Lecture 13: Detectors

• Visual Track Detectors

• Electronic Ionization Devices

• Cerenkov Detectors

• Calorimeters

• Phototubes & Scintillators

• Tricks With Timing

• Generic Collider Detector

Section 3.3, Section 3.4

Useful Sections in Martin & Shaw:

Wilson Cloud Chamber:

Antimatter

Anderson

1933

Evaporation-type Cloud Chamber:

Photographic Emulsions

νµ

νeπ−

µ−e−

Discovery of the Pion

(Powell et al., 1947)

DONUT (Direct Observation of NU Tau) July, 2000

Donald Glazer (1952)

Bubbles form at nucleation sites

in regions of higher electric fields

⇒ ionization tracks

Bubble Chamber

Liquid superheated by sudden expansion

Bubbles allowed to

grow over ∼ 10msthen collapsed during

compression strokehydrogen,

deuterium,

propane

Freon

High beam

intensities

swamp film

Acts as both

target & detector

Slow repetition rate

Spatial resolution

∼100−200 µm

Track digitization

cumbersome

Difficult to trigger

Mechanically

Complex

Electric field imposed to prevent recombination

Medium must be chemically inactive (so as not to gobble-up drifting electrons)

and have a low ionization threshold (noble gases often work pretty well)

Ionization Detectors

signal smaller than initially

produced pairs

signal reflects

total amount

of ionization

initially free electrons

accelerated and further

ionize medium

such that signal is

amplified proportional

to initial ionization

acceleration causes

avalance of pairs

leads to discharge

where signal sizeis independent of

initial ionization

continuous

discharge

(insensitiveto ionization)

minimum

ionizing

particle

heavily

ionizing

particle

E(r) = V0

r log(rout/rin)

Typical Parameters

rin= 10-50 µm

E = 104 V

Amplification = 105

Proportional

Counter

Multiwire Proportional Counter (MWPC)

Typical wire

spacing ~ 2mm

George Charpak

Drift Chamber

Field-shaping wires provide

~constant electric field so

charges drift to anode wires with

~constant velocity (~50mm/µs)

Timing measurement compared

with prompt external trigger can

thus yield an accurate position

determination (~200µm)

use of MWPC in

determination of

particle momenta

Time Projection Chamber (TPC)

n → p + e− + νebut sometimes...

n → p + e− + νe

⇐ occurs as a singlequantum event

⇐ within a nucleus

''double β−decay"

but what if νe = ν

e ?

(Majorana particle)

then the following

would be possible:

n → p + e− + νe

νe + n → p + e−

''neutrinoless double β−decay"

One Application of a TPC:

Example of a radial drift chamber (''Jet Chamber")

Reconstruction of 2-jet

event in the JADE

Jet Chamber at DESY

Angular segment of

JADE Jet Chamber

Spark Chamber

Silicon Strip Detector

electron-hole pairs instead of electron-ion pairs

etched

3.6 eV required to form electron-hole pair

⇒ thin wafers still give reasonable signals and good timing (∼10ns) Spatial resolution ∼10µm

CDF Silicon Tracking Detector

Cerenkov

Radiation

θ

(c/n)t

cosθC= ct/(nvt) = 1/(nβ)

vt

d2Nγ αz2 1

dxdE ℏc β2n2= 1 −( )

# photons ∝ dE ∝ dλ/λ2

⇒ blue light

Cerenkov

Radiation

Threshold Cerenkov Counter:

discriminates between particles of similar momentum

but different mass (provided things aren’t too relativistic!)

m1 , β1 m

2 , β2

= (β22 − β1

2)/β22

β2 = 1 − 1/γ2

= 1 − m2/E2(m12/E12 − m

22/E22)

(1 − m22/E22)

=

(m12 − m

22)

(E2 − m22)

≃

= (m12 − m

22)/p2

1/(nβ1) = 1

1/n2 = β12

just below

threshold

[(1−m22/E22) − (1−m

12/E12)]

(1−m22/E22)

=

length of radiator needed increases

as the square of the momentum!

