- Email: [email protected]
Operation of a silicon vertex detector in the NA14 photoproduction experiment
530
Nuclear Instruments
and Methods in Physics Research A253 (1987) 530-536 North-Holland, Amsterdam
OPERATION OF A SILICON VERTEX DETECTOR IN THE NA14 PHOTOPRODUCTION EXPERIMENT G. BARBER, M. BURCHELL, M. CAITANEO, C. SEEZ, I. SIOTIS and D. WEBSDALE Hackett Laboratory,
Imperial College, London,
J. DIXON, A. DUANE,
R. FORTY, G. HALL,
UK
P. BONAMY, P. BORGEAUD, M. DAVID, P. DUVAL, Y. LEMOIGNE, F. LOUIS, C. MAGNEVILLE, J. POINSIGNON, M. PRIMOUT, J-F. THOMAS and G. VILLET CEN, Saclay, Gif-sur-Yvette,
M. COUNIHAN, Southampton
J. LUI and G. McEWEN
University, Southampton,
R. BARATE CERN,
France
UK
and D. TREILLE
Geneva, Switzerland
The operation and performance of a silicon vertex detector are described. The detector, which consists of a silicon multilayer active target followed by a microstrip chamber, was operated in a 1.2T magnetic field and exposed to the NA14 high intensity photon beam. Results on the precision and efficiency of the detector for reconstruction of the interaction products are presented.
1. Introduction
One aim of the CERN NA14 experiment [l] is to study the photoproduction and decay of heavy flavour states. Mesons and baryons containing heavy quarks (charm and bottom) have lifetimes in the range lo-“lo-l3 s. A device capable of detecting and identifying such short lived decays requires a spatial precision in the location of particle tracks which is better than 20 pm. A silicon vertex detector, comprising a multilayer active target and a microstrip tracking chamber has been constructed. It was operated, in association with the NA14 spectrometer, to analyse the interactions of high energy ((E) = 100 GeV) photons. The schematic layout of the NA14 apparatus is shown in fig. 1. The silicon microstrip chamber is used to reconstruct the trajectories of ionising particles produced in the photon interactions. These trajectories are extrapolated back to the interaction vertex where “offset” tracks can indicate the presence of secondary decay vertices. The reconstructed tracks are then identified with those measured in the spectrometer thus allowing a momentum assignment to the products of primary and secondary vertices. The active target consists of 32 planes of silicon, each plane being 300 pm thick and separated by 016%9002/87/$03.50 0 (North-Holland Physics
Elsevier Science Publishers Publishing Division)
B.V.
200 pm along the beam direction. The active area is 20 cm*, divided into 24 depleted diode strips of 2 mm width. The analogue signal from each strip is recorded and samples the ionisation energy lost by the interaction products. This information is valuable in several respects. The primary interaction can be located from the highly ionising nuclear recoil or breakup. Secondary decay vertices can be distinguished from hadronic secondary interactions in the target (which provide a background source of offset tracks). Finally a measure of the track multiplicity development provides independent evidence for decay vertices occurring within the target. The design and performance of this active target have been described previously [2].
2. The microstrip chamber 2.1.
Design
and assembly
The design criteria are essentially dictated by the experimental requirements and economic realities. The photon beam incident at the target has a large spot size and does not allow us to employ economical geometries such as have been possible by appropriate focussing of the beam in hadronic production experiments [3]. To provide sufficient angular acceptance
G. Barber et al. I A silicon vertex detector
531
Fig. 1. Layout of the NA144 apparatus. H, V: tagging hodoscopes, M: muon veto; A: Si active target, B: AEG vertex magnet; C, D, E, .I: MWPCs; T: Si microstrip detector. F: “crown” lead glass calorimeter; G: Goliath magnet, I,L: Cherenkov counters; K: trigger hodoscopes; N: Imperial College calorimeter; 0: Olga-Penelope calorimeter; P: iron filter; Q, S: p hodoscopes.
