)meter

The BaBar CsI(TlElectromagnetic Calori

Jane TinslayBrunel University

3rd December 1999

Contents

❍ Introduction

❍ Physics of Electromagnetic Calorimetry

❍ Particle interactions with matter❍ Electromagnetic showers❍ Scintillation and radiation damage processes❍ Energy resolution

❍ The BaBar Calorimeter

❍ Design and construction❍ Reconstruction❍ Calibration❍ Current performance

❍ Conclusions

primary particle is

s with the

nto scintillation

l.

all-NaI(Tl),KTev-O4.

Introduction

✴ A device which measures the total energy deposited by a called a calorimeter.

✴ The primary particles energy is degraded by interaction calorimeter material.

✴ A scintillating crystal calorimeter converts this energy i light.

✴ The scintillation light is converted to an electrical signa

✴ Electrical signal proportional to primary energy.

✴ Other experiments using crystal calorimeters- Crystal BCsI, L3-BGO, CLEO, BELLE, BaBar-CsI(Tl) ,CMS -PbW

5.5 charged tracks.

Why does BaBar need a calorimeter ?

✴ Generic B decays contain an average of 5.5 photons and

✴ ~50% of photons < 200 MeV.

✴ Need excellent energy, positionresolution in order to reconstructπ0s.

✴ Need excellent photon detection efficiencies down to low energies

(20 MeV).

✴ Charged, neutral PID.

✴ Crystal calorimeter most suited - best energy resolution

tter

dEdX----

EX0----≅

Particle Interactions With MaElectron/Positron Processes

✴ At low energies, ionisationdominates.

✴ At high energies, bremsstrahlungdominates.

✴ Radiation length,

Nucleus

e-

e-

γBremsstra hlungMean distance over which all

but 1/e of energy is lostdue to bremsstrahlung.

X0716.4gcm

2–A

Z Z 1+( ) 287 Z( )⁄( )ln-----------------------------------------------------------=

ionisation is the

gth is equal to the

✴ Critical energy, Ec = energy at which the energy loss by same as the loss by radiation.

OR

✴ Ec = energy at which the ionisation loss per radiation len electron energy.

Ec800MeVZ 1.2+

----------------------=

Ec610MeVZ 1.4+

----------------------=

Nucleus

Pair Product ion

γ e-

e+

Nucleus

γ

e-

γCompton Scatter ing

Nucleus

γ

e-

Photoelectric effect

Photon Processes

e MeV range, cross reaches a mini-o low energy pho-ay travel a long wayinteracting.

I 0eµt–( )

=

µ 1λ---=

✴ Mean free path at high energies

CsI

λ 97---X0=

✴ In thsectionmum stons mbefore

I

tromagnetic shower

produced by the

e-

γ

e+

e-

γ

e-

e-

γ

e-

e+ e+

γ

2 3

Electromagnetic showers

✴ When a photon/electron/positron enters a crystal an elec is initiated.

✴ A cascade of secondary photons, electrons and positrons particle interactions described previously.

✴ Average shower properties described by simple model.

✴ Define the scale variables:

e-γ

e-

1X0 0

tx

X0-------=

yE0Ec-------=

E0Ec------- y≈

)----

After t generations...

N t( ) 2t

=

E t( )E0

2t

-------=

L23--- N t( ) td

0

tmax∫ ≈=

Number of particles present

Average energy of a shower particle

tmax

E0 Ec⁄(ln

2ln-------------------------=Shower Maximum

Total track lengths of all chargedparticles

m Monte Carlo

i

tainty in the

✴ An over simplified model - more accurate predictions frotechniques.

Ed

td------ E0

bt( )a 1–e

bt–

Γ a( )---------------------------------=

tmax a 1–( ) b⁄ yln C+= =

Longitudinal profile

Where Ce=-0.5, Cγ=+0.5, b =0.5

✴ Photon induced showers are longer because of the uncer position of the first pair production.

1X0.

far depending on

l size to scale withtal dimensions.

er with a radius of

Lateral Profile

✴ Exponential profile.

✴ Up to tmax shower, contained in a cylinder with radius <

✴ Soft photons found near the end of shower - may travel cross sections.

✴ After tmax, multiple scattering of electrons causes latera the Moliere radius - determines optimal transverse crys

✴ Roughly 95% of a shower is contained laterally in a cylind 2Rm

Rm

X0 21.2Mev×

Ec------------------------------------=

16 18

1.0 Gev γ0.5 Gev γ0.1 Gev γ

om front face (X0)

16 18

5.0 Gev e-

5.0 Gev γ

om front face (X0)

0

2.5

5

7.5

10

12.5

15

17.5

20

22.5

0 2 4 6 8 10 12 14

Depth into crystal fr

Ave

rage

ene

rgy

depo

site

d/0.

18X 0 (M

ev)

0

20

40

60

80

100

0 2 4 6 8 10 12 14Depth into crystal fr

Ave

rage

ene

rgy

depo

site

d/0.

