The Backward Silicon Tracker

• 144 strip detectors (number of readout channels = 92.160)

• 48 pad detectors (number of trigger channels = 1536)

Strip detector (640 strips)

Pad detector (32 pads)

Detector system

Applications:

The Backward Silicon Track Trigger for the HERA Experiment H1

Trigger Purposes Trigger Mask Concept

Pad detector module Front-end Electronics Pad readout system

Test with a Calibration Pulse Beam Test and Calibration BST trigger signal BST veto signal

PRO/A readout chip

Interface tothe H1 DAQ

DetectorModules

96 channels

Low noise power supplysystem for the front-end:

VME Cards

Assembling of the BST Layers

Strip detectors

Pad detectors

Deeply Inelastic Scattering event

BST acceptance in the x, Q2 kinematical plane

DIS Measurements with the BST

L1 Trigger Elements: - Track trigger; - Back-to-back tracks; - Background veto.

L2TT information: - θ and φ of the track; - Hit multiplicity.

Sampling phase Readout phase

V

0

trackstrigger dεε)P(VN(V)N

Plateau width

Detector evaluation

Radiation background Radiation Monitor for H1 Radiation Monitor for HERA BST Collaboration

Dose rate measurement with Pads

Intense background components:

• Synchrotron radiation:

][)(

2cmScountsND

gcmMeV /22

The radiation monitor rate dependson the vacuum in HERA, therefore forthe given conditions the backgroundcan be predicted in the first approxi-mation from the current product:

Measuring the cumulative dosebecomes possible with silicondetectors (besides beam losseswhen the energy is released in avery short time) as all particlesdeposit on average the same energy

A linear correlation between the radiationmonitor rate and the ionization current inthe central trackers is used for chambersto control their “turn on” conditions andto explore their high voltage trips.

Correlation with scintillation counter rates

Reaction to the beam currents

Detector smiths

)ε1(ε4εε s3s

4stot

%290εs )( A single layer efficiency

Raw data analysis: the number oftriggered pads is particularly high forthe proton scattering off the rest gasmolecules or off the beam line optics.Online veto: the multiplicity control

helps for early background rejectionwithout affecting events from the eP interaction region.

beam collimator

Fast response time and reduced sensitivity to a neutral backgroundcomponent make the silicon detectors an important part of the H1trigger. A track requirement provides further suppression of the beaminduced background, the main part of which originates beyond the interaction region. Thus the BST trigger offers a significant dataquality improvement for:

Calorimeter trigger rate at low energies becomesenormously high because of the photoproductionbackground. It is kept down by a (hadronic) trackrequirement in the central tracker that introducesa bias to the inclusive DIS data sample. The tracktrigger signal from the BST allows for an efficientand unbiased data taking without rate scaling.

Energy spectrum in DIS Trigger patterns

Influence of a z-vertex spreadon the trigger efficiency

Influence of a XY-beam offseton the trigger efficiency

• Track curvature measurement and finding the particle’s momentum;

• Z-vertex reconstruction and rejection of non-eP tracks and other background.

• Track trigger for an efficient use of a high luminosity of HERA-II;

Inclusive measurementsof a scattered electron with a calorimeter and the BST:

Event kinematics:

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E'1

kP

qPy 2

e

e

);2/(cosE'E4)k'(kqQ 2ee

222

)Q(x,F

y)(11

y)Q(x,F

Q

1

Qx

)Qσ(x, 2L2

22

22

22

4

;sy

Qx

2

Inelastic eP-scattering cross-section:

• DESY Hamburg• Prague Charles University• Institute of Physics AS CR• Rutherford Appleton Laboratory

Max Klein Peter Kostka Thomas NaumannJan Kretzschmar Tomas Lastovicka Mirek NozickaWolfgang Lange Hans Henschel Joachim Meiβner

Rainer Wallny Doris Eckstein Vladimir ArkadovMilan Janata Ulrich Harder Wolfgang EickWolfgang Philipp Olaf Gräber Ilya Tsurin

Bill Haynes H.-C.S-C.

Injected charge (8 fC)

Multiplexed rawdata bit and the trigger signal

Multiplexed raw data bit and the radiation monitor

Phase switch:sampling / readout

Radiation monitor

L2TT information: - θ and φ of the track; - Hit multiplicity.

Radiation monitor(interrupted during theraw data transmissionand then corrected)

Raw data output forthe L2 topological andneural network trigger

Trigger hodoscope:

Two outermost detectors with low constant thresholds definea track and the third detector in the middle is being studied with 5 GeV electrons at DESY-II.

It also extends an acceptance of the H1 triggerfor tracks with a very low Pt momentum.

The trigger masks allow for selectionof tracks which are confined within oneazimuthal sector (22.5 degrees) andcross any three detector layers Merging of more than one sector is possible butit complicates the design.

• F2 measurements at low Q2 in the extended x region

• FL measurements at low Q2 in the high y region (y = 1 – E’/E)

The vertex pointing track trigger is realized by means of a specialdetector structure when the particles from eP collisions traversethe BST layers firing a predefined combination of the silicon pads.A number of those combinations – the so-called “masks” to coverall possible track angles and curvatures has been found empirically.

The detector division in the R-Z plane provides formation of straightroads pointing to the interaction point. With 4 layers and 8 rings thisgives 7 basic masks, but the event vertex spread around the nominalinteraction point leads to a somewhat larger number of masks. In anycase the maximum number of trigger patterns needed is about 60.The trigger efficiency drops for particles whose momentum is below1 GeV (the Pt in the BST angular acceptance is less than 100 MeV).

The PRO/A readout chip was designed in collaborationbetween DESY Zeuthen and IDE AS (Oslo, Norway) andmanufactured in 1.2 um n-well CMOS process by AMS

Specifications:

• 32 channels with digital and analog outputs with controlled ON / OFF function and internal or external trigger thresholds; • Individual / subtraction mode choice for channels for the common mode suppression;

• Dynamic input range 35 dB;• Controlled gain 15…30 mV / fC;

• ENC (noise performance) 570 e;• Noise slope 15 e / pF;

• Shaping time constant 30 ns;• Has a calibration pulse mode.

384 channels

• Manipulating the PRO/A steering codes, setting trigger thresholds;• Synchronization of all detector pulses to HERA clock frequency;• Track validation with masks and computing the track topology;• Accumulating and transmitting the raw data from the silicon pads;

The front-end boards for the detector control and the trigger dataprocessing (the ALTERA chip EP20K300E is a core of each board):

• Monitoring the multiplicity of triggered pads;

Auxiliary functions:

• Monitoring the radiation background;

• Temperature measurements with a CAN chip.

• Bipolar voltages for the analog circuits;• Bipolar and unipolar voltages for the digital circuits and voltage converters;• Bias voltages for the silicon detectors.

The system defines a power-ONsequence which is important forthe PRO/A readout chip !

Electronics in the VME standard unifyall data streams from the front-end andinterface them to the level-1 and level-2 of the H1 central trigger system.

Threshold scan

For each pad of the middledetector a number of triggerpulses was counted as a functionof a threshold voltage applied:

The plateau width is a measure of a MIP signaland the pedestal FWHM is a noise estimate (thelatter was measured with a calibration pulse).The signal-to-noise ratio was control to select good detectors hence the threshold scan wasdone for all silicon sensors prior to the BSTinstallation.

For tracks defined with external scintillationcounters the trigger efficiency of assembly of3 detector modules was (96+/-2)%

was measured in H1 during eP-operation of HERA. The particle’s track in theaperture of the silicon pad detector was reconstructed from two space points:an event vertex and a barycenter of the energy cluster in the calorimeter. Thehigh momentum electrons were selected to exclude the background influence.The trigger masks which require a coincidence of any three layers provide theefficiency of up to accordingly to a formula:

for the DIS electrons

%)295(

Event Z-vertex as measuredwith the BST trigger only. No additional cuts are applied

Electron energies in the SPACAL:• BST L1 trigger only (solid line)• Sector validation (dashed line)

Luminosity data taken prior to 2003 shutdown

Such an event can fulfill randomly anyphysical trigger or even a combinationof several triggers.

The synchrotron radiation fandoes not enter the H1 facilitydirectly but some part of it scatters from absorbers to-wards the detector and does heat the beam line optics that causes a gas evaporation and worsens the vacuum. In turn this increases a

where the latter is the main background component.

• e-gas scattering• P-gas scattering

The instantaneous count rate can be helpful for the HERA crew as afeedback from the detector during injection and the beam steering:

http://h1lumiserver.desy.de:8080/main/h1mon.html

In coincidence with some other counters the radiation monitor can beused for an automatic beam dump in a case of severe radiation load. The trigger algorithm of the pad detector was extended to monitor online

the particle flux through the silicon. The multiplicities of triggered pads are summed up within 1 second, during the next second the result obtained is sent out while the next integral is being prepared.

One silicon sensor (20 cm2)is taken as an area unit forthe radiation monitor. Themaximum rate over 48 paddetector modules is beingdisplayed.

epep IIα)αI(PIdt

dN 0

• DESY Zeuthen

Simply nice picture :-)