Content • Introduction

– Overview of detector systems

– Sources of radiation

• Radioactive decay

• Cosmic Radiation

• Accelerators

• Interaction of Radiation with Matter

– General principles

– Charged particles • heavy charged particles

• electrons

– Neutral particles

• Photons

• Neutrons

• Neutrinos

• Definitions

• Detectors for Ionizing Particles

– Principles of ionizing detectors

– Gas detectors

• Principles

• Detector concepts

Content – Semiconductor detectors

• Semiconductor basics

• Sensor concepts

• Different detector materials

– Readout electronics

– Scintillation detectors

• General characteristics

• Organic materials

• Inorganic materials

• Light output response

• Calorimeters

• Velocity Determination in Dielectric Media

– Cerenkov detectors

• Cerenkov radiation

• Cerenkov detectors

– Transition Radiation detectors

• Phenomenology of Transition Radiation

• Detection of Transition Radiation

• Complex Detector Systems

– Particle Identification with Combined Detector Information

– Tracking

Lecture 9

Scintillating Detectors and

Calorimeters

Scintillation

photodetector

Energy deposition by ionizing particle production of scintillation light (luminescense)

Scintillators are multi purpose detectors:

• calorimetry

• time of flight measurement

• tracking detector (fibers)

• trigger counter

• veto counter

• …

Two material types: Inorganic and organic scintillators

Organic materials sp2-hybridisation:

•2px and 2py mix with s-orbital -orbital

•pz remains unchanged π-orbital

~10-11 sec

Fluorescence

10-8 - 10-9 sec

peak ~ 320 nm Phosphorescence

10-4 sec

non-radiative transition

~ 10-6 sec (Förster transf.)

Pi electron energy levels

Organic scintillators: Monocrystals or liquids or plastic solutions

Monocrystals:

naphtalene, anthracene,

p-terphenyl….

Liquid and plastic

scintillators

They consist normally of

a solvent + secondary

(and tertiary) fluors as

wavelength shifters.

Fast energy transfer via non-radiative dipole-dipole interactions (Förster transfer).

shift emission to longer wavelengths

longer absorption length and efficient read-out device

Wavelength shifting

• no self-absorption

• also used for light

re-direction

The emitted wavelength is always longer or equal to the

incident wavelength. The difference is absorbed as heat in

the atomic lattice of the material.

Practical organic scintillators uses

a solvent

+ large concentration of primary fluor

+ smaller concentration of secondary fluor

+ ......

Organic scintillators

• Organic scintillators have low

Z (H,C)

• Low density (< 2 g/cm3)

• Low g detection efficiency

(practically only Compton

effect).

• But high neutron detection

efficiency via (n,p) reactions.

The most common inorganic scintillator is sodium iodide activated with

a trace amount of thallium [NaI(Tl)].

Energy bands in impurity activated crystal

Inorganic Crystalline Scintillators

often 2 time constants:

• fast recombination (ns - µs) from

activation center

• delayed recombination due to trapping

(100 ms)

BaF2 fast and slow signals

200ns/square 2ns/square

* Strong dependence of the light output

and the decay time with temperature.

* Bismuth germinate Bi4Ge3O12 is the crystalline form of an inorganic oxide

with cubic eulytine** structure, colourless, transparent, and insoluble in water.

** From the Greek eulitos = "easily liquefiable", in allusion to its low melting point.

Inorganic Crystalline Scintillators

Inorganic scintillators

PbWO4 ingot and final polished

CMS ECAL scintillator crystal from

Bogoroditsk Techno-Chemical

Plant (Russia).

Also here one finds 2 time constants: from a few ns to 1 ms.

from C. D'Ambrosio, Academic Training, 2005

Liquified Noble Gases: LAr, LXe, LKr

Common materials

Density (g/cm3)

λemiss (nm)

#photon/MeV

(ns)

NaI(Tl) 3.7 410 40000 230 hygrosc.

CsI(Tl) 4.5 560 45000 1100 hygrosc.

BGO 7.1 480 8000 300

BaF2 4.9 220 / 310

2300/ 10000

0.8 / 630

CeF3 6.2 320 5500 27 rad. hard

plastic 1.03 430 10000 2…5 easy handling

= good = bad

Light collection

Light collection

Optical fibers

corepolystyrene

n=1.59

cladding(PMMA)n=1.49

typically <1 mm

typ. 25 mm

light transport by total internal reflection

n1

n2

6.69arcsin1

2

n

n %1.3

4

d

in one direction

and absorption

length: l>10 m for

visible light

corepolystyrene

n=1.59

cladding(PMMA)n=1.49

25 mm

fluorinated outer claddingn=1.42

25 mm

%3.54

d

multi-clad fibres for improved aperture

Optical fibers for tracking

Scintillating plastic fibers

Capillary fibers, filled with liquid scintillator

Planar geometries

(end cap)

Circular geometries

(barrel)

High geometrical flexibility

Fine granularity

Low mass

Fast response (ns) (if fast read out) first level trigger

(R.C. Ruchti, Annu. Rev. Nucl. Sci. 1996, 46,281)

a) axial

b) circumferential

c) helical

Scintillating fiber tracking

Charged particle passing through a

stack of scintillating fibers (diam.

1mm)

(H. Leutz, NIM A 364 (1995) 422)

Hexagonal fibers with double cladding.

Only central fiber illuminated.

Low cross talk !

Photon Detectors

• Purpose:

– Convert light into detectable (electronic) signal

• Principle:

– Use photoelectric effect to convert photons (g) to photoelectrons (pe)

• Standard requirements:

– High sensitivity, usually expressed as:

• quantum efficiency:

• radiant sensitivity S(mA/W):

– Low intrinsic noise

– Low gain fluctuations

– High active area

gN

NQE

pe(%)

)(

)/(124(%)

nm

WmASQE

l

Main types of photon detectors:

gas-based

vacuum-based

solid-state

hybrid

Photon detectors

Photoemission threshold Wph of various materials

100 250 400 550 700 850 l [nm]

12.3 4.9 3.1 2.24 1.76 1.45 E [eV]

Visible Ultra Violet

(UV)

Multialkali

Bialkali

GaAs

TEA

TMAE,CsI

Infra Red

(IR)

Photon detectors

g

g

e-

Optical

window

Semi-transparent PC

Vacuum

Opaque PC

Substrate e-

3-step process:

• absorbed g’s impart energy to electrons (e) in the material;

• energized e’s diffuse through the material, losing part of their energy;

• e’s reaching the surface with sufficient excess energy escape from it;

ideal photo-cathode (PC) must absorb all g’s and emit all created e’s

The photoelectric effect

Energy-band model in semi-conductor PC

AGph EEWhE g

Band gap EG

g energy Eg

h

e-

(Photonis)

Electron affinity EA

Photoemission

threshold Wph

Standard model NEA material

Gph EW

Negative electron

affinity EA

Bialkali: SbKCs, SbRbCs Multialkali: SbNa2KCs (alkali metals have low work function)

(Hamamatsu)

GaAsP GaAs

CsTe

(solar

blind)

Multialkali Bialkali

Ag-O-Cs

Photon energy Eg (eV)

12.3 3.1 1.76 1.13

QE’s of typical photo-cathodes

Transmission of optical windows

Scintillator-Photomultiplier system

(in-)organic material scintillation light

photomultiplier signal amplification

light guide transmission scint. to tube

Photomultiplier tubes (PMTs)

• Basic principle:

– Photo-emission from photo-

cathode

– Secondary emission (SE)

from N Dynodes:

• dynode gain g 3 – 50

(function of incoming

electron energy E)

• total gain M:

• Example:

– 10 dynodes with g = 4

– M = 410 106

N

i

igM1

Approximately the same as the Photo Electric Effect.

On electron impact, energy is transferred directly to the electrons in

the secondary electron emission material allowing a number of

secondary electrons to escape.

Since the conducting electrons in metals hinder this escape,

insulators and semiconductors are used.

Materials in common use

are:

Ag/Mg, Cu/Be and Cs/Sb.

Use has also been made of

negative affinity materials as

dynodes,

in particular GaP.

