Detectors for particles and radiation Advanced course for Master students Spring semester 2010 S7139 5 ECTS points Tuesday 10:15 to 12:00 - Lectures Tuesday

Detectors for particles and radiationAdvanced course for Master students

Spring semester 2010 S7139 5 ECTS points

Tuesday 10:15 to 12:00 - Lectures

Tuesday 16:15 to 17:00 - Exercises

Detectors for particles and radiation

February 23 Kreslo, Gornea Introduction, History of instrumentation

March 2 Kreslo, Gornea Particle-matter electromagnetic interactions

March 9 Kreslo, Gornea Gas detectors : counters

March 16 Kreslo, Gornea Gas detectors : tracking

March 23 Kreslo, Gornea Scintillating detectors :counters

March 30 Kreslo, Gornea Scintillating detectors : tracking

April 6 Bay, Gornea Nuclear emulsions

April 13 Kreslo, Gornea Semiconductor detectors : counters

April 20 Kreslo, Gornea Semiconductor detectors : tracking

April 27 Kreslo, Gornea Semiconductor detectors : tracking

April 27 Kreslo, Gornea Cryogenic liquids : tracking

May 4 Kreslo, Gornea Calorimetry

May 11 Kreslo, Gornea Calorimetry

May 18 Kreslo, Gornea Particle Identification

May 25 Kreslo, Gornea Momentum measurements

June 1 Kreslo, Gornea Discussion + Lab demonstration

Image and Logic traditions

History of ‘Particle Detection’

Image Tradition: Cloud ChamberEmulsionBubble Chamber

Logic Tradition: Scintillating CounterGeiger CounterTip CounterSpark Counter

Electronics Image: Spark Chambers Wire Chambers Scintillating trackers Silicon Detectors

Introduction, history of instrumentation

1906: Geiger Counter, H. Geiger, E. Rutherford1910: Cloud Chamber, C.T.R. Wilson1912: Tip Counter, H. Geiger1928: Geiger-Müller Counter, W. Müller1929: Coincidence Method, W. Bothe1930: Emulsion, M. Blau1940-1950: Scintillator, Photomultiplier1952: Bubble Chamber, D. Glaser1962: Spark Chamber1968: Multi Wire Proportional Chamber, G. Charpak1979: Time Projection Chamber, D. Nygren1984: Silicon Drift Detectors, E. Gatti & P. Rehak1997: Gas Electron Multiplier, F. Sauli

Etc. etc. etc.

History of instrumentation: Logic detectors (counters)


E. Rutherford H. Geiger1909

The Geiger counter, later further developed and then calledGeiger-Müller counter

First electrical signal from a particle

pulse


First use of Coincidence


Electronic coincidence circuit




Ionization of Gases

Primary ionization Total ionization

Fast charged particles ionize atoms of gas.Often resulting primary electron will have enough kinetic energy to ionize other atoms.

primarytotal

iitotal

nn

W

xdxdE

W

En

43

ntotal - number of createdelectron-ion pairs

E = total energy loss

Wi = effective <energy loss>/pair

Lohse and Witzeling, Instrumentation In High

Energy Physics, World Scientific,1992

Number of primary electron/ion pairs infrequently used gases for MIP.

Wi - NOT the ionisation potential !!!

Ionization of Gases

Example: Ar

Density ~ 1.7 g/l

E = 1.8 MeV/(g/cm2) ~ 3 keV/cm

Wi = 26 eV/ion

ntotal ~ 100 ions/cm (~25 primary)

Ionization of Gases: first approximation

• The number of primary electron/ion pairs is approximately Poisson distributed.

!)(

m

enmP

nm

The detection efficiency is therefore limited to :

neP 1)0(1det

For thin layers det can be significantly lower than 1.For example for 1 mm layer of Ar nprimary= 2.5 → det = 0.92 .

Variation of the number of electron/pairs:

NOT exactly correct!

iLN

Ln

;nn

Ionization of Gases: second approximation

;nFn

F~ 1 for scintillators F~0.2 – 0.8 for gas detectorsF~ 0.12 for Silicon detectors

• 100 electron/ion pairs created during ionization process is not easy to detect.Typical noise of the amplifier ≈ 1000 e- (ENC) → gas amplification is required !! .

I-

Capacitor with gas at low electric field

Response to a primary ionization

Particle

Ar+ e-

Ar+ e-

E

Recombination losses q0=A*Q0

Primary ionisation Q0

e-

Attachment

losses q=q0e -(D/λ)

D, drift distance

I-

I-

+V

Gas amplification: capacitor with gas

Alfa-particle

Beta-particle

+V

0.2 mm

“Ionisation” mode, i.e. no amplification yet…


+V

0.2 mm

“Proportional” mode, linear amplification…

Single Wire Proportional Chamber

xrxE ennenn 00 or

1

Multiplication of ionization is described by the first Townsend coefficient

dn = ndx – mean free path

is determined by the excitation and ionization cross sections of the electrons in the gas. It depends also on various and complex energy transfer mechanisms between gas molecules.There is no fundamental expression for → it has to be measured for every gas mixture.

Amplification factor orGain

Ar-CH4

A. Sharma and F. Sauli, NIM A334(1993)420

Photoemission

In the avalanche process molecules of thegas can be brought to excited states.

Ar *11.6 eV

Cu

e-

cathode

De-excitation of noble gasesonly via emission of photons;e.g. 11.6 eV for Ar.This is above ionizationthreshold of metals;e.g. Cu 7.7 eV.

When gain exceeds about 108 - new avalanches → increase of the discharge current


+V

0.2 mm

“Saturated” mode, logarithmic amplification, saturation…

Photoemission starts…

γ


+V

0.2 mm

“Geiger” mode, only counting is possible, info about primary ionization is lost!

Strong photoemission…

γ


+V

0.2 mm

“Saturated” mode, logarithmic amplification, saturation…

Strong photoemission, ion impact ionisation…

γ

Gas ionization chamber – Operation Modes

• ionization mode – full charge collection, but nocharge multiplication;gain ~ 1

• proportional mode – multiplication of ionizationstarts; detected signal proportional to original ionization → possible energy measurement (dE/dx);secondary avalanches have to be quenched;gain ~ 104 – 105

• limited proportional mode (saturated, streamer) –strong photoemission; secondary avalanches merging with original avalanche; requires strongquenchers or pulsed HV; large signals → simple electronics;gain ~ 1010

• Geiger mode – massive photoemission; full lengthof the anode wire affected; discharge stopped byHV cut; strong quenchers needed as well

Geiger counter: coaxial geometry

Electrons liberated by ionization drift towardsthe anode wire.

Electrical field close to the wire (typical wire Ø~few tens of m) is sufficiently high for Geiger

mode discharge.

a

rCVrV

r

CVrE

ln2

)(

1

2

0

0

0

0

C – capacitance/unit length

R~1-10MOhm pulse Discharge is quenched

by the current-limiting resistor

Geiger counter: coaxial geometry

Single Wire Proportional Chamber

Electrons liberated by ionization drift towardsthe anode wire. Electrical field close to the wire (typical wire Ø~few tens of m) is sufficiently high for electrons(above 10 kV/cm) to gain enough energy to Ionize further → avalanche – exponential

increase of number of electron ion pairs- the proportional operation mode.

Cylindrical geometry is not the only one able to generate strong electric field:

parallel plate strip hole groove

a

rCVrV

r

CVrE

ln2

)(

1

2

0

0

0

0

C – capacitance/unit lengthanode

e- primary electron

Cr

a

drrn

nM exp

0

SWPC – Choice of Gas

In the avalanche process molecules of thegas can be brought to excited states.

