A scintillating tile hodoscope with WLS fibre readout

Nuclear Instruments and Methods in Physics Research A 355 (1995) 35 l-358 NUCLEAR INSl’FlUMEN’fS

& METNODS IN PHYSICS

A scintillating tile hodoscope with WLS fibre readout *

M. Beck a, K.H. Brenzinger a, W. Briickner ‘, T. Hailer a, S. Paul a, B. Povh a*b, A. Simon h, * , A. Wenzel a

a ~~-Planck-Iast~tut fir Kernphysik, Heidelberg, German-v h Physikalisches Institut der lJnil,ersitiit, Heidelberg, Germany

Received 1 September 1994

Abstract We describe the construction and performance of a hodoscope consisting of 127 hexagonal scintillating tiles covering

hermetically a surface of about 1 m diameter. Each tile is read out on one face by five wavelength-shifting (WLS) fibres that are embedded and glued into the surface of the scintillator. The fibres serve the double purpose of primary light collection and subsequent light transmission to high gain photomultipliers located outside the active hodoscope area. Each individual

tile detects the passage of a minimum ionizing particle with an efficiency > 99%. Taking into account the small gaps due to the light-tight wrapping of each tile, the global efficiency of the hodoscope is 2 98%. As a test beam application of this

detector, we present measurements of the albedo from a compensating lead/scintillating fibre calorimeter which was installed behind the scintillating tile hodoscope.

1. Introduction

Detectors made of scintillating plastic material are among the most commonly used in high energy physics experiments. Applications of scintillator plates range from fast stand alone detectors for position-, time-of-fight- or multiplicity-measurements up to more complex detectors such as sampling calorimeters, in which they form the active medium for the detection of shower particles. The produced light is normally collected either by light guides or WLS bars, coupled to one or several lateral edges of the scintillator plates. However, such configurations are not always adequate, especially if the requirements on the detector hermeticity are severe. One possible solution is

based on the use of scintillating fibres which are exploited both for light production and light guidance to the opto- electronic readout elements [l]. An alternative way is to use scintillating tiles and WLS fibres that are embedded into grooves along the edges [2] or on the surface [3-6) of the tiles. This method has been applied in the past to calorimetry only [7]. Here, the amount of light produced by the copious number of shower particles in the active material is comfortably high, and the light losses and light

” Supported by the German Bund~sminister~um fir Forschung und Technologie, contract # 05 HD5241.

* Corresponding author. Address: CERN, PPE division, CH-

1211 Geneva 23, Switzerland. E-mail: [email protected].

output fluctuations caused by absorption processes in the WLS fibres are usually acceptable. Nevertheless, if the light has to be routed to the optoeiectronics over several

meters, a coupling to clear fibres is appropriate. We present in this paper the first use of such tiles with

WLS fihre readout as a hodoscope for detection of single

minimum ionizing particles. In this configuration. the production and collection of a sufficient amount of light by the WLS fibres become important issues. The geometry

and granularity of this hodoscope is predetermined by the requirement to map exactly the hexagonal tower structure of the SPACAL [8] lead/scintillating fibre calorimeter, which is actually used as hadron calorimeter in the hy- peron beam experiment WAS9 [9] at CERN. Operating for the first time in the 1993 run, the scintillating tile hodoscope (SCITIL) has been used as a charge identification detector, in conjunction with the spaghetti calorimeter in the framework of a high energy neutron trigger. This specific application of the hodoscope will not be presented here; it will be the subject of a separate paper.

2. Construction of the hodoscope

The requirements on the hermeticity of the tile structure as well as its mechanical mounting suggested to equip the individual tiles with an optical readout on one face only. In this configuration, the tiles can be attached with the other face to a thin plate and the complete detector can be

016~-9~02/9S/~~9.50 0 1995 Elsevicr Science B.V. All rights reserved

SSDf 0168-9002(94)01141-9

353 ._A M. Beck et al. / Nucl. Ins&. und Me&. in Phy.s. Re,s. A 355 !1995) 35f-3%

(BCF-91 d=2mm)

fSCSN38 th=lOmm)

- 43mm-

Fig. 1. Schematic layout of a ~~inIitlator tile.

installed very close to the SPACAL front face resulting in a system of two detectors with matching granularities. WLS fihres provide a flexible and space-saving optical readout to photomultipliers (PM) that have to be located outside the sensitive detector area of about I m’. In addition, this arrangement allows for a maximum mechanical decoupling of the individual tiles (modular detector).

The optical and electronic performance of this hodoscope is imposed by its use as a charged particle veto detector in the WA89 neutron trigger. An efficiency close to 100% necessitates a clear separation of the signal of a minimum ionizing particle from the pedestal in a charge

sensitive ADC spectrum. furthermore, as a hit pattern has to be generated by this hodoscope at the trigger level, the photomultiplier pulses (after 45 m of signal cable) should be well above the minimum discrimination thresholds available with usual electronics (typically 20-30 mV1. The pulse heights from single photoelectrons should stay well below this threshold.

2.1. Layout of an individual tile

Fig. 1 shows the geometry of an individual tile and its WLS fibre readout. A hexagonal structure is cut out from a 1 cm thick scintillator plate (SCSN-38, Kyowa) and subsequently polished along the edges. The length of each side is 43.0 mm. Considering that a scintillator thickness of 1 cm corresponds to 0.013 interaction lengths (A,), only

1.5% of the high energy neutrons, which we want to identify. will preshower in the hodoscope thereby faking a

charged particle. Light produced by an ionizing particle passing through

the scintillating tile is absorbed by five WLS fibres (BCF- 01, Bicron) of 2 mm diameter and the reemitted green light is guided by total internal reflection to the readout. The surface offered by the fibres to the absorption of primary light hitting the upper face of the scintillating tile is quite high (a fraction of _ 10%). In view of further maximizing the light collection, the fibres are aluminized on one end to reflect the secondary light emitted in the direction opposite to the readout. The first centimetres of these fibres are embedded into 2 X 2 mm’ grooves milled into the surface

of the tile. They are fixed by an epoxy glue (EPO-TEK 302, Polyscience AG) that has a refractive index similar to the one of the scintitlator. The five readout fibres are arranged such that they converge at one corner where they

Fig. ?. The various assembly steps of a scintillating tile and its readout.

M. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. A 355 (I 995) 351-358 353

also emerge from the scintillator surface. This fibre layout

covers the scintillator surface sufficiently uniformly and allows - close to the tile - an easy bundling of the fibres

for common coupling to the electronic readout. This specific configuration, together with the flexibility of the WLS fibres, permits an independent assembly of the indi-

vidual tiles to a full detector. Depending on their radial position inside the hodoscope, the tiles are equipped with

fibres of different lengths, varying from 30 to 80 cm. At their readout end, the fibres are glued into a cylindrical guide and are finally polished. They are coupled via optical grease to the l/2 in photocathode of a high gain,

low noise photomultiplier (R647, HAMAMATSU, g = 4 X lo’, dark current < 1000 c&./s). Fibre guide and PM are housed within an aluminium tube for mechanical protection and light-tightness; a weak spring ensures a perma- nent optical contact.

Each tile has to be made light-proof in order to prevent cross-talk between neighbouring tiles. Taking into account the constraint of an ideal mapping with the SPACAL granularity, the lateral scintillator dimensions of the tiles are slightly reduced with respect to those of the calorimeter towers, leaving about 0.2 mm space for coating on each side. These dead areas have to be minimized, as their insensitiveness to charged particles will fake the passage of a neutral particle in our specific trigger application. The various coating and readout assembly steps are shown on the photograph of Fig. 2. The bare tile is first covered with a thin aluminium foil to reflect light escaping from the tile for subsequent absorption by the readout fibres. Rubber hoses are put over all WLS fibres separately as well as over the whole bundle for light-tightness and mechanical protection. The overall light-tight wrapping of the tile is

done with one layer of black plastic and tape; remaining critical spots around the emerging fibres are covered with black silicon sealing compound.

2.2. Light collection scheme

Given the geometrical dimensions of the tiles, various studies have been done in order to collect a sufficiently high amount of light reaching the PM cathode.

The optical matching between the SCSN-38 emission spectrum and the BCF-91 absorption spectrum is almost complete [lo]. The emission spectrum (peak value at 500 nm in the green) of the WLS fibres is reasonably well separated from the absorption spectrum, except for a small overlap region at shorter wavelengths between 460 and 480 nm. For a selected fibre diameter of 2 mm and for WLS fibre lengths between 30 and 80 cm as in our specific application, an optima1 range of dye concentra- tions can be found. A high concentration is preferable w.r.t. the efficient primary light absorption. By way of contrast, the fibre self-absorption favours a lower dye concentration.

Fig. 3. Attenuation curve of the WLS fibres, measured with beam particles hitting the central part of a tile. The signal height is the value of the Landau peak in the ADC spectrum.

