Basic definitions and introductory remarks • Ionization ...iaselli/Fisica dei rivelatori/Identificazione_1-Nappi.pdfR. Fernow, Introduction to Experimental Particle Physics, Cambridge

Dottorato in Fisica Maggio 2005“Fisica dei rivelatori” Identificazione delle particelle

E. Nappi

• Basic definitions and introductory remarks

• Ionization energy loss

• Time of Flight

• Cherenkov radiation

• Transition radiation

Advised textbooks:R. Fernow, Introduction to Experimental Particle Physics, Cambridge University PressR.S. Gilmore, Single particle detection and measurement, Taylor&Francis, 1992G. F. Knoll, Radiation Detection and Measurement, John Wiley and Sons, New York W. R. Leo, Techniques for Nuclear and Particle Physics Experiments, Springer, 1994


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Complete event analysis (based on the reconstruction of conservation laws): 4-momenta of secondary particles

Deflection in a magnetic field

(+ sign of particle’s charge)

Calorimetry(destructive measurement, effective for neutral particles only)

PID measurement2

242 cpcmE +=

“m” uniquely identifies the internal quantum numbers of the particle

Example:

(p,E)


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Very useful for neutral particles and leptons because of their peculiar interactions with media: electron quickly produces an em shower, µ travels through the entire detectorHadronic showers from π, K, p all look alike and calorimeter energy resolution is not enough to allow measuring mass from m2=E2-p2

example: p=2 GeV/c, Eπ= 2.005 GeV, EK= 2.060 GeV


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The lateral spread of the shower is mainly governed by the multiple scattering of the electrons (Moliereradius RM ).95 % of the shower is contained inside a cone of size 2RM

Various complex processes involved:hadronic and electromagnetic

components

Hadronic shower

charged pions, protons, kaons ….Breaking up of nuclei (binding energy), neutrons, neutrinos, soft γ’smuons …. → invisible energy

neutral pions → 2γ →electromagnetic cascade( ) 6.4)(lnn 0 −≈ GeVEπ

Large energy fluctuations → limited energy resolution

Hadronic showers are much longer and broader than electromagnetic ones !


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Identification method: calculate the invariant mass with all possible daughter candidates

( )2

jj

2

i i2 cpEc1massinvariant M

−== ∑∑

No PID one K identified

two Ks identified φ Κ+Κ−

mφ=1020 MeV/c2

Decay vertex may be reconstructed if it is far from interaction point and daughters are charged

Combinatorial background is

often criticalPID mandatory


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Branching ratios: Bd→π+π− = 0.7×10−5, →K± πm = 1.5×10−5

Bs→K+K− = 1.5×10−5, →K± πm = 0.7×10−5

PID PID

LHCbLHCb

Purity=13%Purity=84%Efficiency=79%


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Bs → Ds KMajor background: Bs → Ds π (No CP violation)

PID PID

LHCbLHCb


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70’s: Hydrogen bubblechamber

1978: BEBC

A Look at the Past A Look at the Past


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A “Modern” Approach to PID

A “Modern” Approach to PID

ALICE at LHC

Silicon trackers +TPC (PID with energy loss)Ring Imaging Cherenkov detector

TOFTRD


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Basic LayoutBasic Layout

magnetic field|p|,charge

Layers of silicon detectors with excellent position (0(10 µm)) and double track (0(100 µm)) resolution near the primary collision region •detection of secondary vertices (short-lived strange and heavy flavorparticles)• impact parameter resolution σ(rφ) ~ 50 µm for pt ~ 1 GeV/c• primary vertex resolution: ~ 10 µm• momentum resolution improvement• PID with energy loss

TPC, away from the interaction region, at more moderate particle densities • tracking (δp/p at the level of 1% for low momenta)• PID with energy loss

e.m. calorimeterTOF and TRD

RICH


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Measuring the Particle VelocityMeasuring the Particle Velocity

( )( )

2

222

21

221

22122

221

222

2

2

2)(

0

mdm

factor) (Lorentz ;

pcmm

cpmm

pdp

pdpd

mcEmcp

−≅

∆

∗+∆

=−

≅

+

=

==

ββ

βββββ

ββγ

γγβ


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0,1 1 10 100 1000

Particle Identification Techniquesp (GeV/c)

