1 Particle Interaction with Matter and Detectors

Particle Interaction with Matter and Detectors
❍ Contact :
[email protected]
« Laboratoire Univers et Particules de Montpellier », UM2, Bat 13, 4th floor
❍
« New directions in science are launched by new
tools much more often than by new concepts.
The effect of a concept-driven revolution is to
explain old things in new ways. The effect of a
tool-driven revolution is to discover new things
that have to be explained »
Freeman Dyson
F. Dyson
1923 -
« Instrumentation for the 21st
century. No one does it better than
physicists when it come to
innovation for instrumentation, and
thus the future of all scientific fields
rests on our hands. »
Michael S. Turner
M. S. Turner
1
Nobel Prizes For Instrumentation
Alfred Nobel chose well
1927: C.T.R.
Wilson, Cloud
Chamber
1954: Walther
Bothe, Coincidence
method
1939: E. O.
Lawrence,
Cyclotron
1960: Donald
Glaser, Bubble
Chamber
1948: P.M.S.
Blacket, Cloud
Chamber
1968: L. Alvarez,
Hydrogen Bubble
Chamber
1950: C. Powell,
Photographic
Method
1992: Georges Charpak,
Multi Wire Proportional
Chamber
2
Particle Interaction with Matter and Detectors
❍ Aim of lectures :
 Importance of high quality detectors to achieve high quality results ⇒ examples of important
discoveries made possible by detector progress
 Basics of detectors: definition of specification and performance parameters, basic principles,
limitations, achieved performances (relative to ideal)
 Examples for detector systems: design ideas, performance, results, how have they been
optimized? ⇒ understand literature on detectors and provide ideas and tools for design and
optimisation of detector for specific applications
 Detectors: interdisciplinary ⇒ same detector concepts for: nuclear-, particle-,
state-, ac- celerator physics + medicine, bio- Research, …
astro-, solid
❍ Plea : I will need feedback!
Questions : What was good ? What was bad ? What was missing ? More detailed derivations ?
More detectors ? More… Less…
❍ Many ideas and plots taken from discussions/presentations of R.Klanner, J.Tinslay, C.Joram,
P.Collins, M.Danilov, R.Forty, F.Sauli, L.Serin, M.Titov, Ph. Schune, O.Ullaland, …
« Every effect of particles or
radiation can be used as a working
principle for a particle detector. »
Claus Gruppen
3
Physical contants
4
Literature

Particle Data Group, Rev.Part. Phys. PLB 592(2004) 1 (pdg.lbl,gov)

W.R.Leo : Techniques for Nuclear and Particle Physics Experiments, Springer

G.F.Knoll : Radiation Detection and Measurement, Wiley 1999

C.Grupen : Particle Detectors, Cambridge University Press, 1996

K.Kleinknecht: Detectors for particle radiation, Cambridge University Press

D.Green : The Physics of Particle Detectors, Cambridge University Press, 2000

W.Blum, L.Rolandi: Particle Detection with Driftchambers, Springer, 1994

T.Ferbel : Experimental Techniques in High Energy Physics, Addison-Wesley 1987

E.Segré : Nuclei and Particles, W.A. Benjamin, inc

R. Bock, Particle Detector Brief Book : http://rkb.home.cern.ch/rkb/PH14pp/node1.html

R. Fernow, Introduction to Experimental Particle Physics, Cambridge University Press, 1989
5
Some units and conventions
Energy E :
measured in eV
Momentum p measured in eV/c or eV
Mass m0
measured in eV/c2 or eV
1 eV is a small energy.
1 eV = 1.6 10-19 J
mbee = 1g =5.8 1032 eV/c2
vbee = 1 m/s => Ebee = 10-3 J = 6.25 ·1015 eV
ELHC = 14 · 1012 eV
However,
LHC has a total stored beam energy
1014 protons x 14 · 1012 eV ~ 108 J
or, if you like,
one 100 T truck
at 100 km/h
From C.Joram, SSI 2003
6
The Standard Model
Matter - fermions (Spin ½) : Three generations of quarks and leptons
Leptons :
Quarks :
1
2
3
Q= + 1 |e|
e
0.511 MeV
μ
106 MeV
τ
1.78 GeV
Q= 0 |e|
υe
~ < 3 eV
υμ
~ < 0.19 eV
υτ
~ < 18 eV
Q= + 2/3 |e|
u
5 MeV
c
1.5 GeV
t
172 GeV
Q= - 1/3 |e|
d
8 MeV
s
100 GeV
b
5 GeV
NB : Neutrinos νi can be seen in
the dedicated detector only, or
sometimes indirectly.
Probability of interaction PInt.
with matter is small.
"I have done a terrible
thing. I invenated a particle
that cannot be detected.”
W. Pauli
Free quarks have not been
observed (jets).
New quantum number :
Color = R, G, B
Quarks form hadrons cf. QCD :
mesons (qq) bosons : π + = ud, ψ = cc (charmed), K = su (strange)
or
baryons (qqq) fermions : p = uud, n = udd (nucleons), Λ = uds (hyperons)
NB :
All of them also have the corresponding Antiparticles !!!
Mass generation via Higgs mechanism : mH ~ 125.3 GeV
7
The Standard Model
Interactions - bosons (Spin 1) :
- 4 types of interaction (“force”) known in Nature :
Gravitational, Weak, Electromagnetic, Strong
- Interaction via exchange of virtual field quanta aka gauge bosons :
Interactions described
using
« Feynman diagrams »
Examples :
Electromagnetic
interaction
Field Quantum
Gravitational
Graviton ?
mg < 7 10-32 eV
Weak
Z0, W±
mZ0= 91.1876 ± 0.0021 GeV
mW± = 80.385 ± 0.015 GeV
Electromagnetic
photon γ
mγ < 10-18 eV
Strong
Gluons g
Weak
Strong
8
Special relativity kinematics
Relativistic factor of a particle of velocity v and mass m in a reference frame R :
Momentum and energy of free particle :
}⇒
Momentum
Energy
Kinetic energy
4-Momentum :
Invariant square of a 4-vectors :
⇒
9
Special relativity kinematics
Particle collisions :
Conservation laws :
Momentum
Energy
⇔ 4-momentum conservation
With :
10
Cross section σ
Cross section σ or differential cross section dσ/dΩ is an expression of the probability of
interaction. In a very thin slice of matter, this probability is proportional to the thickness of the slice
and to the number of potential target particles per unit volume in the material :
Flux F = number of incident unit/
area / unit particles / time
σ has the dimension area.
Differential cross section :
Total cross section :
11
Luminosity
Beam spot
area A1
Beam spot
area A2
The interaction rate,
Φ1 = N1 / t
Rint = (N1 N2) / (A t) = σ L
Φ2= N2 / t
where L is the luminosity in cm-2s-1.
The integrated luminosity ∫L dt is given in cm-2.
L
cm-2s-1
∫L dt
( T=1 month )
pb-1
1012 p/s on
10 cm liquid H
4 x 1035
106
108 π + p/s on
10 cm liquid H
4 x 1031
100
LEP e+ e- CERN
1.6 x 1031
50
pp (sps) CERN
1029
0.3
pp Tevatron
Fermilab
1030
3
Hera e- p DESY
Hambourg
1.5 x 1031
50
LHC CERN
1034
30 000
n = 1012 p/s on l = 10 cm liquid H
( ρ H = 0.07 g cm-3 ) :
Rint (s-1) = n x ns x l x σ
ns = ρ N / A
L= Rint / σ = n x ρ N / A x l
L = 1012 x 0.07 x 6 1023 x 10
⇒ L ~ 4 x 1035 cm-2s-1
σ (e+e- → μ + μ - ) ~ 50 pb
at 50 GeV
⇒ N ~ 2500 evts/month
12
Cross section σ
Geometric cross section (Nuclear physics) :