(up to ~250 mrad) the area of the vertex detector was chosen to be 50 mm x 50 mm. A Monte Carlo simulation which included the effects of multiple Coulomb scattering, delta-rays and diffusion of the primary ionisation in the silicon indicated that a strip pitch of 50 pm would be sufficiently fine to recognise charmed decays. The reconstruction of charged trajectories in space requires coordinates in at least three projections, and we chose an arrangement of ten planes stacked to give four coordinate pairs ( y, z) orthogonal to the beam direction followed by a (u, v) pair at 33” respectively to the y and z axes. This arrangement provides redundancy in both orthogonal projections and in the oblique view which is used to match projections in space. The mechanical mounting of the silicon wafers must be stable such that relative plane displacements and rotations during the course of the experiment do not exceed a few pm. A further constraint in the mechanical design is imposed by the aperture of the upstream spectrometer magnet. To achieve the required acceptance the vertex detector must be situated inside this magnet which limits the overall dimensions of detectors, preamplifiers and cables to 500 mm transverse to the beam. The microstrip detectors were prepared by ion implantation of 1000 p+ strips at 50 pm pitch on 3 in. wafers of n-type silicon. The wafers were 460 pm thick with a resistivity of 7500 R cm and were operated with a depleting voltage of 90V. The detectors were processed at the laboratories of Micron Semiconductor (UK) Ltd, and mounted on a 1.6mm thick fibre-glass printed circuit board (PCB) as shown in fig. 2. The silicon strips are ultrasonically bonded with 25 pm Al wires to conductive tracks on the PCB. Adjacent strips are bonded to alternate sides of the
PCB, so the bond separation and the PCB track pitch are 100 pm. This can be achieved without resort to special techniques. The strips are fanned out to the edge of the PCB where commercial 50-way ribbon cable connectors are soldered to the board at a pitch of l/20 in. Just inside these connectors a 3 mm wide copper ribbon is glued across the conducting strips and isolated from them by 100 pm of mylar. This is used to couple capacitively a test signal into each preamplifier channel. It is used for systematic checks on the complete electronics chain, verification of gains and thresholds on individual channels, etc. The channel to channel uniformity of the test signal was better than 10% when compared with the signal pulse height from minimum ionising particles. Each detector plane was electrically tested before assembly. The leakage current of individual strips was measured and found to have a typical value in the range 0.5-2 nA at 100 V reverse bias. All planes had a few strips with anomalously high leakage currents, often in excess of 1 PA. These strips were identified and the leakage current stabilised by inserting a 10 MR series resistor in the connection to the preamplifier. Strips treated in this way are dead for the purposes of particle detection but are not likely to deteriorate during long term operation to such an extent that the detector cannot be fully biased. The testing procedure allowed the detection of nonisolated strips. Usually this was due to shorted tracks on the high resolution section of the PCB and could be repaired by carefully separating the tracks with a scalpel. Some nonisolated strips were due to a fault in the silicon processing and were not repaired. After these operations each detector plane had typically 995 out of 1000 strips in working order. The alignment of the detector planes required preciVI. NEW APPLICATIONS
G. Barber et al. I A silicon vertex detector
532
Fig. 2. The detector stack during assembly.
sion and stability. The stability was provided by a 20 mm stress-relieved aluminium cast plate. The PCBs were mounted on four 6 mm diameter stainless steel studs, threaded into a titanium support and recessed into the main Al plate. Titanium was chosen to match the thermal expansion coefficient of the fibre glass PCBs. The base plate was fixed to the travelling bed of a high precision jig borer. This device was fitted with a microscope and had a nominal precision of 2 pm. The alignment of the silicon strips was achieved by locating the PCB on the studs using eccentric bushes. The bushes were then rotated to obtain an alignment precision of better than 3 pm over the length of the 50mm strips. Once aligned the PCB was secured in position by means of spacers which were threaded on the studs. The planes were built up in this way ensuring a high precision in the parallelism and orthogonality of the strips. No attempt was made to measure the relative displacement of strips in different detector planes; this was determined subsequently using high energy charged particles. Fig. 2 illustrates the detector stack during assembly. 2.2.
