Comparative study of WLS fibres for the ATLAS tile calorimeter

UCLEAR PHYSIC~

PROCEEDINGS SUPPLEMENTS Nuclear Physics B (Proc. Suppl.) 61B (1998) 106-111

ELSEVIER

Comparative study of WLS fibres for the ATLAS Tile Calorimeter A. Gomes ~, M. David a, A. Henriques ~'b, A. Mai& a LIP and FCUL, Av. Elias Garcia 14 1°, 1000 Lisbon, Portugal ~,b CERN, Geneva, Switzerland The Wave Length Shifting (WLS) fibres are one of the most important components of the ATLAS barrel hadronic tile calorimeter (Tilecal). The fibres collect the light produced in the injection molded scintillating tiles and transport it to the photomultipliers. Parameters like attenuation length and light yield are important, as well as flexibility and radiation hardness. Comparative results of WLS fibres produced by Bicron, Kuraray and Pol.Hi.Tech are presented. The performance of the fibres BCF91A from Bicron and S048 from Pol.Hi.Tech was significatively improved, but the most performant are still the double clad Y l l fibres from Kuraray.

1. I N T R O D U C T I O N The Tilecal concept (fibres running paralel to the edge of scintillating tiles) was proposed in 1991 [1] and since then several prototypes have been built. This calorimeter is made of iron plates interleaved with scintillating tiles read out by 1 m m diameter WLS fibres [2] [3]. The scintillating plastic tiles for the Tilecal are produced by injection molding technique at I H E P / P r o t v i n o , Russia [4]. The base material is transparent granulated polystyrene doped with 1.5% of P T P and 0.04% of P O P O P . Fibres from Bicron, Kuraray and Pol.Hi.Tech companies are being tested and used to build prototypes.

times of 10 ns or less are available. • Fibre to fibre response fluctuation should be small (of the order of 5% or less). • Fibres should be flexible and insensitive as possible to mechanical stress. • Fibres should be radiation hard, at least for a total dose of 400 kGy. • Fibre response to charged particles should be small. This is achieved by doping the fibres with an Ultra Violet Absorber (UVA). 3. E X P E R I M E N T A L

RESULTS

• The light from the fibre should be transmitted through the fibre with attenuation as low as possible.

Fibre response to the blue Tilecal scintillator is measured using a bi-alkali EMI9813KB photomultiplier tube (PMT), integrating the current I(x) with a digital multimeter [5]. The light in the scintillator is excited by electrons from a 9°Sr /3-source. The same/3-source is used to evaluate the WLS fibre response to charged particles. Position of the light (and/3) source, movement of the scanning table and measurement of the current, is computer controlled by a Macintosh equipped with L A B V I E W software [6]. In a first approximation the fibre light output can be described by the equation:

• Fibre response should be fast enough to avoid pile-up of signals. Fibres with decay

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2. G E N E R A L

REQUIREMENTS

For the Tilecal the lengths of the fibres are in the range from 85 to 220 cm. The general requests that are guiding this work are the following: • Fibres should have good efficiency in converting blue light from scintillator to a longer wavelength that should match the photodetector q u a n t u m efficiency.

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Figure 2. At left it is shown the light output for Yll(200)MS fibres with different UVA concentrations, illuminated with Tilecal scintillator light (black symbols) and with 9°Sr fl-source (open symbols). At right it is shown the ratio [I(140) with UVA] / [I(140) no UVA] for several fibres, as function of UVA concentration for scintillator (black dots) and electron response (open dots). The S048-100 UVA concentration was not specified.

108

A. Gomes et al./Nuclear Physics B (Proc. Suppl,) 61B (1998) 106 l l l

s L Lat t and Lat t are the short and long attenuation lengths, and Ios+ IOL is the light yield.

All light output results are presented in arbitrary units. As absolute reference, the number of photoelectrons produced by 150 GeV muons in the center of a 200x100x3 m m 3 Protvino scintillating tile read laterally by two Yll(200)MS fibres at about 2 m from light mixers in front of XP2012 PMTs is about 1.5 photoelectron. The fibres are mirror aluminized at the end opposite to the P M T with typical reflectivity values of 70%. 3.1. L i g h t o u t p u t a n d a t t e n u a t i o n l e n g t h Since 1992, fibres with various concentrations have been systematically tested. Optimal concentrations for each of the selected type of fibres were found. Till a certain limit, the light yield increases with increasing dopant concentration. First, the light transmitted through the fibre is not attenuated, but after a certain dopant concentration the increase in light yield is obtained at expenses of decreasing the attenuation length. In Fig. 1-1eft are shown the light output I(x) of the several typical fibres tested for the Tilecal calorimeter: Bicron BCF91A and BCF99-28, Y l l from Kuraray and S048-100 from Pol.Hi.Tech. In Fig. 1-right the same experimental results are shown but with the I(x) values normalised to the correspondent values for the BCF91A fibres (fibres used to instrument the first Tilecal prototype, and taken here as reference). The Yll(200)MS fibres have attenuation length of the order of 3 m [7], and give the best light output, at least for distances greater than 1 m. BCF99-28 fibres have higher dopant concentration than BCF91A giving more light without degradation of the long attenuation length (~ 2.6 m) [7]. Pol.Hi.Tech produces already fibres (S048-100-N4) with characteristics similar to the BCF91A. However for some of these fibres other parameters need also to be optimized, such as UVA concentration, mechanical fragility and radiation hardness.

3.2. Fibre cladding Light yield can be critical for calorimeters using small density of WLS fibres, as it is the case of Tilecal. Using multiclad fibres, not only improve-

inent on optics is achieved, but also the fibres are better protected by the multicladding. Kuraray was the first to produce double clad fibres, and more recently Bicron and Pol.Hi.Tech have also developed the technique. Multiclad Yll(200)MS fibres were used to instrument some of the Tilecal prototypes and the Module 0. In Fig. 1 it is shown the improvement by using a multiclad instead of a single clad fibre. When compared with single clad fibres they show an increase of more than 40% in light output for distances to the P M T greater than 1 m. 3.3. F i b r e s w i t h U V absorber Experimental tests on the first Tilecal prototypes [3], have shown that muons or pions impinging on the crack region between scintillators produce an enhancement of the calorimeter signal, leading to appreciable non-uniformities. The anomalous signal was expected to be from the scintillation or Cerenkov light produced in the fibres. This effect can be reduced with the addition of small concentrations of UVA to the mixture used in the commercial fibres. Fibres with UVA were obtained from each of the producers. The response of the fibres to charged particles (electrons from the 9°Sr /3source) impinging directly on the fibres and to light from a Tilecal blue scintillator as function of the UVA concentration is compared. Fig. 2-right shows the ratio of the light output, with/without UVA dopant, at 140 cm for Y l l , BCF99-28 and S048 fibres as function of the UVA concentration. Results of the light output response as a function of the distance to the P M T for Y l l fibres with and without UVA is shown in Fig. 2-1eft. The UVA used in the Y l l fibres is effective in suppressing the direct response by a factor 3, while keeping the response to the scintillator practically unchanged for a concentration of 1000 ppm. Instrumentation of the Tilecal calorimeter with Yll(200)MS fibres doped with 600 ppm UVA, improved considerably its uniformity (< 2 % globally) [8]. The UVA concentration in the Bicron fibres needs some optimization in order to reduce the 10% loss of scintillator light while achieving a factor 2.5 in the supression of direct response. S048-

A. Gomes et al./Nuclear Physics B (Proc. Suppl.) 61B (1998) 106 111

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UVA fibres are also effective in the suppression of fibre direct response while keeping the scintillator response. However the fibres used in the test were not yet optimized in light yield and the concentration of UVA was not specified. Therefore further investigation is necessary. 3.4. M e c h a n i c a l s t r e s s Flexibility of the WLS fibres for the Tilecal calorimeter is mandatory. The fibres are curved to diameters of about 10 em and they should survive during 10 years. The light loss for fibres bent at a curvature diameter of 10 cm is negligible for the BCF91A, Y l l ( 2 0 0 ) M S and S048 fibres. For a curvature diameter of 5 cm, the BCF91A show a light loss above 50% as it is shown in Fig. 3, while Y l l ( 2 0 0 ) M S and some types of S048 show a neg-

ligible light loss fibres kept bent showed that the be considerable

[7] [9]. Other studies made with at a 5 cm diameter for one year light loss in these conditions can [9]. Further tests are underway.

3.5. R a d i a t i o n h a r d n e s s Samples of almost all the candidate fibres to be used in Tilecal have been irradiated in a 6°Co source or in a nuclear reactor [10]. 3.5.1. I r r a d i a t i o n w i t h a 6°Co s o u r c e The irradiation facility is equipped with several lead shieldings in order to obtain large regions of uniform radiation field (about 2 m x 0.5 m) and 3 different dose rates: position A with a dose rate of 40 G y / h , position B with 11 G y / h and position C with 5.5 G y / h . The smallest dose rate is still much higher that the expected one for the Tilecal at the ATLAS experiment (14 m G y / h ) ,