( 1 - 1/(β22n2) ) = ( 1 - β

12/β22)

helium 3.3x10−5 123

CO2 4.3x10−4 34pentane 1.7x10−3 17.2

aerogel 0.075−0.025 2.7−4.5H2O 0.33 1.52

glass 0.75−0.46 1.22−1.37

Medium n−1 γ (thresh)

light detectors

on inner surface

Muon Rings

liquid

radiator

gaseous

radiator

Ring Imaging CHrenkov

detector

Above some ''critical" energy, bremsstrahlung and pair production dominate over ionization

EC~ (600 MeV)/Z

t = 0 1 2 3 4

Depth in radiation lengths

Maximum development

will occur when E(t) = EC:

# after t radiation lengths = 2t

Avg energy/particle: E(t) = E0/2t

Assume each electron with E > EC

undergoes bremsstrahlung aftertravelling 1 radiation length, giving

up half it’s energy

Assume each photon with E > EC

undergoes pair production aftertravelling 1 radiation length, dividing

it’s energy equally

Neglect ionization loss above EC

Assume only collisional loss below EC

log(E0/EC)

log(2)tmax=

Calorimeters

Nmax= E

0/EC

•••• Depth of maximum increases logarithmically with primary energy

•••• Number of particles at maximum is proportional to primary energy

•••• Total track length of particle is proportional to primary energy

•••• Fluctuations vary as ≃≃≃≃ 1/√√√√N ≃≃≃≃ 1/√√√√E0

Typically, for an electromagnetic calorimeter:∆E 0.05E √E

GeV

≃

For hadronic calorimeter, scale

set by nuclear absorption length

Scale is set by radiation length: X0≃ 37 gm/cm2

iron ⇒ Λnuc= 130 gm/cm2

lead ⇒ Λnuc= 210 gm/cm2

~ 30% of incident energy is lost

by nuclear excitations and the

production of ''invisible" particles

∆E 0.5E √E

GeV

≃

Examples of Calorimeter Construction:

Photomultiplier Tubes (PMTs)

A Typical ''Good" PMT:

quantum efficiency ∼ 30%collection efficiency ∼ 80%signal risetime ∼2ns

Scintillator

Inorganic Usually grown with small admixture of impurity centres.

Electrons created by ionization drift through lattice,

are captured by these centres and form an excited state.

Light is then emitted on return to the ground state.

Most important example ⇒ NaI (doped with thallium)

Pros: large light output Cons: relatively slow time

response (largely due

to electron migration)

Organic Excitation of molecular energy levels.

Medium is transparent to produced light.

Why isn’t light self-absorbed??

interatomic spacing

potential energy

ground

state

excited

state

Pros: very fast Cons: smaller light

output

NaI (Tl) 2.2 250 410 3.7

CsI (Tl) 2.4 900 550 4.5

BGO ∼0.5 300 480 7.1(Bi

4Ge3O12)

anthacene 1.0 25 450 1.25

toluene 0.7 3 430 0.9

polystyrene 0.3 3 350 0.9+ p-terphenyl

Scintillator Relative Decay λmax Densitylight yield time (ns) (nm) (gm/cm3)

organic{inorganic{

Some Commonly Used Scintillators:

some ways of coupling plastic

scintillator to phototubes to

provide fast timing signal :

t = Lc/β

1/β = ( 1 − 1/γ2 )−1/2

β2 = 1 − 1/γ2

≃ 1 − 1/(2γ2)

∆t ≃ Lc/2 (1/γ22 − 1/γ1

2)

= Lc/2 ( m22/E22 − m

12/E12 )

≃ Lc/2 ( m22 − m

12 )/E2

Time Of Flight (TOF): An Application of Promt Timing

(used to discriminate particle masses)

∆t = Lc (1/β1 − 1/β2)

High Energy Particle

Detectors in a Nutshell: