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
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
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)
* 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
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
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
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
Triggering device
Scintillation is fast perfect for triggering on particle beam
e.g. finger counters, veto panels, etc.
often used in test beams
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: