Electronics, trigger and DAQElectronics, trigger and DAQin high energy physicsin high energy physics

Introduction to HEP experiments

Electronics

Trigger

Data acquisition systems

E. SantovettiUniversita' di Roma Tor Vergata

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Particle acceleratorsParticle accelerators● In scattering experiments, a high energy particle beam, produced in an

accelerator is directed onto a target or toward a highly focused beam coming from opposite direction

Electrons are liberated from high voltage triode tubes

Protons are produced by ionizing hidrogen

● First step of acceleration by a linear sequence of resonant cavities (acceleration) and focusing quadrupole magnets. Then...

● Linear accelerators or circular colliders

– Linear accelerators: bunches very clean and high quality beams, no energy loss by synchrotron radiation, good energy resolution. But very long, a lot of magnetic elements and short pulses

– Circular machines: Less space, less elements and higher luminosity (1013 particles/pulse). Beam quality not very good, beam spot larger, more halo and worst energy resolution.

● Secondary beams: kaons, pions, antiprotons and neutrinos can be produced if an extracted proton beam hits a primary target. Then different particles can be selected at different energy (NA and WA experiment at CERN)

●

DE [keV ]=88.5⋅E4[GeV ] /r [m ]

Definitions and goals of triggers

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Energy balance in scattering experimentsEnergy balance in scattering experiments● In fixed target experiment part of the energy of the incoming particles is

not available for the interaction but wasted in boosting the particles in the forward direction. Storage rings or colliders are used to increase the energy available.

W 2=s=(p1+ p2)2

squared energy available in the collision

Consider the case of a 450 GeV proton colliding on a fixed target

s=mp2+ mp

2+ 2Em p=754GeV →W=√ s=27.5GeV

The energy available for the reaction is:

M x=W−2mp=25.6GeV=2×12.8GeV

the same energy that can be reached in a symmetric collider with 12.8 GeV per beams.

E ft=2E sr2 /m

● Three different types of colliders can be distinguished:

Hadron-hadron, lepton-lepton and hadron-lepton

Definitions and goals of triggers

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Colliders examplesColliders examples

Collider Laboratory Particles Bean energy Luminosity Crossing

Tevatron FNAL p pbar 1000 2.5*1031 3.5

CESR CERN e+ e- 6 6*1032 0.22

PEP II SLAC e+ e- 9+3.1 3*1032 2.3

LEP CERN e+ e- 100 2.4*1031 11

Hera DESY e- p 27.5e+920p 2*1031 0.096

SLC SLAC e+ e- 50 0.8*1030 8300

KEK B KEK e+ e- 8+3.5 1034 0.002

DaΦne LNF e+ e- 0.51 3*1032 0.0027

LHC CERN p p 3500 1034 0.025

ILC ? e+ e- 0.5 – 2.0 1034

SuperB ? e+ e- 9+3.1 1036

Definitions and goals of triggers

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Luminosity at fixed target experimentLuminosity at fixed target experimentIn fixed target experiment, the number of interactions is proportional to the reaction cross section σ, the particle flux, the target atoms density and the target lenght.

It is usual define the target constant F (dimension of an area) as:

F=1/(ρa⋅l)=A /(N A⋅ρ⋅l)

For the target above and a flux of 1010 particles per second the luminosity is:

Then the rate of a give reaction is N events

s=σ

N flux /s

Fσ×L

For a liquid hidrogen target of 11 cm lenght

F=1 g /(6.022×1023×0,071 g /cm3

×11cm)=2.1×10−24 cm2=2.1b

The luminosity (L) is a measure of the sensitivity and give directly the number of events per seconds for a cross section of 1 cm²

L=4.8×1033cm−2s−1=4.8nb−1 s−1

Definitions and goals of triggers

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Luminosity at collider experimentLuminosity at collider experimentIn collider experiment the luminosity is a function of several parameters.

F=Nb2 f kb/(4πσ

2)

All this parameters depend on the energy and on the beam dynamics.

Increasing the number of particles per bunch to much can enlarge the beam size and cause instability due to beam beam interaction or mirror charge on the beam pipe.

Difficult calculate precisely the luminosity, for a given storage ring, starting from the machine parameters.

Nb: number of particles per bunch

kb: number of bunches per beam

f: revolution frequency

σ: beam radius at the crossing point

Definitions and goals of triggers

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Time structure of acceleratorTime structure of acceleratorFixed target experiment: particles are accelerated by radio-frequent electric field (sine wave) and then they are organized in bunches. Because of this timing structure, an experiment receives particles for only a fraction of the overall time (duty cycle).

Colliders: both rings are filled with particles that are then accelerated to the nominal energy. Time to fill ranges from 5 minutes to few hours for e+e- or pp. The time to accelerate can go from few minutes to half an hour. After tuning (and squizing near the IP's), collisions occur for several hours until the intensity of the beam is so low that a new filling procedure is required

Duty cycle = available beam time / total time= duration of a bunch × number of bunches per second

machine Palse duration Pulses/s Duty cycle

SLAC lin. Acc. 1.2 μs 360 0.04%

DESY 1 ms 50 5%

The limit to the bunch duration comes from the fact that the acceleration from the resonant cavities has to occur at the top of the sine wave. Pulses to long result in a worsening of the energy resolution. This can be partially solved by using higher resonant modes.

Definitions and goals of triggers and filters

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Event rateEvent rateIn fixed target experiment, the event rate is directly proportional to the flux of the incoming particle.

At energy ~10 GeV pp cross section is 40 mb. It is quite easy produce high intensity proton beam (1013 protons per spill). Assuming a spill duration of 10 s and F=2.1 b

Lpeak=1013/(10s×2.1b)=0.48×1012b s−1

=0.48×1036 cm−2 s−1

The number of events to record per each spill is 19.2×109 that is huge (total cross section).Usually we measure the differential cross section in a certain angle, in this case the measuredCross section drops by up to ten order of magnitude → ~2 events per spill

In collider experiments event rate is proportional to the luminosity delivered by the machine. Typical values are 1030 → 1034. At p-pbar collider the luminosity is limited by the antiprotons beam intensity

Assuming L = 1030 σtot

(p-pbar) ~ 60 mb → 6×104 events/s (Minimum bias events).

For Z0 production, σ = 2 nb → 17 Z0 events/day

In e+ e- collider, the cross section is very smallThe single photon exchange cross section can be evaluated by the process e+ e- → μ+ μ-

σμμ=4 πα

2

3s=

21.9nbGeV 2

E beam2

The ratio of hadron production to μ+ μ- production is R=3∑i=1

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Q i2=5

At LEP (E = 55 GeV, L = 1.5×1031) the resulting event rate is

N /s=L×σ 1.5×1031cm−2 s−1×4×21.9×10−33cm2

/552=0.0005/ s

The background is dominated by theBeam-gas interaction and showers inThe beam pipe, 103-104/s

Definitions and goals of triggers and filters

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IntroductionIntroduction

Why do we care about electronics?as physicists?as computer scientists?

The Readout ChainShaping, AmplifyingDigitizing, Transmitting, Noise...Timing and Synchronization

SystemsPower, Cooling & Radiation

Electronics

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Physicist stop reading here....Physicist stop reading here....It is well known that....

”Only technical details are missing”

Electronics

A physicist is someone who learnedElectrodynamics from Jackson

Werner Heisenberg, 1958

∇⋅E=ρϵ0

∇⋅B=0

∇×E=−∂B∂ t

∇×B=μ0 J+ μ0ϵ0

∂ E∂ t

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Detector Front End Electronics Detector Front End Electronics (FEE)(FEE)

Electronics

The front end electronics is the first electronic stage the signal encounter just after the detector

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Looking at ATLASLooking at ATLASElectronics

Tracking chamberin a solenoidal field

Hadronic calorimeter

EM calorimeter

Muon chambers in a toroidal field

Beam line

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TrackingTrackingElectronics

Separate tracks by charge and momentum

Position measurement layer by layer

● Inner layers with silicon pixels and strips (high density): presence of hits determines position

● Outer layers with ”straw” drift chambers, needs time to determine the position

By measuring the curvature we can measure momentum and charge of charged particles (always B field)

For neutral particles we need other detector

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CalorimetryCalorimetryElectronics

Particles generate showers in calorimeters

● Electromagnetic calorimeter (yellow): absorbs and measures the energies of all electrons and photons

● Hadronic Calorimeter (green): absorbs and measures the energies of hadrons, including protons and neutrons, pions and kaons

Amplitude measurement

Position measurement provided by segmentation of the detector

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Muon chambersMuon chambersElectronics

Electrons formed along the track drift towards the central (anode) wire

The first electron to reach the high-field region initiates the avalanche, which is used to derive the timing pulse.

Since the initiation of the avalanche is delayed by the transit time of the charge from the track to the wire, the detection time of the avalanche can be used to determine the radial position.

● Principle also used in straw tracker, need fast timing electronics

Ionization gas

anode wire

cathode

charged track

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Summary of measurementsSummary of measurementsElectronics

Si Tracking position to ~10 μm accuracy in r-φ (through segmentation) timing to 25 ns accuracy to separate bunch crossings

Straw Tracker position to 170 μm at r > 56 cm

EM calorimeter energy via LAr ionization chambers position through segmentation

Hadron calorimeter energy via plastic scintillator tiles position through segmentation

Muon System signal via ionization chambers, position through timing measurement

Although these various detector systems look very different, they all follow the same principles. Sensors must determine

● presence of a particle● Magnitude of signal● Time of arrival

Some measurements depend on sensitivity, i.e. detection threshold, e.g.: silicon tracker, to detect presence of a particle in a given electrode.

Others seek to determine a quantity very accurately, i.e. resolution, e.g. : calorimeter – magnitude of absorbed energy; muon chambers – time measurement yields position

All have in common that they are sensitive to:● signal magnitude● fluctuations

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The front end electronicsThe front end electronicsElectronics

Front-end electronics is the electronics directly connected to the detector (sensitive element). Its purpose is to:

● acquire an electrical signal from the detector (usually a short, small current pulse)● tailor the response of the system to optimize:

➢ the minimum detectable signal➢ energy measurement (charge deposit)➢ event rate➢ time of arrival➢ insensitivity to sensor pulse shape

● digitize the signal and store it for further treatment

shaper

preamplifier

Discrimination digitization, buffering and logic processing

Incoming radiation

Detector sensor

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The readout chainThe readout chainElectronics

frequency

time

Input amplitude

Time and amplitude measurements

sensor

Amplifier

Analog filter

shaper

range compression

clockB

uffe

r

Bu

ffer

Ana

log

sign

al (

char

ge)

samplingZero soppression

Digital signal (#Byte)

First step is devoted to handle the analog signal

Second step regard possible digitization of some information

Buffer is a parking area for the data. Discussed later

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The readout chainThe readout chainElectronics

frequency

time

Input amplitude

Time and amplitude measurements

sensor

Amplifier

Analog filter

shaper

range compression

clock

Bu

ffer

Bu

ffer

Ana

log

sign

al (

char

ge)

sampling

Zero soppression

Digital signal (#Byte)

time

Bu

ffer

thrtrigger

trigger

ADC gate

TDC start

Different detectors can have a different signal treatment of the analog and digital part

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Example: photomultiplierExample: photomultiplierElectronics

Photomultiplier has high intrinsic gain (amplification) → no preamplifier is required

Pulse shape does not depend on signal charge → measurement is called pulse height analysis

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The signalThe signalElectronics

The signal is usually a small current pulse varying in duration (from ~ 100 ps for a Si sensor to O(10) μs for inorganic scintillators)

There are many sources of signals. Magnitude of signal depends on deposited energy inside the active material and excitation energy

Asignal=Δ Edep

Eex

Signal Physical effect Exitation energy

Electrical pulse (direct) ionization 30 eV for gases 1- 10 eV for semiconductors

Scintillation light Exitation of optical states 20-500 eV

temperature Exitation of lattice vibration meV

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Acquiring the signalAcquiring the signalElectronics

Interesting signal is the deposited energy → need to integrate the current pulse● on the sensor capacitance● using an integrating preamplifier● using an integrating Analog Digital

Converter (ADC)

The signal is usually very small → need to amplify it● with electronics● by signal multiplication (photomultiplier)

V i=Qs

Cd+ C i

Not so practical since the output response depends on the sensor capacitance

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Acquiring the signalAcquiring the signalElectronics

Feedback amplifier with gain –A

Assume infinite input impedance (no current flows into the amplifier)

Input signal produces Vi at the input of the amplifier generating –AVi on output

All charge must build up on feed back capacitance

● Charge gain depends only on Cf• Cf × A needs to be large compared to Cd

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Fluctuations and noiseFluctuations and noiseElectronics

There are two limitations to the precision of signal magnitude measurements

Fluctuations of the signal charge due to an absorption event in the detector

Baseline fluctuation in the electronics (”noise”)

Often one has both and they are independent from each other so their contributions add in quadrature

Noise affects all measurements – must:

Maximize the signal to noise ration S/N

Δ E=√Δ Enoise2

+ Δ E fluc2

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Signal to noise ratio S/NSignal to noise ratio S/NElectronics

signal

signal

baseline noise

baseline noise

baseline

baseline baseline baseline

baselinebaseline

signal+baseline

signal+baseline

Need to optimize Signal over Noise Ratio (SNR)

signal+baseline

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Signal fluctuationsSignal fluctuationsElectronics

A signal consists of multiple elementary events (e.g. a charged particle creates one electron-hole pair in a Silicon strip)

The number of elementary events fluctuates as

where F is the Fano factor (0.1 for Silicon)

In an ionization chambers the deposited energy distribution is the Landau function (much broader than Poissonian). Primary ionization is Poissonian than there is the secondary one

Δ N=√F⋅N

Δ E=Ei⋅Δ N=√F EiE

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Full width half maximum (FWHM)Full width half maximum (FWHM)Electronics

The FWHM = 2.35 σ for a Gaussian distribution

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Electronic noiseElectronic noiseElectronics

Thermal noise: created by velocity fluctuations of charge carriers in a conductorNoise power density per unit bandwidth is constant: white noise → larger bandwidth → larger noise (see also next slide)

Shot noise (Schottky)created by fluctuations in the number of charge carriers when they pass a p-n junction (e.g. tunneling events in a semi-conductor diode)proportional to the total average current and independent from the frequency (white)

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Important conceptsImportant conceptsElectronics

The Bandwidth BW of an amplifier is the frequency range for which the output is at least half of the nominal amplification

The rise-time tr of a signal is the time in which a signal goes from 10% to 90% of its peak-value

For a linear RC element (amplifier):

For fast rising signals (tr small) need high bandwidth,

but this will increase the noise (see before) → shape the pulse to make it “flatter”

BW⋅t r=0.35

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Noise and detector capacitanceNoise and detector capacitanceElectronics

For a given signal charge Qs

Assume the amplifier has an input noise voltage Vn, then:

The SNR is inversely proportional to the total capacitance to the input. Thicker sensor gives more signal but also more noise

V s=Qs

Cd+ C i

SNR=Qs

V n(Cd+ C i)

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Pulse shaperPulse shaperElectronics

A pulse shaper has to broaden the signal around the peak in order to reduce input bandwidth an hence noise

rise time

time (ns)time (ns)

Not to much otherwise we cannot distinguish anymore close pulses (”pile-up”). As usual we have to find a compromise

sensor pulse shaper output

In this case the reshaping is done by using a RC and CR filters

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Analog / digital / binaryAnalog / digital / binaryElectronics

After amplification and shaping the signals must at some point be digitized to allow for DAQ and further processing by computers

1. Analog readout: analog buffering; digitization after transmission off detector

2. Digital readout with analog buffer

3. Digital readout with digital buffer

Binary: discriminator right after shapingBinary trackingDrift time measurement

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Analog to digital conversionAnalog to digital conversionElectronics

There is clearly a tendency to go digital as early as possibleThis is extensively done in consumer goods

The “cost” of the ADC determines which architecture is chosenStrongly depends on speed and resolution

Input frequencies must be limited to half the sampling frequencyOtherwise this will fold in as additional noise.

High resolution ADC also needs low jitter clock to maintain effective resolution

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Time measurementsTime measurementsElectronics

Time measurements are important in many HEP applications

Identification of bunch crossing (LHC: 25ns)Distinguishing among individual collisions (events) in continuous beam like experiments (or very short bunch interval like CLIC: ~250ps)Drift time

Position in drift tubes ( binary detectors with limited time resolution: ~1ns)Time projection chamber (both good time and amplitude)Time Of Flight (TOF) detectors (very high time resolution: 10-100ps)

Time walk: time dependency on amplitudeLow threshold (noise and pedestal limited)Constant fraction discrimination

Works quite well but needs good analog delays (cable delay) which is not easy to integrate on chip.

Amplitude compensation (done in DAQ CPU’s)Separate measurement of amplitude (expensive)Time measurements with two thresholds: 2 TDC channelsTime over threshold (TOT): 1 TDC channel measuring both leading edge and pulse width

Time Over Threshold (TOT) can even be used as a poor mans ADC

ATLAS Pixel

th1

th2

time

time

th

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A constant fraction discriminatorA constant fraction discriminatorElectronics

Constant fraction discriminator (CFD) is largely used to discriminate analog signals avoiding jitter due to different pulse amplitude

The input signal is is supplied to two circuits, a normal (threshold) discriminator and a constant fraction discriminator. An output pulse is produced from the logic AND of the normal discriminator and the CFD

The output timing of the normal discriminator shows time walk and it should only act as enable for the CDF-output

The input signal is split in two parts. One part is attenuated by a factor 5 and subtracted from the delayed input pulse. The amount of delay is selectable by cable

the resulting bipolar signal crosses the baseline at a fixed, but selectable, time with respect to the start of the pulse

The operation principle of the CFD part does not depend on the selected cable delay. However, the cable delay should be chosen such that the output of the CFD determines the timing of the logic AND

The moment at which the threshold discriminator fires depends on theamplitude of the pulse. If the cable delay of the cfd is too short, the cfd fires too early (tcfd). For small input pulses, the timing is determined by the threshold discriminator and not by the cfd part

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Time to digital conversionTime to digital conversionElectronics

Digital countersLarge dynamic range

Good and cheap time references available as crystal oscillators

Synchronous to system clock so good for time tagging

Limited resolution: ~1ns

The switch is closed between the two times, start and stop, and the capacitor is charged. The resulting voltage increase is proportional to the time interval star-stop

Charge integration (start-stop)Limited dynamic range

High resolution: ~1-100 ps

Sensitive analog circuit needing ADC for final conversion.

Sensitive to temperature, etc. So often needs in-system calibration

Can be combined with time counter for large dynamic range

ADC

start

stop

timestart stop

V

ΔV to ADC