Electronics
The electronics chain must provide an amplified signal from the passage of a minimum ionising particle traversing 460 pm of silicon. Since the particles do not
in general traverse at normal incidence the signal charge can be shared and will be detected by more than one strip. It is therefore necessary to have a signal/noise ratio sufficient to set a threshold for particle detection at a level well below 50% of that expected from a minimum ionising particle (Imin). A preamplifier with an equivalent noise charge lower than 2500 electrons rms would satisfy this requirement. The shaping time must be short (1.50 ns) to avoid problems related to the high rate of background electromagnetic processes associated with the photon beam. The MSD2 design [4], a thick film hybrid transimpedance preamplifier using a bipolar input transistor was chosen. Hybrids, containing four such preamplifiers mounted on 1 in.’ ceramics were supplied by the Laben (Milano) company. They have proved to be cheap, reliable and, having a low power consumption (20 mW per channel), can be closely packed with the hybrids separated by 0.1 in., appropriate for standard cable connection (see fig. 3). The gain of the preamplifiers had been set at the factory by laser trimming. Hybrids were selected having a gain within 10% of the sample mean and with an equivalent noise charge below 2000 electrons (50 ns gate). The noise increases with increasing capacitance at the input of the preamplifier. For this reason it was essential to mount the hybrids as close as possible to the detector. The main contribution to the measured
G. Barber et al.
I A silicon vertex detector
Fig. 3. One of the four preamplifier
Fig. 4. The assembled
microstrip
detector
533
blocks.
and preamplifiers. VI. NEW
APPLICATIONS
G. Barber et al.
534
I A silicon vertex detector
MSD
SHIELDED 3M RIBBON CABLE
I
3
ECL OUTPUT
I
T
5Ons THR”ESHSTROBE
BIVAS Fig. 5. Electronics scheme
of 30 pF per channel is provided by the PCB and 20cm of ribbon cable which connects it to the preamplifier hybrids. The following stage in the electronics chain involves a shaping amplifier and an ADC or discriminator. The provision of an ADC for each channel is expensive and a capacitive charge division readout technique is not appropriate in this application due to the high rates involved. We therefore chose a discriminator solution, which is again a thick film hybrid (MSD3, designed at CERN [5]) and manufactured by the Telecontrolli (Napoli) company. The threshold characteristics of all MSD3s were measured and the hybrids selected according to gain and offset voltage. This was done so that hybrids could be grouped and supplied with a single threshold for the group. The output from the discriminators is a balanced ECL signal which is multiplexed and interfaced to the data acquisition computer using a commercial wire chamber readout system, the Lecroy PCOS 2731 latch. This unit contains an internal delay, variable between 300 and 750ns, thus eliminating the need for long cables. It also provides, under CAMAC control,the threshold voltage for the MSD3 discriminators. Fig. 4 shows the assembled detector and preamplifier blocks, and fig. 5 displays the scheme of the electronics chain. The OR-gate was imposed by financial constraints. Each 2731 latch serves two channels. The OR-scheme is chosen in such a way as to minimize the association of “image” hits into reconstructed “image” tracks. input capacitance
3. Detector performance 3.1. Characteristics
of operation
in the photon
beam
The microstrip detectors were operated continuously in the photon beam during a four week run. They were fully depleted at 90V and the leakage currents,
initially in the range 2-3 PA per plane, steadily rose to 8-12 PA. This rise was not due to temperature change but was dependent on exposure to the beam; an integrated dose corresponding to 5 X 10” equivalent minimum ionizing particles per cm*. One week after the run the currents had fallen to their original values. Preliminary measurements were taken using a charge sensitive ADC readout on a sample of 250 channels. These indicated that the signal/noise ratio for minimum ionising particles was 14 : 1 in a 50 ns gate width. It was also verified that the charge deposited was indeed detected by a single strip; the small amount of charge sharing observed being compatible with diffusion of the primary ionisation and delta-rays. For operation with discriminators the signal to noise ratio is important in choosing a threshold which yields a high efficiency for particle detection and at the same time gives a low population of “noise” hits. With a threshold corresponding to 0.35 Imin and an efficiency of 98.7% the number of noise hits, measured with an out of time strobe, was about 6 in 10000. This compares with about 120 hits for a typical photon interaction producing about 10 charged tracks which traverse the detector. The charge sharing is indicated in the distribution of cluster sizes (fig. 6) which shows that 90% of the hits are isolated hits. The double and triple clusters are compatible with diffusion and delta-rays whilst the occasional very large clusters probably result from particle interactions in the silicon. Throughout the run the system behaved reliably with 9937 out of 10 000 channels fully operational. The test pulse facility was regularly used to check threshold settings on the discriminators and it proved invaluable in locating problems in the electronics chain. 3.2. Spatial precision
and efficiency
The spatial precision has been estimated using data
535
G. Barber et al. I A silicon vertex detector
*lO 200
3
r
240
175
200
150 r s 2
125
u
100
GS
S
160
0 c
x100
75
120
B :: &
/
80
50 25
\
4 4
hl 8
12
Cluster
I
I
16
20
-2oQ-100 Residual
size
(a)
350
-
E
300
-
2
250
-
cn 200 ::
-
-0
1
correlation
I
I,I,flLL -10 M
Fig. 8. Angular
I 0
Azimuthal
cc
u e
300
t -
5
250
-
;
200
-
:: :: *
150-
(pm)
angle
c&ion
I
10
0
of tracks
measured
O-20
(mrad)
correlations
(b)
350
150-
0 -20
3
from other detector planes indicate a spatial precision of 16 pm rms. This result, which is independent of the angle of traversal (up to 150 mrad), is comparable with the theoretical value SO/a pm. From the same plot we can extract the efficiency to register a hit. Averaged over all planes we estimate an efficiency of 98.7%. This figure includes nonworking channels and losses due to the software clustering algorithm. Again, we observe no fall in efficiency for track angles up to
Dip angle
\
g
plane
2oc
strip tracks.