Secondary Electron Emission

Mainly determined by the fluctuations of the number m(d) of

secondary e’s emitted from the dynodes;

Poisson distribution:

Standard deviation:

fluctuations dominated by 1st dynode gain;

!)(

m

emP

m d

d

d

dd

d

d

1m

Pulse height

(H. Houtermanns,

NIM 112 (1973) 121)

GaP(Cs) NEA dynodes EA<0

SE

co

effic

ient

d

E energy

(Photonis)

1 pe

2 pe

3 pe

Coun

ts

(Photonis)

1 pe

Noise

CuBe dynodes EA>0

Pulse height

Coun

ts

SE

co

effic

ient

d

E energy

(Photonis)

Gain fluctuation of PMT’s

Dynode configurations of PMT’s

Traditional Position-sensitive

Mesh

Metall-channel

(fine-machining

techniques)

PMT’s are in general very sensitive to magnetic fields, even to earth field (30-60 µT).

Magnetic shielding required.

(Hamamatsu) “Continuous”

dynode chain

Pb-glass

Kind of 2D PMT:

+ high gain up to 5 104;

+ fast signal (transit time spread ~50 ps);

+ less sensitive to B-field (0.1 T);

- limited lifetime (0.5 C/cm2);

- limited rate capability (mA/cm2);

(Burle Industries)

Pore : 2 mm

Pitch: 3 mm

The Micro Channel Plate (MCP)

Photo Multiplier Tube - dynodes and anode + Silicon Sensor = HPD

Hybrid

Photo

Diode

p+

n+

n

+ -+ -

+ -

V

photocathode

focusing

electrodes

silicon

sensor

electron

~ 4 - 5000 electron-hole pairs Good energy resolution

[Kinetic energy of the impinging electron]

[work to overcome the surface]

Electron-hole pairs =

[Silicon ionization energy]

Hybrid Photo Detector

But…

• Electronic noise, typically of the order of 500 e

• Back scattering of electrons from Si surface:

18.0Siback scattering

probability at E 20 kV

20% of the electrons deposit only a fraction o<1 of their initial

energy in the Si sensor .

continuous background (low energy side)

C. D’Ambrosio et al.

NIM A 338 (1994) p. 396.

3 parameters:

-

- <npe>

- Si

Hybrid Photo Detector

Solid-state photon detectors

• Photodiodes:

– P(I)N type

– p layer very thin (< 1 µm), as

visible light is rapidly absorbed by

silicon

– High QE(80% at 700 nm)

– No gain: cannot be used for single

photon detection

• Avalanche phtodiode:

– High reverse bias voltage: typ.100-

200 V

– due to doping profile, high internal

field and avalanche multiplication

– High gain: typ. 100-1000

Light absorption in Silicon

APD SPAD

Special photo diodes

Avalanche PhotoDiode

• Bias: slightly below breakdown

• Linear-mode: it’s an amplifier

• Gain: limited < 1000

Single-Photon Avalanche Diode

• Bias: well above breakdown

• Geiger-mode trigger device

• Gain huge

• Passive quenching by serial resistor at

output (simple but slow ~ 200 ns)

• Active quenching via additional CMOS

circuitry faster

APD/SPAD quantum efficiency

Triggering device

Scintillation is fast perfect for triggering on particle beam

e.g. finger counters, veto panels, etc.

often used in test beams

Calorimeters

Calorimeter Types

• Homogeneous calorimeters: – detector = absorber

– good energy resolution

– limited spatial resolution (particularly in longitudinal direction)

– only used for electromagnetic calorimetry

• Sampling calorimeters:

– detectors and absorber separated only part of the energy is

sampled.

– limited energy resolution

– good spatial resolution

– used both for electromagnetic and hadron calorimetry

Homogeneous calorimeters

• Scintillators (crystals)

• Cherenkov radiators

Scintillator Density

[g/cm3]

X0 [cm] Light

Yield

g/MeV

(rel. yield)

1 [ns] l1 [nm] Rad.

Dam.

