Transcript pptx - Institute of Nuclear and Particle Physics

Particle Detectors
Tools of High Energy and Nuclear Physics
Goal: to detect Individual Particles
and reconstruct their 4-vectors
Thanks to H. Fenker (Jlab) and B. Surrow (MIT)
Introduction: detector tasks
• Position measurement: localize hits of a charged particle
(eg: wire chambers, segmented scintillators or calorimeters)
• Momentum measurement: by measuring the particle
deflection in a magnetic field (eg: magnetic spectrometer)
• Energy measurement: deposition of energy in a localized
volume (eg: calorimeters)
• Particle identification: mass and charge of the particle
(eg: Cerenkov detectors)
• Triggering: select events of interest
• Data acquisition system: readout of an event and storage
after positive decision.
Introduction: detector performances
• Time
– Response time: time which is required to produce a signal after the
passage of a particle in the detector (from ~10 ns to ~100 ms).
– Deadtime: time that must elapse following the passage of a particle
before the detector is ready for the next particle.
• Efficiency
That is (event registered)/(events emitted by source)
The efficiency is the product of at least two quantities:
– Intrisic efficiency: (event registered)/(events impinging on detector)
– Geometric efficiency: (event impinging on detector)/(event emitted by
source)
• Resolution/Accuracy
Example of position detectors performance
• Avalanche multiplication
• Detectors that see the
electrons
– Wire chambers
– Time Projection Chamber
– Gas Electron Multiplier
• Detectors that see the light
– Scintillators
– Cerenkov Detectors
– Calorimeters
Outline of class
 Interactions of Particles with Matter
 Charged particles
 Photons
 Using the Interactions:Particle Detectors
 Avalanche multiplication
 Detectors that sense Charge
 Detectors that sense light
 Aside: magnetic spectrometers
 Putting it all together
Interaction of particles with matter
Charged particles:
Photons:
Energy and trajectories get
degraded as they pass
through matter:
• Ionization
• Bremsstrahlung
• Cherenkov radiation
Flux gets decreased as they go
through matter. All or nothing
interactions
• Photo-electric effects
• Compton scattering
• Pair production
Outline of class
 Interactions of Particles with Matter
 Charged particle
 Photons
 Using the Interactions:Particle Detectors
 Avalanche multiplication
 Detectors that sense Charge
 Detectors that sense light
 Aside: magnetic spectrometers
 Putting it all together
Charged particles- Ionization
Ionization
Ion
 Atomic electron is knocked free from
the atom.
 The remaining atom is now an ion or
left in an excited state (will decay by
emitting a photon)
Free Electron
Charged
Particle
Electric Field
Ionization: Bethe-Bloch formula
é
dE
2mec 2b 2g 2Tmax
dù
2
2 2 Z 1 1
2
= 4pN A re mec z
ln
-b - ú
2ê
2
dX
A b ë2
I
2û
 where , , relate to particle speed,
z is the particle’s charge..
 The other factors describe the medium
(Z/A, I, ), or are physical constants.
 dE/dx units is MeV cm2/g
 E= dE/dx * x /
 A value to keep in mind is 2 MeV.g-1.cm2
Charged Particles: Bremsstrahlung
 Radiation of real photons in the
Coulomb field of a nuclei of the
absorber.
Photon
Electron
dE
rN A
= 4a
Z(Z +1)re2 ln(183Z -1/ 3 )E
dX
A
Nucleus
 Define X0 the radiation length:
length during which the particle
looses a fraction e of its initial
energy
dE dX
-
E
=
X0
 Critical energy (Emc) is the energy
at which Bremstrahlung loss
equal ionization loss
Emc(e- Cu)=20 MeV
Emc(m- Cu)=800 GeV
Charged particles: Multiple Coulomb Scattering
(both for ionization and radiation)
Detectors scatter particles even without much energy loss…
 MCS theory is a statistical description of the scattering angle arising
from many small interactions with atomic electrons.
 MCS alters the direction of the particle in average.
 Most important at low energy.

Q =0
sQ =
13.6 MeV
z
bcp
[
x / X 0 1+ 0.038 ln( x / X 0 )
]
 is particle speed,
z is its charge,
X0 is the material’s Radiation Length.

0

Charged particles- Cerenkov radiation
The electric field of a particle has a long-range interaction with material as
it passes through a continuous medium.
It does create a shock wave that causes the material to emit light when it’s
speed is large enough
v = c > c/n
where n is the index of refraction of the material
1/n

Also the light is emitted at the angle  = cos-1 (1/n)
Photon energy ~ few eV (UV to visible) (not an efficient way to loose energy)
Cerenkov light produced by fuel elements of a nuclear power plant.
Outline of class
 Interactions of Particles with Matter
 Charged particle
 Photons
 Using the Interactions:Particle Detectors
 Avalanche multiplication
 Detectors that sense Charge
 Detectors that sense light
 Aside: magnetic spectrometers
 Putting it all together
Interaction of photon with matter
Photo-electric effect
Compton scattering
Pair production
Interaction of photon with matter
Total probability for  interaction: s = s pe + s C + s pair
Probability m per unit length or total absorption coefficient
Such that the flux of photon is
Absorption length=
(absorption coeff)-1
I = I0e - mx
æ NA r ö
m =s ç
÷
è A ø
Interactions of Particles with Matter Summary
 When particles pass through matter they
usually produce either free electric charges
(ionization) or light (photoemission).
 Most “particle” detectors actually detect the
light or the charge that a particle leaves
behind.
 In all cases we finally need an electronic
signal to record.
Outline of class
 Interactions of Particles with Matter
 Charged particle
 Photons
 Using the Interactions:Particle Detectors
 Aside: Avalanche multiplication
 Detectors that sense Charge
 Detectors that sense light
 Aside: magnetic spectrometers
 Putting it all together
Particle Detectors… Avalanche
Multiplication
When a particle passes through matter, it creates just a few
electrons/ions or photons
But
the best we can do is to detect signals of the order of nV in a 100 Ohm
resistor which correspond to
I= U/R= 10-9 /100 = 10-11 A = 108 electrons. s-1
Which means:
The detectors need to amplify the charge produced by particles going
through matter. By giving the charges a push, we can make them
move fast enough so that they ionize other atoms when they collide.
After this has happened several times we have a sizeable free charge
that can be sensed by an electronic circuit.
Particle Detectors: amplification with
the photo multiplier
Secondary Emission
Energetic electrons striking some
surfaces can liberate MORE
electrons. Those, in turn, can be
accelerated onto another surface …
so on.
Photoelectric Effect:
A photon liberates a
single electron
PMTs are commercially produced and very sensitive.
•One photon --> up to 108 electrons!
•Fast! …down to ~ few x 10-9 seconds.
Particle Detectors: amplification with the E-Field
E(r) =
V
r ln(rc /ra )
• V is the voltage
• rc radius of the outer cathode plane
• ra radius of the inner anode
Close to the wire the E field is very
Intense, the charged particle is
Accelerated and
as a result creates secondary
Particles.
Particle Detectors… Gas Electron Multiplier (GEM)
Gas Ionization and Avalanche Multiplication again, but…
 … a different way to get an intense electric field,
 … without dealing with fragile tiny wires
--V
GEM
~400v
0.002”
To computer
http://gdd.web.cern.ch/GDD/
Particle Detectors: bypassing the amplification
(new methods)
Dense Material => Lots of Charge.
Typically no Amplification
Semiconductor
Silicon
Diamond
Noble Liquid
Liquid Argon Calorimeter
Strips
Pixels
Drift
Electrons are knocked
loose in the silicon
and drift through
it to electronics.
Readout strips may
be VERY NARROW
0.001”
0.012”
Signals to
Computer
Particle detectors
• Avalanche multiplication
• Detectors that see the
electrons
– Wire chambers
– Time Projection Chamber
• Detectors that see the light
– Scintillators
– Cerenkov Detectors
– Calorimeters
Particle Detectors: Gas Filled Wire Chamber
Let’s use Ionization and Avalanche Multiplication to build a
detector…
 Make a Box.
 Fill it with some gas: noble gases are more likely to ionize than
others. Use Argon.
 Insert conducting surfaces to make an intense electric field: The
field at the surface of a small wire gets extremely high, so use
tiny wires.
 Attach electronics and apply high voltage.