from the photon interaction products. They were not taken under special test conditions and apply to high multiplicity events with tracks crossing detectors at angles up to 200 mrad. The detector is situated in a uniform magnetic field of 1.23 T. Fig. 7 shows a typical residuals plot of the displacement of the closest hit to a straight line fit through points from the three other planes (in the projection with no magnetic bending). Similar plots obtained
-
100
Fig. 7. Residuals distribution from straight line fits to micro-
Fig. 6. Cluster size distributions.
400
0
-10
0
10
20
AQ (mrod) in microstrips
and in the spectrometer. VI. NEW
APPLICATIONS
536
G. Barber et al. I A silicon vertex detector
150 mrad. The relative displacement of the detector planes is adjusted to centre these residual plots. Once determined, it was not necessary to change these values throughout the run, indicating that no relative mechanical displacements exceeding 2 pm occurred. 3.3. Charged particle tracking in the microstrips The function of the microstrip detector is to find tracks which appear offset from the production vertex. Tracks are first found in two projections. These are fit to a straight line in the nonbending projection and to a parabola in the bending projection. The maximum permitted sagitta is 145 pm. This corresponds to removal of tracks with momenta below 1.7 GeVlc, which, due to Coulomb scattering, are likely to yield a false offset when extrapolated to the vertex. The track projections are then matched using the u and v planes, and extrapolated to the active target plane where the nuclear recoil is detected. A sample of 15% of the events have an offset track compatible with a secondary vertex: these are filtered for further analysis which includes the event reconstruction in the spectrometer. The angular correlation of tracks measured in the vertex detector and in the spectrometer is clear from fig. 8 which shows the difference in dip and azimuthal angles as measured in the two systems. The rms value of the difference is 0.7 mrad.
4. Conclusions A 10000 channel microstrip detector has been successfully operated in a high intensity 100 GeV photon
beam. A spatial precision of 16 pm rms, close to the ideal figure for a strip pitch of 50 pm, was achieved in high multiplicity interactions. No problems specific to the operation in a photon beam or in a magnetic field were encountered. In fact the magnetic bending is used to advantage in the data analysis. The operation of 10 000 channels of electronics presented no particular problems of scale. The uniformity of the analogue electronics was important here. Although greater flexibility was available we were able to use a single threshold for the complete system. The essential requirement of associating tracks measured in the vertex detector with those in the spectrometer has been demonstrated.
Acknowledgements
We thank L. Andersson, C. Bishop, P. Brambilla, P. Jarron, P.G. Rancoita, A. Rochester, L. Sohet and L. Toudup for the support and technical assistance which they have brought to this project. References
A program of Heavy Flavour Photoproduction NA-14 proposal: CERN/SPSC/82-73, SPSC/P109 Add2 (1982). PI R. Barate et al., Nucl. Instr. and Meth. A235 (1985) 235. [31 R. Bailey et al., Contribution to the International Europhysics Conf. on High Energy Physics, Brighton (1983) MPI-PAEIExp El.121 (1983). 141 P. Jarron and M. Goyot, Nucl. Ins&. and Meth. 226 (1984) 156. [51 P. Jarron and L. Sohet, private communication.