Elastic scattering of point “classical” particles by a sphere of radius r
thin target ≡ no overlap
total area A
F particle per time units
No of interactions per units of time per targets P = F × π r2
by definition σ = P / F ⇒ σg = π r2
Nucleus : R0~ 10-14 m ⇒ σnuc = 10-28 m2 : 1 barn = 10-24 cm2
Nuclear physics : 0.16 b ≾ σg ≾ 2.7 b
NB : for inelastic processes and specific quantum-mechanical phenomena, the effective cross
sections may differ considerably from these values ...
13
Cross section σ
Consider a target of thickness e (cm) and N (cm-3) atoms per unit volume.
NB : N = ρ N / A where N ≡ Avogadro number, ρ≡density
Probability of interaction for one particle P(e) :
P(e) = N e σ
P(e) = 1 - exp(- N e σ)
for thin targets (N e σ <<1)
for thick targets
As dF = - F dP = - F N σ de ⇒ F = F0 exp(- N e σ) ⇒ P(e)= ΔF/F0 = 1 - exp(- N e σ)
Then, per unit of time, the number n of interacting particle is :
n = ΔF = F P(e) = F N e σ = ν F σ
Where ν = N e is the number of atoms per surface units
the thickness of targets is often given in g/cm2 : e' = ρ e
⇒ ν = N e' / A
14
Example 1 : Interaction with CMBR.
The Greisen-Zatsepin-Kuzmin (GZK) limit
The universe is opaque for cosmic-ray protons when the resonnant reaction with CMBR
photons becomes energetically allowed :
Estimate of the cut-off energy Ep :
4-momenta of the incoming particle as measured by an observer at rest :
(p hits the γ moving along the x-axis with momentum q at angle θ in the xy-plane)
( bosons
are not
fermions :
<E>=3/2 kT )
NB : Natural units (c=1) are most commonly used
15
Mean free path for nucleons above the GZK limit
The mean free path for a particle in a scattering region λ is defined as the average distance
travelled by the incident particle before hitting a scatterer.
Consider a particle incident on a region of area A containing n scatterers per unit volume :
Each scatterer has a cross section σ
⇒ Probability for a particular scatterer being hit p = σ / A
At least one scatterer is reached ⇒ N p = 1 with N=A λ n ⇒
λ = 1/ (n σ)
Cross section for pγ CMB reaction : σ pγ ~ 10-28 cm-2
Density of microwave photons n = 420 (1+z)3 cm-3
⇒ mean free path of protons with Ep ~ 1020 eV : λ GZG = 2.4 1025 cm = 8 Mpc
NB :
1/ Distance of nearest star (Proxima Centory) ~ 1.3 pc (1 pc=3 1018 cm)
Size of our galaxie ~ 0.1 Mpc (distance between galaxies ~ 1 Mpc)
Size of galaxie clusters ~ 10 Mpc
Distance to Virgo cluster ~ 16.5 ± 0.1 Mpc
2/ Mean energy loss per interaction ΔE = x E ⇒ E1= E0-ΔE = E0 (1-x) ⇒ En= E0 (1-x)n
If x ~ 20 % , almost all the energy is lost after around 100 Mpc ( E12 ~ 7 % E0 )
16
Effects on UHECR spectra
Φ = Φ 0 × E-γ
← 1 part./m2/s
E-2.7
Ground based
experiments
1 part./m2/yr →
E-3
Satellites + balloons
1 part./km2/yr →
E-3.3
All-particle energy spectrum of cosmic rays.
At low energies, the flux of primary protons is
shown.
UHECRs spectra as measured with the
four detectors that have the largest
exposures, namely Yakutsk-AGASA,
Auger, and HiRes
17
Example 2
A/
Over one million of neutrino of 1 GeV crossing the earth, how many will interact ?
σ = 0.7 10-38 cm2 /nucleon, earth radius R=6000 km, density ρ = 5 g/cm3, <A> = 20
B/
1/
Compute the geometric cross section of lead 82208Pb ?
(radius of a nucleon r0=1.3 10-13 cm)
2/ Assuming that the cross section of neutrons at ~ 100 MeV interacting with lead is of the order
of the geometric cross section, compute (in g/cm2) the highest thickness for which the lead target
can be considered as thin.
3/
How many atomic ranges does this correspond (ρ Pb= 11.3 g/cm3) ?
4/ What should be the thickness a lead screen to reduce a neutron beam (at ~ 100 MeV) by a
factor of 100 ?
18
Normal distribution
Mean :
A.Moivre
1667-1754
Variance :
Full Width at Half Maximum :
The 68-95-99.7 rule (or three-sigma rule) : for a normal
distribution, nearly all values (99.73%) lie within 3
standard deviations (σ = √var(x)) of the mean.
If x is an observation from a normally distributed
random variable :
Γ
Thus for a daily process, a 6σ event is
expected to happen less than once in a million
years... This gives a simple normality test: if a
6σ event have been recorded, then a normal