 We’re done!!
Straw Tube Tracker for the COSY-TOF Experiment Julich Institute (Germany)
Central Drift Chamber (Hall D Jlab)
Straw chambers: better resolution
y
2D solution:
The wire touched gives X position
info
X
The time of transit between the two
amplis gives the Y position info
Multi-Wire Gas Chamber
Best 1D solution:
-Use an external trigger to start a clock
-Measure the time it takes for the
electron to drift from the initial ionization
to the wire.
stop
start
Resolution ~ 10 mm
TPC: 3D position information.
Time Projection Chamber (TPC): Drift through a Volume
Just a box of gas with
Electric Field and
Y
Readout Electrodes
Readout elements only on one surface.
Ionization Electrons drift to Surface for
Readout
electrodes
Amplification
Charge Collection
Readout Electrode Position gives (x,y)
Time of Arrival gives (z).
Particle
track
Z
Cathode
Anode
(gain)
X
Particle detectors
• Avalanche multiplication
• Detectors that see the
electrons
– Wire chambers
– Time Projection Chamber
• Detectors that see the light
– Scintillators
– Cerenkov Detectors
– Calorimeters
Particle Detectors… Cerenkov Counter
If =v/c > 1/n, there will be light.
If not, there won’t.
Can be used for trigger system
If you know the momentum of the
particle: can be used for PID
Cerenkov
Counters –
sensitive to 
TRD Counters –
sensitive to 
 = v/c
= p/E
= (1- 2)-1/2
= E/m
Momentum (GeV/c)
Rich
detector
: Ring
Light is emitted at the angle
 = cos-1 (1/n)
imaging
Cherenkov
Scintillators
Scintillation Counters are probably the most widely
used detectors in Nuclear and High Energy Physics.
Scintillator material are special material that
-emit light when traversed by energetic particles and
-can shift the wavelength of this light
to be harnessed by PMTs
They can be solid, liquid (even gas)
They can be molded in all kind of shapes
Solids
Liquid
Saint Gobin Inc
Sample experiment
MIT/Bates
Detecting scintillation
In Air.
Particle Detectors: Calorimeter
Used to measure energy.
Based on Bremsstrahlung effect
Suppose an initial photon of energy E0
After t radiation lengths, the average energy of secondary is:
The shower stops when E(tmax ) = E c
t max =
E(t) = E 0 /2t
ln(E 0 / E c )
~ 10X 0
ln2
The number of secondaries is Nmax = exp(t max ln2) = E 0 / E c
The energy resolution of the calorimeter therefore goes as
EM (or hadronic) calorimeters are used because:
- They can detect charged or neutral particle
- They produce a really fast signal (10-100 ns) that is ideal for making
trigger decision
Alice’s ZDC calorimeter
CERN
Hall A/DVCS calorimeter
(50*30 cm2)
JLAB
Outline of class
 Interactions of Particles with Matter
 Charged particle
 Photons
 Using the Interactions:Particle Detectors
 Avalanche multiplication
 Detectors that sense Charge
 Detectors that sense light
 Aside: magnetic spectrometers
 Putting it all together
Particle Detectors:
aside: magnetic spectrometer
Nature lets us measure the
Momentum of a charged
particle by seeing how much
its path is deflected by
a magnet.
Just as light of different wavelength is
bent differently by a prism...
p(GeV) = 0.3 B(T)r(m)
(x2,y2)
(x1,y1)
Magnet
MAMI (Germany)
IN frame
Usually a
“short” target
OUT frame usually equipped with
a Vertical Drift Chamber (VDC)
Central trajectory
Four quantities measured in the IN frame and 4 in the OUT/VDC frame.
The best is to design the magnet to be insensitive to xIN and
with <x|> as the main coefficient
Outline of class
 Interactions of Particles with Matter
 Charged particle
 Photons
 Using the Interactions:Particle Detectors
 Avalanche multiplication
 Detectors that sense Charge
 Detectors that sense light
 Aside: magnetic spectrometers
 Putting it all together
Putting it all Together: A Detector System
Hall C/JLAB
Putting it all Together: A Detector
System
QWEAK: conceptual overview
• Elastic e-p scattering on liquid hydrogen target
• Toroidal magnet to provide momentum dispersion
• Collimator system to select elastic events only
• Lower energy inelastic events bent outside of the detector acceptance
Outline of class
 Interactions of Particles with Matter
 Charged particle
 Photons
 Using the Interactions:Particle Detectors
 Avalanche multiplication
 Detectors that sense Charge
 Detectors that sense light
 Aside: magnetic spectrometers
 Putting it all together