Nuclear Instruments
and Methods in Physics Research A253 (1987) 530-536 North-Holland, Amsterdam
OPERATION OF A SILICON VERTEX DETECTOR IN THE NA14 PHOTOPRODUCTION EXPERIMENT G. BARBER, M. BURCHELL, M. CAITANEO, C. SEEZ, I. SIOTIS and D. WEBSDALE Hackett Laboratory,
Imperial College, London,
J. DIXON, A. DUANE,
R. FORTY, G. HALL,
UK
P. BONAMY, P. BORGEAUD, M. DAVID, P. DUVAL, Y. LEMOIGNE, F. LOUIS, C. MAGNEVILLE, J. POINSIGNON, M. PRIMOUT, J-F. THOMAS and G. VILLET CEN, Saclay, Gif-sur-Yvette,
M. COUNIHAN, Southampton
J. LUI and G. McEWEN
University, Southampton,
R. BARATE CERN,
France
UK
and D. TREILLE
Geneva, Switzerland
The operation and performance of a silicon vertex detector are described. The detector, which consists of a silicon multilayer active target followed by a microstrip chamber, was operated in a 1.2T magnetic field and exposed to the NA14 high intensity photon beam. Results on the precision and efficiency of the detector for reconstruction of the interaction products are presented.
1. Introduction
One aim of the CERN NA14 experiment [l] is to study the photoproduction and decay of heavy flavour states. Mesons and baryons containing heavy quarks (charm and bottom) have lifetimes in the range lo-“lo-l3 s. A device capable of detecting and identifying such short lived decays requires a spatial precision in the location of particle tracks which is better than 20 pm. A silicon vertex detector, comprising a multilayer active target and a microstrip tracking chamber has been constructed. It was operated, in association with the NA14 spectrometer, to analyse the interactions of high energy ((E) = 100 GeV) photons. The schematic layout of the NA14 apparatus is shown in fig. 1. The silicon microstrip chamber is used to reconstruct the trajectories of ionising particles produced in the photon interactions. These trajectories are extrapolated back to the interaction vertex where “offset” tracks can indicate the presence of secondary decay vertices. The reconstructed tracks are then identified with those measured in the spectrometer thus allowing a momentum assignment to the products of primary and secondary vertices. The active target consists of 32 planes of silicon, each plane being 300 pm thick and separated by 016%9002/87/$03.50 0 (North-Holland Physics
Elsevier Science Publishers Publishing Division)
B.V.
200 pm along the beam direction. The active area is 20 cm*, divided into 24 depleted diode strips of 2 mm width. The analogue signal from each strip is recorded and samples the ionisation energy lost by the interaction products. This information is valuable in several respects. The primary interaction can be located from the highly ionising nuclear recoil or breakup. Secondary decay vertices can be distinguished from hadronic secondary interactions in the target (which provide a background source of offset tracks). Finally a measure of the track multiplicity development provides independent evidence for decay vertices occurring within the target. The design and performance of this active target have been described previously [2].
2. The microstrip chamber 2.1.
Design
and assembly
The design criteria are essentially dictated by the experimental requirements and economic realities. The photon beam incident at the target has a large spot size and does not allow us to employ economical geometries such as have been possible by appropriate focussing of the beam in hadronic production experiments [3]. To provide sufficient angular acceptance
G. Barber et al. I A silicon vertex detector
531
Fig. 1. Layout of the NA144 apparatus. H, V: tagging hodoscopes, M: muon veto; A: Si active target, B: AEG vertex magnet; C, D, E, .I: MWPCs; T: Si microstrip detector. F: “crown” lead glass calorimeter; G: Goliath magnet, I,L: Cherenkov counters; K: trigger hodoscopes; N: Imperial College calorimeter; 0: Olga-Penelope calorimeter; P: iron filter; Q, S: p hodoscopes.
(up to ~250 mrad) the area of the vertex detector was chosen to be 50 mm x 50 mm. A Monte Carlo simulation which included the effects of multiple Coulomb scattering, delta-rays and diffusion of the primary ionisation in the silicon indicated that a strip pitch of 50 pm would be sufficiently fine to recognise charmed decays. The reconstruction of charged trajectories in space requires coordinates in at least three projections, and we chose an arrangement of ten planes stacked to give four coordinate pairs ( y, z) orthogonal to the beam direction followed by a (u, v) pair at 33” respectively to the y and z axes. This arrangement provides redundancy in both orthogonal projections and in the oblique view which is used to match projections in space. The mechanical mounting of the silicon wafers must be stable such that relative plane displacements and rotations during the course of the experiment do not exceed a few pm. A further constraint in the mechanical design is imposed by the aperture of the upstream spectrometer magnet. To achieve the required acceptance the vertex detector must be situated inside this magnet which limits the overall dimensions of detectors, preamplifiers and cables to 500 mm transverse to the beam. The microstrip detectors were prepared by ion implantation of 1000 p+ strips at 50 pm pitch on 3 in. wafers of n-type silicon. The wafers were 460 pm thick with a resistivity of 7500 R cm and were operated with a depleting voltage of 90V. The detectors were processed at the laboratories of Micron Semiconductor (UK) Ltd, and mounted on a 1.6mm thick fibre-glass printed circuit board (PCB) as shown in fig. 2. The silicon strips are ultrasonically bonded with 25 pm Al wires to conductive tracks on the PCB. Adjacent strips are bonded to alternate sides of the
PCB, so the bond separation and the PCB track pitch are 100 pm. This can be achieved without resort to special techniques. The strips are fanned out to the edge of the PCB where commercial 50-way ribbon cable connectors are soldered to the board at a pitch of l/20 in. Just inside these connectors a 3 mm wide copper ribbon is glued across the conducting strips and isolated from them by 100 pm of mylar. This is used to couple capacitively a test signal into each preamplifier channel. It is used for systematic checks on the complete electronics chain, verification of gains and thresholds on individual channels, etc. The channel to channel uniformity of the test signal was better than 10% when compared with the signal pulse height from minimum ionising particles. Each detector plane was electrically tested before assembly. The leakage current of individual strips was measured and found to have a typical value in the range 0.5-2 nA at 100 V reverse bias. All planes had a few strips with anomalously high leakage currents, often in excess of 1 PA. These strips were identified and the leakage current stabilised by inserting a 10 MR series resistor in the connection to the preamplifier. Strips treated in this way are dead for the purposes of particle detection but are not likely to deteriorate during long term operation to such an extent that the detector cannot be fully biased. The testing procedure allowed the detection of nonisolated strips. Usually this was due to shorted tracks on the high resolution section of the PCB and could be repaired by carefully separating the tracks with a scalpel. Some nonisolated strips were due to a fault in the silicon processing and were not repaired. After these operations each detector plane had typically 995 out of 1000 strips in working order. The alignment of the detector planes required preciVI. NEW APPLICATIONS
G. Barber et al. I A silicon vertex detector
532
Fig. 2. The detector stack during assembly.
sion and stability. The stability was provided by a 20 mm stress-relieved aluminium cast plate. The PCBs were mounted on four 6 mm diameter stainless steel studs, threaded into a titanium support and recessed into the main Al plate. Titanium was chosen to match the thermal expansion coefficient of the fibre glass PCBs. The base plate was fixed to the travelling bed of a high precision jig borer. This device was fitted with a microscope and had a nominal precision of 2 pm. The alignment of the silicon strips was achieved by locating the PCB on the studs using eccentric bushes. The bushes were then rotated to obtain an alignment precision of better than 3 pm over the length of the 50mm strips. Once aligned the PCB was secured in position by means of spacers which were threaded on the studs. The planes were built up in this way ensuring a high precision in the parallelism and orthogonality of the strips. No attempt was made to measure the relative displacement of strips in different detector planes; this was determined subsequently using high energy charged particles. Fig. 2 illustrates the detector stack during assembly. 2.2.