110

A. Gomes et al. /Nuclear Physics B (Proc. SuppL ) 61B (1998) 106-1ll

but a compromise between low dose rate and time for the test has to be established. Fibres with a length of 2 m were irradiated with the following dose profiles: a negligible dose rate between 0 and 50 cm (<2 G y / h for position A and <0.3 G y / h for positions B and C) and a mean dose of 1.4 kGy in the last 150 cm for all the irradiation positions. The summary of the results for the relative light output after/before irradiation R(x) at a distance x=180 cm for the irradiations with the 3 dose rates is shown in Table 1. The last two columns of Table 1 show respectively R(180) measured for dose rate C after 60 days of recovery, and Rc(180) calculated using the attenuation lengths of the fibres (taken from single exponential fits) before and after irradiation. The measured and calculated values for each fibre are very similar, showing that for these long fibres (length > 1 m) the main damage comes from degradation of the attenuation length. No dose rate dependence up to -t-5% was observed in the permanent damage (60 to 90 days after the end of irradiation). Figure 4 illustrates one set of measurements done with the smallest dose rate, and the radiation damage of fibres without UVA and with UVA are compared. Among the fibres without UVA, the Yll(200)MS fibres are the least damaged losing between 7% and 11% in light output at x=180 cm. The BCF99-28 fibres lose between 14% and 18% and the S048-100-N4 fibres lose 38% to 43%. For the fibres with UVA, the Yll(200)MS fibres with 1000 ppm of UVA are the less damaged losing between 10% and 13%, the BCF99-28 fibres with 600 ppm of UVA lose between 21% and 26% and the S048-100 fibres lose about 22%. The fibres without UVA are radiation harder than the fibres with UVA, except for the S048. 3.5.2. I r r a d i a t i o n in a n u c l e a r r e a c t o r Single and double clad BCF91A (1995 production) and Yl1(200) fibres (1993 production), were irradiated in the mixed field (80% 7 + 20% neutrons) of the Portuguese Research Nuclear Reactor (RPI). Five 150 cm long fibres of each type were tested. A non-uniform dose between 100

Table 2 Ratio R(130) for fibres irradiated with nculrons + 7's. Average of 5 fibres of each type. Fibre type

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and 150 cm with a 'peak' total dose of 10 kGy and dose rate of about 15 k G y / h was applied. Table 2 summarizes the preliminary results obtained for the ratio R(x) at x=130 cm. The recovery process was monitored between 1 hour and 20 days after the end of the irradiation. The initial damage is very strong (of the order of 70-80%). The fibres recover in about 6 days to the permanent damage, since no difference was observed between 6 days and 20 days after the end of irradiation. The permanent damage is 28% for the Yll(200) fibres (single and double cladding), and somewhat smaller for the BCF91A fibres (21% for double cladding and 24% for single cladding). Single and double clad fibres show similar permanent damages. 4. S U M M A R Y

WLS fibres produced by Bicron, Kuraray and Pol.Hi.Tech present properties to be used in hadron calorimetry. All three companies offer fibres with reasonable light yield and attenuation length. Optimization of the multiclad and UVA will improve the performance of the calorimeter. Pol.Hi.Tech fibres need to be radiation harder, while keeping the other properties already shown. 5. A C K N O W L E D G M E N T S

This work was supported by J N I C T / P o r t u g a l . REFERENCES

1. O. Gildemeister et al., Proc. II Int. Conf. on Calorimetry in HEP, Capri 1991

A, Gomes et al,/Nuclear Physics B (Proc. Suppl.) 61B (1998) 106-111

III

Table 1 Ratio R(180) for fibres irradiated with a 6°Co source. Average of 5 fibres of each type. Fibre type A, 4 h 0.78 0.72 0.84 0.82 0.77 0.39 0.58

BCF9928 No UVA BCF 600 ppm UVA BCF91A Yll(200)MS Y l l 1000ppm UVA S048-100-4 S048-100 UVA

R(180 cm), position and time after irrad. C, 60 d A, 9 0 d B, 4 h B, 6 0 d C, 4 h 0.82 0.86 0.74 0.85 0.78 0.74 0.79 0.67 0.77 0.72 0.74 0.81 0.76 0.83 0.79 0.89 0.90 0.83 0.93 0.86 0.87 0.87 0.81 0.90 0.84 0.62 0.57 0.54 0.78 0.77 0.71

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ATLAS Tech. Proposal, C E R N / L H C C / 9 4 43, 15 December 94 3. A. Ariztizabal et al., NIM A349 (1994) 384397 4. V . K . Semenov, Proc. IX Conference on Scintillators, Kharkov 1986, p. 86 5. M. David et al., ATLAS Internal Note, TILECAL-NO-034, 1994 6. B. Tom~ et al, Test bench for quality control of scintillating and WLS fibres, L I P - 5 / 1 0 / 9 4 7. A. Maio et al., WLS fibres for calorimetry comparative studies, Proc. of the VI Inter. Conf. on Calorimetry in HEP, Frascati, 1996. 8. A. Henriques, Response of the ATLAS TILE-

CAL Prototype to Muons and M. Bosman, The ATLAS Hadronic TILECAL, VI Inter. Conf. on Calorimetry in HEP, Frascati, 1996. B. Girolamo and E. Mazzoni, Presentation in ATLAS weeks, 1995, 1996. 10. M. David et al., Dose rate effects in WLS fibres, VI Inter. Confi on Calorimetry in HEP, Frascati, 1996. .