[Gy]

Comments

NaI (Tl) 3.67 2.59 4104 230 415 10 hydroscopic,

fragile

CsI (Tl) 4.51 1.86 5104

(0.49)

1005 565 10 Slightly

hygroscopic

CSI pure 4.51 1.86 4104

(0.04)

10 310

36 310

103

Slightly

hygroscopic

BaF2 4.87 2.03 104

(0.13)

0.6 220

620 310

105

BGO 7.13 1.13 8103 300 480 10

PbW04 8.28 0.89 100 10 440

10 530

104

light yield =f(T)

Material Density

[g/cm3]

X0 [cm] n Light yield

[p.e./GeV]

(rel. p.e.)

lcut [nm] Rad.

Dam.

[Gy]

Comments

SF-5

Lead glass

4.08 2.54 1.67

102

SF-6

Lead glass

5.20 1.69 1.81

102

PbF2 7.66 0.95 1.82 2000

103

Not available

in quantity

Two main types: Scintillator crystals or “glass” blocks (Cherenkov radiation).

photons. Readout via photomultiplier, -diode/triode

Example ECAL - homogeneous

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

10500 blocks (10 x 10 x 37 cm3, 24.6 X0),

PM (barrel) or PT (end-cap) readout.

002.006.0)( EEE

Spatial resolution (intrinsic) 11 mm

at 6 GeV

Principle of pre-sampler or pre-

shower detector

Sample first part of shower with

high granularity. Useful for g/0,

e/g, e/ discrimination.

Usually gas or, more recently, Si

detectors

Sampling calorimeters

• Sampling calorimeters = absorber + detector

• MWPC, streamer tubes

• warm liquids (TMP =

tetramethylpentane,

TMS=tetramethylsilane

• cryogenic noble gases: mainly LAr

• scintillators, scintillating fibres,

silicon detectors

Shashlik readout

Energy resolution of a calorimeter

C

totalE

EN 0

C

cut

E

ETFT )(det

00 X

E

ET

C

0detdet

det 11)()(

ETT

T

E

E

E

cb

E

a

E

E

)(

total number of track segments

total track length

detectable track length (above energy Ecut)

holds also for hadron calrimeters

More general: Also spatial and angular

resolution scale like 1/E

stochastic term (see above)

constant term

• inhomogenities

• bad cell inter-calibration

• non-linearities

noise term

• electronic noise

• radioactivity

• pile up

Quality factor!

Sampling calorimeters

• Sampling fluctuations

• Pathlength fluct. + Landau fluct.

detectors absorbers

d

d

TN det

d

X

E

EF

c

0)(

0)(

1)(

X

d

E

E

FN

N

E

E c

wide spread angular

distribution of (low energy) e

In thin gas detector layers

the deposited energy

shows typical Landau tails

Hadronic cascades

• A hadron calorimeter shows in general different efficiencies for the

detection of the hadronic and electromagnetic components h and e:

• The fraction of the energy deposited hadronically depends on the

energy response of calorimeter to hadron shower becomes non-

linear

eehhh EER

Hadronic cascades

• How to achieve compensation?

– increase h: use Uranium absorber amplify neutron and soft

photon component by fission + use of hydrogeneous detector

high neutron detection efficiency

– decrease e: combine high Z absorber with low Z detectors.

Suppressed low energy photon detection ( Z5)

– offline compensation: requires detailed fine segmented shower

data event by event correction.

Example ECAL - sampling

• ATLAS electromagnetic Calorimeter

Accordion geometry absorbers immersed in Liquid Argon

Liquid Argon (90K)

+ lead-steal absorbers (1-2 mm)

+ multilayer copper-polyimide

readout boards

Ionization chamber.

1 GeV E-deposit 5 x106 e-

• Accordion geometry minimizes dead

zones.

• Liquid Ar is intrinsically radiation hard.

• Readout board allows fine

segmentation (azimuth, pseudo-

rapidity and longitudinal) acc. to

physics needs

Test beam results:

Example HCAL sampling

CMS Hadron calorimter

Cu absorber + scintillators

Scintillators fill slots and are read out

via fibres by HPDs

2 x 18 wedges (barrel)

+ 2 x 18 wedges (endcap)

1500 T absorber

%5%65

EE

ETest beam

resolution for

single hadrons