Electronics
The electronics chain must provide an amplified signal from the passage of a minimum ionising particle traversing 460 pm of silicon. Since the particles do not
in general traverse at normal incidence the signal charge can be shared and will be detected by more than one strip. It is therefore necessary to have a signal/noise ratio sufficient to set a threshold for particle detection at a level well below 50% of that expected from a minimum ionising particle (Imin). A preamplifier with an equivalent noise charge lower than 2500 electrons rms would satisfy this requirement. The shaping time must be short (1.50 ns) to avoid problems related to the high rate of background electromagnetic processes associated with the photon beam. The MSD2 design [4], a thick film hybrid transimpedance preamplifier using a bipolar input transistor was chosen. Hybrids, containing four such preamplifiers mounted on 1 in.’ ceramics were supplied by the Laben (Milano) company. They have proved to be cheap, reliable and, having a low power consumption (20 mW per channel), can be closely packed with the hybrids separated by 0.1 in., appropriate for standard cable connection (see fig. 3). The gain of the preamplifiers had been set at the factory by laser trimming. Hybrids were selected having a gain within 10% of the sample mean and with an equivalent noise charge below 2000 electrons (50 ns gate). The noise increases with increasing capacitance at the input of the preamplifier. For this reason it was essential to mount the hybrids as close as possible to the detector. The main contribution to the measured
G. Barber et al.
I A silicon vertex detector
Fig. 3. One of the four preamplifier
Fig. 4. The assembled
microstrip
detector
533
blocks.
and preamplifiers. VI. NEW
APPLICATIONS
G. Barber et al.
534
I A silicon vertex detector
MSD
SHIELDED 3M RIBBON CABLE
I
3
ECL OUTPUT
I
T
5Ons THR”ESHSTROBE
BIVAS Fig. 5. Electronics scheme
of 30 pF per channel is provided by the PCB and 20cm of ribbon cable which connects it to the preamplifier hybrids. The following stage in the electronics chain involves a shaping amplifier and an ADC or discriminator. The provision of an ADC for each channel is expensive and a capacitive charge division readout technique is not appropriate in this application due to the high rates involved. We therefore chose a discriminator solution, which is again a thick film hybrid (MSD3, designed at CERN [5]) and manufactured by the Telecontrolli (Napoli) company. The threshold characteristics of all MSD3s were measured and the hybrids selected according to gain and offset voltage. This was done so that hybrids could be grouped and supplied with a single threshold for the group. The output from the discriminators is a balanced ECL signal which is multiplexed and interfaced to the data acquisition computer using a commercial wire chamber readout system, the Lecroy PCOS 2731 latch. This unit contains an internal delay, variable between 300 and 750ns, thus eliminating the need for long cables. It also provides, under CAMAC control,the threshold voltage for the MSD3 discriminators. Fig. 4 shows the assembled detector and preamplifier blocks, and fig. 5 displays the scheme of the electronics chain. The OR-gate was imposed by financial constraints. Each 2731 latch serves two channels. The OR-scheme is chosen in such a way as to minimize the association of “image” hits into reconstructed “image” tracks. input capacitance
3. Detector performance 3.1. Characteristics
of operation
in the photon
beam
The microstrip detectors were operated continuously in the photon beam during a four week run. They were fully depleted at 90V and the leakage currents,
initially in the range 2-3 PA per plane, steadily rose to 8-12 PA. This rise was not due to temperature change but was dependent on exposure to the beam; an integrated dose corresponding to 5 X 10” equivalent minimum ionizing particles per cm*. One week after the run the currents had fallen to their original values. Preliminary measurements were taken using a charge sensitive ADC readout on a sample of 250 channels. These indicated that the signal/noise ratio for minimum ionising particles was 14 : 1 in a 50 ns gate width. It was also verified that the charge deposited was indeed detected by a single strip; the small amount of charge sharing observed being compatible with diffusion of the primary ionisation and delta-rays. For operation with discriminators the signal to noise ratio is important in choosing a threshold which yields a high efficiency for particle detection and at the same time gives a low population of “noise” hits. With a threshold corresponding to 0.35 Imin and an efficiency of 98.7% the number of noise hits, measured with an out of time strobe, was about 6 in 10000. This compares with about 120 hits for a typical photon interaction producing about 10 charged tracks which traverse the detector. The charge sharing is indicated in the distribution of cluster sizes (fig. 6) which shows that 90% of the hits are isolated hits. The double and triple clusters are compatible with diffusion and delta-rays whilst the occasional very large clusters probably result from particle interactions in the silicon. Throughout the run the system behaved reliably with 9937 out of 10 000 channels fully operational. The test pulse facility was regularly used to check threshold settings on the discriminators and it proved invaluable in locating problems in the electronics chain. 3.2. Spatial precision
and efficiency
The spatial precision has been estimated using data
535
G. Barber et al. I A silicon vertex detector
*lO 200
3
r
240
175
200
150 r s 2
125
u
100
GS
S
160
0 c
x100
75
120
B :: &
/
80
50 25
\
4 4
hl 8
12
Cluster
I
I
16
20
-2oQ-100 Residual
size
(a)
350
-
E
300
-
2
250
-
cn 200 ::
-
-0
1
correlation
I
I,I,flLL -10 M
Fig. 8. Angular
I 0
Azimuthal
cc
u e
300
t -
5
250
-
;
200
-
:: :: *
150-
(pm)
angle
c&ion
I
10
0
of tracks
measured
O-20
(mrad)
correlations
(b)
350
150-
0 -20
3
from other detector planes indicate a spatial precision of 16 pm rms. This result, which is independent of the angle of traversal (up to 150 mrad), is comparable with the theoretical value SO/a pm. From the same plot we can extract the efficiency to register a hit. Averaged over all planes we estimate an efficiency of 98.7%. This figure includes nonworking channels and losses due to the software clustering algorithm. Again, we observe no fall in efficiency for track angles up to
Dip angle
\
g
plane
2oc
strip tracks.