π-K identification rangesTR+dE/dx

Cherenkov

dE/dx

TOF150 psFWHM

electron identification

The applicable methods depend strongly on the particle momentum (velocity) domain of interest

PID techniques are based on the detection of particles via their interaction with matter: ionization and excitation (Cherenkov light & Transition Radiation)


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identifiedAABB

BidentifiedAAB

totalAidentifiedAAA

NN

NNefficiency

−≠

−→

−−→

∑==

==

/ioncontaminat,

ε

ε

higher efficiency -> larger contamination

(example: ALICE-ITS simulation)

purity= 1-contamination


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Momentum (GeV/c)10-1 1 101 102 103

10-2

10-1

1

10

Det

ecto

r le

ngth

(m)

ToF (100ps@FWHM)RICH

TR+dE/dx

dE/dx

3σ separation for π/K

Liquid-SolidAero

gel

Gases

Separation Power

AB

BA SSnσσ

−== power separation

N.B. in case of samples with different population:at a given separation power, the resulting contaminationof the largest populated sample of particles in the other species will be larger by a factor equal to the ratio between the relative populations


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Basic processes occurring when a charged particle traverses a mediumbeing surrounded by a cloud of virtual photonsthat interacts with atoms in the medium

• ionization and excitation of the atoms of the medium (secondarily produced electrons could further ionize the medium)

• radiative phenomenaCherenkov radiationTransition radiation

Overall effect: the particle loses energyDetection of the energy lost is the physical

basis of many of the techniques used in charged particle detectors

Energy Loss MechanismsEnergy Loss Mechanisms

Ionization trail: particle’s trajectory and velocity information

δ-ray


E. NappiCharged Particle-Matter InteractionsCharged Particle-Matter Interactions

e-

θ

khh , photonvirtual

ω

0, mpparticle ∫∫∞

ω

∞ σ−=

v dEdpddpEdEn

dxdE

/

2

0

(photon)

time

space

A Batom

fastchargedparticle

Modern approach (unitary description in terms of matter properties): Allison and Cobb (1980)• Charged particle moves in a dielectric medium through which virtual photons propagate • The particle loses energy by doing work against the field created by the medium polarization

atom

ionization by close collisions δ-electrons

exci

tatio

n

ioni

zatio

n by

di

stan

t col

lisio

n

belo

w e

xcita

tion

thre

shol

d

energy transfer/photon exchange

rel.

prob

abili

ty Schematically !(after Gilmore)

photons are virtual, their energy and momentum are independent E≠pc• the integral must be performed over both energy and momentum separately• virtual photon behaviour approximated with a combination of cross sections for the interactions of real photons allowing to perform the momentum integration for virtual photons

ω= hEkp h=

∫∞

−=0

dEdEdEn

dxdE σ

density of atoms: n=ρNA/A


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Classical approach : Bethe-Bloch equation modified to include the Fermi effectAverage specific energy loss:

Valid only for particles with m>me

• dE/dx does not depend on m but on the charge z• Non relativistic region: dE/dx ∝1/β2

(more precisely as β-5/3)• Minimum: at βγ = 3÷4 (Minimum Ionizing Particle)

• At high βγ: dE/dx ∝lnγ2 (relativistic rise)• Density effect: δ(βγ)

(medium polarization reduces long range effects)saturates at βγsat: 230 Ar

68.4 CH455.3 C2H642.4 C4H105.6 Si

22

2ln2130710

2

2max

222

2

2

−−−⋅−=

ZCδβ

IEγβcm

βztρ

AZ.

dxdE e

Ionization Energy LossIonization Energy Loss

2121 cm MeV gmipdxdE −÷≈

Z=atomic number of the medium;I~Z•12 eV=effective ionization potential;Emax=max energy transfer (I ≤ dE ≤ Emax)


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( )222 ln1 γβ

β∝

dxdE

βγmp =m from simultaneous measurement of p and dE/dx

Fermi plateau is a few percents above the minimum in solid and liquid media, 50-70% in high Z noble gases at STP -> PID in the relativistic rise region only possible in gases!π/K separation (2σ) requires a dE/dx resolution of few percents

Particle ID using the specific energy loss dE/dx

Average energy loss in 80/20 Ar/CH4 (NTP)(J.N. Marx, Physics today, Oct.78)

( )dxdEdxdEdxdEn

B

BA

///

σσ−

=

(dE/

dx)/

(dE/

dx) m

in

:”cross-over” regions (as wide as + 100 MeV/c)ambiguites-> complementary PID mandatory


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<dE/dx> is practically measured by evaluating ∆E in a short interval δxthis is not necessarily the average energy lost in the given slice of material-> the distribution shows large fluctuations and Landau tail

Fluctuations in the energy loss dE/dx

Most interactions involve little energy exchange -> the total energy loss from these interactions is a Gaussian (central limit theorem). Few interactions involve large energy exchange-> Landau tail

Because of the high energy tail, increasing the thickness of the detector or choosing high Z material does not improve σ(dE/dx). Indeed the relative width of Gaussian peak reduces but probability of high energy interaction rises -> tail gets bigger

(B. Adeva et al., NIM A 290 (1990) 115)

1 wire 4 wires

L: most likely energy lossA: average energy loss


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(M. Aderholz, NIM A 118 (1974), 419)

Samples must not be too many:for each total detector length L, there existsan optimal N

Rule of thumb: at least N=100 for a total tracklength of 3-5 m/atm

• Choose material with high specific ionization• Divide detector length L in N gaps of thickness T.• Sample dE/dx N times• Calculate truncated mean, i.e. ignore samples with (e.g. 40%) highest values• Also pressure increase can improve resolution. Drawback: reduced relativistic

rise due to density effect !

Improve dE/dx resolution and fight Landau tails

Thick absorber: large chance of high energy δ ray production cancels the reductionof fluctuations -> (dE/dx)A – (dE/dx)B < Landau fluctuationsUsual method of measuring dE/dx is:


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Particle Separation

– dE/dx resolution (A.H. Walenta et al. Nucl. Instr. and Meth. 161 (1979) 45)

n: number of sampling layers,t: thickness of the sampling layer (cm)p: pressure of the gas (atm)

Remarks:• σ does not follow the n-0.5 dependence owing to the Landau

fluctuations;• if the total lever arm (nt) is fixed, it is better to increase n;

so long as the number of produced ion-pairs is enough in each layer.

( )dxdEBdxdEAdxdEBAN DS /

)(/)(/);(.. σ−

=

( ) 36.032.047.043.0 )(/ −÷−−÷− ⋅∝ ptndxdEσ

dE/dx & Separation Power


E. Nappi

Time Projection Chamber → full 3-D track reconstruction• x-y from wires and segmented cathode of MWPC• z from drift time -> precise knowledge of vD (LASER calibration + p,T corrections)

• dE/dx

Gate open Gate closed∆Vg = 150 V

Drift over long distances → very good gas quality required

Space charge problem from positive ions, drifting back to medial membrane → gating

TPC: basic principle


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80’s: 6.4 TeV Sulphur - Gold event (NA35)

TPC

Tracker evolutionTracker evolution

STREAMER CHAMBER

2000: STAR


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• Gas: P10 ( Ar-CH4 90%-10% ) @ 1 atm, 50,000 Liters

• Voltage : - 31 kV at the central membrane 148 V/cm over 210 cm drift path

420 cm

Self supporting Inner Field Cage:Al on Kapton using Nomexhoneycomb; 0.5% rad length

STAR TPC


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Two-track separation 2.5 cmMomentum Resolution < 2%

Space point resolution ~ 500 µmRapidity coverage –1.5 < η < 1.5

A Central Event Typically 1000 to 2000 tracks

per event into the TPC

STAR TPC


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Anti - 3He

dE/dx PID range:

~ 0.7 GeV/c for K/π~ 1.0 GeV/c for K/p

PID via dE/dx with the STAR TPC

12

πK

p d

eµdE

/dx

(keV

/cm

)

0

8

4

Gas: P10 ( Ar-CH4 90%-10% ) @ 1 atm


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Pb+Pb @ 158 GeV/nucleon

NA49 TPCs


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Field Cage Inner Vessel

drift gas90% Ne - 10%CO2

gas volume88 m3

Central membrane frame

ALICE TPC

6x105 channels, corresponding

to 6x108 pixels in space

560 cm


E. Nappi

Field Strips


E. Nappi

TPC Assembly


E. Nappi


E. NappiNch(-0.5<η<0.5) = 8000slice: 2o in θ

Projection of a slice (2o in θ)

Nr. of Pixels:570,132 pads x 500 time bins

Projection of the entire drift volume into the pad plane; dNch/dy = 8000(~ 2 x 104 charged particle tracks)

