
The only parameters left are the measured signals S,.rt the positions of the excitation x and x’ and the attenuation length -4. The attenuation length can be derived therefore from:
5. Results Six types of scintillating fibers from four different manufacturers (Bicron, Pol.Hi.Tech, Kuraray and Plastifo) were tested with the described setup. Fig. la shows the attenuation length as a function of wavelength of two versions of the BCF-12 fiber from Bicron. Both versions consist basically of the same materials, but in one, denoted by BCF-12 + , high purity materials and a more careful production process were used. The form of the distributions is characteristic for all fibers tested here, with a minimum at 440 nm and an increase towards greater wavelengths. The minimum can be explained by self-absorption on the dyes in the fiber, as pure polystyrol, which is the basic material, does not show
S,( x. A)&( x’, A) A( A) = 2(x’ -x)/In
S,( x, A)&( x’, A) ’
All unknown parameters like the initial excitation intensity, Z,, the gain and efficiency factors cancel out. Moreover, the measurements can be repeated at several other excitation points for consistency tests. A further advantage is the fact, that only the attenuation mechanisms between the two excitation points contribute to the measurement, whereas attenuation processes between the excitation points and the fiber ends cancel out. The attenuation length is measured on approximately 2 m long samples for wavelengths from 400 nm to 600 nm with a stepsize of 10 nm. The fiber is excited at five points at distances of 0 cm, 530 cm and _t 60 cm from the middle of the fiber by 200 laser shots at each position. The signals from the left and right end photomultipl~ers are fed into a Q-ADC of a CAhJAC data acquisition system, which also controls the measurement. In advance of every laser shot the pedestal of the ADC is measured. All measured data are stored on magnetic tape for offline evaluation.
4. Measurement of the spectral light output For the measurement of the spectral light output, the fiber is excited by the laser at a distance of 10 cm from one fiber end and the spectral light output is measured by the spectrometer on the near side. This is to minimize effects on the light spectrum due to attenuation processes
400
450
SO0
550
Wavelength [n?l Fig. 1. (a) Attenuation length of Bicron fibers BCF-12 + (circles) and BCF-12 (squares) as a function of wavelength. (b) Light output of BCF- 12 + as a fun~~on of wavelength (solid line). The scaIe is ~ormaIized to the maximum yield at 470 nm. The dashed lines show the calculated light output for I m, 5 m and 10 m fiber length.
G. Drexlin et al. /Nucl. Instr. and Meth. in Phys. Res. A 360 (1995) 245-247
-
4
Om
i .,: 400
450
Fig. 2. (a) Attenuation length of function of wavelength. (b) Light as a function of wavelength (solid the maximum yield of BCF-12+ show the calculated light output length.
500
550 Wavelength
[:I?]
Pol.Hi.Tech fiber 0046-100 as a output of Pol.Hi.Tech 0046-100 line). The scale is normalized to at 470 nm. The dashed lines for 1 m, 5 m and 10 m fiber
The error bars denote the statistical and sytematical errors, which are only on the order of a few cm in the blue region, where in the green-red region due to the poorer photostatistics the errors approaches the order of 1 m. Above 440 nm the BCF-12 -t- reaches on average a 25% better attenuation length than the ordinary BCF-12 fiber. In this spectral region the attenuation is mostly due to reflection losses and scattering and depends therefore mainly on the purity of the materials and the quality of the surface between core and cladding. Fig. lb shows as a solid line the light output of the BCF-12 + fiber over the wavelength, where the intensity is normalized to the value at 470 nm. The light output curves of all other tested fibers are also normalized with respect to the light output of the BCF-12 + at 470 nm to obtain comparable absolute units. The light output is maximal from 420 nm to 460 nm and decreases then almost exponentially. The minimum of the attenuation length is very near to the maximum of the light output, so that the spectral component with the highest intensity is the most this effect.
247
attenuated. As the length of the fiber increases, the maximum light output is shifted towards greater wavelengths. This is demonstrated in Fig. lb by the dashed lines which denote the calculated light output for 1 m, 5 m and a 10 m long fibers with respect to the spectral attenuation length. This effect has to be considered when selecting the photomultipliers for a fiber detector, favouring the use of greenextended photocathodes. The light output curve for the ordinary BCF-12 is essentially the same but approximatly 20% lower. The fiber Kuraray SCSF81 shows essentially the same behavior as the Bicron BCF-12 + . The spectral attenuation length of a Pol.Hi.Tech 0046100 fiber is shown in Fig. 2a. The attenuation length below 500 nm is much lower than the Bicron BCF-12 + or the Kuraray SCSF81, but the light output curve of the Pol.Hi.Tech 0046-100 fiber in Fig. 2b is on average 25% higher than the BCF-12 + and shows its maximum at 460 nm. However, recently Pol.Hi.Tech has reported a much improved attenuation length of this fiber [S]. We also tested the Plastifo S 101 D scintillating fiber, the only fiber without an optical cladding. Due to high reflection losses caused by the missing cladding we found a maximal spectral attenuation length which is less than 3 m, where all other tested fibers easily achieved values up to IO-20 m.
6. Conclusions With the described experimental setup it is possible for the first time to measure the spectral attenuation length, the spectral light output and homogeneity [4] of scintillating fibers and hence to derive quality criteria of fibers independent from both the length of the fiber and characteristics of the light detector. Moreover the data allow detailed investigations of the different attenuation processes in the fibers. The experimental setup itself is relatively simple and the test of a fiber can be performed in a short time so that it could also be used for the control of the fiber quality during the production line.
References [I] PrZsions [2] [3] [4] f5.j
Beugungsgitter, Technische Daten, Carl Zeiss Oberkochen (1992). J.B. Birks, The Theory and Practice of Scintillation Counting (Pergamon, Oxford, UK, 1964). J.B. Birks, Photophysics of Aromatic Molecules (Wiley, London, 1970). D. Hunkel, Diplom Arbeit, Universit%t Kartsruhe (1994). V. Eberhard, Pol.Hi.Tech s.r.1. S.P. Turanese Km. 44,400 i-67061 Carsoii (AQ), private communication.
V. TRENDS IN ~~RIME~Y