Diapositiva 1

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Calorimetri elettromagnetici a cristalli per la
fisica delle alte energie
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Indice

• Introduzione
• Interazioni di elettroni e fotoni con la materia
• Sciami elettromagnetici
• Calorimetria elettromagnetica di precisione
• Calorimetri a cristalli
• Il calorimetro di CMS a LHC

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Modern milestones
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Crystal Ball _at_ SPEAR - Stanford

• 672 60 NaI crystals
• PM read out
• Eg range 0.1? 1 GeV

The first crystal calorimeter pioneering most of
the features of modern barrel calorimeters

• energy resolution
• 3.5 _at_ 300 MeV
• 2.6 _at_ 1 GeV
• solid angle 93 over 4p

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Crystal Ball cc system transitions
1974 J/Y discovery
precision in g energy see peaks
charmonium spectroscopy ee- ? ? ??X
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Replicas for ? at CESR

• CUSB (NaI) and CUSB II (NaIBGO)

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Large Hadron Collider Higgs hunt
Only precision in g detection will tell a peak (H
?gg signal) from a huge background
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Calorimeters a simple concept
particle showers
electric
optical
thermic
acoustic
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Calorimeters some features
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Energy losses by e g
In matter electrons and photons loose
energy interacting with nuclei and atomic
electrons

• Electrons
• ionization (atomic electrons)
• bremsstrahlung (nuclear)
• Photons
• photoelectric effect (atomic electrons)
• Compton scattering (atomic electrons)
• pair production (nuclear)

Above 1 GeV radiative processes dominate energy
loss by e/?
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Electrons

• ? ? Z ? ? ln E/me

• ? ? Z(Z1) ? ? A/X0 Egt1 GeV, ? ? ln
  E/me Elt1 GeV

Lunghezza di radiazione X0 spessore di materiale
che riduce lenergia media di un fascio di
elettroni di alta energia di un fattore e. Per
materiali densi Xo 1 cm.
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Electrons

• Critical energy Ec

(solids, liquids)
Strongly material dependent, it scales as 1/Z
(eg. 7 MeV for lead)
Gli elettroni irraggiano fotoni finchè la loro E
non diventa minore dellenergia critica
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Photons

• photo-electric effect

? ? Z5 , E-3.5

• Compton scattering

? ? Z , E-1

• pair production occours if E? gt 2mec2

• ? Z (Z1) ? lnE/me for Elt 1GeV, constant E
  gt1GeV
• Probability of conversion in 1X0 is e-7/9
• Define a m.f.p. Lpair 9/7 X0 (g disappears)

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Electromagnetic showers
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Electromagnetic showers
Above 1 GeV the dominant processes,
bremsstrahlung for e and e- and pair production
for g, become energy independent
Through a succession of these energy losses an
e.m. cascade is propagated until the energy
of charged secondaries has been degraded to the
regime dominated by ionization loss (below Ec)
Below Ec a slow decrease in number of particles
occurs as electrons are stopped and photons
absorbed
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EM showers a simple model

• In 1X0 an e loses about 2/3 of its E
• and a high energy g has a probability
• of 7/9 of pair conversion
• Assume X0 as a generation length
• In each generation the number of
• particles increases by a factor 2

E0
E(tmax) Ec E0 / 2tmax Ec
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EM showers longitudinal profile
tmax 1.4 ln(E0/Ec)
Ntot ? E0/Ec
Longitudinal containment
t95 tmax 0.08Z 9.6
Shower parametrization
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EM showers transverse profile
Transverse shower profile

• Multiple scattering make electrons move away
  from shower axis
• Photons with energies in the region of minimal
  absorption can travel
• far away from shower axis

Molière radius sets transverse shower size, it
gives the average lateral deflection of critical
energy electrons after traversing 1X0
75 E0 within 1RM, 95 within 2RM, 99 within
3.5RM
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EM showers transverse profile
Central core multiple scattering
Peripheral halo propagation of less attenuated
photons, widens with
depth of of the shower
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EM showers energy loss detection
The energy deposited in the calorimeters is
converted to active detector response

• Evis ? Edep ? E0

• Main conversion mechanism
• Cerenkov radiation from e
• Scintillation from molecules
• Ionization of the detection medium

Different energy threshold Es for signal
detectability
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EM calorimeters energy resolution
Intrinsic limit
Detectable signal is proportional to the total
track length of e and e- in the active material,
intrinsic limit on energy resolution is given by
the fluctuations in fraction of initial energy
that generates detectable signal

• maximize fs
• minimize Z/A

Fix E0
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EM calorimeters energy resolution
Compare processes with different energy threshold
Lowest possible limit
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Crystal calorimeters
Among different types of calorimeters those with
scintillating crystals are the most precise in
energy measurements

• Excellent energy resolution (over a wide range)
• High detection efficiency for low energy e and g
• Structural compactness
• simple building blocks allowing easy mechanical
  assembly
• hermetic coverage
• fine transverse granularity
• Tower structure facilitates event reconstruction
• straightforward cluster algorithms for energy
  and position
• electron/photon identification

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Classic calorimeters a comparison
resolution ()
Energy (GeV)
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Crystals building blocks
These crystals make light!
Crystals are basic components of electromagnetic
calorimeters aiming at precision
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Scintillation a three step process
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Scintillating crystals
Edep ? e-h Es b Eg bgt1 Neh Edep / bEg
Eg
Efficiency of transfer to luminescent centres
radiative efficiency of luminescent centres
?g Ng / Edep SQNeh / Edep SQ/ bEg

• S, Q ? 1 , bEg as small as possible
• medium transparent to lemiss

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Scintillating crystals
Variation in the lattice (e.g. defects and
impurities) ? local electronic energy levels in
the energy gap
If these levels are unoccupied electrons moving
in the conduction band may enter these centres

• The centres are of three main types
• Luminescence centres in which the transition to
  the ground state
• is accompaigned by photon emission
• Quenching centres in which radiationless thermal
  dissipation of
• excitation energy may occur
• Traps which have metastable levels from which
  the electrons may
• subsequently return to the conduction band by
  acquiring thermal
• energy from the lattice vibrations or fall to
  the valence band by
• a radiationless transition

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Scintillating crystals
PbWO4 lexcit300nm lemiss500nm
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Scintillator parameters
relative importance depends on the application
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Crystals for HEP experiments
The perfect crystal does not exist the search
must go on!
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Crystals growing technology
Optimization and equalization of crystals
properties may take few years of efforts
Czochralski method
seed
RF heating
It is not a supermarket object!!!
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Crystal calorimeters

• Large number of channels
• uniformity of crystals, quality control
• Long period of operation
• response stability, monitoring
• Dynamic range
• appropriate electronics
• High magnetic field
• solid state photo-detectors
• High Luminosity pile-up, multiplicity
• fast response, small transv. dimens. (RM)
• Radiation environment
• radiation hardness

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Crystal calorimeters
Design guidelines and major improvements

• CMS
• Best electromagnetic resolution up to TeV
  energies
• Extremely good lateral granularity
• Fast response
• PWO (new) increase the light
  output
• develop APD
• improve radiation resistance
• Belle BaBar
• Excellent resolution down to 100 MeV
• Optimize p0 detection
• CsI(Tl) minimize electronic noise
• improve uniformity
• KTeV
• Minimize uncertainty on acceptance for K?p0p0
  
• Control absolute energy scale
• CsI (pure) optimize uniformity and response
  linearity

H ???
B ? n?? X B ? e X
CP
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Volume for H energy multiplicity calo

• To resolve nearby particles with a fixed angular
  separation D?
• (the transversal dimensions of a crystal is ? Rm)

r
Dx rD?
Dx Rm
D?
l Xo

• r ? Rm/D?
• Vcrystals Rm2Xo
• Small D? ? large dimensions and costs
• Issue find new dense materials
• (side effect ? high refraction index)

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EM calorimeters energy resolution
Energy resolution of a calorimeter can be
parametrised as

• a the stocastic term accounts for any kind of
  Poisson-like fluctuations
• natural merit of homogeneous calorimeters
• several contributions add to the intrinsic one
• b the noise term responsible for degradation of
  low energy resolution
• mainly the energy equivalent of the electronic
  noise
• contribution from pileup the fluctuation of
  energy entering the
• measurement area from sources other than the
  primary particle
• c the constant term dominates at high energy
• its relevance is strictly connected to the small
  value of a
• it is mostly dominated by the stability of
  calibration
• contributions from energy leakage, non
  uniformity of signal
• generation and/or collection, loss of energy in
  dead materials,

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Energy resolution stocastic term
In scintillating crystals the only intrinsic
source of fluctuations is photostatistics
where the Light Yield of the scintillator is only
one factor, as Npe/GeV (g/GeV)?(light
collection eff.)?(geometrical PD
eff.)?(photocathode eff.)   Other sampling-like
sources of fluctuations Lateral
containment Material in front A kind of biased
sampling
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Light collection efficiency
?K? Kr wrapping
?R? (1 R)
Reflection in the transition radiator-PD
?LY ? ?a ? ?K ? ?r ? ?R
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Energy resolution constant term
c (leakage)?(intercalibration)?(system
instability)?(nonuniformity)To have c ? 0.5
all contributions must stay below 0.3
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Energy resolution constant term

• Intercalibration
• requires several steps before, during and after
  data taking
• test beam precalibration
• continuous monitor
• absolute calibrations by physics reactions
  during the
• experiment lifetime

L3 BGO experience
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Energy resolution constant term

• System instabilities
• temperature dependence of the LY (in /oC) at 18
  oC
• stability of the read-out chain
• Electronics gains
• e.g. Avalanche Photo Diodes dM/dV 3/V
• 
  dM/dT -2.3/oC
• global radiation damage effects

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Energy resolution constant term

• Longitudinal non uniformity
• Intrinsic non uniformities
• inhomogeneities in active medium
• variation of doping concentration
• Temperature gradient when LY f(T)
• Light collection (first observed in L3)

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Non uniformity of light collection

• Non linearity of the response
• (can be corrected)
• smearing of the response at fixed
• energy due to shower fluctuations
• (can not be corrected)

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Position resolution

• Reconstruction of invariant masses of particles
• decaying into photons, electron identification
  using
• match with track measured in tracking devices
• Impact position of showers is determined using
  the
• transverse (and longitudinal) energy
  distribution in
• calorimeter cells
• Method based on center of gravity (COG)
  calculation
• works for projective geometry and particles
• coming from the interaction vertex
• calorimeter cell size d ? 1RM
• Typical resolutions few mm/?E

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Position resolution