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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:
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Wilson Cloud Chamber:
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Antimatter
Anderson
1933
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Evaporation-type Cloud Chamber:
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Photographic Emulsions
νµ
νeπ−
µ−e−
Discovery of the Pion
(Powell et al., 1947)
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DONUT (Direct Observation of NU Tau) July, 2000
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Donald Glazer (1952)
Bubbles form at nucleation sites
in regions of higher electric fields
⇒ ionization tracks
Bubble Chamber
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Liquid superheated by sudden expansion
Bubbles allowed to
grow over ∼ 10msthen collapsed during
compression strokehydrogen,
deuterium,
propane
Freon
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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
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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
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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
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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
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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
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Time Projection Chamber (TPC)
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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:
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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
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Spark Chamber
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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
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CDF Silicon Tracking Detector
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Cerenkov
Radiation
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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
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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)
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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
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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
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•••• 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
≃
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Examples of Calorimeter Construction:
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Photomultiplier Tubes (PMTs)
A Typical ''Good" PMT:
quantum efficiency ∼ 30%collection efficiency ∼ 80%signal risetime ∼2ns
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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
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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 :
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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)
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High Energy Particle
Detectors in a Nutshell: