Download LECTURE 17 ELECTROMAGNETIC INTERACTIONS PARTICLE DETECTORS

LECTURE 17
ELECTROMAGNETIC INTERACTIONS,
PARTICLE DETECTORS
PHY492 Nuclear and Elementary Particle Physics
Particle Detection
For the detection of a particle, there are many requirements;
one needs to know;
- particle identification
- energy
- position
- momentum
- time
etc…
In reality, experimentalists combine several detectors Gas detectors – convert the ionization produced by the passage of
a charged particle into an electronic signal
→ tracking detectors
Spectrometer – tracking detector in a magnetic field
→ momentum & position measurement
Scintillation counters – excellent time resolution
→ trigger of the system
Solid-state detectors – employs the properties of semi-conductors
→ excellent position & energy resolutions
Cerenkov counters – measure a velocity of a charged particle
→ particle identification February 17, 2014 PHY492, Lecture 17 2 Particle Detection Basics
Scintillation light generated by
excitation from particle charge
Gas ionization created by
passing particle charge
Pressurized gas
Plastic doped with scintillating molecules or
crystal scintillating material
Electronic Readout
Light collected and transferred to
photo-detector via fiber optics.
Semiconductor ionization created
by passing particle charge
Ions are collected on anode wires (red) and
signal recorded via electronic readout
Energy measurement via calorimetry
Dense calorimeter material
Particles ionize thin silicon layers and this
charge can be collected to identify a “track”
Strong force dominates
Electromagnetic force dominates
quark
electron
Using dense materials sensitive to
different interactions, stop particles and
measure energy.
By placing in a solenoidal magnetic field, Lorentz
force 1on
February 7, particle
2014 measures momentum
PHY492, Lecture 17 3 Gas Detectors
Most of gas detectors detect the
ionization produced by the passage of
a charged particle through a gas.
Cathode(-) particle + -
+ -
Anode(+) + -
gas (eg. Ar) electron-ion pair production typically
needs an energy of 30±10eV There are several regions for detector
operation depending on the Voltage applied;
recombination region, ionization chamber
region, region of proportionality, Geiger-Muller region … February 17, 2014 PHY492, Lecture 17 4 Ionization Chamber
Ionization chamber:
At low applied energies, the output signal is very small, because
electron-ion pairs recombine before reaching the electrodes
( recombination region ). When the voltage increases, the numbers of
pairs increases to a saturation level, which means a complete collection
( region of ionization chamber ).
Generally, used as beam monitors
(energy, timing resolutions are poor) Eg. Cylindrical shape with an inner anode of
radius ra and an outer cathode of radius rc V R
V V
Electric field E(r) is given by r · ln(r /r ) c a
February 17, 2014 PHY492, Lecture 17 5 Wire Chambers
Wire chamber :
Wire chambers are operated at the voltage in the proportional region,
which is beyond the region of operation of the ionization chamber.
In the proportional region, a cylindrical arrangement will produce
electric field strengths of order 104-105 V/cm near the wire.
With the high electric field, secondary ionization will be induced by the
primary electron-ion pairs.
MWPC
Multi Wire Proportional Chamber
The planes (a) have anode wires into
the page and those in plane (b) are at
right angles.
The wire spacings are typically 2mm.
A positive voltage applied to the anode
wires generates a field.
February 17, 2014 PHY492, Lecture 17 6 Time Projection Chambers (TPC)
Time Projection Chamber (TPC) :
One of the most advanced applications of proportional and drift chamber.
Electrons formed along the track of an ionizing particle drift under the
electric field E towards one of the endcaps along helical trajectories. 2m long
1m in diameter February 17, 2014 PHY492, Lecture 17 7 Beyond the region of proportionality
The Geiger-Muller region :
The output signal is independent of the energy loss of the incident
particle. Detectors working in this region are called as the Geiger-Muller
counters.
Beyond :
The avalanche develops moving plasmas or streamers.
Recombination of ions then leads to visible light that can be made to
generate an electrical output.
Detectors are called streamer and spark chambers.
February 17, 2014 PHY492, Lecture 17 8 Scintillation Detectors
Scintillation light generated by
excitation from particle charge
Plastic doped with scintillating molecules or
crystal scintillating material
Light collected and transferred to
photo-detector via fiber optics.
Scintillation detectors:
The charged particles lose their energies due to excitation and
ionization in the medium of the detector.
In special materials, called scintillators, one can expect to obtain
a small fraction of the excitation energy as visible (or UV region) light.
February 17, 2014 PHY492, Lecture 17 9 The Photomultiplier Tube
It's all in the name!
n 
The Photomultiplier Tube (PMT or Phototube) is designed to
amplify the signal of a single photon
n 
Based largely on the photo-electric effect
n 
”Gain” in signal is achieved by collecting electrons through a
series of voltage drops between ”Dynodes”.
February 17, 2014 PHY492, Lecture 17 10 Semiconductor Detectors
Solid-state detectors (Si, Ge) operate through the promotion of electrons
from the valence band of a solid to the conduction band as a result of
the passage of the incident particle.
The advantage of the solid-state detectors is the good energy resolution,
because the energy loss required to produce electron-hole pair is only
3-4eV, compared to 30eV required in a gas detector.
Energy required to produce
electron-hole pair
eV
Scintillator
100-300
Gas detector
10-30
Germanium
2.9
Silicon
3.7 February 17, 2014 PHY492, Lecture 17 11 Silicon Ionization Detectors
Semiconductor ionization created
by passing particle charge
Particles ionize thin silicon layers and this
charge can be collected to identify a “track”
By placing in a solenoidal magnetic field, Lorentz
force on particle measures momentum
February 17, 2014 PHY492, Lecture 17 12 >
Cerenkov Counters
>
Cerenkov effect When a charged particle with velocity “v” traverses a medium
of refractive index “n”, and if “v” is greater than the speed of light
in the medium “c/n”, a part of the energy from the excited atoms
in the vicinity of the particle appears as coherent radiation emitted
at a characteristic angle θ.
charged particle
velocity v X θ = cos-1(c/nv) Cone surface onto which
light is emitted when particle is at X
The number of photons N(λ)dλ radiated per unit path length in a
wavelength interval dλ is given by N(λ)dλ = 2πα ( 1 - 1/β2n2)dλ/λ2 < 2πα ( 1 - 1/n2)dλ/λ2 February 17, 2014 PHY492, Lecture 17 13