Nr. of hits = 19,431,047

Challenge: Track Density in Pb-Pb


E. Nappi

TPC dE/dx performanceTPC dE/dx performanceAt dN/dy = 8000 At dN/dy = 4000

σ dE/dx = 10 % σ dE/dx = 7 %

σ dE/dx =10%


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Drift Velocity Control:Pressure (mbar)

5.44

5.45

Drif

t vel

ocity

(cm

/µs)

1010 1020

• Lasers for coarse value• Fine adjustment from tracking

TPC: experimental issues

• mechanical tolerances (gain and electrical field)• stability of high voltage power (gain)• space charge effects (track distortion)• gating efficiency (background)• temperature, pressure (drift velocity)

examplesfrom STAR


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6.9

3.0

2.8

4.6

6.4

7.5

6.6Calc.(%)

7.0Ar/CO2 /CH4 =89/10/110.8372Drift ch.MKII/SLC*

2.8Ar/CH4 /iC4H10 =88.2/9.8/2 41.0159Jet ch.OPAL*

3.0Ar/CH4=80/ 208.50.4183TPCPEP*

4.5Ar/CH4=90/ 1010.4338TPCALEPH*

5.7Ar/C2H6=50/5011.451Drift ch.CLEO II

7.2He/C4H10=80/2011.440Drift ch.Babar

5.1He/C2H6=50/5011.552Drift ch.BelleMeas.(%)Gasp (bar)t (cm)nType

• Higher pressure gives better resolution, however, the relativistic rise saturate atlower βγ. 4 – 5 bar seems to be the optimal pressure• Higher content of hydro-carbons gives better resolution (Belle and CLEO II).

Landau distribution (FWMH); 60 % for noble gas, 45% for CH4,33% for C3H6

( ) 32.043.0 )(41.0/ −− ⋅= ptndxdE calcσdE/dx Detector Performance

* Data from M. Hauschild (NIM A 379(1996) 436)


E. Nappi

L=particle’s path between

two counters

t=time to traverse L

For two particles:

For known momentum p:

In the non-relativistic limit (β~0.1):

Time of flight: basicsTime of flight: basics

tLv == speed particle

( ) ( )[ ]

( ) ( )mmtmm

cmLtm

pLmm

pLt

cpcmcpcmpcL

pcE

pcE

cLt

cL

vvLttt

cvvvmmm

∆=−=∆⇒∆=−=∆

+−+=

−=∆

−=

−=−=∆

==≅=≅

1212

2/122421

2/1224222

12

121212

1212

1111

β

ββ

β

Consequently, for a time resolution of ∆t=200 ps and a flight path L=1 m, it is possible to discriminate between low-energy particles to better than 1% level of accuracy

12

22

−==Ltcp

cpm

γβ


E. Nappi

12

22−=

Ltcpm

Combine TOF with momentum measurement2

42

LdL

tdt

pdp

mdm

+γ+

=Mass resolution

TOF difference of two particles at a given momentum

−≈

+−+=

−=∆ −

22

212

2222

2221

2121

2/1/111 mm

pLcpcmpcm

cL

cLt

ββ

Time of flight for relativistic particlesTime of flight for relativistic particles

For momenta above some GeV/c the resolution in mass discrimination is almost lost

+−

+=

∆=

222211

pcm

pcm

cLtN BA

ttAB

t σσσ


E. Nappi

In ALICE, the time resolution

of TOF is 100 ps

3σ separation equivalent to

300 psdifference

π/K up to 2.2 GeV/c K/π up to 3.7 GeV/c

L=4 m

Momentum limit at 3 σMomentum limit at 3 σ


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From Theory to Practice

From Theory to Practice

TOF PID asenvisaged in

ALICEfor Pb-Pb collisions


E. Nappi

TOF: experimental issuesTOF: experimental issues

Start and stop countersfast detectors:

plastic scintillators (well assessed technology)gaseous detectors (old technology, new advances)

Specific signal processing (timing+charge measurements)pulse height analysis->digital conversion to stop a fast digital clock

discriminators (specifically designed for slewing correction)TDCs

Calibrations – corrections for cable lengths, counters delay time….Continuous stability monitoring

start counter stop counter

particle


E. Nappi

production of scintillation light (luminescence)

