Scintillation + Photo · PDF filemolecules osmall fraction of energy released as ... NIM A 387 (1997), 186) ... In the end, all energy converted into

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Accelerators and Experiments 2008Scintillation & Energy Measurement Kenneth Österberg VII/1

Scintillation + Photo Detection

Inorganic scintillators

Organic scintillator

Readout & fiber tracking

Photo detectors


Scintillation

Scintillation

Two material types: Inorganic & organic scintillators

(generally)high light output lower light output

but slow but fast

photodetector

Energy deposition by ionizing particle excitedmolecules small fraction of energy released asoptical photons (scintillation light or luminescense)

Scintillators multi purpose detectors

calorimetrytime of flight measurementtracking detectors (fibers)trigger countersveto counters…..



3 different scintillation mechanisms:1a. Inorganic crystalline scintillators (NaI, CsI, BaF2...)most common: sodium iodide with thallium trace [NaI(Tl)]

due to high density& high Z inorganicscintillators well suitedfor charged particle &photon detection.

light output ofinorganicscintillatorswavelengthdependent

energy bandsin crystalsactivated byimpurity tracesoften 2 timeconstants:• fast recombi-nation (10 ns -1 s) from acti-vation centers• delayedrecombinationdue to trapping( 100 ms)



Light output of inorganic crystals also shows strongtemperature dependence

1b. Liquid noble gases (LAr, LXe, LKr)

A

A+

A2*

A2+

A

A

e-

ionization

collisionwith g.s.atoms

excitedmolecule

ionizedmolecule

de-excitation anddissociation

UV130nm (Ar)150nm (Kr)175nm (Xe)

A*excitation

A2*

recombination

also here one finds 2 time constants: one at few ns &a second at 10 1000 ns but both at same wavelength.

BGO

PbWO4

(From Harshaw catalog)



LAr 1.4 1.295)120-1705 / 860LKr 2.41 1.405)120-170LXe 3.06 1.605)120-170

5) at 170 nm2 / 853 / 22 1.00

14

2.87

A lead tungstate(PbWO4) crystal forCMS electromagneticcalorimeter (goodradiation tolerance &high density ataffordable cost)

Light yield normalized to NaI(Tl) 40k photons / MeVNB! light output dependent on light collection efficiency &quantum efficiency of photo detector (see following pages)

sensitive to humidity?

wavelength of maximum emissionProperties of some inorganic crystal scintillators

for a MIP


Organic scintillators

Scintillation basedon the twoelectrons of theC C bonds(“aromatic rings”).

Organic scintillators have low Z (H,C) low photondetection efficiency but high neutron efficiency via(n,p) reactions. Reasonable for charged particles.

2. Organic scintillators: liquid or plastic solutionsoriginal emitted light in UV range

Normally made of plastic base/solvent & primary fluor+ secondary (& tertiary) fluors as wavelength shifters.Fast energy transfer from plastic base/solvent to fluorsvia resonant dipole-dipole interactions (”Förstertransfer”) since short distances between molecules

shift emission to longer wavelengthslonger absorption length & more efficient read-out


Organic scintillators

Some widely used plastic bases/solvents & fluors:

After mixing components together plastic scintillatorsproduced by a complex polymerization method.Very easy to shape.

Most organic scintillators have similar properties:density ( ) 1.03 – 1.20 g/cm3

refractive index (nD) 1.58decay time ( decay) 1-12 nslight yield 10k photons / MeV (25 % of NaI(Tl))

plastic base/solvent primary fluor secondary fluorLiquidscintillators

TolueneXylene

DPOPBD

BBOBPO

Plasticscintillators

PolyvinyltoluenePolystyrene

DPOPBD

TBPBBODPS

Schematicrepresentationof wave lengthshiftingprinciple

(C. Zorn,Instrumentation In HighEnergy Physics, World

Scientific,1992)

