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ELSEVIER

Nuclear Instruments and Methods in Physics Research A 360 (1995) 212-218 NUCLEAR

INSTRUMENTS & METHODS IN PHYSlCS RESEARCH

Sec!~on A

Recent developments in tile/fiber calorimetry

Sebastian White

Brookharen Nat~ona~~abffrato~,~pton, New York 11973, USA

Abstract At the ‘93 Elba Calorimeter Conference, final results on R&D for the CDF plug upgrade were presented. Much new

work was also reported on alternate schemes which allow for higher sampling frequency and simple, modular construction, Some highlights in waveshifter fiber developments are presented here with emphasis on differences between the Shish-Kebab

and the CDF design.

1. Introduction

It has now been approximately 10 years since the development of high quality plastic optical fibers loaded with scintillating and waveshifter fiuors was first exploited in the design of sampling calorimeters. Today, scintillating fibers have been used in a number of calorimeters and imaging shower detectors.

During the same time, geometries were tested [l-3] in which wavelength shifter (WLS) doped fibers read out

scintillator/absorber stacks exhibiting many advantages over conventional light guides or waveshifter bars. For example, the tile-fiber technique results in relatively small

deadspots and, because of developments in fiber connec- tors and splicing techniques, signals can be transferred to long, low loss, clear fibers and to a remote PMT.

2. Tile and fiber geometry

At least three of the basic geometries that could be considered were tried right from the start.

- Embedded fiber: Both fibers and scintillator plates

are oriented in a plane normal to the incident particle direction (Fig. la>. In the CDF plug calorimeter, the fiber is held in a groove milled in the shape of a “g” on one face of the scintillator. Longitudinal segmentation is then

relatively straightforward and a natural mechanical subunit is a single layer grouping of many towers (“pizza

wedges”). Existing EM calorimeter designs have up to - 20 sampling layers.

- “Bgyan”: Fibers and scintillator plates are stacked parallel to the particle direction (Fig. lb). In a recent test [4], thin scintillator and absorber layers (0.5 mm Pb and 1.2 mm scintillator/fiber) were also folded into a zig-zag (“accordian-like”) pattern to eliminate channeling of par- ticles between the absorber plates.

- Shish-Kebab: scintillator plates are oriented normal to the particle direction but fibers penetrate through holes running along the particle direction (Fig. 2).

In the latter two cases the natural mechanical subunit is a small group (i.e. l-4) of calorimeter towers. An arbitrary number of sampling layers could be added without compli-

cating the fiber readout. Because of the simple readout path, timing resolution is preserved. These designs do not provide a simple option for longitudinal segmentation.

2.1. Common features

There are many aspects which remain unchanged from one geometry to another.

In most cases tile/fiber EM calorimeters use a

polystyrene based scintillator with para-terphenyl and POPOP as the scintillators with peak emission at 415 nm. The WLS fiber absorbs blue light escaping the scintillator

through an air gap. The fiber is also made with a polystyrene core and

loaded with a green waveshifter (i.e. Y-7, Y-11 or K-27 with peak emission at 500 nm) at a concentration of order 100 ppm. Green light is reemitted isotropically so the trapping efficiency just depends on the index of the fiber’s outer cladding material relative to polystyrene (n = 1.59). With common acrylic (n = 1.49) claddings this efficiency is 3.1%. Fluorinated acrylics having lower indices (n _ 1.42) can be used to form a second cladding layer increas- ing the trapping efficiency to 5%.

Typically, the fiber diameter is about 1 mm which is a

standard diameter plastic communication fiber. The fiber cladding accounts for lo-20 pm of the fiber radius.

Since the original tile fiber tests, photostatistics have improved by about an order of magnitude to a 10 photo- electron yield from a (typical) 4 mm thick scintillator layer. This improvement has been significant, not only for basic performance (i.e. for energy, position and timing resolution) but also because it has made possible compro- mises in light yield favoring spatial uniformity or lower

fiber densities. The above green waveshifters are relatively slow (7defay

- 10 ns) so in applications requiring fast response either

016%9002/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved .SsDf 0168-9002(95)00094-l

S. ~ife/~ucl. Instr. and Me& in Phys. Res. A 360 (199.5) 212-218

special green fluors (i.e. PHENIX uses “G-2” [17] with Q._ = 2.8 ns) or scintillator and fiber fluors with shorter wavelength (blue stint/blue fiber) are substituted.

In contrast to scintillating fiber calorimeters, where fibers account for 25-50% of the calorimeter volume, waveshifter readout fibers only occupy = 1% of the calorimeter.

In a tile/fiber calorimeter, light enters the readout fiber through many possible paths. For example, in the Shish- Kebab calorimeter, the light produced in a single EM shower is captured through 1000-2000 independent tile/fiber joints.

3. Physics and performance issues

3. I. Energy resolution

The CDF plug upgrade EM calorimeter uses 4.5 mm Pb absorber and 4.0 mm scintillator in each sampling layer.

a

Fig. 1. (a) Generic view of the optical fiber readout from a u-tile

within a mechanical subunit of the CDF endplug calorimeter.

Fibers from tiles in different layers of the same calorimeter tower

are combined at the remote PMT. (b) Principle of the Bayan

calorimeter readout.

Fig. 2. Assembly sequence of a PHENIX 4-tower Shish-Kebab

module.

The energy resoiution contribution from sampling fluctua- tions grows roughly as:

oi

-E- sampling = -& = (4.0%&) jI&. , (1)

where E is the energy loss in the sampling layer in units of MeV. With the CDF construction, a _ 12% and a fourfold increase in sampling frequency is required to reach an a of 6%. The physics measurements of H + yy and of directly produced photons in heavy ion collisions are examples where a sampling coefficient of I 10% are required. Consequently alternatives to the embedded fiber technique have been considered.

3.2. Timing resolution

Calorimeter timing resolution plays an important role in the physics of recently proposed experiments, both for particle i.d. [5,6] and vertex reconst~ction [12]. Timing measurement from the energy signal in a calorimeter cell is subject to contributions from shower depth fluctuations. In the case of the Shish-Kebab geometry, for example, the arrival time of the signal at the PMT (using BCF 92 fibers)

V. TRENDS IN CALORIMETRY

Fig. 3. Temporal scheme for obtaining longitudinal segmentation

in Shish-Kebab calorimeters. T,,~ are scintillator and waveshifter decay constants.

varies with depth as v = 16 cm,/ns. So for an EM shower

with longitudin~ fluc~ations of 1 X0 we expect:

fft I intrinsic = X0

c - (16 cm/ns) -0.1 ns. (2)

In addition, depending on the timing scheme, statistical fluctuations in the signal lead to a term of the form:

CT, = 7 (3)

where 7x, the signal risetime, depends on the PMT and on the scintillator and waveshifter decay constants as illus- trated in Fig. 3. The parameter k is approximately equal to

1.

Fig. 4. CDF study of tile response unifo~ity for different differ- ent groove depths with the embedded fiber technique.

3.3. Position resolution

Position resolution of the EM shower centroid is also determined by statistical contributions and is best near a boundary between cells.

The Delphi Small Angle Tile Calorimeter (STIC) [I I] measures the Bhabha scattering distribution for luminosity monitoring and is designed primarily to achieve precision

in the shower position measurement. It is a Shish-Kebab calorimeter whose internal alignment is maintained by

using large area absorber plates and segmenting only the scintillator.

Similarly, the CMS Shish-Kebab calorimeter is re-

quired to deliver a y pointing accuracy of uH (: 70

mrad/ dm in order to associate y’s with the inter- action vertex. This was demonstrated [13] using the calorimeter shower osition measurement (which had o;,~

IO.9 cm/ E(GeV) m a 5.2 X 5.2 cm’ tower) in con- &----+ junction with a preshower tracker.

4. Light yield and uniformity

4.1. CDF

Response variations within the calorimeter can arise from light attenuation in the waveshifter fibers (typically L,,, = 2.5 m). Also near to the fiber-to-scintillator joints,

there is a local increase in light collection efficiency which can be compensated for by suppressing the scintillator output.

Earlier schemes for reducing these nonuniformities re-

lied on masking techniques to equalize response within the calorimeter. These schemes have now been completely

eliminated by CDF. An aspect of the CDF design which deals with attenua-

tion is the development of splices and connectors [lo] having > 90% transmission and + 4% reproducibility. Within each scintillator plane (Fig. la), the WLS fiber is

spliced to clear fiber (L,, - 7 mf as it exits the CT tile. A multifiber connector at the edge of the “pizza pan” allows it to be disconnected from the fiber runs as a stand alone module.

Still, because of remaining differences in pathlength and tile dimensions, the depth variation in response has to be tuned out to I 10%. This is accomplished by varying the amount of waveshifter fiber in successive layers of the hadron calorimeter [ 161.

Fig. 4 shows the results of a study of response uniform- ity within a tile, required to be I 2%. In the final design, by using a 1.6 mm groove depth in the 4 mm thick scintillator plate, response non-unifo~ities near the fiber are eliminated.