distribution most likely does not provide a
good model for the magnitude or frequency of
large deviations ...
19
Examples for major discoveries
made possible by detector progress
“Good” science needs “good” detectors (and “good” sources)
Advances in detectors → advances in science
(appreciated → Nobel prizes for detector achievements)
Particle (nuclear) physics led the way in detector (and other e.g. WWW) developments which had
major impact on other fields of science, technology, medicine and everyday’s life.
Theory
Analysis
Simulation
Simulation
Experimentation
20
Discovery of positron by C.Anderson
(Nobel prize 1936)
http://prola.aps.org/abstract/PR/v43/i6/p491_1
Cloud Chamber (C.T.Wilson Nobel prize 1927)
C.Anderson
1905 - 1991
Di
co
ve
ry
ph
oto
23 MeV
positron
6 mm lead
plate
Incoming
63 MeV positron
Adiabatic expansion → Saturated vapour
Charged particles → ionisation
→ condensation of droplets
Hypothesis (discovery !) : particle with
mass ~ me and charge +1, the positron
First anti-particle, first instance where
theory indicates the existence of a new
particle that is subsequently found.
B⊗
- ionisation
→ elementary charge
- curvature in mag.field → sign of charge +
qvBR = mv2
measure momentum
- energy loss in 6mm Pb (+ charge and momentum)
→ mass < 20 x me
exclude proton (2000 x me)
- 1300 tracks, 15 show evidence of positrons
21
Imaging techniques: Nuclear Emulsions
❍
A nuclear emulsion plate is a photographic plate with a thick
emulsion layer and uniform grain size.
 It records the tracks of charged particles passing through
 It produce a cumulative record
 The plates must be developed before the tracks can be
observed.
❍
In 1937, Marietta Blau and Hertha Wambacher discovered
nuclear disintegration stars due to spallation in nuclear
emulsions exposed to cosmic radiation at a height of 2,300
meters above sea level.
❍
First evidence of the decay of the Kaon
into 3 Pions was found in 1949 in
Nuclear emulsion
22
Imaging Detectors: the Bubble chamber
(1960 Nobel Prize)
❍ A bubble chamber is a vessel filled with a
superheated transparent liquid (Hydrogen at T=30K).
Discovery of Omega
A charge particle initiate boiling.
❍ Urban history: Glaser was inspired by the bubbles
in a glass of beer.
D.A. Glaser
1926 -
❍ In a 2006 talk he did experiments using beer to fill
early prototypes.
❍
The size of the chambers grew quickly:
 1954: 2.5'' (6.4cm)
Discovery of Omega, Ω- = sss , Confirmation of the quark model
 1954: 4'' (10cm)
 1956: 10'' (25cm)
 1959: 72'' (183cm)
 1963: 80'' (203cm)
 1973: 370cm
Some disadvantages:
 It cannot be triggered
 Low rate capability
 The photographic
readout: for data analysis one
had to look through millions of
photos
❍
BNL, First Pictures 1963, 0.03s cycle
23
First neutral current events.
Garagamelle collaboration (1973).
A.Lagarrigue, A.Rousset, P.Musset
⇒
F.J. Hasert et al. Phys. Lett. 46B (1973) 121-124
Bubble chamber (D.A. Galser, Nobel prize 1960)
The bubble chamber Gargamelle (20 tons of freon) at
the moment of installation into the magnet coils.
First neutral current event, seen in the Gargamelle
bubble chamber :
One candidate found in 360,000 anti-neutrino event
(pictures!)
24
Electronics detectors
❍
In the 70ies the logic (electronic) detectors took over
 Geiger counters
 Scintillator + photomultipliers
 Spark counters
❍ The particle is not “seen” but its nature and existence
“deduced” via a logic experiment (coincidences, triggers,
detection of decay products )
❍ Spark Camber : The Spark Chamber was developed in
the early 60ies. Schwartz, Steinberger and Lederman used it
in discovery of the muon neutrino.
❍ A charged particle
traverses the detector and
leaves an ionization trail.
The scintillators trigger an
HV pulse between the
metal plates and sparks
form in the place where the
ionization took place.
❍ Allows computer
reconstruction
25
Discovery of intermediate vector bosons W±,Z0, UA1
and UA2 at CERN in anti-p p interactions
(1984 Nobel prize C.Rubbia, S.v.d.Meer)
C.Rubbia, van der Meer : upgrade SPS into SppS
(Super proton-antiproton Synchrotron)
• √s = 540 GeV
• 3 p + 3 p bunches, 1011 particles per bunch
• L ~ 5 x 1027 cm-2 sec-1
• development of large volume gaseous detector
with FADC read-out
• Hard to store sufficient p with small phase space
→ stochastic beam cooling
• first collisions in December 1981
S.v.d.Meer
1925 - 2011
e
+
C.Rubbia
1934 -
Z0 → e+ e-
eThe UA1 detector during assembly
mW± = 82.1 ± 1.7 GeV,
mZ0 = 93.0 ± 1.7 GeV
W-→eν :
ν via missint pT
26
Discovery of neutrino oscillations +
detection of neutrinos from SN1987A
(2002 Nobel prize M.Koshiba with R.Davis)
M.Koshiba, 1926 -
Original intention = proton decay :
τ p > 1034 years, Kamiokande (1996-2008)
→ Neutrino oscillation:
- Neutrinos have mass
- Lepton number is violated
→ Neutrino Astrophysics
600 MeV electron
(MC)
Large area photomultipliers
collaboration with industry
(Hamamatsu)
Large volume (50 ktons) H20
Cerenkov detector 12,000 PMTs
27
Discovery of the Higgs boson at CERN in p p interactions
Kibble, Guralnik, Hagen, Englert and Brout
(2013 Nobel prize ?)
Englert, F.; Brout, R.
Higgs P.
Kibble T.W.B.
: http://prl.aps.org/abstract/PRL/v13/i9/p321_1
: http://prl.aps.org/abstract/PRL/v13/i16/p508_1
: http://prl.aps.org/abstract/PRL/v13/i20/p585_1
LHC accelerator + experiments :
40 000 000 croising/s, 4 Peta octets/yr
1500/2000 tracks in a typical event
In order to increase the number of observed interesting events :
- Improved acceptance and reconstruction (and trigger) efficiency
- 4π vs. forward experiment
- Gap free detector
- Unambiguous track reconstruction vs. Efficiency
- eg, Si-detectors with μm resolution :
1981: 50 cm2 Si-detectors
2007: CMS = 200m2 Si-detectors!
NB : Bubble chambers → multiwire proportional chamber
P.Higgs
Georges Charpak = Nobel Prize 1992
1929 –
G.Charpak
1924 – 2010
28
4 July 2012 CMS / ATLAS seminar
held jointly at CERN
Gluon fusion
Higgs
Strahlung
Vector boson
fusion
Top
fusion
← Candidate Higgs decay to 4
e± (ATLAS 2012)
Mass distribution for
the 2-γ channel.
Need to reconstruct tracks (EM clusters for photons)
and measure momenta, energies and identify
particles (charge and mass Hypothesis) !
mH =125.3 ± 0.4 (stat) ± 0.5 (sys) GeV/c
29
Requirements detectors for present and future accelerator experiments
Requirements: (Physics) ⊗ (Parameters of event source [accelerator])
Future after Higgs « discovery » : LHC : Higgs properties = mass,
BR, couplings , etc … Standard or not ? NP ? → 2020
Future : HL-LHC ( L= 5 x 1034 cm-2 sec-1 , 200-300 fb-1/yr) ? HE-LHC
(16.5+16.5 TeV, 20 T magnets) ? ILC (Self-coupling, if NP) ?? LEP3
(e+e- collider at ~ 240 GeV – Higgs-strahlung) ?
30
The sky is opaque at γ-rays : γ-ray astronomy is a
domain of balloons, rockets, satellites!
Detection technique : Pair conversion is the
dominating interaction process for γ-rays (>10 MeV)
Balloon,
rockets,
satellites
H2O CO2 O3
Transparent
A particle detector in space for
gamma-ray astronomy ?
The Fermi Observatory !
O2
O3
Balloon,
rockets,
satellites
Oxygen and Nitrogen
GBM
Gamma-ray Burst
Monitor
NaI and BGO Detectors
8 keV - 30 MeV
Observations of transient
events
LAT
Large Area Telescope
20 MeV to >300 GeV
LAT FoV
All sky survey mode
GBM FoV
KEY FEATURES :
Huge field of view : LAT ~ 20% of the sky (~2.4 sr) at any instant; in sky survey mode, expose
all parts of sky for ~30 minutes every 3 hours. GBM: whole unocculted sky at any time.
Huge energy range : Total of >7 energy decades
31
The GeV-TeV Gamma-ray Sky
Star
Forming
Regions
Supernova
Remants
Pulsars
Fermi-LAT Sky map (2-year, >1GeV)
DM
&
NP
Comes from many standard
astrophysical contributions
Galaxies
Unidentified
??
Modular pair-conversion telescope
4 x 4 array of identical towers
The LAT
ACD
γ
~1.8 m
Incoming
γ
Conversion
(γ in e+/e-)
in W foils
CAL
e+
e-
TKR
Precision Si-strip tracker :
Si-strip detector, W converter foils,
80 m2 of Si active area,
1.5 radiation lengths on-axis.
Hodoscopic CsI calorimeter :
array of 1536 CsI(Tl) crystals in 8 layers.
8.6 radiation lengths on-axis.
Segmented Anti-Coincidence Detector :
89 plastic scintillator tiles and 8 ribbons.
charged particles veto (0.9997 average
detection efficiency).
Incoming direction
reconstruction by
tracking the
charged particles
+identification
Energy
measurement with
e.m. Calorimeter
+ shower imaging
Fermi LAT Collaboration, APJ 697, 1071 (2009)
33
The LAT design
γ
TKR :
18 tracking planes.
Each TKR layer constists of
2 Si layers rotated by 90°
(X,Y) which contains several
thicknesses of W.
0
TKR
12 layers Si +
3% X0 of W
Thin W
e
+
Thick W
e-
Blank (no W)
4 layers Si
+ 18% X0 of W
2 layers Si (no W)
17
0
EM
showers
8 layers CsI
CAL
7
CAL :
Each CAL layer consists of 12 CsI(Tl)
Cesium Iodide (CsI), Thallium doped
crystal (96 crystals per tower, 95 kg).
Hodoscopic, imaging configuration =>
Energy leak correction.
PIN diode read-out on each end.
Large dynamic range (5x105).
Low power consumption.
Minimal dead time (less than 20 μs).
ΔE/E < 10 %
Electromagnetic
Shower leakage
The LAT Calorimeter
The LAT TKR
CDE Components
576 SSD
55K channels
228 µm pitch
EM dual photodiode
Optical Wrap
PIN Diode
(each end)
Bond
CsI Crystal
Wire
leads
End Cap
Readout Cables
Position measurement from
left-right light asymmetry
35
Getting data to the Ground
Instrument Triggering and Onboard Data Flow
Hardware Trigger
On-board Processing
Hardware trigger based on special signals
from each tower; initiates readout
Function:  “did anything happen?”
 keep as simple as possible
Combinations of trigger primitives:
x
x
x
 TKR: 3 x•y pair layers
in a row
workhorse γ trigger
 CAL:
LO – independent
check, energy info.
HI – indicates high
energy event:
Upon a trigger, all subsystems
are read out in ~27µs
Instrument Total Rate: <3 kHz>*
*using ACD veto in hardware trigger
Onboard filters: reduce data to fit within downlink,
provide samples for systematic studies.
Flexible, loose cuts
 Signal/background
zThe FSW filter code is
can be tuned
wrapped and embedded in
the full detector simulation
γ rate: a few Hz
Leak a fraction of
otherwise-rejected events to
the ground for diagnostics,
along with events ID for
calibration
Total Downlink Rate: <~400 Hz> **
On-board science analysis:
transient detection (bursts)
Spacecraft
**current best estimate, assumes compression, 1.2 Mbps allocation. 36
36
Requirements for gamma-ray experiments.
γ-ray atronomy needs (at least!) :
- Good direction reconstruction of the - Large Effective Area – Aeff :
incoming γ-ray (PSF)
Not all entering γ s pair-convert
- Good energy determination
- Large Field of View – FoV
Point-Spread-Function :
2D Point Source Image at 275 MeV
Energy determination :
Issues:
Low Energies : Energy loss in Tracker is critical.
Combining the Tracker with the Calorimeter.
Use Tracker as a (poor) Sampling Calorimeter
Count Hits and Apply Correction for Inter-Tower Gaps.
High Energies : Shower Leakage Corrections are critical
Measured longitudinal profile allows estimation of shower
leakage event-by-event.
SLAC Test Beam Data
PSF Characterized by 68% & 95%
Containment
Shower Tail
escapes out
backside
37
Impact of systematics on
gamma-ray astronomy.
Incoming
γ
Conversion
(γ in e+/e-)
in W foils
Incoming direction
reconstruction by
tracking the
charged particles
+identification
∆θ(deg)
Impact of multiple
scattering on PSF
Energy
measurement with
e.m. Calorimeter
+ shower imaging
Impact of shower
leakage E/E
38
Example of trending LAT-CAL performance and
mapping crystals with atmospheric muons
(SLAC 2005)
LAT cosmic ray run
Methodology :
1/ Collect CR muons crossing the LAT towers
Tkr tracks
2/ Use reconstructed tracks from tkr to select
TaB tracks and compute path length H
3/ Compute corresponding deposited energy for
vertical tracks : △E' = △E × cos θ →
histograms per logs
“noTaB” hit
“TaB” hit
CAL logs
μ μ
△E = dE/dx × H
h
θ
H
△E' = dE/dx × h
4/ For each log, fit a simple Landau function
(black line)
5/ For each module, plot distribution of
Most Probable Values (red histogram) and
compare with MC expectations :
<MPV> = 11.46 ± 0.02 MeV (MC)
39
Trending CAL performance and mapping crystals
Distribution of MPV from
Landau fit for each tower
Map of number of hits per bin
<MPV> = 11.46 ± 0.02 MeV (MC)
Map of mean energy per bin
40
A very simple detector :
key components of a typical scintillation counter
Detection of incident particle (lets say X or gamma-ray) with scintillator + PMT
means relations between :
- Incident particle properties (energy, intensity, …)
- Measured quantities (current intensities, frequency of impulsions, ...)
Chain :
Incident gamma-ray particle → production of charged particles in the scintillator (interaction of
gamma-ray with matter) → energetic electrons
Energy loss from ionizing radiation i.e. ionization and/or excitation of the atoms/molecules of the
material (interaction of charged particles with matter)
Scintillation : emission of visible (or near-visible) light in the cintillators = class of materials which
“scintillate” when excited by ionizing radiation.
NB : Limunescence photons are usually emmited with with time dependence → need integration of
the the signal
Transmission of the scintillation light in the material (i.e. The material must be transparent to its own
radiation)
Collection by total internal reflection
Photomultiplicator tube : Detection of the scintillation light by the photodetector and conversion into
an electrical pulse (amplitude proportional to the number of simultaneously detected photons)
Electronic chain : analysis of the signal
Need good unterstanding of stastistical distributions :
- fluctuations in PMT signal amplitude → effects in energy resolution
- fluctuations in arrival time of the signal (temporal resolution)
41
A very simple detector :
key components of a typical scintillation counter
multiplicator
optics
scintillator
Principle : energy loss → scintillation light → light transport → electric signal
photomultiplicator
Tube (PMT)
measures the light
from the crystal
Key characteristics :
Detector efficiencies
depends on :
light efficiency of the
scintillator
k ~ 13 % (NaI(Tl))
Optical efficiency
Ω ~ 70 %
quantum efficiency of the
photocatod ρ ~ 20 %
Optical efficiency of the
collecting area ν ~ 90 %
photomultiplicator gain
M ~ 106 – 109
electronic amplifier + electronic equipment to
count and quantify the amplitude of the signals
produced by the photomultiplier
scintillator = transparent crystal (phosphor plastic or organic liquid that fluoresces when
struck by ionizing radiation
42
Scintillators
Scintillators are multi purpose detectors :
 calorimetry (HEP, γ-rays detectors e.g. Fermi)
 time of flight measurement (10s psec)
 tracking detector (thin fibers)
 trigger counter (logic “shielding”)
 veto counter
 calorimetry
.....
Two material types:
❍ Organic scintillators : Monocrystals or liquids or plastic solutions
Lower light output but fast
Short decay time (~ns)
Long attenuation length (~m)
Low density (~1g/cm3)
Modest light yield (max 10000 gamma/MeV)
Cheap (1 euro/cm3)
❍ Inorganic crystalline scintillators (NaI, CsI, BaF …)
High light output but slow
Slower decay time (wrt. Org. Scint.)
Higher light yield (wrt. Org. Scint.)
→ good energy resolution
High density, high Z :
→ High stopping power
→ high conversion efficiency
Expensive (e.g. LYSO ~ 100 euros / cm 3)
Response of (some) scintillators is non-linear (Birk’s law 1951)
43
Properties of some
organic scintillators
Aromatic hydrocarbon compounds
containing benzene ring structures
Scintillation :
 based on excitation (and
consequent de-exitation) of
molecular electronic levels.
 inherent to molecular
property → independent on the
physical state (solid, liquid,
vapor, ...)
Fluorescence (τ~10-8- 10-9 s) :
I=I0 exp(-t/τ)
Phosphorescence (τ>10-4 s)
Delayed fluorescence (τ ~ s)
Organic scintillators ~ low Z (H,C) :
→ Low γ detection efficiency
(practically only Compton effect).
→ High neutron detection efficiency
via (n,p) reactions.
44
Properties of some
inorganic scintillators
Scintillation due to the electronic band
structures in crystals. Energy bands in
impurity activated crystal :
Most common inorganic scintillator =
sodium iodide activated with a trace
amount of thallium [NaI(Tl)].
45
Strong dependence of the light output
and the decay time with temperature and magnetic field.
*
NIMA 312 (1992) 451
Temperature :
magnetic field
* 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.
→ for precision pulse-height measurement quite some care has to be taken!
46
Radiation Damage

All crystals suffer from radiation damage - change in crystal response.

Not thought to be a damage to the scintillation mechanism.

The formation of colour centres in the crystal produces absorption bands.
 Colour centres are formed when an impurity atom is displaced from itslattice
position by ionising radiation, into which an electron can drop, causing absorption
bands.

Results in an overall loss in light output.
 I f the photon attenuation length in a given crystal isn’t long enough, radiation
damage will produce a non-uniform light output.

Every crystal is different - impurities introduced during manufacture.

Crystal non-uniformity introduces a constant term to the energy resolution.

Non-uniformity isn’t always bad - the ‘compensation for leakage’ effect.
47
Photo Multiplier Tubes, Light
Collection and Photon Detection
 Mainly rely on photo-electric effetc.
The cathode could be any metal, but would
more likely be an alkali.
 Photo emission from photo cathode.
Q.E. = Np.e./Nphotons
 secondary emission from dynodes.
 dynode gain g = 3 - 50 (f(E))
 total gain M :
Basic principle :
Anode
Photo
Cathode
light
Dynodes
Example : 10 dynodes with g=4 ; M = 410 ≈ 106
PMTs come in many sizes and layouts
- dynode configurations
- multi-channel (pixel) PMTs
- less sensitivity to B-field
48
(External) QE of typical semitransparent photo-cathodes
12.3
3.1
Photon energy Eg (eV)
1.76
1.13
Bialkali: SbKCs, SbRbCsMultialkali: SbNa 2KCs (alkali metals have low work function)
49
Vacuum PMs can resolve single photons
Fluctuations dominated by gain of first dynode Poisson ~1/(gain1)1/2
→ special first dynodes:
single p.e.very useful for calibration and monitoring of detector
50
Scintillator Readout
Scintillator → light → photodetector
Liouville’s theorem: phase space of light conserved (area x angle) :
examples for “flat” light guides (using total internal reflection):
51
Parameters characterizing detectors
A „perfect detector“ which cannot be calibrated is “pefectly useless”
(detector) ⊗ (readout) ⊗ (calibration) ⊗ (analysis) all have to be understood !
❍ Generic detector:
❍ Efficiency:
 acceptance : (recorded events)/(emitted by source): [geometry x efficiency]
 efficiency/sensitivity : (recorded events/particles passing detector)
 peak efficiency : (recorded events in acc.window/particles passing detector)
❍ Response (resolution) :
Ex : spectrum from monoenergetic radiation.
Response to 661 keV γs :
Ge-detector
organic scintillator
Resolution generally defined as 1
standard deviation (1σ) for a
Gaussian distribution, or the
FWHM (Δz).
For a Gaussian 1 σ= FWHM / 2.36
Chapter 5: W.R.Leo
52
❍ Reponse (resolution) continued:
 fact that response function is complicated is frequently ignored → wrong results !!
 “good detector” aims for Gaussian response (with little non-Gaussian tails)
Calibration by N events with energy E
mean:
rms resolution (σ):
for Gauss functions (separate two peaks) :
frequently <S> is not the best choice: e.g. Landau distribution: σ→ ∞
(median, truncated mean, are sometimes better choices ! )
❍ Time response:
 delay time: time between particle passage (event) and signal formation
 dead time: minimum time distance that events can be recorded separately
(depends on properties of detector and electronics (“integrating” or “dead”) and
on resolution criteria)
 pile up effects: overlapping events cause a degradation of performance
 time resolution: accuracy with which “event-time” can be measured
53
Detector Optimization
❍
❍
❍
❍
❍
❍
❍
Which kind of “particle” we have to detect?
What is the required dimension of the detector?
Which “property” of the particle we have to know?
 Position
 Lifetime
 Quantum numbers
 Energy
 Charge
What is the maximum count rate?
What is the “time distribution” of the events?
What is the required resolution ?
What is the dead time?
54
Interactions of particles with matter
no interactions
→ no particle detection
interactions → limit detector performance (efficiency, resolution, particle ident.)
interactions → limit lifetime of detector (radiation damage)
understanding interactions is needed for “daily life” of experimentalist + other
applications (radiation protection, medical diagnosis and therapy,...)
Charged particles :
Neutrons :
X- and γ-rays :
“continuous” interactions with electrons (e.g. ~10 μm in solids)
Bethe-Bloch formula valid for “heavy” particles (m≥mμ).
Electrons and positrons need special treatment
(mproj=m target), in addition Bremsstrahlung !
“rare” interactions producing charged particles ( ~10 cm in solids)
interactions producing e,γ (100 μm – 10 cm – f(energy + material))
Signals in particle detectors are mainly due to ionisation :
Gas chambers
Silicon detectors
Scintillators
Direct light emission by particles travelling faster than the speed of light in a medium :
Cherenkov radiation
Similar, but not identical :
Transition radiation
55
EM interaction of particles with matter
Interaction with the
atomic electrons.
Incoming particles
lose energy and the
atoms are
excited or ionized.
Interaction with the atomic
nucleus. Particles are
deflected and a
Bremsstrahlung photon
can be emitted.
If the particle’s velocity is > the velocity of
light in the medium → Cherenkov
Radiation.
When a particle crosses the boundary
between two media, there is a probability
≈1% to produce an X ray photon →
Transition radiation.
56