Ar *11.6 eV

Cu

e-

cathode

De-excitation of noble gasesonly via emission of photons;e.g. 11.6 eV for Ar.This is above ionizationthreshold of metals;e.g. Cu 7.7 eV.

new avalanches → permanent discharges

Solution: addition of polyatomic gas as aquencher

Absorption of photons in a large energy range (many vibrational and rotationalenergy levels).

Energy dissipation by collisions ordissociation into smaller molecules.

ELASTIC IONIZATION

SUM OF EXCITATION

ELASTIC

IONIZATION

excitation levels

vibrational levels

S. Biagi, NIM A421 (1999) 234

S. Biagi, NIM A421 (1999) 234

SWPC – Signal Formation

drdr

dV

lCV

Qdv

0

Avalanche formation within a fewwire radii and within t < 1 ns.Signal induction both on anode andcathode due to moving charges(both electrons and ions).

Electrons collected by the anode wire i.e. dr isvery small (few m) – almost no induction signal

Ions have to drift back to cathode i.e. dr is large(few mm). Signal duration limited by total ion drifttime.

Need electronic signal differentiation to limit dead time.t (ns)

0 100 200 300 400 500

v(t)

300 ns

100 ns

50 ns

+

-

+

- +

Multiwire Proportional Chamber

Simple idea to multiply SWPC cell : Nobel Prize 1992

First electronic device allowing high statistics experiments !!

Normally digital readout :spatial resolution limited to

12

dx

for d = 1 mm x = 300 m

Typical geometry5mm, 1mm, 20 m

G. Charpak, F. Sauli and J.C. Santiard

CSC – Cathode Strip Chamber

Precise measurement of the second coordinate

by interpolation of the signal induced on pads.

Closely spaced wires makes CSC fast detector.

Space resolution

CMS

= 64 m

Center of gravity of inducedsignal method.

RPC – Resistive Plate Chamber

Ec luste rs

resistive electrode

resistive electrode

gas gap

HV

GND

readout strips

readout strips

HV

GND

MRPC

Multigap RPC - exceptional time resolutionsuited for the trigger applications

Rate capability strong function of the resistivityof electrodes in streamer mode.

useful gap

= 77 ps

Time resolution

2 mm

A. Akindinov et al., NIM A456(2000)16

Limitations of Gas Detectors

Avalanche region → plasma formation (complicated plasma chemistry)

•Dissociation of detector gas and pollutants•Highly active radicals formation•Polymerization (organic quenchers)•Insulating deposits on anodes and cathodes

Classical ageing

Anode: increase of the wirediameter, reduced and variablefield, variable gain and energyresolution.

Cathode: formation of strongdipoles, field emmision andmicrodischarges (Malter effect).

Limitations of Gas Detectors

DischargesField and charge density dependent effect.Solution: multistep amplification

Insulator charging up resulting in gain variable with time and rateSolution: slightly conductive materials

Space charge limiting rate capabilitySolution: reduction of the lenght of the positive ion path

Solutions: carefull material selection for the detector construction and gas system,detector type (GEM is resitant to classical ageing), working point,non-polymerizing gases, additives supressing polymerization (alkohols, methylal),additives increasing surface conductivity (H2O vapour), clening additives (CF4).

Computer Simulations

MAXWELL (Ansoft) electrical field maps in 2D& 3D, finite element calculation for arbitrary electrodes &

dielectrics

HEED (I.Smirnov)energy loss, ionization

MAGBOLTZ (S.Biagi) electron transport properties: drift, diffusion, multiplication, attachment

Garfield (R.Veenhof) fields, drift properties, signals (interfaced to programs above)

PSpice (Cadence D.S.) electronic signal

In the next lecture

Spark chamber

Gas Electron Multiplier

Drift tubes

Time Projection Chamber

Documents

Detectors for particles and radiation Advanced course for Master students Spring semester 2010 S7139 5 ECTS points Tuesday 10:15 to 12:00 - Lectures Tuesday