S. White / Nucl. Insw, and Meth. in Phys. Res A 360 w95) 212-218 215

Depth response variation in the Shish-Kebab results from attenuation in the fibers. Fiber loops at the front of the Phenix modules are used to effectively double the attenuation length. The final concentration of waveshifter fluor in the Phenix fibers was chosen by balancing the convicting demands of m~imum photostatistics and atten- uation length. The effective attenuation length of a module is 5.5 m. with BCF99-29a.

Light yield and response uniformity within a tile are strongly dependent on the light trapping efficiency of the tiles and their wrapping material, particularly at wave- lengths of 330-415 nm. Fig. 5 shows an example of measurements carried out by the Phenix group to screen reflector materials. The final choice (TYVEK 1055B [20]) resulted in a 50% increase in light yield over material (“stack paper”) used in earlier tests. In the Phenix con- struction, tile edges are vacuum aluminized and the calorimeter modules (Fig. 2) are made by stacking 66 alternate layers of 4 mm scintillator, paper reflectors and 1.5 mm lead absorber plates.

5. Particle identi~cation

In the Phenix experiment at the RHIC heavy ion col- lider, the electromagnetic calorimeter will be used primar- ily to measure and identify electron pairs in high multiplic-

125

Fig. 5,

‘00 300 400 500 mJc7 700 800

wavelength (“In)

Spectral reflectance measurement of different surface reflectors used to select among TYVEK paper layers.

Time of flight resolution versus energy deposit obtained with BCF-99-29a WLS fiber and an XP2081B photomultip~er.

ity collisions. The EM calorimeter backs an axial field magnetic spectrometer which covers 180” in azimuth and is centered on 9 = 0, The calorimeter is at a radius of 5 m from the interaction point and most of the coverage con- sists of Shish-Kebab modules. Over a limited area, the calorimeter will use high resolution crystals (BaF,) with a factor of _ 2 better resolution than the u,/E = 7-S% at 1 GeV obtained with the Shish-Kebab modules. The crystals will extend the reach of direct y to lower pt.

Timing resolution for electromagnetic showers was tested using various choices of fluors. The timing resolu- tion as a function of the total energy deposit in the channel under study is well described by the form a, = u 63 b/ dm with b = 80 ps. for a light yield of 3500 phot~le~ron/GeV (Fig. 6). The data plotted in Fig. 6 were taken using a Phillips XP2081b PMT which has risetime (5 ns) and q.e. at 500 nm (2 15%) similar to those of PMTs developed for Phenix 1191. Timing signals were derived from a leading edge discriminator with threshold corresponding to 50 MeV and slew corrected with a simple empiricat form.

5.1. Longitudinal segmentation

The Shish-Kebab energy resolution can be used in conjunction with momentum determination, from tracking, to discriminate pions from electrons (only 1% of which give consistent shower energies within 2a at 1 GeV.

V. TRENDS IN CALORIMETRY

0-

S. White/N&. Insir. and Meth. in Phys. Res. A 360 (1995) 212-218

-

Fig. 7. Fraction of the signal observed in the delayed gate for 1 GeV/c incident beam particles. A 10: 1 pion rejection is achieved with 95% electron efficiency, with the optimal gate delay.

Longitudinal segmentation of the Phenix calorimeter,

whereby the first 6X, are separately readout, was calcu- lated to give a factor of 10 rejection against n * relative to electrons due to differences in shower depth profile. The most practical scheme tested to carry out this segmentation uses two seperate gated integrators and scintillators in the back compartment which had POPOP replaced with a doping concentration of di-phenylanthracene yielding a 12

ns decay constant (vs. 3.5 ns) but the same total light output (Fig. 6). Distributions of signal fraction appearing

in the delayed gate (6 ns) for 1 GeV incident pions and electrons are plotted in Fig. 7.

With the optimal choice of gate delay, e/n separation equaled that predicted for independent readout. The final PHENIX design, however does not use longitudinal seg- mentation.

6. Photodetectors

The long wavelength characteristic of WLS fibers has led to the use of “green extended” bialkali tubes which

can give - 30% improvement over regular bialkali tubes

at 500 nm. The total photocathode area needed for tile/fiber

calorimeters is relatively small (1 cm diameter). In cases where only a few fibers are readout per channel, such as

the CDF shower maximum detector [16], multichannel PMTs could be used. However, these were found to have

lower effective quantum efficiency by as much as a factor

of 3.

6.1. Operation in magnetic fields

The CDF tile/fiber design allows remote location of

the PMTs thereby solving the problem of magnetic field immunity.