18X 0 (M

ev)

Damage

ermined by

energy levels -

rgy levelsity

CsI(Tl) Scintillation and RadiationMechanisms

Scintillation Mechanism

✴ CsI(Tl) is an inorganic scintillator.

✴ Scintillation mechanism depends on energy states - det the structure of the crystal lattice.

✴ Well defined valence and conduction bands.

✴ Impurities, called activators (Thalium) added to modify increase the probablility and λ of scintillation light.

Exciton

Valence band

Conduction band

Hole

Electron

Modified enedue to impur

Exciton band

previous diagram).

- Impurity is in aded to excite it to a

✴ Fluorescent light with a relatively short decay time (see

✴ Phosphorescent light (afterglow) with a long decay time metastable state. Extra energy (eg, thermal energy) nee level where it can return to ground state.

✴ Issues:

Transparent to own radiation Decay time Radiation hardness Light Yield Wavelength of scintillation light Radiation length Ease of manufacture Cost

stal response.

ism.

absorption bands.

Radiation Damage✴ All crystals suffer from radiation damage -change in cry

✴ Not thought to be a damage to the scintillation mechan

✴ The formation of colour centres in the crystal produces

placed from itsctron can drop,

long enough,ut.

manufacture.

e energy resolution.

r leakage’ effect.

✴ Colour centres are formed when an impurity atom is dis lattice position by ionising radiation, into which an ele causing absorption bands.

✴ Results in an overall loss in light output.

✴ If the photon attenuation length in a given crystal isn’t radiation damage will produce a non-uniform light outp

✴ Every crystal is different - impurities introduced during

✴ Crystal non-uniformity introduces a constant term to th

✴ Non-uniformity isn’t always bad - the ‘compensation fo

calorimeter will

nergy

E

BE----

2C

2+ +

Energy Resolution

✴ A beam of mono-energetic photons incident fired into ahave a spread in measured energy.

✴ Energy resolution :σEE

-------

E

σ✴ Contributions:

Fluctuations in scintillation photons stats Fluctuations in photo-electron stats Noise-coherent+incoherent Leakage fluctuations Calibration Front material (efficiency) backgrounds non-uniform light collection

σE----

2 A

E--------

2=

stal wrappings, two

3mr

E----------

22mr

2+

A = Shower+readout fluctuationsB = NoiseC = Calibration, leakage, non-uniformity, dead material

BaBar

✴ Achieved through high crystal light yield, reflective cryphotodiodes, very low electronic noise.

σE----

2 0.01

E4----------

2

0.0122

+=

σθ( )2 =

Angular resolution

✴ Determined by the transverse crystalsize and average distance to the interactionpoint.

The BaBar CalorimeterDesign and Construction

with 120 crystals inφ.3*7 crystals.ow - 6 crystals inθ)tric tonnes.

ϑ( )os 0.715≤ (CM)

180.9 cm

e+e–

91 cm15.8°

140.8°

IP26.9°

112.7 cm

EMC locatedasymetricallyabout interaction point.Non-projective crystal geometry by15-45mr in θ - minimise lost photons.

0.775– ϑ( )cos 0.962≤ ≤

Barrel :

5760 crystals.48 θ rows, each 280 modules of (except for last rWeighs 23.5 me

0.916– c≤

(lab)

(CM)0.916– ϑ( )cos 0.895≤ ≤

n.n 20 modules.

contains 41

wrt the vertical.er of crystals inφ

0-120.

ge in CM:

lead-shielding.

etric tonnes

ϑ( )cos 0.895≤

Endcap:

Conic Sectio820 crystals i

Each modulecrystals.

Tilted at 22.7o

8 θ ring-numbvaries from 8

Angle covera

Inner ring of

Weighs 3.2 m

0.718≤

the back barrel.

nd side faces),

plifiers.

ystal (negligible).

Crystals:

✴ Trapezoidal in shape.

✴ Range in length from 17.5 X0 in the EndCap, to 16 X0 in

✴ Typical front face dimensions 4.7*4.7 cm2

✴ Typical back face dimensions 6.0*6.0 cm2

✴ Tolerances: 250 µm transversely, 1mm longitudinally.

✴ Crystal wrappings: two 150 µm layers of Tyvek (front a one layer of 30 µm aluminium foil.

✴ Read out by two silicon PIN diodes, connected to pream

✴ Implements two independent electronics chains - reliability + signal/noise.

✴ With digital filtering incoherent noise = 150 KeV per cr

✴ Noise dominated by beam backgrounds.

ing/polishing

✴ Crystals are tuned for uniform light collection - roughen surface.

PbWO4

8.280.89

2.2

5-15440-500

0.05

no

Properties CsI(Tl) CsI

Density (g cm-3) 4.53 4.53Radiation

length (cm)1.85 1.85

Moliere radius(cm)

3.8 3.8

Decay time (ns) 940 10Peak Emission

λ (nm)565 320

Light yield(Photons/Mev*103)

50-60 2-10

Hygroscopic slightly slightly

uorocarbon whichng in 6.13 Mev

stals.

nearity.

re dose.