The effective absorption curve for the selected fibre

readout is shown in Fig. 3. It is obtained by exposing a scintillating tile to a test beam and cutting the WLS fibre bundles from their maximum length of 80 cm gradually down to 20 cm. The double exponential fit to the data yields a short absorption length in the range of 20-30 cm, originating from the strong self-absorption at shorter wavelengths, and a weaker absorption of the remaining light at larger wavelengths {abso~tion lengths of the order of 2-3

ml. The absorption curve indicates no necessity for cou-

pling to clear fibres. The earliest possible and robust way of fusing the WLS fibres to clear fibres would be after their common bundling at a length of 20 cm. For the tiles

with the shortest fibre lengths (30 cm), the light losses due to absorption are only 10% and would not justify a coupling to clear fibres where the unavoidable losses at the interface would be of the same order. For the central tile with the longest readout length (80 cm), the absorption amounts to about 30%. For this worst case, the signals are still safely separated from the pedestal. The absolute light loss - as far as the pulse heights are concerned - can be compensated by an increase of the PM high voltage of typically 50 V (I 2%). An optical readout for the whole detector, using only WLS fibres, is the easiest and most robust solution, avoiding potential sites of fracture at the interfaces in the case of clear fibre coupling.

We also varied the number and geometrical arrangement of the WLS fibres on different prototype tiles, using

the same PM system. A higher number of fibres or a larger diameter offers more absorption surface to the scintillator light, thus increasing the light collection probability. How- ever, the mechanical efforts will be considerably larger and the ~exibility of the whole system will decrease rapidly. We considered the selected configuration with five 2 mm diameter fibres to be a good compromise in this respect. Compared to other prototypes with only three fibres, we observed a higher light output (approximately a factor 5/3

as naively expected by pure geometrical arguments) and a uniformity of the tile which was slightly improved, How-

ever, the dependence on the geometrical layout of the fibres (e.g. a parallel fibre arrangement on the surface rather than a converging one) was only very weak so that we chose the more convenient converging configuration,

2.3. Assembly of the whole detector

The full-size hodoscope is finally buih of 127 tiles in an hermetic arrangement as shown on the photograph of

Fig. 4. The entire detector including its readout is mounted on

an aluminium frame which is fixed to the SPACAL calorimeter. The tiles are attached by double-sided adhe-

sive tape to a hard foam board. The PMs are mounted on the front and back side of the aluminium frame. This setup allows fast access and easy exchange of individual modules. HV and signal cables are distributed to panels on the

left and right side of the frame. The SCITiL hodoscope is kept at a fixed position of 15

cm in front of the SPACAL calorimeter. Except for the

two side panels, the whole hodoscope is covered by a black plastic foil for overall light-tightness. The hodoscope modules are connected via 45 m long signal cables to 16fold CAMAC discriminator modules (LRS 4413, Le Croy). Immediately following the discriminators is a sys-

tem of pattern units (PCOS III, Le Croy) that store the hodoscope hit distribution for later event readout. For the

recording of the analogue signals during test operation,

single tiles were temporarily disconnected from their dis-

criminator channel and fed into a charge sensitive ADC (LRS 1885F. Le Croy).

3. Detector performance

The resuhs presented in the following sections were obtained in the X3 test beam at CERN-SPS with electrons and pions in the energy range from 2 to SO GeV. The SCITIL/SPACAL detector system was installed on a movable platform that could be positioned in both horizontal and vertical directions to better than 0.5 mm. Beam chambers located several metres upstream allowed the prediction of the impact of a particle on the detectors with a precision I 1 mm. The rate of particles was very low, typically a few thousands per spill. Off-line cuts ensured a single and well reconstructed incoming beam particle for

each event. We first determined the signal ~~~0~~~~ of the ho-

doscope modules by illuminating one half of a tile (the fibre layout is symmetric with respect to the central fibre). Beam spots of 2 X 2 mm” were defined by software as scanning units over the surface. Variations in the light output were less than 30% over a whole tile (Fig. 5). The response was maximal and practically constant over the central part of a module and the border regions which are reached by WLS fibres. The most significant signal drop

occurred in the region where the WLS fibres emerge out of the scintiilator, an area which is not well covered by fibres.

Fig 4. Front view of the full-size SCITIL detector. For experimental operation, the whole detector is covered additionally with a black

for further light-tightness.

foil

M. Beck et al. /Nucl. Instr. and Meth. in Phys. Res. A 355 (1995) 351-358 355

horizontal coordinate [nun]

O’.‘.. ‘1. “““““’ ““““’ ” ’ “” -8 -7.5 -7 -6.5 -6 -5.5 -5 -4.5 -1 -3.5 -3

z-coordinate [cm]

Fig. 5. 2D-uniformity scan (spots of 2 X 2 mm’) across one half of Fig. 7. Local efficiency scan across two scintillator tiles. The

a scintillator tile. The signal heights are normalized to the maxi- nonzero response in the tile neighbouring the impact module is

mum value; white areas reflect missing or statistically insignifi- due to the albedo effect caused by the hadron calorimeter (30 GeV

cant data samples. e- data).