from the photon interaction products. They were not taken under special test conditions and apply to high multiplicity events with tracks crossing detectors at angles up to 200 mrad. The detector is situated in a uniform magnetic field of 1.23 T. Fig. 7 shows a typical residuals plot of the displacement of the closest hit to a straight line fit through points from the three other planes (in the projection with no magnetic bending). Similar plots obtained
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Fig. 7. Residuals distribution from straight line fits to micro-
Fig. 6. Cluster size distributions.
400
0
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0
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AQ (mrod) in microstrips
and in the spectrometer. VI. NEW
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150 mrad. The relative displacement of the detector planes is adjusted to centre these residual plots. Once determined, it was not necessary to change these values throughout the run, indicating that no relative mechanical displacements exceeding 2 pm occurred. 3.3. Charged particle tracking in the microstrips The function of the microstrip detector is to find tracks which appear offset from the production vertex. Tracks are first found in two projections. These are fit to a straight line in the nonbending projection and to a parabola in the bending projection. The maximum permitted sagitta is 145 pm. This corresponds to removal of tracks with momenta below 1.7 GeVlc, which, due to Coulomb scattering, are likely to yield a false offset when extrapolated to the vertex. The track projections are then matched using the u and v planes, and extrapolated to the active target plane where the nuclear recoil is detected. A sample of 15% of the events have an offset track compatible with a secondary vertex: these are filtered for further analysis which includes the event reconstruction in the spectrometer. The angular correlation of tracks measured in the vertex detector and in the spectrometer is clear from fig. 8 which shows the difference in dip and azimuthal angles as measured in the two systems. The rms value of the difference is 0.7 mrad.
4. Conclusions A 10000 channel microstrip detector has been successfully operated in a high intensity 100 GeV photon
beam. A spatial precision of 16 pm rms, close to the ideal figure for a strip pitch of 50 pm, was achieved in high multiplicity interactions. No problems specific to the operation in a photon beam or in a magnetic field were encountered. In fact the magnetic bending is used to advantage in the data analysis. The operation of 10 000 channels of electronics presented no particular problems of scale. The uniformity of the analogue electronics was important here. Although greater flexibility was available we were able to use a single threshold for the complete system. The essential requirement of associating tracks measured in the vertex detector with those in the spectrometer has been demonstrated.
Acknowledgements
We thank L. Andersson, C. Bishop, P. Brambilla, P. Jarron, P.G. Rancoita, A. Rochester, L. Sohet and L. Toudup for the support and technical assistance which they have brought to this project. References
A program of Heavy Flavour Photoproduction NA-14 proposal: CERN/SPSC/82-73, SPSC/P109 Add2 (1982). PI R. Barate et al., Nucl. Instr. and Meth. A235 (1985) 235. [31 R. Bailey et al., Contribution to the International Europhysics Conf. on High Energy Physics, Brighton (1983) MPI-PAEIExp El.121 (1983). 141 P. Jarron and M. Goyot, Nucl. Ins&. and Meth. 226 (1984) 156. [51 P. Jarron and L. Sohet, private communication.