Features

Radioactive source system

✴ Thin tubes on the front face of the calorimeter carry a fl has been excited by neutron irradiation - decays resulti photons.

✴ Used as a calibration and monitoring tool.

Light pulser system

✴ Xenon flash lamps feed light onto the rear face of all cry

✴ Used as a monitoring and diagnostic tool - electronics li

RadFETs

✴ 56 in barrel, 60 in End Cap - small devices which measu

Modules made from 300 µm CFC,and supported from the rear byan aluminium strongback.Mounted in an aluminium sup-port cylinder-supported off coil.Cooling and cables located at the back of the modules.

20 MeV.

k for local maxima.

ed under a

Reconstruction

✴ Digis->Clusters->Bumps->Cands

✴ Digis: Crystals with >1 MeV (TDR=0.5 MeV).

✴ Cluster: Collection of adjacent digis. Summed energy >

✴ Bump: Split each cluster up into one or more bumps-loo

✴ Cand: Energy+position corrected bump or cluster, select particular particle hypothesis.

PID

✴ Charged: e-, µ, π − shower shapes, E/P

✴ Neutral: γ, merged π0, K0L - shower shapes

ree calibrations:

onse using a charge

an individual crys-

de out of clusters ofy due to clustering,

anges - eg, radiation

h should

Calibration

✴ Scintillation light -> final particle energy depends on th

✴ Electronics calibration: Linearises the electronics respinjection system.

✴ Inter-Crystal calibration: Determines the response of tal. Calibrates to the deposited energy. Time dependent.

✴ Cluster calibration: Applied to candidate particles macrystals. Sets the final energy scale. Corrects for lost energleakage. Time independent.

✴ Monitoring - light pulser + calibration methods track chdamage.

Inter-Crystal Calibration

✴ Different response depending on incident energies whic (if the crystals are reasonably uniform) be small.

✴ Fixes calibration points along a calibration curve.

Energy

Techniques:

✴ Bhabhas✴ High energy point (3->9 GeV)✴ Well known topology.✴ High rate (200 hits/crystal = ~8 hours

data taking at nominal luminosity)

✴ Source✴ Low energy point (6.13 MeV)✴ Fast (doesn’t depend on luminosity !).✴ Has seen changes in crystal response.

✴ Radiative Bhabhas - intermediateenergies.

cons

tant

n - data.

Cluster Calibration

✴ Theta, particle type dependent.

✴ MC derived correction.

✴ Pi0Calibrator - shifts pi0 mass peak to correct positio

✴ Polynomial derived correction.

✴ Should be relatively stable over time.

ht pulser).

wer supplies,

eV.

dose of ~50 rads int is 10 krad over 10

Current Status

✴ Problems at startup:✴ Coherent noise.✴ Electronics non-linearity (diagnosed by lig

= High digi cut = worse resolution.

✴ Major improvements with coherent noise-torroids on po oscillating capacitor removed.

✴ Digi cut now at 1 MeV (was at 5 MeV), TDR aim is 0.5 M

✴ Pi0 cluster calibrator derived from data implimented.

✴ Beam backgrounds not as bad as expected - accumulated hottest crystals (improved vacuum, collimators). Budge years

✴ 10% difference in source and bhabha constants.

E measured / E expected1 1.1 1.2 1.3 1.4 1.5

BABAR

(fwd brl)

E measured / E expected deposited0.6 0.7 0.8 0.9 1 1.1 1.2 1.3

En

trie

s/B

in

0

200

400

600

800

1000

1200

1400

1600

1800Constant = 1674

Mean = 1.016

Sigma = 0.02214

EMC Bhabha Clusters

Forward Barrel

BaBarConstant = 1674

Mean = 1.016

Sigma = 0.02214

0.5 0.6 0.7 0.8 0.9

En

trie

s

0

500

1000

1500

2000

2500Constant = 2227 +- 14.12

Mean = 0.9963 +- 0.0001326

Sigma = 0.02441 +- 4.441e-05

Constant = 2227 +- 14.12

Mean = 0.9963 +- 0.0001326

Sigma = 0.02441 +- 4.441e-05

MC E/E_expected

mγγ (GeV)0.5

ar

.2 MeV

MeV

0 0.1 0.2 0.3 0.4

Ent

ries

0

500

1000

1500

2000

2500

3000

3500

π0 Mass E γγ > 500 MeV

BaB

π0-mass = 134

π0-width = 7.7

(MC gives 4-6 MeV width)

y the

r performance

Conclusions

✴ CsI(Tl) Crystal calorimeter should deliver the necessar photon/electron energy resolution required.

✴ Detector is functioning - big improvements with detecto - still things to understand...

lmut Marsiske)

5, lectures by Rich-

ernow

.

Bibliography

SLUO Lecture Series - Lectures 13, 14 (Jim Brau), 15 (He

PDG - Passage of particles through matter

Techniques and Concepts of High Energy Physics VI -p.32ard Wigmans.

BaBar Physics Book, TDR

Introduction to Experimental Particle Physics - Richard F

Radiation Damage: Papers by Ren-yuan Zhu, H. Chowdry