DPO = diphenyloxazolePBD = phenyl-biphenylyloxadizole


Scintillator readout

Scintillator readoutReadout to be adapted to geometry & emissionspectrum of scintillator (need to shift wavelength?).Geometrical adaptation:

optical fibers & light guides: light transport by totalinternal reflection (& outer reflector for light guides)

wavelength shifter (WLS) bars

Photo detector

primary particle

UV (primary)

blue (secondary)

greensmall air gap

scintillator

WLS

adiabatic light guide

optical fiber

corepolystyrene

n=1.59

cladding(PMMA)n=1.49

25 m

fluorinatedouter claddingn=1.42

25 m


Scintillating fiber tracking

Scintillating fiber trackingScintillating plastic fibersCapillary fibers, filled with liquid scintillatorHigh geometrical flexibility & low massFine granularity & good hermeticity (=“no holes”)Fast response (ns) (if fast read out) 1st level trigger

60 m

Hexagonal fibers with double cladding. In figure onlycentral fiber illuminated low cross talk !

3.4 m

Number of photonsproduced by atraversing minimumionizing particle notvery high so needefficient readout.1 mm fiber< 2000 photons.Taking into accountfiber attenuation,capturing & photodetector efficiency10 20 photons remain (must

be careful not to loose signal).

D0 central tracker at Tevatron:1.6 -2.5 mlongpolysterenefibers.


Photo Detectors

purpose: convert light into detectable electronics signalprinciple: Use photoelectric Effect to convert photonsto photo electrons (multiplied by some mechanism)

standard requirementhigh sensitivity, usually expressed asquantum efficiency Q.E. = Nphoto electrons / Nphotons

Main types of photo detectorsgas based (use photosensitive additive e.g. TMAE)vacuum based (photosensitive photo cathode)solid state detectors (e.g. GaAs)

Photoemission threshold Wph of various materials

100 250 400 550 700 850 [nm]

12.3 4.9 3.1 2.24 1.76 1.45 E [eV]

VisibleUltraViolet(UV)

Multialkali

Bialkali

GaAs

TEA

TMAE,CsI

InfraRed(IR)


Photo Detectors

Photo Multiplier Tube (PMT)

main phenomena:• photo emission from

photo cathode.• secondary emission

from dynodes.dynode gain: g = 3 50

total gain:

For example: 10 dynodeswith g = 4 M = 410 106

N

iigM

1

PMT’s very sensitive tomagnetic fields, even toearth field (30-60 T).metal shielding required.

(Philips Photonic)

e

photon

Photoelectric effect in photo cathode 3-step process:- absorbed ’s give energy to e’s in photo cathode (PC)- e’s diffuse through PC, losing part of their energy- e’s reaching surface with enough excess energy escapeideal photo cathode absorb all ’s & emit all created e’semission threshold Wph > Eg (band gap) + Ea (electron affinity)

Photo cathode:Multialkali: SbNa2KCsBialkali: SbKCs, SbRbCs

quartz

dynode: Ag/Mg, Cu/Beor Cs/Sb


Quantum efficiencies of typical photo cathodes

Photo Detectors

Energy resolution of PM’s: determined by fluctuations ofthe number of secondary electrons emitted by dynodes(follows a Poisson distribution with expectation value ).

Relative resolution:nn

nn

n 1 (fluctuations atthe first dynodemost important!!)

n

up to 8x8 channels.size: 28x28 mm2.active area 18x18 mm2 (41%).bialkali PC: Q.E. = 20% at

max = 400 nm. Gain 106.

Multi anode PM´s e.g Hamamatsu R5900 series

(Hamamatsu)

GaAsP GaAs

CsTe(solarblind)

MultialkaliBialkali

Ag-O-Cs

Photon energy E (eV)12.3 3.1 1.76 1.13


Photo Detectors/Large area applications

Hybrid photo diodes (HPD) – large window inwave length for photon acceptance

Photo cathode likein PMT, V = 10 20 kV

31056.3

20eV

keVW

VeGSi

V

photocathode

focusingelectrodes

siliconsensor

electron

photo electronacceleration + silicondetector (pixels or strips)

(LHCb - DEP)

Pixel-HPD, 80mm Øcross-focused

3 x 61 pixels

test beam data, 3 HPDs

Cherenkov ring imaging with HPD’s

can do real singlephoton counting !!Q.E. 10 - 30 %


Photo Detectors/Small area applications

Visible Light Photo Counter (VLPC)

vacuum tube

photo cathode

anode (grid)

dynode

40 m

m

G 7 10. Work in axialmagnetic fields of 1 TUsed by OPAL & DELPHIfor readout of lead glassendcap calorimeters.

High reverse biasvoltage 100 200 V.High internal fieldavalanche multiplication.G 100 107

Q.E. 70 %sub-ns response time

(sketches from J.P. Pansart, NIM A 387 (1997), 186)Avalanche Photo Diodes (APD)

Photo triodes = single stage PMT

•-•+Photon

IntrinsicRegion

GainRegion

SubstrateDriftRegion Spacer

Region

Hole drifts towardshighly doped driftregion & ionizes adonor atom freeelectron. Multiplicationby ionization of furtherneutral donor atoms.

•h•e•e

• Operation at low bias voltage (7 V)• High infrared sensitivity requires liquid He cooling• Q.E. upto 90% in a small wavelength range (500-600 nm).• Gain up to 50.000 but also noise prone system!

IEEE NS-30 No. 1 (1983) 479


Basic principlesInteraction of chargedparticles & photonsElectromagnetic cascadesNuclear interactionsHadronic cascades

Homogeneous calorimetersSampling calorimeters


Calorimetry

Calorimetry:Energy measurement by total absorption,combined with spatial reconstruction.

Calorimetry a “destructive” method

Detector response (ideally) E

Calorimetry works both forcharged (e & charged hadrons)neutral particles (neutral hadrons, )

Basic mechanism: formation ofelectromagnetic showers (e , & 0’s)hadronic showers (charged & neutral hadrons)

In the end, all energy converted intoionization or excitation of detector material.

An electromagnetic part to measure e , & 0’s& an hadronic part to measure all other particles

”only” measurement of neutrals


Interaction of charged particles

Z,A

chargedparticle

Energy loss by Bremsstrahlung

Emission of real photonsby a charged particleaccelerated in Coulombfield of nuclei of absorber

Effectively plays a role only for e± & ultra-relativistic ’sFor electrons:

Taking also into account the interactions with theatomic electrons ( Z)

EZdxdENB

Ioniz

ln!

3/122

220

23 183ln)(

)(4Z

EznZcm

cdxdE

e

)/183ln(4

)/183ln(4

3122

0

0

312

2

ZrZN

AX

XE

dxdE

ZErA

ZNdxdE

eA

eA

)/287ln()1(4.716

0 ZZZAX

radiationlength

[g/cm 2]

(>1 TeV)


Interaction of charged particles

(Leo)

Radiation length of material defined for electrons !!

For a compound:

Critical energy Ec

For electrons one finds approximately:

For example Ec(e ) in Fe (Z=26) = 22.4 MeVFor muons:

Ec( ) in Fe (Z=26) 1 TeV

bremsstrahlung need only be taken into account for e !!

N

jjj

Xw

X 1 00

1 wj: mass fractionof components

energy loss(bremsstrahlung+ ionization) ofelectrons &protons in copper

Ionizc

Bremsc E

dxdEE

dxdE

92.0710

24.1610 gasliquidssolid

ZMeVE

ZMeVE cc

2

)()(e

cc mm

eEE

Ec


Interaction of photons

Interaction of photonsTo be detected, a photon has to create chargedparticles and/or transfer energy to charged particles(note photon interaction with matter fundamentallydifferent w.r.t. charged particles since photonseither absorbed or large angle scattered)

3 mechanisms: photo-electric effect, Comptonscattering & pair production

• Photo-electric effect:

Only possible in close neighborhood of a third collisionpartner (the nucleus) photo effect releases mainlyelectrons from the K shell ( 80 %).

cross section shows strong modulation when EEionization

shell. At high energies ( 1)

e-

X+X

eatomatom

)Thomson(32 238

254

7

21

eeTh

e

eTh

Kphoto r

cmE

Z

14 542 ZreKphoto

5Zphoto



independentof energy !

• Compton scattering:

Assume electron as quasi-freecross section: Klein-Nishina formula, at high energies:

atomic Compton cross-section:

• Pair production

Only possible in Coulombfield of a nucleus (or an e ) if

cross section (high energy approximation)

'' ee

cos111EE

)2ln( 21

2ee

cr

ec

atomicc Z

Z

e+

e-nucleuseenucleus

22 2cmE e

At high energiesenergy sharingbetween e+ & emore asymmetric.

31

183ln974 22

ZZrepair

00 7

9197 X

XNA

pairA

the interaction length for photons



...)or( )(

00

PairComptonPhoto

Xx xXeIeIxXI

: massattenuationcoefficient

Comptonscattering

Pairproduction

Photo-electriceffect

4M

eV

In summary for photon interactions with matter: (todescribe the statistical process of photon attenuation):

: linearabsorptioncoefficient

gcmA

Ni

A

i

i 2

(Grupen)

(Kleinknecht)

Dominating effectin each domain ofthe photon energy

atom number Zof absorber plane


Reminder: basic electromagnetic interactionse+ / e

Photoelectric effect

Compton effect

Pair production

Ionisation

BremsstrahlungE

dE/d

x

E

dE/d

x

E

E

E

Electronshower in a

cloud chamberwith leadabsorbers

A real electromagnetic shower:

NB!

fore

ener

gylo

ss

NB!

for

inte

ract

ion

prob

abili

ty


Electromagnetic cascades

2lnln 0

maxcEEt

c

t

t

tttotal

EEN 0

0

)1( 2122max

max

process continues until E(t) < Ec

after t = tmax dominating processes ionization,excitation,Compton scattering & photo-electric effect absorption.

Simple qualitative model

e±

consider onlybremsstrahlung& pair production(OK for highenergy e/ ).for simplicity:X0 pair & e 2

t(max): shower depthin radiation lengths(at maximal numberof shower particles)

ttN 2)(tEtE 2/particle)( 0

em

em

em

emem

t = # of X0 or pair


Electromagnetic cascades

Longitudinal shower development:

98% energy containmentsize of a calorimeter grows only logarithmically with E0

max%98 5.2 tt

Example: 60 GeV electron in leadglass, Ec = 11.8 MeV tmax 8.0,t98% 20.0; X0 2 cm, RM 3.6 cm

][MeV210 cmX

ER

cM

Transverse development of electromagnetic shower(mainly caused by multiple scattering of electrons &positrons with detector material): cone containing 90(95) % of the shower energy has a radius of 1(2) RM

Molière radius

40 cm

14 cm

(Ce = 0.5;C = +0.5)

(PDG)

jc

CEEt 0em

max ln

showermaximum(in reality) at

em

em

btetEdtdE )( em

0


Energy resolution

Energy resolution of a calorimeter (intrinsic limit)

c

total

EEN 0 total number of track segments

0

11)()(ENN

NEE

Ecb

Ea

EE)(

holds also forhadron calorimeters

stochastic termdepends onthe number ofcreatedsecondary tracksegments

More general:

constant term

inhomogenities,bad cell inter-calibration,non-linearities

noise term

electronic noise,radioactivity,pile up

quality factor !!

also spatial and angular resolution scale like 1/ E

calorimeters relative energyresolution improves with E0 !!


Nuclear InteractionsThe interaction of energetic hadrons (charged/neutral)determined by inelastic (& elastic) nuclear processes.

For high energies (> 1 GeV) the cross-sectionsdepend only little on the energy & on the type of theincident particle (p, , K…).

