calorimetry in particle physics experimentspersonalpages.to.infn.it/~arcidiac/calo_em.pdf · r....

Calorimetry in particle physicsexperiments

Unit n. 4Electromagnetic calorimeters

Roberta Arcidiacono

R. Arcidiacono Calorimetria a LHC 2

Lecture overview

Main techniques in:● Homogeneous calorimeters● Sampling calorimeters● Some examples here and there...


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



● 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


� 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)


Cherenkov light charact. Cherenkov photons spectrum Quantum efficiency of bialkali PMT


� 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.


� 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


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


SuperKamioKande

Detector during water filling


SuperKamioKande

solar-n 5- 20 MeVatm-n 100 MeV – 10 TeV

μ-like ring seen in the SK event display


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



● 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)



● 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


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


Optical characteristics of some crystals

http://www.hep.caltech.edu/~zhu/papers/12_nss_hhcal.pdf

UV absorption edge


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


Famous Crystal Calorimeters


Light Output Temperature Coefficient


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...

http://indico.cern.ch/event/125222/contribution/6/2/material/slides/0.pdf


Crystals for the futureFor PWO

For LYSO very modest changes: the crystal of the future!


Intermezzo: LHC FILLhttps://op-webtools.web.cern.ch/vistar/vistars.php


Intermezzo: LHC FILL


BaBar Calorimeter

BaBarBaBar

Homogeneous ECAL CsI(Tl) crystals

Ee+ = 3.1 GeV , Ee- = 9 GeV


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



● Types:Types:

– ČČerenkov calorimeterserenkov calorimeters



– Semiconductor calorimetersSemiconductor calorimeters

scintillation light

electron-hole pairs

Čerenkov light

charge collection


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



● 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


Noble Liquid Characteristics

LAr mostly employed in sampling calorimeters – low costLKr preferred for homogeneous cal.LXe would be even better BUT very expensive



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).


Sampling Calorimeters

● Types:Types:


– Gas calorimetersGas calorimeters

– Solid-state calorimetersSolid-state calorimeters


scintillation light

charge collection

charge collection

charge collection


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



● 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)


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


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



● 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



● 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


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%


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


From the PDG book

calorimetry in particle physics experimentspersonalpages.to.infn.it/~arcidiac/calo_em.pdf · r....

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