Scintillation CountersScintillation Counters

Dynodes Anode

e l e c t r i c a lp u l s e

Photocathode

photon

photoelectron

∼ 106 secondary electrons

particle

scintillator light guide photon detectorMatches the scintillatorshape to the PMT’s round face and transports photons (total internal reflection & external reflector)

convert photons to electronsthus providing an electrical signal

Nphoton~ 2 •104/cm

1 photon/100 eV

fish-tail

QE = Np.e./Nphotons

dynode gain = 3-2010 dynodes with gain=4

M = 410 ≈ 106

cm/MeV2~dx/dE


E. Nappi

ScintillatorsScintillators

Two types:

Inorganic crystals (high density and Z materials: NaI, CsI,…)good light yield, too slow for TOF application (OK for e.m. calorimeter)

Organic scintillators (low Z material: polystyrene doped with fluorescent molecules to shift light from UV to visible & monocrystals: naphtalene, anthracene, p-terphenyl….)Excitation at molecular level

The light yield is lower than for inorganic scintillators because of recombination and quenching effects of the excited molecules. Fast, suited for TOF application

photons / MeV Decay time

CsI(Tl) 50000 800 ns

Pilot U 11000 1.4 nsDensity (g/cm3)

1.03

4.5

representativescintillators


E. Nappi

• Excitation: A0->A1 (∆E=EA1-EA0=absorption spectrum)• Vibrational energy transfer to other molecules nearby: A1->B1• Scintillation: B1->B0 (∆E=EB1-EB0=emission spectrum)• Decay from the vibrationally excited ground state to energetic minima: B0->A0

Because of the energy lost by vibrational quanta:emission and absorption spectra are shifted in wavelength

scintillator is transparent to the light it produces

Organic ScintillatorsOrganic Scintillators


E. Nappi

Number of photo-electrons

25.0~ >< QE

Photon detector transit time spread limits the TOF performance:

• Line-focus type PMT : 250 ps (Philips XP2020)

• Fine-mesh type PMT : 150 ps ( Hamamatsu R2490-05)

• Micro-channel Plate : 55 ps (Hamamatsu R2809U)

Design Issues

( ) λλλλ dQEDeNN Lphpe ⋅⋅⋅⋅= ∫ − )()(phl

Nph∼104/cmL=scintillator length, lph(λ)=photon attenuation lengthD=photon collection efficiency (including geometry factors)


E. Nappi

Expected timing resolution for long counters

From W. B. Atwood (SLAC) 1980:

pet N

cmLcmps )()(87~ 2/1 ⋅⋅ −σ

14390250R6680BC4084 x 6 x 255Belle420180180XP2020SCSN382 x 3 x 300R. Stroynowski240210300R1828BC4124.2 x 13 x 400TOPAZ125110270XP2020BC4085 x 10 x 280E. Nappi

5350200R1828SCSN234 x 3.5 x 100T. Sugitate110140180R1332SCSN383 x 20 x 150T. Tanimori

60120200XP2020NE1143x 15 x 100G.D.Agostini

σt(exp)σt(meas)λatt (cm)PMTScintillatorCounter size (cm)

(T x W x L)Exp. application

Overall Time Resolution


E. Nappi

EXAMPLES:Scintillator based TOF EXAMPLES:Scintillator based TOF

grid:

Small, but thick scintillators8 x 3.3 x 2.3 cm

long scintillators (48 and 130 cm), read out on both sides

From γ conversion in scintillators

Flight path=15 m


E. Nappi

PID with TOF only

Combined PID: TOF + dE/dx (TPC)

T rel.

= T

/ Tπ

NA49:TOF + dE/dxNA49:TOF + dE/dx


E. Nappi

Central Arm Detectors

Finely segmented high resolution TOF at mid-rapidtyKeep the occupancy level < 10 %

1500≅dy

dNch segments1000≅

~ 100 cm2/segment

∆φ = 45 deg. , ∆η = 0.7

•Scintillator: Bicron BC404• decay constant : 1.8 ns• attenuation length : 160cm

•PMT : Hamamatsu R3478S• Rise time : 1.3 ns• Transit time : 14 + - 0.36 ns

• Consists of 960 plastic scintillators• Flight path= 5 m• PMT readout at both ends of scint. (1920 ch.)

385cm

200cm

200cm

PHENIX TOFPHENIX TOF

TOF

start timing


E. Nappi

Prism light guideto reduce dead space

PMT

Scintillator slat

PHENIX Preliminary

pK+

K-

π+

π-

p

e+

e-

pK+

π+(a.u

.)

PID cut

PHENIX Preliminary

m2 [GeV/c2]

w/o PID cut

TOF intrinsic timing resolution ~120 ps has been achieved without slewing correction

PHENIX TOF PERFORMANCEPHENIX TOF PERFORMANCE


E. Nappi

1949: J. Keuffel (Caltech) planar spark counters1970: Y. Pestov (Novosibirsk): 1st example of resistive plate chamber: glass electrode (Pestov glass)+ metal electrode

Excellent time resolution ~ 50 ps or better!

Many drawbacks:• long tail of late events• mechanical constraints (high pressure)• non-commercial glass• nasty gas composition (contains butadiene)

ALICE R&Dtest beam: σt ≈ 40 ps !

pressure vessel

TOF with fast gaseous detectorsTOF with fast gaseous detectors


E. Nappi

Gas Amplification in Parallel Plate ChambersGas Amplification in Parallel Plate Chambers

cathode

anode

Uniform and high electric fieldElectron avalanche according to Townsend: N = No eαx

If set minimum gas gain at 106 (10 fC signal) and maximum gain as 108 (streamers/sparks produced above this limit), then sensitive region first 25% of gap

Only avalanches initiated close to anode produce detectable signal on pickup electrodes

A parallel plate chamber cannot perform as a fast gas detector:

• time jitter ≈ time to cross gap ≈ gap size/drift velocity• electron drift velocity ∼cm/µs -> few µm gap• low detection efficiency (for 1 e-ion pair about 30 eV is needed) !!


E. Nappi

E

Volts !

E

5 mV/div

20 mV/div

20 mV/div

50 mV/div

E

From Avalanche to SparkFrom Avalanche to Spark

As soon as the number of electrons in the avalanche reaches ~10 8(Raether’s criterium):the space charge becomes so relevant to balance the external field, the subsequent recombination of electrons and ions generate UV photons that initiate other avalanches (streamer) up to the spark regimeFAST (signal formation driven by UV light rather by slower electrons) but high dead time


E. Nappi

τdischarge << τrecovery= ρε ~ 10 ms

Recovery time long electrodes behave as insulators while electrons reach the anode the electrical field is quenched locally (a small region of almost 0.1 cm2

will appear “dead” for ~ 10 ms)

Ci

R

Ci

R

Ci

R

Ci

R

Ci

R

Ci

R

Ci

R

Ci

R

Ci

R

Ci

R

HV

Resistive Plate ChambersResistive Plate Chambers

Pestov idea:use as anodic electrodea high resistivity glass !!Concept extended to RPCswith both electrodes withhigh resistivity

particle


E. Nappi

Requirements:(a) Small gaps to achieve a high time

resolution(b) Very high gas gain (immediate

production of signal)(c) Possibility to stop growth of avalanches

(otherwise streamers/sparks will occur)

C. Williams – INFN Bologna (1999):add boundaries that stop avalanche development. These boundaries must be invisible to the fast induced signal -external pickup electrodes sensitive to any of the avalanches

Designing a Fast Gaseous Detector Designing a Fast Gaseous Detector


E. Nappi

MULTIGAP RESISTIVE PLATE CHAMBER

Stack of equally-spaced resistive plates with voltage applied to external surfaces

Pickup electrodes on external surfaces (resistive plates transparent to fast signal)

Anode 0 V

(-2 kV)

(-4 kV)

(-6 kV)

(-8 kV)

Cathode -10 kVFlow of electrons and negative ions

Flow of positive ions

Internal plates electrically floating!

In this example: 2 kV across each gap (same E field in each gap) since the gaps are the same size - on average - each plate has same flow of positive ions and electrons (from opposite sides of plate) - thus zero net charge into plate. STABLE STATE


E. Nappi

Anode 0 V

(-2 kV)

(-4 kV)

(-6 kV)

(-8 kV)

Cathode -10 kV

-6.5 kV Low E field - low gain

High E field - high gain

Decreased flow of electrons and increased flow of positive ions - net flow of positive charge. This will move the voltage on this plate more positive than -6.5 kV (i.e. towards 6 kV)

Internal plates take correct voltage - initially due to electrostatics but kept at correct voltage by flow of electrons and positive ions - feedback principle that dictates equal gain in all gas gaps

