The CLEO II.V silicon vertex detector

EISEVIER

Nuclear instruments and Methods in Physics Research A 386 (1997) 32-36

The CLEO 1I.V silicon vertex detector

William Ross *

Department of Physics and Astronomy, University of Oklahoma, Norman. OK 73019, USA

NUCLEAR INSTRUMENTS & METNODS IN PHYSICS RESEARCH

Section A

Abstract A description of the new silicon tracker installed in the CLEO interaction region is given. This device consists of three

layers of double-sided AC-coupled silicon microstrip detectors arranged in an octagonal array. The wafers are readout by 416 CAMEX64/JAMEX64 VLSI amplifier chips. A description of the data acquisition and monitoring systems developed to read out the SVX is provided. A summary of performance and some results are presented.

1. Introduction

In the fall of 1995, a three-layer silicon tracking device was installed as part of an upgrade to the CLEO-CESR interaction region. Other elements of this effort included a water-cooled Be beampipe, a moving tungsten shields system, and a refurbished inner drift chamber. ‘Bvelve PIN radiation monitors, located at f 13 cm from CLEO center, were also installed.

The silicon tracker (SVX) replaced a six-layer straw-tube chamber, and represented the first major CLEO subdetector upgrade since the installation of a segmented CsI calorime- ter in the fall of 1989 [ 11. The CLEO upgrade took place in the context of a larger CESR effort aimed at delivering luminosities on the order of 6 x 103* cm-*/s. The finer seg- mentation of the SVX over its predecessor is expected to improve both tracking and secondary vertex reconstruction. Comparisons between CLEO II data and SVX Monte Carlo simulations suggest improvements of factors of 2-4 in the resolutions for track parameters perpendicular to the beam, and an order of magnitude improvement in z resolution.

2. Silicon microstrip detectors

The wafers used in the construction of the SVX are AC- coupled, double-sided detectors manufactured by Hama- matsu Photonics [ 2 J , and consist of 300 pm of bulk silicon, with a 0.2 ,um SiOz layer coupled to aluminum readout traces. For this application, the P side is measuring r-4, while the N side measures the z coordinate. The sense strips on the r-4 side consist of p-type material implants.

* Address: Wilson Lab, Cornell University, Ithaca, NY 14853. USA, fax + I 607 255 8062, e-mail [email protected].

The z-side sense strips are separated by barrier implants of the same material. The r-q5 implant pitch is 28 pm, with every fourth strip instrumented. The z-side pitch is 105 pm, with every strip read out. ‘Bvo types of wafers were manufactured, one with 189 instrumented strips (used in layers 1 and 3), and another with 252 instrumented strips (used exclusively in layer 2). Non-instrumented strips contribute by means of charge sharing.

The dimensions of the wafers are approximately 2 cm x 5 cm. This aspect ratio, coupled with the unalterable ClJAMEX 100 pm input pitch, forces the three-fold gang- ing of sense vs readout strips on the z-side, leading to the use of “double-metal” architecture. Thus the readout traces are parallel to the sense direction on the r-4 side, and or- thogonal on the z-side. A cross-sectional view of a wafer, taken in the r-4 direction, is shown in Fig. 1.

Measured strip capacitance is on the order of 9 (27.5) pF/strip for the r-4 (z) side for a Ll wafer, scaling by 1.25 for an L2 detector. Most of the z side capacitance is due to the double-metal construction on that side. Biasing is accomplished by means of poly-Si resistors on the z-side, and by way of punch-through resistors on the r-4 side. Operating bias voltages are f40 V.

D-WV- II-tYp. .ul..

iXd.ll+. arip.

\

300~ Bulk Silicon

Fig. I. SMD cross-sectional view.

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U! Ross/Nucl. Instr. and Meth. in Phys. Res. A 386 (1997) 32-36 33

Fig. 2. UJAMEX circuit diagram

3. The UJAMEX VLSI amplifiers

The wafer readout traces are wire-bonded to CAMEX64 (or in the case of L3N, JAMEX64) VLSI amplifier chips. These devices were developed at MPI Munich [ 31, primar- ily for use on the ALEPH experiment, and consist of 64 channels of quad-sampling (switched capacitor filter) inte- grating amplifiers coupled to a sample-and-hold circuit. A 6-bit analog multiplexer controls which channel is read out. A simplified circuit diagram is shown in Fig. 2. The input pitch for both devices is 100 pm. Gain has been measured to be 12.6 mV/fC, and the ENC to be 350 + 30 x pF, for CAMEXs, both measurements using four samples. These devices are run asynchronously with respect to the CESR beam crossing, constantly sampling the baseline in the ab- sence of triggers. This sampling stops in the event of a min- imum bias trigger (LO). Signal sampling only occurs, how- ever, if more selective trigger criteria (Ll ) are satisfied. The rates for these two types of triggers are 60 kHz are 30- 40 Hz, respectively. CLEO currently runs with three signal samples, at six beam crossings per sample.

