Silicon strip vertex detectors at LEP

Nuclear Instruments and Methods in Physics Research A 342 (1994) 218-232 North-Holland

NUCLEAR INSTRUMENTS &METHODS IN PHYSICS RESEARCH Section A

Silicon strip vertex detectors at LEP Andreas S . Schwarz

Max-Planck-Institut für Physik, Werner Heisenberg Institut, Föhringer Ring b, D-8000 Miinchen, Germany

The very successful application of silicon strip detectors for the study of short-lived particles at fixed target experiments that . started in the early 1980s coincided with the preparatory discussions of the large e ' e - collider experiments at LEP and the SLC introduced to study the production and decay of the Z" resonance . Consequently, as early as 1982 proposals were put forward to employ this new type of detector also in a colliding beam environment . The design, construction and performance of the detector systems built for LEP will be described .

1. Introduction The very successful application of silicon strip detectors in fixed target experiments in the early 1980s for the study of charmed particles prompted the proposal to use these devices also in a colliding beam environment . The primary motivation for the construction of the silicon strip detectors for experiments at the e +e - colliders LEP (CERN, Switzerland) and SLC (SLAG, California, USA) was the potential to tag the presence of heavy flavour (charmed and beauty) hadrons in the decay of the Z" resonance produced at these colliders . A silicon strip detector is a specialized application of a p-n diode [1] . When a p doped piece of silicon (high concentration of holes as quasifree charge carriers) is brought into contact with a piece of n doped silicon (high concentration of electrons as mobile charge carriers) the diffusion of holes to the n side and electrons to the p side creates a space charge region depleted of mobile charge carriers at the contact zone of the two materials . Under the application of a reverse bias voltage to the two regions (positive potential to the n side and negative potential to the p side) this space charge region is increased . When a charged particle traverses the depleted region in the silicon bulk it generates electron-hole pairs along its path by ionization in a very narrow tube (about 22000 electron-hole pairs for a 300 p,m thick silicon detector [2]). In the electrical field the holes and electrons drift to the p' and n + electrodes respectively and cause a change in the induced surface charges, thus generating a short :,irrent pulse at the electrodes . During the transport of the charge cloud it broadens by diffusion .

il is now straightforward to create a position sensitive device by subdividing the p'--n diode into many individual diodes creating a strip or pad pattern of the p + or n + region . On the p + side this poses no additional complication and in fact all silicon strip detectors (based on n bulk silicon) that provide one din",;nsional position information have been built based on this design . Fig . 1 shows a schematical representation of a generic silicon detector. In order to fully exploit the intrinsic precision of a silicon detector it is desirable to employ a strip pitch that is comparable to or smaller than the width of the integrated projected charge distribution due to diffusion (typically of the order of 5-10 wm). This will allow one to interpolate the particle position with several samples probing the shape of the charge distribution . If the strip pitch P is much larger than the diffusion width of the charge distribution, the spatial resolution will approach the geometrical limit qp,,y" = P/ 12 . The diode-to-diode pitch most commonly used lies between 20 and 50 p.m. In the following, the detection principles of silicon will be sketched and some basic design considerations for a silicon strip detector system at a collider will be outlined . The various systems proposed for LEP will be descr 1belÎ 1Ît the next Jeclion .

2. The experiments Table 1 lists the silicon strip vertex detector systems that have been constructed (or presently are under construction) for colliding beam experiments . The four experiments built for LEP will be discussed in the

0168-900/94/$07 .00 ,0 1994 - Elsevier Science Q.V. All rights reserved SSDI 0168-9002(93)E0955-R

A.S. Schwarz /Nacl.

Particle

E i

&ctr. and Meth. in Phys. Res. A 342 (1994) 21e8-?32 Signal Charge

'Ar,

16

219

A I Strips E) s=o n

",ITT" MIT"MM

n+ A II Plane

Fig. 1 . A schematical representation of a generic silicon detector . The graph to the right depicts the dependence of the electric field strength as a function of the position in the silicon bulk .

following. For information on the other detectors the reader is referred to the references listed in Table 1 . 2.1. DELPHI

Since the LEP e + e - collidcr is a storage ring, all collimators and masks for shielding the experimental area from synchrotron radiation and stray beam particles must be located outside the beam stay-clear region so as not to interfere with the operation. of the machine. When LEP started in 1989 the beam pipe radius was set conservatively at 8.5 cm. This was subsequently reduced to the present radius of 5.5 cm. Detailed studies based on the experience at the storage ring PEP and measurements done at LEP [14,151 indicate that the background distribution and the requirement of a stable machine operation may prevent the radius to be reduced much below this value . Discussions on the construction of a silicon strip vertex detector for the DELPHI experiment at LEP date back to as early as the letter of intent in 1982 [16]. For the startup of LEP in 1989 a few prototype mod-

ules were installed, followed in 1990 with the installation of two complete concentric cylindrical layers of detector modules. The reduced beam pipe radius in 1991 allowed the addition of a third inner layer. Fig. 2 shows the basic layout of the present (1993) DELPHI silicon strip vertex detector in the rdr view as well as in a rotated 3D view [17,181. Each layer consists of 24 so called "ladders" which overlap in the ro view. The layers are positioned at an average radius of 6.3, 9.0 and 11 .0 cm from the beam line. The (active) width of the ladders increases as a function of the radius

Table 1 An overview of colliding beam experiments constructed (or currently under construction) that use silicon strip detectors Experiment

Ref.

Collider

Construction completed SLC (e + e - ) MarklI [31 [41 LEP (e + e - ) DELPHI LEP (e + e -) ALEPH [5] [61 LEP (e' e - ) OPAL [71 LEP (e' e ) L3 TEVATRON (pp) CDF [81 [91 ARGUS DORIS-11 (e+e - ) Under construction CLEO-11 1101 [111 KEDR [121 DO [131 HI

CESR (e + e -) VEPP-4 (e ' e - ) TEVATRON (pp) HERA (ep)

vfs- [GeV] 94/Z" 94/Z" 94/Z" 94/Z" 94/Z" 1800 10/ TOS) 10/ T(4S) 10/ TOS) 1800 26(e)x820(p)

wa "v Fig. 2. The basic layout of the present (1993) DELPHI silicon strip vertex detector in the r(b view as well as in a rotated 3D view . III. COLLIDER APPLICATIONS

220

A .S. Schwarz / Nuct Instr, and Meth .

in Phys. Res. A 342 (1994) 218-232

from 1 .92 to 3 .20 cm to accommodate the increased circumference . The coverage of the full solid angle is 84% (80% and 73%) for the first (second and third) layer. The overlap in the ro view within a given layer is 10% . Each ladder consists of two electrically independent half-modules and each module is built up of two silicon strip detectors. The strips are oriented parallel to the direction of the beam and provide the r(A coordinate of a particle hit. The two detectors are daisy chained together by aluminum ultrasonic wire bonding. The readout is performed with custom designed VLSI charge sensitive amplifier chips (MX3) [19] wire bonded to the detectors and mounted onto ceramic thick film hybrids providing the necessary signal and voltage distribution for the detectors and the electronics, Three (four/five) chips are needed to readout the first (second/ third) detector layer. The electrical connection of the hybrids to the outside world is done via a flat cable made out of copper laminated Kapton. The total number of analog readout channels is 73 728 for the complete 3-layer detector. The ladders are mounted onto two hemi-cylindrical aluminum end rings. The assembled half cylinders are equipped with electrical and mechanical shielding and are inserted independently of each other into the central DELPHI region .