In addition, an even finer scan (spots of 1 mm width) was performed in a direction perpendicular to one of the fibres. A drop of the scintillator response (Fig. 6) can be observed for particles traversing the tile in the region of the 2 mm deep grooves housing the WLS fibres. This drop reflects qualitatively the reduction in scintillator thickness compared to the rest of the tile.

The level of nonuniformity was taken into account for an adequate calibration of the hodoscope modules, in order to guarantee an operation with maximum possible efficiency over the whole detector surface. The operating conditions were first established on a module with a long fibre readout and thus worse signal transmission, with the beam directed into its central area. Within its long-term operation limit, the PM high voltage was adjusted to provide analogue pulse heights at the Landau peak of = 75 mV. In this way, the minimum pulse heights of = 60 mV were safely above the common 35 mV threshold of the discriminator system, taking into account the maxi-

1, I

‘I.5 I .., 11, -10 -5 0 5 10

orthogonal distance (fibre, beam impact) [mm]

Fig. 6. lD-uniformity scan in steps of 1 mm across a region perpendicular to the WLS fibre labelled no. 4.

-module 87

module 88

- 87.88 combined

mum nonuniformity of 30%. For this PM gain, the ampli-

tudes from single photoelectron pulses were around 5 mV. Thus, a rough estimation based on amplitude comparisons would point to _ 15 photoelectrons being detected by a minimum ionizing particle traversing the tile.

The calibration of this particular module was trans- ferred to all the remaining hodoscope modules by adjust- ing the photomultiplier gains in such a way that the ADC spectra had their Landau peaks located at the same channel as for the initial module.

In the operation mode as described above, we determined the single tile eficiencies by comparing the number of observed hits in the individual tiles to the number of beam particles impinging on a central region of 2 X 2 cm’ size. We found a mean value over all the hodoscope tiles of 99.5% 5 0.2%. One can fold this mean efficiency value with the purely geometrical efficiency (98.97%) of the hodoscope, taking into account the gaps between the various SCITIL modules due to their individual coating. In this way, we obtained a value for the global detector efficiency of 99.5% X 0.9897 = 98.5%. Another estimation, although less representative for the detector as a whole, can be derived from a local efficiency scan across the border of two modules. The result of this measurement is shown in Fig. 7 and yields a uniform efficiency inside the module, up to several mm close to the edges. We integrated this curve over the gap region and subsequently extrapolated to the complete surface. The detector efficiency value that we obtained this way was 97.9%.

In summary, the SCITIL hodoscope will contribute to the performance of a neutron trigger with an efficiency of 2 98% for rejecting charged particles. The other aspect is the contribution of the hodoscope to the efficiency for neutral particle identification. As pointed out already in Section 2.1, we expect 1.5% of the neutrons to preshower in the tiles, this potentially faking a charged particle which will lead to a trigger veto. A more serious problem arises

AX2 counts

Fig. 8. Pedestal subtracted ADC spectrum of minimum ionizing

particles (30 GeV e- 1 traversing the central area of a tile. See text

for explanations of the fits.

from albedo particles i.e. shower particles in the hadron calorimeter being backscattered and leaking out through the front face of the ~lorimeter. These particles, if charged. will produce hits in the SCITIL hodoscope. We present a determination of the size of this effect as an appfication of the hodoscope in the context of the present paper.

4. Application: albedo measurement for a lead/ scintillating fibre calorimeter

The effect of charged albedo particles from the SPACAL calorimeter on the SCITfL hodoscope can be

directly seen in the charge spectrum of minimum ionizing particles through a tile (Fig. 8). A single, pure Landau fit (broken line) does not describe the data very well. A better fit to the measured spectrum is obtained with a sum of a single Landau distribution (describing the beam particle) and the folding of two Landau distributions (describing the beam and an additional albedo particle). In addition, both distributions are folded with a Gaussian function to account for the detector resolution. From the relative weight of the two distributions we extracted a value for the albedo from 30 GeV electron showers of about 7% in the very backward direction covered by the impact tile. Using this value, we can also estimate [IO] that an artificial increase of the single tile efficiencies, as determined prior to the present albedo considerations, is i: 0.2%.

A second manifestation of the albedo is the observation of additional hits in hodoscope tiles around the impact module. Such hits were seen to be correlated in time to the incoming beam particles and were absent for random triggers. For shower particles being backscattered from the calorimeter located 1.5 cm downstream of the hodoscope, one expects their signals to arrive several ns later than the signal from the beam particle (typically 2 ns for electromagnetic showers and 4-5 ns for hadronic showers). This is indeed what we observed: delay curves taken for the full hodoscope showed a time shift between the impact and the t~eighbouring modules, increasing towards larger distances from the impact.

In the following, we investigated in detail the charged albedo for electromagnetic and hadronic showers with

2 lo L___,_d.L. .,--...,-,_i,_? ET 0 2 il 6 8 ? 2 i 6 8

r-i

I c: ii_L.___ r-_-iii-r. ., “, i..L.~-

0 2 4 6 ‘? 0 2 4 6 E

multiplicity

Fig. 9. Multiplicity distributions for electromagnetic and hadronic shower albedo at a mean transverse distance of 20.3 cm from the beam

impact. The data are compared to fits assuming Poisson statistics (shaded distributions).

M. Beck et al. / Nucl. Instr. and Meth. in Phys. Res. A 355 (I 995) 351-358

transverse distance to beam impact [cm]

Fig. 10. Number of charged albedo particles per cm’ and event for

various transverse distances from the beam impact. Data are

shown for electron showers.

Fig. Il. Identical to Fig. 10, except that data are shown for pion

showers.

electrons and pions of different energies (2, 10, 20, 30, 40 and 50 GeV). For this we searched for hits in a set of modules equidistant to the impact module (or approximately equidistant, as the hexagonal modules are not exactly positioned at circular shells around the central one). The azimuthal hodoscope segmentation was exploited to determine the number of charged albedo particles at a certain radial distance. The resulting multiplicity distributions are shown in Fig. 9 for electromagnetic and hadronic shower particles for the case of a set of 18 modules at a mean distance of 20.7 cm. The data are overlayed with the result of a fit assuming Poisson statistics. The number of charged albedo particles originating from electron showers follows very well a Poisson distribution. This observation

is valid over the whole range of energies and radial distances that are covered by our measurements. The corresponding spectra for hadronic showers exhibit tails towards higher multiplicities. These deviations from Poisson

distributions are increasing with energy and reflect grow- ing correlations in the angular distribution of albedo particles from hadronic showers towards higher energies. It should be noted that for all sets of equidistant moduies, the multiplicity distributions fall rapidly so that pileup in single tiles can be safely ignored.

The mean multiplicities are shown for different energies in Figs. 10 (electron data) and 11 fpion data) in terms of the number of observed albedo particles per cm’ and event for various transverse distances from the beam impact. For a given distance from the impact point, the albedo is linearly increasing with energy for both types of showers. Pion and electron data are differing in their absolute size: at a distance of 15 cm from the SPACAL front face, the charged particle albedo originating from hadronic showers is about 50% lower than in the electromagnetic case. The apparent radial decrease of albedo is governed to a large extent by solid angle effects which cannot be corrected as the origins of the albedo particles in

a shower are not known. This effect can, however, be assumed to be stronger for electrons than for pions due to the smaller depths of electromagnetic showers in compari- son to hadronic showers.

We also determined the charged albedo in form of a hit probability per hodoscope module (area of 48.07 cm’) as a function of the radial distance from the impact point. These probabilities are at the level of several % in the region enclosing the impact module and show qualitatively the same radial, energy and shower type dependence as the data above which explicitly include the multiplicity information. The value of 6% for 30 GeV electron showers is consistent with the nonzero response in a module neighbouring the impact module, as already observed for the scan across two modules in Fig. 7. The albedo hit probabilities are globally compatible with other measurements at low energies (I-10 GeV) in a more restricted setup [3].

In view of the negative influence of albedo on the

identification of neutrons by means of SCITIL hodoscope information, tests have been done to suppress at least some fraction of the albedo. For both types of showers a copper plate of 2 mm and a Plexiglas plate of 1 cm have been alternatively put between the SCITIL back face and SPACAL front face in order to absorb part of the albedo particles and energy. No significant effect was found for either method.

Acknowledgement

We wish to thank G. Snow and his colleagues from UA6 for their help and advice in the initial phase of this project. We are indebted to J. Zimmer for his ever precious help and technical support at all stages of the detector construction. We also thank P. Leitenberger for his contribution in the beginning of the project and C. Scheel for her participation in the tests. We acknowledge the help and advice of R. De Salvo.

358 M. Beck et al. / Nwl. Instr. and Meth. m Phys. Res. A 355 II 995) 351-358

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Documents

A scintillating tile hodoscope with WLS fibre readout