A number of devices have been tested to readout Shish-Kebab calorimeters in high magnetic field environ-

ments, including phototetrodes [ll], silicon photodiodes [13,7] and avalanche photodiodes [7].

e- 150 GeV

r e- 150 GeV

UJE = 1.3x

* 140 150

4

\

_!!L_ 3 170 0

sisnal WI

Fig. 8. Lineshape obtained with Si photodiode (upper curve) and PMT readouts in the CM.5 Shish-Kebab prototype tests.

S. W~~ite / Nucl. Instr. and Meth. in Phys. Res. A 360 (19951 212-218 217

Because of the photodetector is mounted directly on the

calorimeter, its response to minimum ionizing particles is a

potential problem. This is particularly serious in the case

of Si photodiodes where a “direct hit” from shower leakage corresponds to - 5 GeV of photoelectron re-

sponse from the calorimeter. An example of the effect of punchthrough on the lineshape in CMS electron test data is

shown in Fig. 8. The same problem can appear with phototubes at a

much reduced level (with consequences mostly for timing measurements) as can be seen from the hadron data in Fig.

6. In the region of the straight-through peak (230 MeV Eequiv), large fluctuations in timing from 1 GeV pions were

observed using the Phillips XP2081B tubes. It was found

that Cherenkov light produced in the 2.5 mm thick window

(amounting to 17 photoelectrons) would occasionally trig- ger the discriminator 1.5 ns earlier than the calorimeter

signal.

Supermodule fiber fonout

x64 to individuo! FTlOdUkS

7. Calibration and monitoring

Monitoring of the CDF Plug calorimeter relies on

moveable ‘“‘Cs sources which could be used to map

localized damage from radiation as well as electronic gain tracking.

Fig. 9. Principle of the PHENIX monitoring system showing the

lossy fiber illuminating four towers of a single module.

For the Phenix monitoring system, radiation damage is not an issue. Instead, a technique is chosen which allows

direct excitation of the scintillators by a pulsed UV laser (see Fig. 91. A quartz or acrylic fiber [18] is damaged so as to have a controlled loss profile along the calorimeter

depth. Laser data are taken continuously to monitor both gain and timing changes in the readout.

shown in Fig. 2. Modules are then incorporated in a 6 X 6 supermodule which provides mechanical integrity.

A secondary goal of the calibration system is to obtain a 510% gain balance, for trigger purposes, using cosmic ray muons while still in the ~mmissioning phase. Re- cently, there has been some operational experience with a

large Shish-Kebab array in a running experiment (BNL E-865 [9]) and this goal now seems practical.

In the STIC design, with unsegmented absorber plates, response varies by less than 2% accross a tower boundary.

An interesting further step away from modular construc- tion involves also eliminating optical segmentation of the scintillator plates. Bench tests of cross talk in a Shish- Kebab scintillator show that the effective attenuation length is - 7 cm in a 4 mm thick tile and this can be made smaller in a number of ways including using thinner

scintillator.

Modules are now under test which will use this idea with plastic [14] and liquid [15] scintillators.

8. Mechanical integration 9. Conclusions

The mechanical construction of Shish-Kebab tile/fiber calorimeters has been greatly simplified by the original development at INR and IHEP, Russia of mass fabrication techniques of absorber piates and injection molded scintil- lator with premade ~netration holes for the fiber [S].

The use of smalf absorber and scintillator parts has some drawbacks, particularly in high energy applications where response non-uniformity could dominate the energy resolution. The intrinsic non-uniformity at module bound- aries is minimized in the PHENIX design, where the module is held together, under compression, using 0.1 mm thick steel tensioning sheets on the side of the module as

Tile/fiber sampling calorimeters are now being built which take advantage of the many advances in fiber technology and photodetectors. A stochastic term of

7%/ dm in energy resolution and < 100 ps/

dm are obtained along with similar pe~ormance for spatial resolution.

Acknowledgement

This work was supported by the US Dept. of Energy under contract DE-AC02-76CH0016.

lntegroting

PbSc Module stock

Quartz roa . (02mm)

V. TRENDS IN CALORIMETRY

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1161 G. Apollinari, P. de Barbaro and M. Mishina, ibid.

[17] Bicron Inc., 12345 Kinsman Rd., Newbury, OH 44065-9677.

Bicron products BCF-92, BCF-99-29a and BCF-99-29b des-

ignate fibers with x1,x2 and x4 relative G-2 concentrations

used in the PHENIX tests.

[18] Infolite F-200, Hoechst Celanese Corp., Summit, NJ 07901.

[19] The FEU-115M PMT was developed at the Moscow Electric

Light Plant, Moscow, Russia.

[20] E.I. DuPont & Co., Chestnut Run Plaza, Wilmington, DE

19880-0705.