silicon microstrip-detectors 20 cm
BPT
Figure5.4: BPT attached to the BPC Northmo dule. Shown isthe BPT as usedin
1997 with twoplanes of silicon microstripdetectors (X1 and X3) [Mo98a].
analysis, only lower and upp er thresholds are needed for the BPC energy information at the
rst-leveltrigger. Therefore,a discriminator is used to check the BPC South sumsignals and
the BPC North and South backtile sum signals. All digitized signals are sent to the GFLT
to b e used in various FLT applications. The timing information for b oth BPC mo dules are
derivedusing conventionalLeCroy discriminatorswhose resp ectivelogic output signals are fed
into 4-bit TDCs (Time-to-DigitalConverter) with a5ns step at the GFLT.
Several second and third-leveltrigger slots include BPC information. At these trigger levels,
a mo diedversion of the BPC reconstruction co de providesmoredetailed informationthan is
availableat therst-leveltrigger. This includesthe reconstructedenergy,p osition,and shower
sizefor b oth BPC mo dules. Figure 5.3 shows the BPC triggerconguration for 1997. A more
detaileddescription of the trigger slots used inthis analysis isgivenin section9.
5.3 BPT design
The Beam Pip e Tracker (BPT) was designed to supplement the BPC which was describ ed in
the last two sections or a new BPC comp osed of a matrix of lead tungstate (PbWO
4
)
crys-tals [Ca96 ] [Me99] [Ge99]. As a tracking systemindep endentfrom the ZEUS central tracking
chamb ers, it is designed to provide an indep endent Z-vertex reconstruction, reduce
photopro-b etweenBPC Northand thecorresp onding b eampip e exitwindow. Each detectoradds 0.32%
of a radiation length X
0
of inactive material b etween interaction p oint and BPC. The three
(two) detectors are oriented insuchaway as to determinethe X-(Y-)co ordinates of the tracks
intersectingthem. The supp ort structure has dimensionsof X Y Z =6:37:041:0 cm 3
and iscovered inmetaland plasticfoils inorderto isolateitoptically andelectromagnetically.
In July 1997 the rst two microstrip detectors were mounted inside a carb on-bre structure,
which in turn was attached via a sp ecial ange to the front face of the BPC as depicted in
gure 5.4. Carb on-bre is used b ecause it is robust and adds little inactive material in front
of the BPC. By construction, the relative alignment of the silicon detectors is known within
50 m. The two detectors have strips oriented along the Y-axis and are used to reconstruct
the X-co ordinate of intersectingtracks. Due to the lo cation of the BPC the resolution in is
dominated by the resolution in X. The BPT as installedin 1997 was exp ected to increasethe
resolution in and thereforein Q 2
. After a few weeks of commissioning (seesection 5.5) data
taking started inearlySeptemb er1997.
TheBPT microstripdetectors aresingle-sided andconsistof N-typ e silicon. Eachdetectorhas
an active area of 5.76 5.76 cm 2
and is (30015) m thick. The active area consists of 576
implanted p +
strips with a pitch of p = 100 m. The exp ected spatial resolution is given by
=p=
p
12 30 m. The strips are numb eredfrom0 to 575 starting fromthe strip closest to
the b eam pip ein the case of the verticalstrips and from the one at the b ottom in the case of
horizontalstrips. Thestrips areAC-coupledto the readout electronicsto suppress signalshifts
due tothe darkcurrent,whichincreasesifthe detectors are exp osed to radiation. Thisis done
by a layer of silicon oxide b etweenthe p +
implantationand the aluminiumreadout strips. A
guardring is used to bias the detector through the punch-through eect.
The front-end electronicsisrotated by 90 o
with resp ect to the silicon detectors. It is mounted
inside the carb on-bre supp ort structure on multi-layer Printed Circuit Board (PCB), which
serve as mechanical supp ort and distribute p ower and signal lines. Co oling is provided by a
copp erpip eof 1mm 2
circulatingwater of20 o
C. 50 mthickfanout cablesof upilexsubstrate
are usedto connectthefront-endelectronicsto the detectorstrips. Electroplatedcopp er strips
covered by a thinlayerof gold are used to provide go o d electricalcontact. Anoverviewof the
BPT sp ecications isgivenin table 5.2.
It was necessary to removethe metaltub e includingthe TLDs and the silicon dio des in front
of the BPC North, in order to connect the BPT supp ort structure to this BPC mo dule: b oth
deviceshaveb een movedinside the carb on-bre structure.
5.4 BPT readout and trigger
The BPT readout isof the binary typ e. Ifthe pulse heightof a givenreadout channelexceeds
the threshold, this channel is marked as hit. The strip and detector identiers of each hit
channelare stored. The front-end electronicsand readout of the BPT isidenticalto that used
by theZEUSLPS [La93 ][Co96 ],whichwas brieydescrib edinsection4.2. TheBPThas b een
includedinthe readout and calibrationschemeof theLPS [Mo98a ]. 64 BPTchannelsare read
out by the samechainof tworeadout chips. Dueto spaceconstraints,only eightpairs of chips
could b e mounted for eachdetector. The 128 silicon strips of each detectorfar away from the
b eam pip e were connected to a pair of chips in groups of two, which reduces the numb er of
readout channels to 512. The readout channels are numb ered from 0 to 511 for each plane.
The analog amplierand comparator chip (TEKZ)is connected to the silicon strips. For each
Size (Height Width Depth) (6 6 0.03) cm 3
Bulkmaterial N typ e high purity silicon
Resistivity (8{10) kOhmcm
Thickness (300 15) m
Fulldepletion (FD)voltage 30 V typically
Activearea (58 58) mm
N. of channels 576
Elementpitch 100 m
Elementwidth 80 m
Readout AC
Guard ring included
Metalization (Al) (8000 1000) A
o
Oxide edge width 1125 mb etweenlast guard ring and edge
Op erational voltage 1,5 FD
Elementcapacitance 35 pF/cm
2
Total leakagecurrent (FD) typ. 100 nA max 500 nA
Dynamic biasingresistor >100 MOhm
Radiation hardness >200 krad (Co
60
)
Table 5.2: Sp ecications of the BPT silicon microstripdetectors.
shaping timeof 32 ns ensuresthat eacheventisassigned to the correctHERA bunchcrossing.
The digital output of the TEKZ is transferred to the DigitalTimeSlice Chip (DTSC), which
stores the data until a GFLT decisionhas b een made. The BPT information was not used in
the ZEUStrigger selection.
5.5 Commissioning of the BPT
The average energy loss for minimumionizingparticles(MIPs) insilicon is ab out 39 keV/100
m[Le87 ]. For the silicon microstrip detectors used in the BPT an energy dep osition of 120
keV is exp ected. Since the average energy required to create an electron-hole pair in silicon
at 20 o
C is 3.6 eV, for a MIP approximately 30,000 electron-hole pairs are created in one
BPT detector,which corresp onds to acharge of 4.8 fC. Fromthe design of the BPT readout,
simulations,andtest measurementsb eforeinstallation,the thresholdsof the detectorsfordata
taking were estimatedto b e of the order of 1.5 fC.After installation of the BPT it was found
that thresholds b elow1.6 fC resultedin articialnoise ina large numb erof channels [Pe99].
Before the BPT was used in the data taking, itstime delay w.r.t. the ZEUSreadout and the
thresholds for b oth detectors had to b e determined. The pro cedures used in b oth cases are
describ ed b elow. The readout of the LPS detectors is synchronized to the HERA clo ck. In
orderto makesurethatsignalsfromtheBPTareassignedtothecorrectHERAbunchcrossing,
the timedelayb etweenthe LPS detectors and the BPT readout was determined. Sp ecial runs
were taken in August 1997 to determine the correct time delay, which comp ensates for the
dierent cable lengths of the BPT compared to the LPS detectors. In the absence of noise
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
300 325 350 375 400 425 450 475 500
delay (ns)
BPT Efficiency
Figure 5.5: BPT eÆciency as a function of the delay time w.r.t. the GFLT. The
eÆciency is dened as the fraction of events with hits in b oth BPT planes from a
sample of well-measured BPC p ositrons. The delay time selected for data taking
was 420 ns.
highlyenergetic p ositron inthe BPC is slightly ab ove 1. In most cases the particletrajectory
only intersects one strip, but in a small fraction of events it might hit the area b etween two
readout strips and thus cause a signal in b oth strips. Noise and background are exp ected to
increasethisvalue. Sp ecialrunsweretakeninAugust1997to determinethecorrecttimedelay.
A mo died BPC trigger (FLT 52, SLT DIS 2, TLT DIS 18, see section 9) was used to select
eventstriggered by energy dep osition inthe BPC. The data was taken in the high luminosity
trigger mo de with the mo dication b eing that the prescale factors for FLT 52 and TLT DIS
18 were changed from 64 to 8 and 9999 to 1 resp ectively. In order to use a well-measured
p ositron sample with a low numb erof background events, only eventswith a p ositron energy
ab ove 20 GeV measured in the BPC were selected. This corresp onds to the kinematicregion
oflowy (seesection3.4), wherephotopro duction background issmallandthe currentjetis far
awayfromthe BPC.Thethresholds forb oth BPTdetectors weresetto 2.4fC to minimizethe
amountof noise inthe measurement. The eÆciency
BPT
was dened as the fraction of events
N
BPC
with at least one hitin the BPT N
BPC+BPT :
BPT
= N
BPC+BPT
N
BPC
(5.3)
Figure 5.5 shows the eÆciency as a function of time delay. The optimal value for the data
taking was determinedto b e 420 ns.
The sameBPC triggerused ab ove was used to determinethe threshold for b oth detectors for
datataking. Thethresholdsarerequiredtob elowenough sothatfewsignaleventsare rejected
and thusthe detectors are as eÆcientas p ossible. On the other hand if the thresholds are to o
lowthe noisewillincreasewhichmightleadto additional reconstructedtracks. Usingthetime
p ositrons in the BPC:
Energy : 15 <E
BPC
<30 GeV
X-p osition: 5:2<X
BPC
<8:0 cm
Y-p osition: 2:5<Y
BPC
<2:5 cm
Showersize:
X
<0:7 cm,
Y
<0:7 cm
Z-vertex : 50<Z
VTX
<50 cm
The Z-vertex was taken from the CTD and the BPC quantitieswere reconstructed using the
algorithms develop ed in the context of the 1995 analysis [Su98 ], [Mo98]. In addition, several
noisychannelsin the BPT weremasked(1,189, 190, 197 in planeX1, 0, 1,386, 392-402,
485-511inplane X3)andthe totalnumb erofhits inb othplanes wererequiredto b elessthan200.
TheeÆciencyforone planetodetectap ositronisdenedintwosteps. Firstthe reconstructed
vertexandthep ositionofthedetectedp ositronintheBPCareusedtoestimatethehitp osition
inb oth BPTplanes. The particletrajectoryisassumedto b eastraight lineb etweenthe event
vertexand the BPC. The eectof the magneticeld is ignored. If the closest hitin one plane
is less than 0.2 cm from the extrap olated line, then this hit is used further. This hit and the
vertex p osition are used to get a b etter estimate of the hit p osition in the other plane, since
theBPT resolutionisb etter thanthat ofthe BPC.Again the particletrajectoryisassumedto
b e astraight lineb etweenthe two p oints. The N
Extrap olation;J
eventswith a prediction for ahit
inplane J are used to determinethe eÆciency
BPT;J
of this plane.
BPT;J
isdened as
BPT;J
= N
Found;J
N
Extrap olation;J
(5.4)
N
Found;J
are the events with the closest hit in plane J b eing less than 0.2 cm away from the
prediction. Figure 5.6 shows
BPT;J
and the numb er of hits N
Hits;J
p er plane J p er
bunch-crossing forb othBPTplanesas afunctionofthe thresholdsetinDACunits. Fromcalibration
measurementsthe conversionofthethresholdinDACunitsintofCwasdeterminedtob egiven,
to go o d approximation, by[Pe99]:
threshold (fC)' A
1
(mV) threshold (DAC)
A
2
(mV/fC)
(5.5)
The parameters A
1
and A
2
were determined for each detector. The mean values found were
A
1
=140mVand A
2
=185 mV/fC.The dierentamountof noiseinthetwoplanesrequireda
higherthreshold for planeX3 of3.5 DACunits (2.6 fC)than for planeX1 with3.1 DACunits
(2.4 fC).
5.6 BPT data quality monitoring
The BPT is included in the ZEUS data quality monitoring (DQM). In the online DQM, bias
voltage, temp erature, and strip o ccupancy of the detectors are monitored. This allows the
shiftcrew to identifydeador noisyreadout channels. The oineDQM consists of an analysis
program inthe frameworkof the ZEUS analysis package EAZE(see section9.2). During data