4. Mechanical design

The 96 wafers which comprise the SVX are arranged in separable three-layer octants. Fig. 3 shows two cross- sectional views of the detector. Within an octant, each layer is supported by low-mass Kevlar U-beams, the ends of which are supported by Be0 support structures. These in turn are mounted on carbon-fiber hemicylinders, termed “clamshells”. Cooling is assured by heat exchange with PF200 fluid running in copper tubes along the bases of these support structures. The layers are located at radii of 23.5, 32,47, and 48 mm. Layer 3 is double length, double width. Be0 hybrids, manufactured by Pacific Microelec- tronics [ 41, are glued to the ends of the wafers. Ll and L3 (L2) hybrids support 3 (4) CAMEX chips. Also mounted on the hybrids are buffers for chip output, as well as tem- perature sensors. Fig. 4 shows a picture of a completed octant assembly before mounting in the clamshell.

2

Double

4.69 cm Radius

Fig. 3. SVX cross-sectional views.

5. Data acquisition

The hybrids are connected via flexible circuits [5] to receiver boards ( 1 receiver board per half-octant) , located at f21 cm from the detector center, also mounted on the carbon-fiber tubing. These constitute the first elements of the SVX data acquisition (DAQ) system.

As mentioned above, the AC-coupling on the wafers is afforded by a 200 nm thick 502 layer. Perforations. or “pinholes”, in this layer exist at the 2% level in the SVX. These allow the formation of DC paths into the C/JAMEX chip inputs, thereby saturating channels, and rendering sec- tions of the wafer unusable. This problem is alleviated if the C/JAMEX input is held at a voltage close to the wafer face voltage. The SVX DAQ is designed to level-shift all control and output signals, as well as supply voltages, to an operating voltage within 2 V of wafer bias voltage on the z-side, and within 20 V of bias on the r-4 side.

The SVX DAQ consists of the following elements: - Receiver Boards: These funnel all control and output sig-

nals, as well supply voltages, into the interaction region. - SC1 ( Sequencer-CAMEX-Interface) boards: Level-shift

all control and output signals to/from SVX and create and monitor necessary supply voltages.

- Databoards: sixteen 68040 CPU-controlled VME boards. Perform common mode noise subtraction and data sparsification before inclusion into CLEO event .

- Sequencer: Produces necessary C/JAMEX control signals, triggers digitization, interfaces with CLEO trigger.

- Slow Control: Provides monitoring of >2000 quantities. - Power Supplies: Distributed HCll CPU system allows

1. VERTEX DETECTORS AT e+e- COLLIDERS

34

: c*

WI -1

CAMPI :

i i i Rcvaa;

I-ACP i la BIT

CRWPP IArc& .j .: LmcBI i : e TIT0 : : : x31

i data

I-AC% . .

isvD ~Ctat. ,in Pit

Fig. 5. Diagram of SVX data acquisition.

direct local control and error handling. In all, 26 208 channels are instrumented, making the SVX

the largest CLEO subdetector. A schematic of the SVX DAQ is shown in Fig. 5

up to a dose of - 170 krad has been received. Under current running conditions, the SVX typically receives -5 rad/pbb’ of integrated luminosity

7. The radiation shields system 6. Running environment

Within the next year, the CESR peak luminosity is expected to reach 6 x 103* cm -*/s, with currents of -600 mA in the storage ring. The currently peak luminosity is -3.5 x

103*, with a typical instantaneous value of -3 x 103’ cm-*/s. Most of the radiation incident on the SVX is in the form

of particles, with only around 10% due to synchrotron radiation from the beams. This poses a problem, since CAMEX chips are radiation soft, with significant damage occurring at integrated doses of w-20-35 krad received with power on [ 61. JAMEX chips are harder, with no damage occurring

In order to protect the SVX against synchrotron radiation (but not particle backgrounds), an elaborate shielding system, shown in Fig. 6, has been installed. This device consists of a set of two fixed and two movable tungsten cylinders wrapped around beam pipe. The two movable cylinders are actuated by stepping motors (located outside of CLEO) by way of stainless-steel cables running into the interaction region. A series of infrared sensors assures that the shield positions are always known during the open-close cycle. In normal operation, the shields are open during HEP running, and closed for CESR injection.

U! Ross/Nucl. Instr. and Merh. in Phys. Res. A 3X6 (1997) 32-36

Table I Efficiencies, signal/noise

PEEK-Fmsrrd berylliiom beam pipe

stranded stairdam skel cable

,r,yer 1 hybrid

fixed luqnteo shield

/ front delrln bauiuinp

Fig. 6. Retractable shields

and noise for SVX layers

Noise [ ENCl SIN e I%1 - r-4 r-4 z r-f#J

LI 790.f50 1960 f 150 20 IO 0.957 h 0.008 0.799 * 0.014

L2 1190.*75 2180 f 150 13 IO 0.947 f 0.008 0.791 f 0.014

L3 1440.f70 2150 f 130 IO 9 0.854 f 0.014 0.759 * 0.015

Given the large amount of cosmic ray data taken during pre-installation testing and the -1 W’ (Y(4S) and con- tinuum) of data taken with colliding beams, several SVX

8. Performance and results Gaussian fits to these yield u,_* = 90 pm and CT: = 190 pm The primary vertex position distributions have been obtained using hadronic events. These are shown for x,y and z in Figs. 8 and 9, respectively. The widths of these distributions

performance parameters have been measured. Using cosmic rays, the S/N values of Table 1 were measured. The hit effi- ‘6c ‘r ciencies were obtained using e+e- + e’e- , ~‘J_L- events.

The bad channel count now stands at -9%. This is due in I part to the fact that sparsification algorithms and constants are still being optimized, thereby artificially inflating the 1zc number of unusable channels.

Global alignment of the detector is now complete, which leaves the SVX aligned at the 25 pm level. Calculation of internal alignment constants is now beginning. Two tracking algorithms are being tested. The first combines a stand-alone SVX tracker with matching to drift chamber tracks. In this scheme. after a match has been made, a global refit is performed by a Kalman fitting algorithm. The second tracking method relies on extrapolating drift chamber tracks into the SVX and then performing a global refit.

Both of these schemes are currently being optimized. The existence of working tracking packages has allowed various basic checks to be performed. At a basic level, miss distances from two-track events have been plotted using p-pairs. The r-4 and ; miss distance distributions are shown in Fig. 7.

50 00.10~~ -lOOO.1C~ ^C’?.‘, .

Fig. 7. p-pair miss distance distributions.

1. VERTEX DETECTORS AT e+e- COLLIDERS

W Ross/Nucl. Instr. and Meth. in Phys. Rex A 386 (1997) 32-36

c

-YkWlpLdtiO~ - Y beam position wlo SVX - Double gah fst

-XbePmposition - x beam position w/o W-X -Double Saws&n Lit

-30.00~10-~ -10.00.10- -0 ml8 -0 004 0.000

Fig. 8. Primary vertex y (I.) and x (r.) distributions.

are gx = 293 ,um, a, = 145 pm, and crz = 1.05 cm. The actual beam spot dimensions are 10 km in y x 300 ,um in x.

9. Summary

-0 05 0 3c 0.05 0 12

Fig. 9. Primary vertex z distribution.

for z. Improvements in tracking and vertexing are expected to benefit all aspects of the CLEO physics program Given the CESR upgrades which took place at the same time as SVX installation, -5 tb-’ of data are expected to be taken throughout the lifetime of the SVX. With 1 fb-’ of data already accumulated, analysis efforts are just beginning.

A silicon tracker has been installed in the CLEO interaction region and started taking data in the fall of 1995. This detector consists of three layers of double-metal DSSDs in an octagonal configuration. Readout is by way of 416 VLSI UJAMEX chips. Given that all wafers can be read out from their ends, the readout electronics are placed outside of the interaction region. Based on comparisons between CLEO II measurements and SVX Monte Carlo simulations, resolutions are expected to improve by x 2-4 for r-4, x 20

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[31

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[61

Y Kubota et al.. Nucl. Instr. and Meth. A 320 (1992) 66. Hamamatsu F’hotonics, K.K., 1126 Icbino-Cho, Hamamatsu City, 435 Japan. H. Becker et al., IEEE Trans. Nucl. Sci. NS-38 (1988) 246; and W. Battler et al., Nucl. Instr. and Meth. A 315 (1992) 420. Pacific Microelectronics. 10575 Southwest Cascade Blvd., Portland, OR 97223, USA. Flextron Systems Inc., 835-T Pennsylvania Blvd.. Feasterville, PA 19047, USA. J.P. Alexander et al., Nucl. Insu. and Meth. A 337 (1993) 171.

The CLEO II.V silicon vertex detector

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