In analogy to radiation length X0 a hadronicinteraction length can be defined (inelastic only)

similarly a hadronic collision length (inelastic + elastic)

Nuclear interactions

n

p

+

0

-

hadronZ,A

7.00

3.0 since AAN

Ainel

inelAI

mbAinel 3507.0

0

secondary multiplicity ln(E)

Inelastic: excitation & finally breakup of nucleusnuclear fragments + production of secondary particles.

IctotalA

c AN

A31

transverse size: pt had 0.4 GeV


Nuclear interactions

Material Z A [g/cm3] X0 [g/cm2] I [g/cm2]

Hydrogen (gas) 1 1.01 0.0899 (g/l) 63 50.8Helium (gas) 2 4.00 0.1786 (g/l) 94 65.1Beryllium 4 9.01 1.848 65.19 75.2Carbon 6 12.01 2.265 43 86.3Nitrogen (gas) 7 14.01 1.25 (g/l) 38 87.8Oxygen (gas) 8 16.00 1.428 (g/l) 34 91.0Aluminium 13 26.98 2.7 24 106.4Silicon 14 28.09 2.33 22 106.0Iron 26 55.85 7.87 13.9 131.9Copper 29 63.55 8.96 12.9 134.9Tungsten 74 183.85 19.3 6.8 185.0Lead 82 207.19 11.35 6.4 194.0Uranium 92 238.03 18.95 6.0 199.0

0.1

1

10

100

0 10 20 30 40 50 60 70 80 90 100

X0

I

X 0,

I[c

m]

Z

For Z > 6: I > X0

I and X0 in cm


Interaction of neutrons and neutrinos

dA

NN Adet

Interaction of neutrinosNeutrinos interact only weakly tiny cross-sectionsFor their detection we need again charged particles.Possible detection reactions:

• + n + p = e, ,• + p + n = e, ,

Cross-section for the reaction e + n e + p is of theorder of 10 43 cm2 (per nucleon, E few MeV).

Detection efficiency

1 m Iron:Neutrino detection requires very (!!) big detectors(“light year size”) or big & massive detectors (ktons) +very high neutrino fluxes (“long beam line experiments”).Event rates typically low (~ 10) e.g. SN1987a detectedby Kamiokande experiment with handful of events.

In collider experiments fully hermetic detectors allowto detect neutrinos indirectly:• sum up all visible energy & momentum.• attribute missing energy & momentum to neutrino.example UA1: W+ e+ + e. Reconstruct transversemomentum of the e from missing transversemomentum of the whole event (“missing ET & pT”).

17det 105


Hadronic cascades

Hadronic cascades (showers)

hadronic + electromagneticcomponent

some energy of shower lost due to neutrinos, muons …

large energy fluctuations limited energy resolution

neutral pions ( 2 , electrons, ’selectromagnetic cascade

visible energy

charged hadrons: pions, protons, kaonsneutral hadrons: neutrons, neutral kaons

visible energy

Various processes involved. Much more complexthan electromagnetic cascades.

(Grupen)


Hadronic showers and instrumental effects

longitudinal shower development

]cm[ln7.0][ln2.0)(

%95

max

bEatGeVEt I

for iron: a = 9.4, b = 39E = 100 GeV

t95% 80 cm

Iron: I =16.7 cm

Hadronic showers are much longer and broader thanelectromagnetic ones !

(B. Friend et al., NIMA136 (1976) 505)Laterally shower consists

of core + halo. 95%containment in acylinder of radius I.

OPAL end cap pre-shower + calorimeter

(C. Beard et al., NIM A 286 (1990) 117)

had

had

had

Material in front of calorimeterShowers start in ‘dead’ material in front of calorimeter(other detectors, solenoid, support structure)Install a highly segmented pre-shower detector in frontof calorimeter- recover lost energy- improved background rejectiondue to better spatial resolution

- improve angular resolution


Calorimetry

Calorimeter types

Homogeneous calorimeters:

Sampling calorimeters:

detector = absorbergood energy resolutionlimited spatial resolution (particularly in

longitudinal direction)only used for electromagnetic calorimetry

detectors & absorber separated onlyfraction of the energy measured.

limited energy resolution

good spatial resolution

used both for electromagnetic & hadronic

calorimetry


Homogeneous calorimeters

Homogeneous calorimeterstwo main types: scintillator crystals or “glass” blocks

(that use Cherenkov radiation, see later).photons. readout via photomultiplier, -diode/triode

scintillators (crystals)

Cherenkov radiators

Scintillator Density[g/cm3]

X0 [cm] LightYield/MeV

(rel. yield)

1 [ns] 1 [nm] Rad.Dam.[Gy]

Comments

NaI (Tl) 3.67 2.59 4 104 230 415 10 hydroscopic,fragile

CsI (Tl) 4.51 1.86 5 104

(0.49)1005 565 10 Slightly

hygroscopicCSI pure 4.51 1.86 4 104

(0.04)10 31036 310

103 Slightlyhygroscopic

BaF2 4.87 2.03 104

(0.13)0.6 220620 310

105

BGO 7.13 1.13 8 103 300 480 10PbW04 8.28 0.89 100 10 440

10 530104 light yield =f(T)

Material Density[g/cm3]

X0 [cm] n Light yield[p.e./GeV](rel. p.e.)

cut [nm] Rad.Dam.[Gy]

Comments

SF-5Lead glass

4.08 2.54 1.67 102

SF-6Lead glass

5.20 1.69 1.81 102

PbF2 7.66 0.95 1.82 2000 103 Not availablein quantity

Relative light yield: rel. to NaI(Tl) readout with PM (bialkali PC)


Homogeneous calorimeters

examplesOPAL Barrel + end-cap: lead glass + pre-sampler

BGO E.M. Calorimeter in L3

10500 blocks (10 x10 x 37 cm3, 24.6 X0),photo multiplier(barrel) or photo triode(end-cap) readout.

002.006.0)( EEE

(OPAL collab. NIM A 305 (1991) 275)

11000 crystals, 21.4 X0,temperature monitoring +control system, lightoutput very sensitive totemperature (-1.55 %/ C)

E/E < 1% for E > 1 GeVspatial resolution < 2 mm

(E >2 GeV)

(L3 collab. NIM A 289 (1991) 53)

Spatial resolution(intrinsic) 11 mmat 6 GeV


Sampling calorimeters

Sampling calorimetersabsorber + detector separated additional samplingfluctuations

d e te c to rs a b s o rb e rs

d

dX

EEF

dTN

c

10

det detectable track segments

0

1Xd

ENN

EE

• MWPC, streamer tubes• warm liquids

TMP = tetramethylpentane,TMS = tetramethylsilane

• cryogenic noble gases:mainly LAr (LXe, LKr)

• scintillators, scintillation fibers, silicondetectors

”gain”factor



ATLAS electromagnetic Calorimeteraccordion geometry absorbers immersed in Liquid Argon

(RD3 / ATLAS)

Liquid Argon (90K)+ lead-steal absorbers (1-2 mm)+ multilayer copper-polyimide

readout boardsionization chamber.

1 GeV E-deposit 5 x106 e-

• geometry minimizes dead zones.• liquid Ar intrinsically radiation hard.• readout board allows fine

segmentation (azimuth, pseudo-rapidity & longitudinal) accordingto the physics needs

Test beam results, e- 300 GeV (ATLAS TDR)

cEbEaEE /)(spatial &angular

uniformity0.5%

spatialresolution5 mm / E1/2


CMS hadron calorimeterCu absorber + scintillators


scintillators fill slots & readout via fibres by HPDs

2 x 18 wedges (barrel) + 2 x 18 wedges (endcap)in total a 1500 ton absorber

%5%65EE

Etest beamresolution forsingle hadrons


Jet clustering


Jet clustering

Corrections usually depend onjet energy & direction. Absoluteenergy calibration using knownprocesses: Z0 e+e & + (forelectromagnetic & hadronic part).

Documents

Scintillation + Photo · PDF filemolecules osmall fraction of energy released as ... NIM A 387 (1997), 186) ... In the end, all energy converted into