The bias resistors consist of polysilicon deposited on the surface of the detectors . They arc routed such that the bias resistance is the same for every strip. The bias voltage is provided via a common bus line [22]. The diode-to-diode pitch of the detectors is 25 tLm . Every second strip is metallized and the readout pitch is 50 p,m utilizing the method of capacitive charge division. Table 2 summarizes the parameters of the three types of silicon detectors .

2.1.1. The silicon strip detectors The silicon strip detectors have been developed in collaboration with the Center for Industrial Research, SI, in Oslo [2(), 21]. In order to avoid pedestal shifts due to varying strip leakage currents they have integrated coupling capacitors and individual bias resistors for each strip . The capacitors are formed by growing a thin oxide (Ad= 100 nm) onto the p + diode strip before the final metallization step in the processing.

2.1.2. The VLSI readout electronics Originally, the DELPHI experiment planned to use the NMOS Microplex chip [23] for the readout . The large power consumption of this chip initiated the design of a chip with similar performance but much lower power consumption, realized in 3 wm CMOS technology: the MX chip. This chip went through various iterations and only the version presently used (the MX3 chip) will be described here [191. The MX3 consists of 128 input channels and includes data storage circuits and a shift register for sequential readout . It measures 6.5 mm (width) x 6.8 mm (length) x 0 .5 mm (thickness) and has a channel pitch of 50 p,m . Like in the Microplex chip each channel comprises a charge sensitive front end amplifier, two sample-and-hold circuits and selectable output drivers. In addition to the features implemented from the original Microplex design it allows one to tune the bandwidth of the amplifier for noise optimization. The noise properties have been measured as a function of the input capacitance and the equivalent noise charge of the chip can be approximated by ENC = 650e + 55e - Cit, [pF]. Extensive radiation tests have been carried out with a "'Co source and the devices showed no measurable change in performance up to an integrated dose of 15 krad. The total power consumption of one analog

Table The basic properties of the silicon strip detectors that are used (DELPHI, ALEPH, OPAL) or will be used (OPAL, L3) by the experiments at LEP. C,, is the interstrip capacity measured in [pF/cm] Manufacturer Reference 2D readout Active area [mm 2 j Thickness [w m] Strip pitch [wmj p' side n' side (n'-P' ) Readout pitch [wmj side n' side r [pF/cm]

DELPHI

ALEPH

OPAL

SI-Oslo [20,211 no 19 .2 x 48 .0 25 .6 x 58 .0 32 .0 x 58 .0 285

CSEM [34,35,36] yes 5o x 50

Micron [43,441 no/yes 36 .3 x 60 .0

280

300 (2 x 250)

300

25 -

25 25

25 -/50

25 25

50 -1 .3

100 100 0 .8-1 .0

50 -,' 100 " 1 .3-1 .6

50 150/2W -0 .8-1 .0

L3 CSEM [7 .361 yes 38 .4 x 70 .4

A .S. Schwarz / MO. Instr. and Meth. in Phys. Res.

DELPHI

Fig. 3. Perspective view of a hemi-cylinder of the DELPHI vertex detector equipped with silicon detector modules . The units of the x, y and z axes are em. channel is 0.5-1 mW compared to - 14 mW for the Microplex chip (see also Table 3). 2.1.3. The mechanical holding stnictu .re

The electrically independent modules are mounted together with a very stable low mass carbon fibre profile glued to the back side of the detectors . The modules are mounted with the help of two screws through each of the ceramic substrates holding the readout chips onto end rings made of aluminum . The position of the detectors is referenced to the end rings with a precision aluminum cylinder (5 mm diameter and 2 mm height) that is placed onto the hybrid surface . Since the detector is designed to be installed after the beam pipe is already inserted into DELPHI, each of the end rings is vertically divided into two half rings. Fig . 3 shows a sketch of a hemi-cylinder equipped with modules [1$]. During installation or removal the two

A 342 (1994) 218-232

half cylinders slide along the beam pipe on rails which are mounted on the inner detector surrounding the vertex detector. The two halves are not touching after final installation . Each of the half cylinders is covered on both the inside and the outside with mechanical protection and shielding made of a 1 mm Rohacell sandwiched between two layers of 20 lim thick aluminum foil- The total thickness of the complete three layer detector for a particle at normal incidence is - 1 .50 of a radiation length . In order to preserve the spatial resolution intrinsic to the silicon strip detectors the whole structure has been measured prior to installation with a 3D measurement machine . In addition. the position of the hemicylinders is checked as a function of time with a capacitive distance measuring system as well as with a light spot monitoring system [4j using the detectors themselves as sensors by illuminating them with the light of a laser diode focused on the detector surface . A final alignment of the detector is performed with particle tracks . 2.1 .4. System performance In 1990 the DELPHI

silicon vertex detector consisted of layer 2 and layer 3 of the present system and accumulated - 130k hadronic Z° decays to be followed in 1991 and 1992 by - 1000k hadronic events taken with the full 3 layer device . The signal-to-noise of the detector (defined as the total cluster pulse height divided by the rms noise of a single readout channel) lies between 13 and 16 for the three layers. This is consistent with the calculation using the known interstrip capacitance of the detector acting as a load to the amplifier input and the noise properties of the MX3 chip (see Tables 2 and 3). Since the detector has a large degree of overlap in the ro projection within a given layer and since it consists of three layers, the intrinsic spatial resolution can be determined without having to rely heaAly on

Table 3 The basic properties of the VLSI readout chips that are (or will be) used by the experiments at LEP . The input capacity C, is measured in [pF] L.3 OPAL ALEPH DELPHI SVX MX5(7) CAMEX64A MX3 Chip name [451 (321 [19] Reference 3 tsm CMOS pm CMOS 1 .5 3.5 wm CMOS 3 ~Lm CMOS Type 128 128 64 channels 128 Number of 6.3 ;< 6 8 x 6.8 6.5 6.4 x 5.;, 6.5 x 6.8 Area [mm'] 48 rows) 50 rows) (2 (2 100 50 (2 rows) Bond pitch [I.m] 1 .3 L -1 -0.5-1 Power/channel [mW] 350 + 58C,n 325 + 23C,n 335 + 35C, 650+55C,~, ENC [e" ] Radiation -20 -500-50) 10-15 -15 hardness [krad] 111 . COLLIDER APPLICATIONS

A.S . Schwarz/ Nucl. Instr. and Meth . in Phys. Res. A 342 (1994) 21 8-232

222

but necessitates the addition of a second electrically insulated readout electrode layer. The addition of a second (double metal) layer complicates the detector design considerably but it offers the additional advantage that the heat generation due to the readout electronics remains localized to a small region on the supporting end rings that can effectively be cooled . The readout strips arc AC coupled to the readout electronics with coupling capacitors integrated into the detector design and a field plate approach is used to interrupt the electron accumulation layer between the n' readout strips [22]. On both sides the individual strips arc biased by a common bias line via polysilicon resistors . The planned strip pitch is 50 p.m on both sides. Since the detectors arc rectangular, the density of the recording strips on the n t side is larger by a factor that is determined by the aspect ratio of the detector geometry. In order to be able to use the same number of readout chips on both detector sides, DELPHI plans to introduce an ambiguity by connecting several strips on the n ' side to the same readout electronics channel . For a detector of size 1 .92 x 7.68 cm= corresponding to 384 strips, 7.68 cm long on the p ' side, there arc 4 times more recording strips on the n' side and the ambiguity to be introduced can be up to fourfold . A possible ganging scheme where the ambiguity is reduced at the cost of spatial resolution can be introduced by reading out the first halt of the detector (corresponding to large polar angles for the incident

the performance of the outer tracking devices . The residual distribution of tracks with an average angle of the particles' incidence on the detector surface of - 5° yields an overall intrinsic spatial resolution of - 6 p.m. The alignment accuracy of the individual wafers is estimated to have reached -000) wm. The missed distance between the ~t' and the p. - track in these events (when the momentum of the track has been constrained to be half the mass of the Z") allows one to estimate the impact parameter resolution to be m t,(w " l.. ) = 21 p.m . The measured dependence of the impact parameter resolution of the particle momentum in the ro plane has been parameterized as (r,; = (24)2 + (69/P ., )` [p,m] where the momentum is measured in CeV/c. Table 4 summarizes the system parameters of the DELPHI silicon strip vertex detector. 2.1.5. Futttm pktns For the running in 1994, the DELPHI collaboration is preparing to replace the modules of the inner and the outer layer of their vertex detector with new ones based on novel silicon strip detectors that provide spatial eoord'Ttatc information from both the p + side and the n ' side of the detector wafer. The strips on the n + side are oriented at an angle of 9()° to the strips on the p + side which remain parallel to the beam direction . The readout of the n ' side strips is also routed to the same region of the module where the readout of the p' strips is located . This allows one to provide two coordinates with one and the same silicon wafer without the introduction of additional material

hablr -1 Some system parameters describing the existing silicon strip vertex detectors at LEP. The momentum in the rcfi plane (p 1 ) is measured in GeV/c . The symbol ® denotes the quadratic addition of the two contributions to the impact parameter resolution (r,, (n .i . : no information available) DELPItI

ALEPII

OPAS.

L3

Reference Number of layers ] )[ ( r2 ) Icm] (r,> [cm] Seelid angle coverage ["d

141

[5)

[6)

171

10 .8 10 .84

7 .5 11 .83

7 .8 0 .92

overlap/ layer I"( Number of detectors Silicon area Im Number of VLSI chips Number of readout channels Averltge thickness Ir .c .'.11! Signal/ noise Intrinsic resolution

111 288 0 .42 57f, 73 728 - 0 .9-2 .0 13-16 6--u

Iwml

Impact parameter (r,, I~Lml

3 6 .3 9 .0 11 .0 0 .84

0.80 0.71

24

2 6.6

00)

4 96 0 .25 1152 73 728 - 3 .6 18 12 (r (h) 12(rz) 27+98,/p .

2 b.1

__

0 .77

75 (150) (1 .15 (t1 .3t1) 125 1000002000) - 1 .6 22 s

n .i .

2 6 .1

-

0.8 .

12 ()(, 0 .30 570 73 728 -2 .7 (r(b) 15-17kr- ) 11 .1 .

A.S . Schivar.- /Nucl. Instr. arid Nfeth. in Pk-s. Res . .4 342 (19W) 21N-232

particles) at a pitch of 50 gm introducing a fourfold ambiguity for this part . The second half of the detector, corresponding to more grazing angles can be read out with a strip pitch of 100 gm where two i3eighboring strips are connected with each other. This does not hurt dramatically, since the spatial resolution for these tracks is anyway worse due to the increased importance of 3 electrons. A nice alternative [24] is to flip one of the two detectors of a module such that the p+ strips of one detector are daisy chained to the n " strips of the next detector. In this way the ambiguity is essentially removed (by the polarity of the current pulse) and the input capacity for the readout electronics is equalized on the two readout sides . Three companies have performed intensive R& D to build these detectors for DELPHI [21,25,26]. The construction of the silicon detector modules for the first and third layer is in progress. The readout chip used for the new modules will be an improved version of the MX3 chip, the MX7 [27] with improved noise performance. The equivalent noise charge of this chip is reported as ENC = 300e - + 20e C, [pF]. For the running at center of mass energies above the W' W threshold (LEP 200) the DELPHI collaboration plans to upgrade the solid an,ic coverage of the vertex detector such that all charged particle tracks with polar angle >_ 25° have at least 3 points measured with the silicon detector . In additic .n, the v::ry forward region down to polar angles of >_ 1 l° will be equipped with a self-sufficient silicon tracker based on silicon strip detectors with 200-300 ium pitch or an array of pixel detectors N%ith - 500 x till() Vm - pixel size [281.

223

:s

:s

-:o

.!

o.

.

5

to

15

Hybrid Fig . -t . An o%er-,le\% of the present configuration of the ALEPti Slilt0n 'trip \ertCX detector The unit , of the axes in the top figure are cm .

2.2. ALEPH Like DELPHI also the ALEPH collaboration proposed the use of silicon strip detectors in a high precision vertex detector already to their letter of intent [29] in 1982. The main difference was that ALEPH considered to install silicon strip detectors with readout strips on both sides of the same wafer essentially from the start [30] aiming for three-dimensional position information for each particle hit . i O a éderv- ~~ r~ri~t~w 1 989 " avaJ"rn fr~ . M0,7UICC u .~. .s -era> ate. inët .ll`l1 intn ltd i(1~ startup phase of LEP . ALEPH to participate in the for the first time at a The run was successful and side and on the n ' side of collider clusters on the p ' the same piece of silicon could be correlated to tracks of charged particles originating from hadronic Z" decays [311. In 1990 a complete inner layer and about 50 1; of a second outer layer were installed . Problems mainly with the peripheral hardware rendered most of the data of this run not useful for physics analysis . But in 1991 a complete and improved 2 layer detector

svstem has been installed and successfully commissioned, making the ALEPH experiment the pioneer of the implementation of silicon detectors providing readout on both sides of the wafer. Fig . 4 shows the configuration of the detector as installed in 1991 . Two layers of silicon strip detectors are arranged in two concentric barrels around the beam pipe with an average radius of = 6.6 cm for the inner layer and = 10_R em tot the outer laver . The active detector areas of adjacent wafers are arrang-ri sucn that they overlap by 2 mm in the rd) projection to cover the full solid angle with active detector area. This corresponds to a coverage of the r4~ solid angle with overlapping wafers of = 4% . Two detectors each are mounted onto one electrical building stone . the module and two modules are again mounted together lengthwise to form the basic mechanical building stone of the svstem, the ladder. The inner layer consists of 9 mechanically independent ladders and the outer layer has 111. COLLIDER APPLICATIONS

224

A.S. Schwarz /Nucl. Instr. and Meth . in Phys.

15 ladders resulting in a total of 48 electrically identical and independent modules . Only one type of ladders has been used for both layers . The solid angle coverage of the detector with active detector area is 84% for the inner layer and 6V/, for the outer layer. The silicon strip detectors have readout strips on both sides. The strips on one side arc parallel to the beam direction and measure the azimuthal angle and the strips on the other side are perpendicular to the beam and measure the coordinate z. On the 4 side the two detectors arc daisy chained together via ultrasonic wire bonding . With the radius r of the individual wafers given by the mechanical holding frame, the position of a single particle hit is determined in cylindrical coordinates r-O-z. The readout of the detectors on both the 0 side and the z side is performed with custom designed VLSI CMOS amplifier chips, the CAMEX64 [32]. Eight (sixteen) chips are used to readout the p ' (n ' ) side. The total number of analog readout channels is 73 728. The CAMEX64 chips arc mounted onto two ceramic pieces (one for each detector side) which also serve as hybrid substrates for the routing of the electrical signals to and from the chips . The contact to the outside world is established with two copper plated Kapton foils ending in a custom designed connector. In order to read out the n + strips the electronics is oriented orthogonal to the electronics on the p+ side (see Fig . 4) and covers, together with the supporting hybrid ceramic, a considerable part of the active silicon area . This is the simplest possible readout solution but introduces additional sources of multiple .nattering as well as a distributed heat load . As a consequence the ceramics for the n + side hybrids have been kept very . The distributed heat load thin (thickness 250 Lm) necessitates the use of gas cooling . 2.2.1. The silicon strip detectors The width of the silicon detector modules is given by the width of the silicon wafers . In order to get the same 100 p,m readout pitch on the z side as on the (~ side without adding additional ambiguities in the pattern recognition, the detectors have been designed as squares . The use of silicon wafers with 3 in. diameter confined the size of the detectors to an overall size ,.f '51 .2 X 51 .2) mm2. The active area covered with strips is (49 .8 x 49.7) mm= on the p + ("junction") side and (49 .7 X 49.0) mm2 on the n' ("ohmic") side. The strip pitch on the junction side is 25 Rm. Every second strip is equipped with a bonding pad. On the ohmic side the n ' implanted strips are interspersed with p + implanted blocking strips. The strips on the junction side and the strips on the ohmic side are oriented at an angle of 90° to each other . The pitch between the p' and the n + strips on the ohmic side is

Res. A 342 (1994) 214-232

25 p.m. Each n + strip is equipped with a bonding pad. The active strip areas of both sides are surrounded with a guard ring structure that is needed for the biasing . The detector is read out at a pitch of 100 p,m on both sides by employing the principle of capacitive charge division [33]. In this way the density of connections between the detector and the readout electronics and the number of analog readout channels with the accompanying power consumption and heat dissipation is reduced . Only every fourth p' strip on the junction side and every second n + strip on the ohmic side are connected to the readout electronics. The design introduces novel biasing schemes [34] for the individual strips on both sides which make it possible to deplete the entire detector volume with the help of only two contacts, one for the p ' side and one for the n + side . This allows an easy implementation of capacitive coupling of the strips to the readout electronics as well as the use of the concept of capacitive charge division on both sides [311. On the p+ side the implanted strips run below the aluminized strips and end a few p.m distance away from a p + guard ring surrounding the active detector area. In the absence of any electronics connected to the readout strips, the floating p + strips are biased via the guard ring structure using the punchthrough effect between two close p + implants . On the n + side the strips are biased by taking advantage of the electron accumulation layer present at the silicon-silicon dioxide interface . Fig . 5 shows the principle of the strip pattern at the edge of the n n" bias contact

p compensation implantation

n' readout strips

Fig. S. The strip pattern on the n' side of the ALEPH silicon strip detectors.

A.S. ScInvar: /NucL Insu. and Meth. in PhYs . Res. .4 342 (1094) 218-232

225

side. The p + side implants between the n + strips (needed to electrically separate the n + strips) arc formed to end in a channel [34,35], thereby defining (at full depletion) an ohmic bias resistor, with a value determined by the sheet resistance of the electron accumulation layer and the length and the width of the channel . In ALEPH the silicon detectors are capacitively coupled to the readout electronics using external capacitors produced on an independent substrate [351. Table 2 summarizes the basic properties of the detectors. 2.2.2. The VLSI readout electronics The CAMEX64 chip is a custom designed VLSI chip with 64 parallel channels arranged in a readout pitch of 100 p,m [32]. Noise reduction has been achieved by employing fourfold double correlated sampling applying the switched capacitor technique [37]. The noise performance has been measured as a function of the input capacity and for various integration and sampling times. Under optimal conditions the equivalent noise charge can be expressed as ENC = 335e - + 35e - Cin [pF]. The overall size of the CAMEX64 is 6.35 mm (width) x 4.95 mm (length) x 0.5 mm (thickness) . The CAMEX64 has been irradiated with an X-ray source both with power off and with power on and the clocking signals operating [38] . For power off during irradiation the noise approximately doubles at an integrated radiation dose of - 140 krad. Under normal operating conditions the noise increases mildly by 201( for an integrated dose of - 1(1 krad where the chip ceases to function . Table 3 summarizes the main characteristics of the CAMEX64 chip. 2.2.3. The mechanical holding structure Fig . 6 shows a three dimensional view of the complete mechanical holding frame (without the shielding) . It consists of two ladder holding rings, a cylindrical shell that holds these together, two cable holding rings and the electrical and mechanical shielding. Since the detector has to be installed around a beam pipe that is already mounted into the ALEPH experiment and integrated into the LEP storage ring, the whole structure consists of two half sectors (one for 4 (7) ladders in the inner (outer) layer and one for 5 (8) ladders in the inner (outer) layer. The two ladder holding rings are connected to each other via a hollow carbon fibre sandwich structure . This structure consists of two halves that form a cylinder when assembled together . The cylinder fills the space between the two layers and is intended to provide a stable and rigid frame. The distance of the ladder surfaces (including the height of the wire bonds) to this cylinder is designed such that the volume avail-

Fig . 6 . A three dimensional view of the complete mechanical holding frame for the ALEPH silicon strip vertex detector . The average radii for the inner and outer layer are r, = 6.6 cm and r  _ 10 .8 cm .

able for the cooling airflow is approximately equal for both layers and the 45 and the z side. The ladder holding rings are made out of carbon fibre to minimize the amount of multiple scattering and the position of a given ladder is determined by three aluminum studs on each side of the holding frame that have been inserted into pre-drilled holes in the carbon fibre; material . The radial position of the ladder is fixed by the surface of the two outer posts on each side . In order to decouple the ladder from the outside world the connectors on the Kapton foils are mounted on a me=chanically separate: cable; holler ring. The rings are made of carbon fibre plates and are only lightly coupled to the ladder holding structure with carbon fibre tubes that are fed through holes in the carbon fibre shell holding the ladder holding rings together . The mechanical holding frame is completed by a shield that encloses the ladders . It consists of a 125 p.m carbon fibre foil laminated with = 25 P.m epoxy to a 25 p,m aluminum foil. It serves as a protection from mechanical as well as electrical interference. In order to be able to mount the detector inside ALEPH, feet are mounted to the cable holder ring and the ladder holder ring in two positions (on the top and on the bottom) . These arc used in the installation to slide the whole cylindrical structure on rails which arc mounted onto the inner wall of the support tube of the inner tracking chamber of ALEPH. The two halves of the detector are fixed to each other with two sets of locks made out of aluminum which are located on the cable holder as well as on the ladder holding rings. These are designed such that they can be operated with a special long key when only one side of the detector remains accessible during the 111. COLLIDER APPLICATIONS

250 7associated and detector Thethe rzI,pulse ~ side pulse ~ up~4250 rz 250 with The side height height is shown charged pulse (topspectrum 500 right) in height tracks the Schwarz ofcorrelation scatter 0 the (in forThe ADC the ALEPFi plot /Nucl pulse rq~counts) rdi (top 250 view ofheight side Instr silicon theoffor left) (bottom r4i the andstrip clus500 and layout of i the PhysFig -inner OPAL Res the chamber) 3Svstetn the height 1991 The height is7has detector Acollected tracks wafer system alignment silicon corrected a0 Also pulse tracking from 342 of (see and observed careful side toversus performance aspectrum (1994) traverse [391 isradiation shown vertex combining Fig 1992 surface tracks height and (including shown chamber alignment the -accuracy for the 218-232 6) signal the detector the FIBRE 1000k iseffective on the traversing correlation zAt for the zfor length One ALEPH side average side intrinsic perpendicular angle the to and the hadronic scatter has clusters ofnoise can pulse silicon 0the point been atboth Silicon material side nicely on detector plot which complete outer figure height resolution Z° associated determined vertex and the layers ofStrip incidence, decays observe time the isthe equivalent the two The resolution detector, 18 ALEPH particle and Vertex 0zof projecwafer pulse with The side side with the the for the a

A.S.

226

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500

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installation particle to .6%

ra N N 0

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0

rip

z

i~! 414~f

0 Fig . . ters left) vertex

N E rrp

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Fig . ;+ .

2.2.4. In detector pulse charged in . . pulse height hits expected sides . both After tracking the tion vertex with residuals

.

. .

.

. . .

CARBON PRESSURE TUBE

.

:1

A.S. Schtivarz /Nucl.

hzstr. and A1eth. in P1tys. Res. A 3-t2 (1994) 218-232

overlap region in one of the two layers . The point resolution for tracks at perpendicular incidence is 12 (12) p.m in the ro (rz) view in the data compared to 8 (10) Rm in the Monte Carlo. The addition of two very high precision points onto the helix of a given charged particle track improves the momentum resolution by - 25% from dp/p = 8 X 10-4p to 1Ip/p = 6 X 10-4p. As a test of the impact parameter resolution, the distance between the tracks produced in Z° - R + A events is measured. Fit to a single Gaussian, the widths of the distributions imply impact parameter resolutions of 15 (29) p,m in the rq$ (r) view to be compared with 107 (820) p,m with the outer tracking alone. The momentum dependence of the three-dimensional impact parameter resolution can be parameterized by ar t, = 27 + 98/p 1 [Rm] where the momentum is measured in GeV/c. Table 4 summarizes the basic system performance of the ALEPH silicon strip vertex detector . 2.2.5. Future plaits The present design of the ALEPH silicon vertex detector offers a solid angle coverage of only 69% when the requirement is made that for a given charged particle track at least two points are measured with the silicon. In addition the ceramic hybrid that is used for the readout of the z coordinate lies in the active region of the detector and increases the contribution from multiple scattering considerably . Consequently, the upgrade plans of the ALEPH collaboration [40] call for a new design of the whole detector which looks similar to the design of the presently commissioned L3 detector . The arrangement of the faces in the ro projection will not change . The ladders will be made longer by the addition of a third detector per module and by using slightly longer detectors . This will increase the solid angle coverage with two points measured for a given track to 87%. The z strips will be read out with copper cladded and patterned Kapton foils. The readout chips under investigation at the moment are the MX7 [27] and the VIKING circuit [41]. The detector is scheduled to be ready for the startup of LEP200 in 1995/1996. 2 .3. OPA 1. The rapid con,;t-!ction of the OPAL silicon vertex detector is a good example to indicate the progress that has been made in this rather novel field. The detector has been proposed and approved for installation early in 1990 and the installation of the complete system took place only one year later, in March 1991 . Serious data taking started in the summer of 1991 . The OPAL detector has already a very precise vertex chamber [42] and the addition of the silicon

2?7

device was only made possible due to the reduction of the beam pipe radius from 8.5 cm in 1989/1990 to 5.5 cm in 1991 . This circumstance reflects in the layout as shown in Fig. 8. In the following, the detector used in the 1991 /A 992 data taking period will be described. It consists of two layers of silicon detector modules identical in design placed at an inner radius of 6.1 cm and at an outer radius of 7.5 cm. There are I I of such modules in the inner layer and 14 in the outer. The diode strips of the silicon detectors arc oriented parallel to the beam direction and only the ro coordinate of a particle hit is measured. There is no overlap of adjacent modules within a given layer. The silicon detector module consists of three - 6 cm long silicon detectors that are daisy chained together. The readout is arranged at one end and the MX5 chip with 128 charge sensitive amplifier channels is used. The chips (five per module) are mounted onto a ceramic substrate that also provides the signal distribution from the outside world to the front end electronics. In total 16000 analog channels are to be read out. Between the detector and the readout chips a small adapter piece made of aluminized g.ass is placed . This is used to help optimize the automatization of the ultrasonic wire bonding by avoiding a varying angle for the wire bonding which would otherwise be necessary due to the geometrical difference between the readout chips and the detector readout pad arrangement. The solid angle coverage of the detector is 83% (77rß ) for the inner (outer) layer. The modules are mounted onto two end rings which contain channels for the water cooling of the electronics. No special cover is added and the shielding is provided by the inner wall of the pressure tube of the OPAL vertex chamber. 2 .3.1. The silicon strip detectors

The detectors have been designed by the OPAL group in collaboration with Micron Semiconductor [43] and are described in detail in ref. [44]. They are capacitively coupled to the electronics using an oxide layer between the p + implant and the aluminum readout electrode. The individual p + diode strips are biased in a similar way as is done for the ALEPH detectors using the punchthrough current between two adjacent p structures (see section 2.2). In order to allow the dynamic resistance between the diode strips and the p guard ring to be tuned, a field effect transistor is introduced with the gate on the field oxide (a "FOXFET"). Fig. 9 shows the basic design of this bias structure. The gate overlaps both the ends of the readout strips (acting as the FET drain) as well as the bias strip (acting as the FET source). Fig. 10 shows the dynamic III . COI,LIDER APPLICATIONS

A.S. Schwarz /Ntic1. Instr. and Meth . in Phys. Res. A 342 (1994) 218-232

228

, V S0 5 Is

AI S i0, l,+ ` nHigh Resistivity Substrate Fig. 9. The bias structure for the diode strips of the OPAL FOXFET silicon strip detectors.

I

a k,

0

T - 24 9% I _ 550 nA 6 I _ 5 nA

S

,MO)

100

10

resistance of the structure as a function of the gate-todrain voltage difference. It increases as the channel is depleted of electrons below the positive fixed oxide charge until an inversion layer is created switching the FET on. The detectors have a diode pitch of 25 l..m and every second strip is read out . Table 2 summarizes the basic parameters of the detectors .

2

,fiI -4

-8

,

I,I -12 -16

-20

vso (v) Fig. 1t) . The dynamic resistance of the FOXFET bias structure [Mill, as a function of the gate-to-drain voltage Vc;n. It is defined as the change of strip-to-drain voltage Ven (with S nA injected current SIs) divided by the current SIS.

2.3.2. The mechanical holding structure The three silicon detectors and the ceramic hybrid circuits of a module are mechanically supported by a reinforced 500 p,m thick Kevlar sheet. The modules are mounted to the two aluminum end rings with precision aluminum positioning pins (Fig. 11) . The cables from the individual modules lead to connectors that arc fed into 7 printed circuit boards which contain the electronics used to drive the signals to the digitizing electronics . The completed cylinder is positioned inside OPAL using positioning skates to the inner pressure tube of the surrounding vertex chamber . For

the installation the detector can be split into two hemi-cylinders. The total amount of material traversed by a particle at normal incidence is - 1.6% of a radiation length .

BE RY1 I I11M VACUUM TORE OUTER DETECTOR LADDER

DETECT OR SUPPORT R ING

.

INNER DETECTOR LADDER D ETECT OR COOLING RING

PRESSURE

TUBE I IR SUPPORT RING

OPAL Fig. I i . The mechanical mounting trame for the OPAL silicon vertex detector. (ICR: interconnect ring which carries ~,dditional electronics, busses and connectors) .

A.S. Schwarz /Nucl. Instr. and Meth . irr Phys. Res. A 3-t2 ( 1994) ?1,s'- 232

2.3.3. System perfonnance In 1991 and 1992 the OPAL silicon detector has taken - 1000k hadronic Z" decays . The observed signal-to-noise ratio for two-strip clusters is 22 : 1 . The impact parameter resolution can be deduced from the missed distance of the two tracks originating from Z t' - IL + W - events . For momentum constrained tracks the derived impact parameter resolution is crt,(p, + I.L - ) 18 gm. Table 4 summarizes the system performance of the present OPAL silicon strip vertex detector. 2.3'.4. Future plans For the year 1993 the OPAL collaboration has installed a completely new set of modules equipped with detectors providing readout of the ro as well as the rz side. This has been achieved by glueing two 250 I.Lm thick FOXFET detectors back to back with the readout routed to one side using 200 Rm thick glass sheets with gold traces . The readout is performed with the MX7 chip, a radiation hardened version of the MX3. The overall detector geometry has stayed unchanged. 2.4. L3 Up to this year, L3 was the only remaining LEP experiment that did not yet have a silicon strip vertex detector. This has changed in 1993 with the installation of a detector system proposed in 1991 [7] . It is based on silicon strip detectors with orthogonal readout on both sides of the silicon wafer. As for the OPAL experiment the installation of this new device was made possible by the reduction of the radius of the LEP beam pipe in 1991 . The ro layout of the detector is shown in Fig . 12, and Fig. 13 shows a schematical view of the proposed ladder design . The system consists of 2 layers of 12 silicon ladders each at a radius of - 6.1 and - 7.8 cm respectively . Each ladder consists of two electrically independent silicon detector modules. Each module has two daisy chained silicon strip detectors which are patterned on both the

Fig. 12. The rcb layout of the L3 silicon strip vertex detector .

y

-A

Hvbrid

Detector

Fig. 13. The ladder design for the L3 silicon strip vertex detector . (a) The ladder design of the inner and the outer layer: (b) a closeup view of one readout end of a ladder . p ' and the n ' side with the strips of the two sides oriented at an angle of 90° to each other . In order to reduce the amount of confusion in the track-to-hit correlation in the pattern recognition the modules of the ladders in the outer layer are tilted with respe=ct to the bcûm direction at a stereo angle of 2°. There is no overlap of adjacent modules in the rO view in the outer layer. The introduced gap corresponds to - 7% of the azimuthal solid angle . Each module is read out on both sides of the wafers with six custom designed VLSI amplifier chips, the SVX [45]. These are mounted to ceramic substrates used also for signal distribution. The detector strips are capacitively coupled to the readout electronics using separate AC coupling chips that are placed between the detector and the electronics (see Fig . 13b) . The total number of analog readout channels for the complete detector is 73 725. The ladders are mounted onto a carbon fibre tubular support structure which consists of two hcmi-cylinders for case of installation into L';. 2.4.1. The silicon strip detectors The L3 collaboration chose to employ detectors with the same bias scheme as designed for the ALEPH experiment (see section 2.2) . The strip pitch of the detectors is 25 lim on the p + side and 50 wm on the n" side. The readout pitch is 50 Wm on the p+ side (which will provide the ro coordinate of a particle hit) III. COLLIDER APPLICATIONS

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A . S. Schivarz / Nucl. h:sir. und Meth. in Phys. Res. A 342 (1994) 214-2.32

which are latched . For each latched channel the analog signal voltage is connected to an output bus and the k hannel address is simultaneously connected to a 7 bit digital bus. Hence the readout time is dramatically decreased and is mainly set by the detector occupancy rather than tl?^ total number of channels. The equivalent noise charge of the SVXD under these operating conditions ("quadruple correlated sampling") is ENC 35()c + 58e C, [pF]. Fig . 14. The routing scheme of the n' strip signals to the readout side in the L3 silicon ladder design using a copper cladded Kapton foil.

and 151)/2(1() jim on the n' side (which will provide tire r: coordinate). The routing of the n' strips to the readout region is achieved with it special copper cladded Kapton foil. Fig. 14 show; u sketch of the proposed scheme. In order to arrive at the same number of analog electronics channels for the p ` and the n' side readout despite the aspect ratio of 4 : 1 the readout pitch is chosen to be 15() Rm for the detector closest to the interaction point and 21)0 ~tm for the second detector where the steeper angle of the track's incidence leads to a wider spread of the deposited charge distribution . Through this choice the introduction of additional atnbiguitics (as is done in the DELPHI design, see subsection 2 .1) is avoided . The basic properties of the dctcetors are listed in Table 2. 2. 4. _' . Me I TS1 readout ch ,ctrwnc .%

The VLSI readout chip chosen for this detector is the SVX chip (version D) [45]. It has been developed for the silicon vertex detector of the CDF experiment at the TEVATRON pP collider at Fermilab [8] . The chip contains 128 charge sensitive amplifier channels followed by a sample-and-hold stage, it threshold storage stage and a comparator and latch. Digital circuitry is used to control a serial multiplexed readout . Accumulated charge from strip leakage current (if present) and the calibration pulse (integrated be fort, the arrival of the signal charge) is used as a threshold , which signals (supert'titposcd onto further strip Îcitl\agC current) must exceed in order for the comparator to eleridc wlietlicr the signal is accepted as a "real" hit or not . If it is accepted, a latch is set for this channel . Since charge from a traversing particle can be spread over more than one strip, not all strips of a charge cluster may trigger their comparator. For these channels the SVX chip includes "neighbour logic" : channels adiaccnt to hit channels may be read out as well . When the chip is read out, the digital control circuitry switches sequentially only to those channels

2.4 .3. The mechanical holding; structure

The structural stability of the ladder is achieved with it carbon fibre profile which is reinforced with ().3 mm thick silicon . The ladders are mounted onto hemicylindrical aluminum alloy flanges which are connected together with a rigid carbon fibre honeycomb structure embedded between the inner and the outer layer (similar to the ALEPH design) . The aluminum flanges are equipped with water cooling pipes to remove the heat produced by the VLSI readout electronics . Since the completed detector is not allowed to touch the beam pipe nor the inner wall of the surrounding time expansion chamber (TEC), it has been decided to embed the detector in a cylindrical annulus which extends from one end plate of the TEC to the other where it is supported . For the installation into L3 the cylindrical structure is split into two halves which are brought around the beam pipe where they are fixed together. The completed cylinder is then fixed to rails which allow it to slide carefully between the TEC and the beam pipe . The basic parameters of the complete detector system are summarized in Table -1. The detector has been installed earlier this year and is being commissioned. 3. Summary The intense R&D work in the last decade on the design and construction of complex silicon strip detector systems for experiments in a colliding beam environment has culminated in the successful commissioning of six silicon strip vertex detector systems during the last two years . The very rapid construction of the OPAL and the L3 silicon detectors shows that the field is maturing and the icchnoiogics are becoming standard. Following the success of the ALEPH experiment in the application of silicon strip detectors providing two orthogonal coordinates with the same piece of silicon, also the other LE13 experiments have made efforts to employ this concept (and variations thereof) for the running period in 1993 . Already now a plethora of physics analyses arc emerging that arc based on the superb spatial resolution provided by these detector systems . These analy-

A.S. Schwarz / Nucl. Instr. and Meth . in PhYs. Res. A 342 (199-)) 218-'2

ses include improvements in the measurement of heavy particle lifetimes, the tagging (or anti-tagging) of hadronic Z" decays containing charm or beauty mesons and the search for exclusive decay modes of these particles. Acknowledgements It is a pleasure to thank my collegues of the different experiments described in this article for their generous support in providing much of the material, in some cases including so far unpublished material . My special thanks go to Hans Dijkstra, William Trischuk and Mike Tyndel of DELPHI, Tony Carter and Roncn Mir of OPAL and Bill Burger of L3. Last but not least I want to thank my collegues in the ALEPH collaboration . References [1] A.S . Grove, Physics and Technology of Semiconductor

Devices (Wiley). [21 See e.g.: M . Aguilar-Benitez et al ., Review of Particle Properties, Phys. Lett . 170 B (1986) 43 and references therein. [31 C. Adolphsen et al ., Nucl . Inst . and Meth . A 313 (1992) 63 . [41 G. Anzivino et al ., Nucl . Instr. and Meth . A 256 (1987) 65 ; G. Anzivino et al ., Nucl . instr. and Meth . A 263 (198 ;0 215, M. Burns et al . Nucl . Instr . and Meth . A 277 (1989) 154; H. Uijkstra et al ., Nucl . Instr. and Meth . A 2N9 (1990) 400; V. Chabaud et al ., Nucl . Instr. and Meth . A 292 (1990) 75 ; H. Borner et al ., Contributed paper to the 26th Rochester Conf. on High Energy Physics, Dallas, Texas, August 6-12, 1992, DELPHI 92-92 PHYS 223 (1992) . [5] B. Mours et al ., The design, construction and performance of the ALEPH 3D silicon strip vertex detector, to be submitted to Nucl . Instr . and Meth . G. Batignani et al ., Nucl . Instr. and Meth . A 236 (1903) 183; G. Batignani et al ., Conf . Record of the 1991 IEEE Nuclear Science Symp ., 2-9 November 1991, Santa Fe . New Mexixo, USA, vol. 1 . p. 438. G. Batignani et al ., Nucl . Phys . B (Proc. Suppl.) 23 (19`)1) 291 and references therein. [61 A. Carter et al ., talk given at the LEPC meeting at CERN, Switzerland, November 1991 . [71 L3 Collaboration, CERN-LEPC 91-5, LEPC P4-Add .1 (1991) . [81 W.C . Carithers et al ., Nucl . Instr. and Meth . A 289 (1990) 388. [9] S. Khan, Contribution to the Common Meeting of the

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ECFA Working Groups on a B-Meson Factory in Europe, CERN, Switzerland, 7 November 1991 ; Contribution to the Int. Conf . on B Factories - The State of the Art in Accelerators, Detectors and Physics, Stanford Linear Accelerator Center, Stanford, .('aiifornia April 6-111, 1942, SLAC-Report -41)1) (1992) . [101 J. Alexander, Contribution to the Int. Conf. on B Factories - The State of the Art in Accelerators, Detectors and Physics, Stanford Linear Accelerator Center, Stan ford, California, April 6-10, 1992, SLAC-Report-4110 (1992) . [III V.V . Anashin et al ., Proc . Int. Symp . on Position Detectors in High Energy Physics, Dubna (1988) p. 58 . 1121 DO-Collaboration, see e.g . J. Ellison, these Proceedings (Int . Symp. on Development and Application of Semiconductor Tracking Detectors, Hiroshima, Japan, 1993) Nucl . Instr. and Meth . A 342 (1994) 33 . [131 H.J . Behrend et al ., Technical Proposal to Build Silicon Vertex Detectors for I-Il, DESY Internal Report, PRC 92/01, H1 (16/92-226 . [141 D.M . Ritson, SLAG-PUB-4317 (1987) invited talk given at the Int. Conf . on Advances in Experimental Methods for Colliding Beam Physics, Stanford, California, March 9-13, 1987 . [15] G. von Holtey, CERN-LEP-BI/88-52 (1988), G. von Holtey and D. Ritson, CERN-LEP-NOTE-614 (1988) 15 pp . [161 DELPHI Collaboration, DELPHI-82/1, January 1982 : DELPHI Letter of Intent, CERN 'LEPC / 83-3 . LEPC/P2 (1983); DELPHI Technical Proposal, DELPHI 83-66/1 . 1171 Throughout the paper we use the same coordinate system: the beam particles define the : direction and the plane perpendicular to the z direction is the r - Y plane . In cNlindrical coordinates the t - %, plane coincides with the rrh plane where (h is the azimuthal angle. The ankle of a particle originating at : = () with the : axis i" the polar angle v. The dip angle is defined as A - y0' - 0[181 U. Amaldi, DELPHI collaboration, DELPHI Status Report presented at the LEPC meeting, DELPHI 919-108 GEN 122 (1991) . [191 J .C . Stanton, IEEE Trans. Nucl . Sci. NS-36 (1) (1989) 522. [201 M . Caccia et al ., Nucl . Instr. and Meth . A 260 (198'0 124. [21] Center for Industrial Research, Box 350 Blindern . N-(131 t Oslo 3, Norway . [221 H. Dijkstra et al ., IEEE Trans. Nucl : Sci. NS-36 (1) (1989) 591 : H. Dijkstra et al .. Contribution to the 26th Int. Conf . on High Energy Physics, Munich . August 4-10. 1988 . [`1] J .T . Walker et al ., Nucl . Instr. and Meth . 2'6 (1984) 2( )i). [241 H. Dijkstra . DELPHI collaboration. private communication . [251 B.S . Avset et al ., IEEE Trans. Nucl . Sci . NS-37 (3) (1990) 1153 : L. Hubbeling et al ., Nucl . Instr. and Meth . A 311) (1991) 197; R. Brenner et al ., Helsinki University preprint . HUSEFT-91-16, submitted to Nucl . Instr. and Meth . A. [26] Hamamatsu Photonics K.K., Hamamatsu City . Japan. [27] J . Ardelean et al ., Nucl . Instr. and Meth . A 315 (1992) Ill . COLLIDER APPLICATIONS

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393. The MX7 is a radiation harder version of the MX6 described in this reference. [281 DELPHI collaboration, Letter of Intent, CERN/LEPC/ 92-5, LEPC/1-11 (1992), P. Delpierre, these Proceedings (Int . Symp. on Development and Application of Semiconductor Tracking Detectors, Hiroshima, Japan, 1993) Nucl . Instr. and Meth . A 342 (1994) 233. [291 CERN-LEP-LI-(1) (1982). [3111 L. Bosisio et al ., ALEPH-NOTE 84/130 (1984). [311 H. Becker et al ., IEEE Trans. Nucl . Sci. NS-37 (1990) 1(11 . [321 W. Buttler et al ., Nucl. Instr. and Meth . A 273 (1988) 778, ICFA Instrumentation Bulletin No. 5 (September 1988). [331 B, Hyams et al ., Nucl . Instr. and Meth . 2115 (1983) 99, U. Koetz et al ., Nucl . Instr. and Meth . A 235 (1985) 481 . [341 P. Holl et al ., IEEE Trans. Nucl . Sci. NS-36 (1988) 251 . [351 G. Batignani et al ., Nucl . Phys . B (Proc. Suppl.) 23B (1991) 297; Ci . Batignani et al ., Proc. 5th Pisa Meeting on Advanced Detectors; Nucl . Insu. and Meth . A 315 (199:) 121 .

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[361 CSEM, Case Postale 41, CH-2007 Neuchatel, Switzerland . [371 G . Lutz et al ., Nucl . Instr. and Meth . A 263 (1988) 163. [381 P. Cattaneo et al ., Nucl . Phys . B (Proc. Suppl.) 23B (1991) 313. [391 W. Wiedenmann et al ., presented at the 1992 Wire Chamber Conf., Vienna, 17-21 February, 1992 and ALEPH Collaboration, Nucl . Instr. and Meth . A 323 (1992) 213. [401 ALEPH collaboration, CERN/LEPC 93-8, LEPC/Pl Add. 1 (1993) . [411 E. Nygard et al . Nucl . Instr. and Meth . A 301 (1491) 506; P. Aspell et al. CERN-PP-91-154 (1991). [421 J.R . Carter et al ., Nucl . lnstr. and Meth . A 286 (1990) 99 . [43] Micron Semiconductor Ltd, Lancing, Sussex BN 158UN, UK . [441 P.P. Allport et al ., Nucl . Inmr . and Meth . A 310 (1991) 155, [451 S. Kleinfelder et al ., IEEE Trans. Nucl . Sci. NS-35 (1988) 171 .