calorimetry in particle physics experimentspersonalpages.to.infn.it/~arcidiac/calo_em.pdf · r....
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Calorimetry in particle physicsexperiments
Unit n. 4Electromagnetic calorimeters
Roberta Arcidiacono
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Lecture overview
Main techniques in:● Homogeneous calorimeters● Sampling calorimeters● Some examples here and there...
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Homogeneous Calorimeters
● Pros:Pros:
– excellent energy resolution● Contras:Contras:
– less easy to be segmented laterally and longitudinally (drawback x position meas, particle ID)
– non-compensating
– large interaction length
rarely used as hadronic calorimeters in accelerator physics, very suitable for neutrino/astroparticle physics
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Homogeneous Calorimeters
● Types:Types:
– ČČerenkov calorimeterserenkov calorimeters
– Scintillation calorimetersScintillation calorimeters
– Noble-Liquid calorimetersNoble-Liquid calorimeters
– Semiconductor calorimetersSemiconductor calorimeters
NB: light ⇒ photoelectrons (by photosensitive device)
σ /E ∝ 1 / √ Npe
so, maximization of light yield is important!
scintillation light
electron-hole pairs
Čerenkov light
charge collection
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� erenkov calorimeters
● Usually employed for particle identification (Čerenkov light is a threshold effect → particle speed v > c/n → for a given momentum, depends on particle mass)
● Provide calorimetric measure when collecting all the light produced in the shower
● Light yield is very small (104 smaller than scintillation light):
– only shower tracks above Čer. threshold produce a signal
– ph <300-350 nm does not match well photocathodes window
(300-600 nm)
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Cherenkov light charact. Cherenkov photons spectrum Quantum efficiency of bialkali PMT
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� erenkov calorimeters: materials
● Acc. Phys:
– Lead glass (PbO) widely used (NOMAD, OPAL). Poor radiation resistance
– Newer material PbF2 has smaller radiation length and higher light output, radiation resistant
● Astro-part Phys:
– water tanks, sea water, polar ice and the atmosphere of the earth for solar, atm, cosmic neutrinos and ultra high energy particles in cosmic rays.
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� erenkov radiators
-85755140-PbO content (%)
21.4*
120
1.5
380
1.85
5.57
SF57
3004503803700.5UV absorption edge (nm)
18
193
0.5
_
11.3
Pb
156120108104Interaction length (g/cm2)
2020**26.5*29.9*Interaction length (cm)
0.931.41.62Radiation length (cm)
1.82-1.61.6Index of refraction
7.86.24.073.47Density (g/cm3)
PbF2HeavySF5F5Material
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SuperKamioKande
50 kton of ultra-pure water + 12000 photomultiplier 1000 m underground.
Designed to study solar/atmospheric neutrino interactions,neutrino oscillations
Calor. sensitive to Electrons > 5 MeV (up to TeV) :σ /E ~ 20% for 10 MeV electrons from neutrinos interactions
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SuperKamioKande
solar-n 5- 20 MeVatm-n 100 MeV – 10 TeV
μ-like ring seen in the SK event display
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Scintillation calorimeters
Relevant quantities:
➢ Scintillation spectrum
➢ Light yield : Photoelectrons / MeV
➢ Light decay time
➢ Refractive index n
➢ Transmission curve
Chain: scintillation light ⇒
photodevice ⇒ photoelectron ⇒ signal
PbWO4
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Scintillation calorimeters
● Scintillators types:
– organic fast response - poor light yield● organic solvent + ≤1% scintillating solute: molecules
excitation transferred to solute; occasionally wavelength shifter is added; very fast process (ns)
● used mainly as active components in sampling calorimeters (not very dense)
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Scintillation calorimeters
● Scintillators types:
– inorganic slow response – large light yield● electron-hole pairs produced in the conduction/valence bands;
photons emitted when electrons return to the valence band; large variation in frequency and response time; use of dopants to increase light yield (Thallium)
● light yield several order of magnitude better than Čerenkov calorimeter, nevertheless minimization of light collection inefficiencies is important
● drawback: crystals are not intrinsically uniform, lots of effort in calibration and stability control
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Commonly used crystals
NaI(Tl) = widely employed in the past, low cost, hygroscopic, long radiation lengthBGO = dense material, not good radiation hardnessPbWO4 = “ , radiation hard, very low light yieldCsI = very popular, fast, short radiation lengthCsI(Tl) = increased light yield, slow response
Quality of mass produced crystals have improved a lot in recent years
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Optical characteristics of some crystals
http://www.hep.caltech.edu/~zhu/papers/12_nss_hhcal.pdf
UV absorption edge
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For LHC?
0.89
Rad.Har 1 10 1 105
radiation hardness (Gray, absorbed radiation equivalent to 1 joule/kg) = Total Ionizing Dose causing damages
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Crystals for the future● http://indico.cern.ch/event/125222/contribution/6/2/material/slides/0.pdf
For HL-LHC and beyond:
Studies of effects of high ionizing dose rates, in particular fast hadrons, on: PWO, CeF3 , LYSO
How the light transmission curve changes, or the scintillation light...
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Crystals for the futureFor PWO
For LYSO very modest changes: the crystal of the future!
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Intermezzo: LHC FILLhttps://op-webtools.web.cern.ch/vistar/vistars.php
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BaBar Calorimeter
BaBarBaBar
Homogeneous ECAL CsI(Tl) crystals
Ee+ = 3.1 GeV , Ee- = 9 GeV
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BaBar Calorimeter
● calorimeter technique dictated by BaBar goal of reconstructing ~10 MeV from B rare decays
● CsI(Tl) very large light output (7000 Pe/MeV)
● long decay time (ms) not a problem: rate ~100 Hz
● 6580 crystals, 17X0 , trapezoidal
face 5x5 cm2
● p0 mass reconstruction
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Homogeneous Calorimeters
● Types:Types:
– ČČerenkov calorimeterserenkov calorimeters
– Scintillation calorimetersScintillation calorimeters
– Noble-Liquid calorimetersNoble-Liquid calorimeters
– Semiconductor calorimetersSemiconductor calorimeters
scintillation light
electron-hole pairs
Čerenkov light
charge collection
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Noble-Liquid calorimeters
● In Noble liquids (Ar,Kr,Xe) charged particles lose ~ In Noble liquids (Ar,Kr,Xe) charged particles lose ~ half energy in ionization half energy in ionization (charge drift – slow signal)(charge drift – slow signal) and and half in scintillation half in scintillation (fast signal)(fast signal)
● Best resolution when collecting both signals● No large scale calor. based on both readout built
● Excellent energy resolution even when collecting only ionization charge
Total Signal N = Nion + Nscint
Fluctuations on Nion (from Binomial Stat) = s(Nion)=√ N* (Nion/N)*(Nscint/N)
s(Nion) ~ 0.4-0.5√N factor 2 better than the expected 1/√N
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Noble-Liquid calorimeters
● Pros:Pros:
– high charge without electron amplification, so better response uniformity
– good radiation resistance
– good uniformity (liquid!)● Contras:Contras:
– requires cryogenics (so extra dead material in front of calorimeter) and purification system
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Noble Liquid Characteristics
LAr mostly employed in sampling calorimeters – low costLKr preferred for homogeneous cal.LXe would be even better BUT very expensive
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Noble-Liquid calorimeters
History:
In early 1970s, introduced liquid argon as active medium.
Successful: has been used in many fixed target and collider experiments (R807/ISR, MARK2, CELLO, NA31, SLD, HELIOS, D0, HERA, ATLAS).
In 1990 D. Fournier introduced a novel design for a LAr calorimeter, the so-called ”accordion” [ no dead space between towers and provides better uniformity of response and fast signal extraction]
The ”accordion” was adopted by NA48 (LKr) and ATLAS (LAr).
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Sampling Calorimeters
● Types:Types:
– Scintillation calorimetersScintillation calorimeters
– Gas calorimetersGas calorimeters
– Solid-state calorimetersSolid-state calorimeters
– Noble-Liquid calorimetersNoble-Liquid calorimeters
scintillation light
charge collection
charge collection
charge collection
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Sampling Calorimeters
● Easy to segment longitudinally/laterallyEasy to segment longitudinally/laterally
– offer better space resolution & particle ID● Common absorbers: Common absorbers:
– lead, iron, copper, uranium● Mostly used as Hadron Calorimeters
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Scintillation calorimeters
● Large number of sampling calorimeters use organic (plastic) scintillators arranged in fibers or plates
● fastfast response
● good light yieldgood light yield
● can be compensated (hadron calorimetry)
● drawbacks: aging, radiation damage, large constant term (non uniformities in light collection chain)
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Gas calorimeters
● low cost; segmentation flexibilitylow cost; segmentation flexibility
● widely employed until recently (e.g LEP)
● modest energy resolution modest energy resolution <<≈≈ 20%/ 20%/√ √ E (GeV)E (GeV) – Landau fluctuactions + path length variation in the active
layer
● due to gas low density, sampling fraction sampling fraction ≪ ≪ 1%1% ⇒ gas operated in proportional mode (large voltage on wire to produce avalanche multiplication – gain 103-105)– modest stability and uniformity of response
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Solid-state calorimeters
● active medium: silicon (most cases), very dense material compact devices
● 3.6 eV to produce electron-hole pair (no gain needed), to be compared to 30 eV for Gas
● high cost (not used in large scale detectors), poor radiation resistance → well, not until now....
● widely used as luminosity monitors for LEP detectors
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Noble-Liquid calorimeters
● as already said stable & uniformstable & uniform response, good good energy resolutionenergy resolution ( <<≈≈ 10%/ 10%/√ √ E (GeV)E (GeV)), radiation radiation hard, easy to calibratehard, easy to calibrate
● but need:
– cryogenic system
– careful control of liquid purity
...we now consider LAr sampling calorimeters
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Noble-Liquid calorimeters
● standard sampling:standard sampling: layers perp. to particle direction; absorbers @ GND, electrods in the LAr gap @HV
● drift time (for 2mm gap at 2kV) ~ 400 ns; signal is integrated for tp ~ 40-50 ns; signal transfer time must be small
● long cable needed ->tiny signal collected
● accordion electrodes:accordion electrodes: layers are parallel to particle direction; accordion geometry prevent particles from escaping through the active gaps without crossing the absorber
● minimize cables and dead spaces inside calorimeter; better signal/noise ratio
electrods
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ATLAS EM calorimeter
Lead-LAr layers in the rapidity region ||<3.2
200000 readout channels. Almost fully analog readout chain
Energy res 10%/√ E (GeV) + 0.17%
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ATLAS EM calorimeter
● 3 longitudinal regions:
– fine strips in direction (4mm)
– 2x4cm2 towers
– 4x4cm2 towers
● Complete Φ symmetry without azimuthal cracks
● Good particle ID
● Liquid response has strong temperature dependence: temperature distribution checked to be uniform to a fraction of degree