Detector length (m)

• 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 Press R.S. Gilmore, Single particle detection and measurement, Taylor&Francis, 1992

G. F. Knoll, Radiation Detection and Measurement, John Wiley and Sons, New York W. R. Leo, Techniques for Nuclear and Particle Physics Experiments, Springer, 1994

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 measurement

2 2 4

2c p c

m

E = +

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

Example:

(p,E)

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 detector Hadronic 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, E_π= 2.005 GeV, E_K= 2.060 GeV

The lateral spread of the shower is mainly governed by the multiple scattering of the electrons (Moliere radius R_M ).

95 % of the shower is contained inside a cone of size 2R_M

Various complex processes involved:

hadronic and electromagnetic components

Hadronic shower

charged pions, protons, kaons ….

Breaking up of nuclei (binding energy),

neutrons, neutrinos, soft γ’s muons …. → invisible energy

neutral pions→ 2γ → electromagnetic cascade

( )

nπ⁰ ≈ E GeV −

Large energy fluctuations → limited energy resolution Hadronic showers are much longer and

broader than electromagnetic ones !

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

( )

j j 2

i i

2 E p c

c mass 1 invariant

M 









−

=

=

∑ ∑

No PID one K identified

two Ks identified

φ Κ

Κ

m_φ=1020 MeV/c²

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

Combinatorial background is often critical

PID mandatory

Branching ratios: B

→π

π

= 0.7×10

, →K

π

= 1.5×10

B

→K

K

= 1.5×10

, →K

π

= 0.7×10

PID PID

LHCb LHCb

Purity=13%

Purity=84%

Efficiency=79%

Bs → D

K

Major background: Bs → D

π (No CP violation)

PID PID

LHCb LHCb

70’s: Hydrogen bubble chamber

1978: BEBC

A Look at the Past

A Look at the Past

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

Basic Layout Basic 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 flavor particles)

• impact parameter resolution σ(rφ) ~ 50 µm for p_t ~ 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. calorimeter

TOF and TRD RICH

Measuring the Particle Velocity Measuring the Particle Velocity

( )

( )

2 2 2 2 2

1

2 2 1

2

2 2 1

2 2 2

1

2 2

2 2

2

2

) 0 (

m dm

factor) (Lorentz

;

p c m m

p c m

m p dp

p dp d

mc mc E

p

≅ −

∆









∗ +

= ∆

−

≅



 

 + 



 



= 



 





=

=

β β β

β

β β

β β γ β γ γβ

0,1 1 10 100 1000

Particle Identification Techniques

p (GeV/c)

π-K identification ranges

TR+dE/dx Cherenkov

dE/dx

TOF150 ps FWHM

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)

identified A

A B B

B

identified A

A B

total A identified

A A

A

N N

N N

efficiency

≠ − −

→

−

−

→

∑

=

=

=

=

/ ion

contaminat

,

ε ε

higher efficiency -> larger contamination

(example: ALICE-ITS simulation)

purity= 1-contamination

Momentum (GeV/c)

10^-1 1 10¹ 10² 10³

10^-2 10^-1

1 10

Detector length (m)

ToF (100ps@FWHM)

RICH

TR+dE/dx

dE/dx

3σ separation for π/K

Liquid-Solid

Aerogel G ases

Separation Power

AB B

A S

n S

σ σ

= −

= separationpower

N.B. in case of samples with different population:

at a given separation power, the resulting contamination of the largest populated sample of particles in the other

species will be larger by a factor equal to the ratio between the relative populations

Basic processes occurring when a charged particle traverses a medium

being surrounded by a cloud of virtual photons that interacts with atoms in the medium

• ionization and excitation of the atoms of the medium (secondarily produced

electrons could further ionize the medium)

• radiative phenomena

¾ Cherenkov radiation

¾ Transition radiation

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

basis of many of the techniques used in charged particle detectors

Energy Loss Mechanisms Energy Loss Mechanisms

Ionization trail: particle’s trajectory and velocity information δ-ray

Charged Particle-Matter Interactions Charged Particle-Matter Interactions

e^-

θ hk

h ,

photon virtual

ω

, m0

particle p ∫ ∫ω^∞

∞ σ

−

= v dEdp

dpE d dE

dx n dE

/

2

0

(photon)

time

space

A B

fast atom charged particle

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

excitation ionization by distant collision below excitation threshold

energy transfer/photon exchange

rel. probability

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

= hω k E

p h=

∫

−

= 0 dE

dE E d dx n

dE σ

density of atoms: n=ρN_A^/A

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

Valid only for particles with m>m_e

• 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)

¾ saturates at βγ_sat: 230 Ar 68.4 CH₄ 55.3 C₂H₆ 42.4 C₄H₁₀

5.6 Si

2 2 ln 2

2 3071 1

0

2 2

max 2 2 2 2

2



 



 − − −

⋅

−

= Z

C δ β

I E γ β c m β

t z A ρ . Z

dx

dE _e

Ionization Energy Loss Ionization Energy Loss

2

2 1

1 MeV g cm dx mip

dE ≈ ÷ ₋

Z=atomic number of the medium;

I~Z•12 eV=effective ionization potential;

E_max=max energy transfer (I ≤ dE ≤ E_max)

( )

2 ln

1 β γ

∝ β dx dE

βγ m p =

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)

(

)

n dE

B

B A

/

/ /

σ = σ −

(dE/dx)/(dE/dx) min

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

<dE/dx> is practically measured by evaluating ∆E in a short interval δx

this 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 loss A: average energy loss

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

Samples must not be too many:

for each total detector length L, there exists an optimal N

Rule of thumb: at least N=100 for a total track length 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 reduction of fluctuations -> (dE/dx)_A – (dE/dx)_B < Landau fluctuations

Usual method of measuring dE/dx is:

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.

(

)

A dx B dE

A N_{S D}

/

) ( / )

( ) /

;

.(

. σ

= −

( ^{dE / dx} ) ^{∝ n}

^{( t ⋅ p )}

σ

dE/dx & Separation Power

Time Projection Chamber → full 3-D track reconstruction

• x-y from wires and segmented cathode of MWPC

• z from drift time ->

• dE/dx

Gate open Gate closed

∆V_g = 150 V Drift over long distances → very good

gas quality required

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

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

TPC Tracker evolution

Tracker evolution

STREAMER CHAMBER

2000: STAR

• 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 Nomex honeycomb; 0.5% rad length

STAR TPC

Two-track separation 2.5 cm Momentum Resolution < 2%

Space point resolution ~ 500 µm Rapidity coverage –1.5 < η < 1.5

A Central Event

Typically 1000 to 2000 tracks per event into the TPC

STAR TPC

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

π

K p d

e µ

dE/dx(keV/cm)

0 8

4

Gas: P10 ( Ar-CH₄ 90%-10% ) @ 1 atm

Pb+Pb @ 158 GeV/nucleon

NA49 TPCs

Field Cage Inner Vessel

drift gas

90% Ne - 10%CO₂

gas volume 88 m³

Central membrane frame

ALICE TPC

6x10⁵ channels, corresponding to 6x10⁸ pixels in space

560 cm

Field Strips

TPC Assembly

N_ch(-0.5<η<0.5) = 8000^{slice: 2o in θ}

Projection of a slice (2^o in θ)

Nr. of Pixels:

570,132 pads x 500 time bins

Projection of the entire drift volume into the pad plane; dN_ch/dy = 8000

(~ 2 x 10⁴ charged particle tracks)

Nr. of hits = 19,431,047

Challenge: Track Density in Pb-Pb

TPC dE/dx performance TPC dE/dx performance

At dN/dy = 8000 At dN/dy = 4000 σ

= 10 % σ

= 7 %

σ

Drift Velocity Control:Pressure (mbar)

5.44 5.45

Drift velocity (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)

examples from STAR

6.9 3.0 2.8 4.6 6.4 7.5 6.6

Calc.^(%)

7.0

Ar/CO₂ /CH₄ =89/10/1

1 0.83 72

Drift ch.

MKII/SLC*

2.8

Ar/CH₄ /iC₄H₁₀ =88.2/9.8/2

4 1.0

159 Jet ch.

OPAL*

3.0

Ar/CH₄=80/ 20

8.5 0.4

183 TPC

PEP*

4.5

Ar/CH₄=90/ 10

1 0.4

338 TPC

ALEPH*

5.7

Ar/C₂H₆=50/50

1 1.4

51 Drift ch.

CLEO II

7.2

He/C₄H₁₀=80/20

1 1.4

40 Drift ch.

Babar

5.1

He/C₂H₆=50/50

1 1.5

52 Drift ch.

Belle

Meas.^(%) Gas

p ^(bar) t ^(cm)

n Type

• Higher pressure gives better resolution, however, the relativistic rise saturate at lower 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

(

)

σ dE/dx Detector Performance

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

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: basics Time of flight: basics

t v = particlespeed = L

( ) ( )

[ ]

( ) ( )

m t m m

c m m t L p m

m L p m

t L

c p c

m c

p c

pc m L pc

E pc E c t L

c L v

L v t

t t

c v v v m m

m ∆

=

−

=

∆

⇒

∆

=

−

=

∆

+

− +

 =



 



 −

=

∆



 



 −

 =



 



 −

=

−

=

∆

=

=

≅≅ =

1 2

1 2

2 / 2 1 2 4

2 1 2

/ 2 1 2 4

2 2 2 1

2

1 2

1 2 1

2

1 2

1 2

1 1

1 1

β β

β

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

2 1

2

2 −

=

= L

t p c

c m p

γβ

2 1

2

2 −

= L

t p c m

Combine TOF with momentum measurement

2 4

2

L dL t

dt p

dp m

dm 



 



 +

γ

 +



 



=  Mass resolution

TOF difference of two particles at a given momentum



 

 −

≈



 



 + − +

=



 



 −

=

∆ _{− 1}² ₂²

2 2

2 22 2

2 12 2

2 1

1 1 1 1 / 1 / 2

m p m

p Lc c m p

c c m

L c

t L

β β

Time of flight for relativistic particles Time of flight for relativistic particles

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

























 +

 −









 +

∆ =

=

2 2 2 2

1

1 p

c m p

c m c

L

N t ^{A B}

t t

AB

t σ σ

σ

In ALICE, the time resolution

of TOF is 100 ps

3σ separation equivalent to

300 ps difference

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

L=4 m

Momentum limit at 3 σ

Momentum limit at 3 σ

From Theory to Practice From Theory

to Practice TOF PID as envisaged in

ALICE for Pb-Pb

collisions

TOF: experimental issues TOF: experimental issues

‹ Start and stop counters fast detectors:

¾ plastic scintillators (well assessed technology)

¾ 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

production of scintillation light (luminescence)

Scintillation Counters Scintillation Counters

Dynodes Anode

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

Photocathode

photon

photoelectron

∼ 10⁶ secondary electrons

particle

scintillator light guide photon detector

Matches the scintillator shape to the PMT’s round face and transports

photons (total internal reflection & external reflector)

convert photons to electrons

thus providing an electrical signal

N_photon~ 2 •10⁴/cm

1 photon/100 eV

fish-tail

QE = N_p.e./N_photons dynode gain = 3-20

10 dynodes with gain=4 M = 4¹⁰ ≈ 10⁶

cm / MeV 2

~ dx / dE

Scintillators Scintillators 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 ns

Density (g/cm³) 1.03

4.5 representative

scintillators

• Excitation: A₀->A₁ (∆E=E_A1-E_A0=absorption spectrum)

• Vibrational energy transfer to other molecules nearby: A₁->B₁

• Scintillation: B₁->B₀ (∆E=E_B1-E_B0=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 Æ scintillator is transparent to the light it produces

Organic Scintillators

Organic Scintillators

¾ Number of photo-electrons

25 . 0

> ~

< QE

¾ 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

( )

e N

N _pe = _ph ⋅

∫

N_ph∼10⁴/cm

L=scintillator length, l_ph(λ)=photon attenuation length

D=photon collection efficiency (including geometry factors)

Expected timing resolution for long counters

From W. B. Atwood (SLAC) 1980:

pe

t N

cm cm L

ps ( )

) (

87

~ ⋅ ^−1/2 ⋅

σ

143 90

250 R6680

BC408 4 x 6 x 255

Belle

420 180

180 XP2020

SCSN38 2 x 3 x 300

R. Stroynowski

240 210

300 R1828

BC412 4.2 x 13 x 400

TOPAZ

125 110

270 XP2020

BC408 5 x 10 x 280

E. Nappi

53 50

200 R1828

SCSN23 4 x 3.5 x 100

T. Sugitate

110 140

180 R1332

SCSN38 3 x 20 x 150

T. Tanimori

60 120

200 XP2020

NE114 3x 15 x 100

G.D.Agostini

σ_t(exp) σ_t(meas)

λ_att (cm) PMT

Scintillator Counter size (cm)

(T x W x L) Exp. application

Overall Time Resolution

EXAMPLES:Scintillator based TOF EXAMPLES:Scintillator based TOF

grid:

Small, but thick scintillators 8 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

PID with TOF only

Combined PID:

TOF + dE/dx (TPC)

T rel.= T / T π

NA49:TOF + dE/dx

NA49:TOF + dE/dx

Central Arm Detectors

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

≅1500 dy

dN_ch

segments

≅1000

~ 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 TOF PHENIX TOF

TOF

start timing

Prism light guide to reduce dead space PMT

Scintillator slat

PHENIX Preliminary

K⁺ p

K^- π⁺

π^-

p e⁺

e^-

K⁺ p 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 PERFORMANCE

PHENIX TOF PERFORMANCE

1949: J. Keuffel (Caltech) planar spark counters 1970: Y. Pestov (Novosibirsk):

1^st 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&D

test beam: σ_t ≈ 40 ps ! pressure vessel

TOF with fast gaseous detectors TOF with fast gaseous detectors

Gas Amplification in Parallel Plate Chambers Gas Amplification in Parallel Plate Chambers cathode

anode

Uniform and high electric field

Electron avalanche according to Townsend: N = N_o e^αx

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

Volts !

E

5 mV/div

20 mV/div

20 mV/div

50 mV/div

E

From Avalanche to Spark From 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 regime

FAST (signal formation

driven by UV light rather by slower electrons) but high dead time

τ

<< τ

= ρε ~ 10 ms

Recovery time longÆ electrodes behave as insulators while electrons reach the anode Æ the electrical field is quenched locally (a small region of almost 0.1 cm² will appear “dead” for

~

C_i

R C_i

R

C_i

R C_i

R

C_i

R C_i

R

C_i

R C_i

R

C_i

R C_i

R

HV

Resistive Plate Chambers Resistive Plate Chambers Pestov idea:

use as anodic electrode a high resistivity glass !!

Concept extended to RPCs with both electrodes with high resistivity

particle

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

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 kV

Flow 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

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

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 3000

time 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 of 33 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 14

Applied HV (kV)

60 80 100 120 140

Efficiency [%]

Resolution [ps]

SPRING 1999

Efficiency [%]

Counts / 50 ps Resolution [ps]

MRPC PERFORMANCE

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

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 m Divided into 18 sectors