MGRPC: OPERATIONAL STABILITY


E. Nappi

Schott A2(0.5 mm thick)

Schott 8540(2 mm thick)

Anode electrode 3 x 3 cm2

Cathode electrode 3 x 3 cm2

Schott A14(0.5 mm thick)

5 cm

Single cell Multigap RPC

0 1000 2000 3000time difference between start counter and MRPC [ps]

1000

100

10

1

Gaussian fit σ = 77 ps

Tail of late signals 29 events / 17893 events

= 0.16 %

-1000-2000

12 kV

Subtract jitter of start counters of33 ps give time resolution of 70 ps

5 gas gaps of 220 micron

60.0

65.0

70.0

75.0

80.0

85.0

90.0

95.0

100.0

8 9 10 11 12 13 14Applied HV (kV)

60

80

100

120

140

Efficiency [%]Resolution [ps]

SPRING 1999

Effic

iency

[%]

Coun

ts / 5

0 ps

Reso

lution

[ps]

MRPC PERFORMANCE


E. Nappi

The red hits/track corresponds to a single particle(π in this case)

Hits in inner tracker

TPC hits

Hits in TOF array

TOF with very high granularity needed!

ALICE TOF


E. Nappi

Along the beam direction

each sector divided into 5 modules

i.e 5 x 18 = 90 modules in total

1674 MRPC strips in total

160 m2 and 160,000 channels

ALICE TOF GEOMETRY

A standard TOF system built of fast scintillators + photomultipliers would cost >100 MCHF

TOF ARRAY arranged as a barrel

with radius of 3.7 mDivided into 18 sectors


E. Nappi

130 mmactive area 70 mm

M5 nylon screw to hold fishing-line spacer

honeycomb panel (10 mm thick)

external glass plates 0.55 mm thick

internal glass plates (0.4 mm thick)

connection to bring cathode signal to central read-out PCB

Honeycomb panel (10 mm thick)

PCB with cathode pickup pads

5 gas gaps of 250 micron

PCB with anode pickup pads

Cross section of double-stack MRPC (5x250 µm gaps per stack)made of resistive plates ‘off-the-shelf’ soda lima glass

Silicon sealing compound

PCB with cathode pickup pads

Flat cable connectorDifferential signal sent

from strip to interface card

Mylar film (250 micron thick)

120 cm

Standard unit detector

there will be ~ 1600 strips


E. Nappi


E. Nappi


E. Nappi


E. Nappi


E. Nappi

404550556065707580

5.0 5.5 6.0 6.5

020406080

100

5.0 5.5 6.0 6.5

Efficiency [%]

Applied voltage [kV]Resolution [ps]

strip 12strip 10

Applied voltage [kV]

strip 12strip 10

Typical time spectrum

Typical performance

1000

800

600

400

200

01500 25002000 3000 35001000

STRIP 10 H.V. +- 6 kV

Time with respect to timing scintillators [ps]

0 500 1000-500-1000

1200

1000

800

600

400

200

0

STRIP 10 H.V. +- 6 kV

Time with respect to timing scintillators [ps]

Entri

es/5

0 ps

Entri

es/5

0 ps

Uncorrected time spectrum

time spectrum after correction for slewing

σ = 66 ps minus 30 ps jitterof timing scintillator = 59 ps

σ = 53 ps minus 30 ps jitterof timing scintillator = 44 ps

10 gaps of 220 micron


E. Nappi

Strip 10 effective voltage 11.4 kV


Strip 10 applied voltage 11.4 kV






Equivalent flux of through-going charged particles [Hz/cm2]

Efficiency [%] Time resolution [ps]

50556065707580859095

100

0 200 400 600 800 1000 1200 1400 16000

102030405060708090

0 200 400 600 800 1000 1200 1400 1600

GAMMA IRRADIATION FACILITY AT CERN

Investigated performance with muon beam while test device irradiated with high flux of 662 keV gammas

TOF: rate capability


E. Nappi

Particle Identification Ranges

Efficiency and contamination at high density (dN/dy = 8000)

B = 0.2 T

B = 0.4 T

Documents

Basic definitions and introductory remarks • Ionization ...iaselli/Fisica dei rivelatori/Identificazione_1-Nappi.pdfR. Fernow, Introduction to Experimental Particle Physics, Cambridge