Transcript PPTX

Methods of Experimental
Particle Physics
Alexei Safonov
Lecture #12
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On transition radiation
ISAAC SARVER
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Transition Radiation
• Take the electric field solutions for a charged particle in
vacuum and in medium.
• Subtract the differences and you have the transition
radiation.
• To work, the foil must be sufficiently thick for the
material to react. Jackson says the thickness is on the
order of 10 microns.
• As we have transition radiation from both surfaces, a
well selected foil thickness and separation distance can
result in coherence effects that improve detection.
• Jackson 13.7, Wikipedia.org “Transition Radiation”
Today and Next Time
• Detectors and Technologies used in modern HEP
experiments
• Tracking devices:
• Gaseous detectors
• Silicon detectors
• Muon detectors
• Gaseous detectors again
• Calorimeters:
• Electromagnetic and Hadron Calorimeters
• Compensation
• Trigger, DAQ etc
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Gaseous Detectors
• Gaseous tracking devices
• Measure positions where charged particle left
ionization to build a track
• Guide electrons and ions to electrodes using electric field to
collect charge
• Typically use charge multiplication
• E.g. an ionization electron, if put in strong electric field, will
accelerate and ionize media on its path liberating more
electrons and creating “avalanches”
• Advantages:
• They can be very “light” (gas is light!)
• You only want to see where the particle went, you don’t want
it to seriously interact with your tracker
• Good precision
• Can identify particle types by measuring how much
they ionize the media at given momentum
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Drift Chambers
• Implementations vary, but
same principle:
• Guide electrons and ions from
ionization to detector sensitive
elements
• Figure out where the particle went in terms
of its position
• Use whatever you can:
• Measure when the signal arrived (time gives
you how far it traveled)
• Measure charge, time
difference between electron
and ion arrival times
• Often stick a lot of sense wires,
layers etc.
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What Matters
• Want charges to be large and come fast
• Pick gas mixtures with low ionization energy
• Easy to ionize
• And with large drift
velocities
• To get signal fast
• And with small transverse
diffusion
• To better measure position
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What Else?
• And you want large E field as v ~E:
• But not too large or you will ionize gas by the electric
field - a lot of noise (or turn it into a spark chamber)
• Many of these desires contradict each other
• Building these is a complex optimization problem
• Resolution is usually limited by ~ 100 microns
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Limitations
• Technically difficult
• Small mistakes in wire positions can cause large
field distortions and make the whole chamber not
working
• “Slow” signals as they have to drift over not
negligible distances
• You still want to get sufficient multiplication to make it
detectable
• This is bad if you have a lot of particles and collisions
happen often
• You don’t want showers to start overlapping, do you?
• Tevatron: 396 ns between crossings, LHC: 25 ns
• Don’t take rate too well
• Charge accumulation (ions) at very high rate, can cause
gain losses, field distortions etc.
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Ionization in Semiconductor
• As charged particle traverses a semiconductor, you
want to create an electron-hole pair
• Need to give the electron enough
energy to cross from valence into the
conduction bend
• In reality need a little more energy as you
also need to spend some on creating a
phonon to preserve momentum
conservation
• Would want a small bandgap as you want to
create many electron-hole pairs without
putting too much material
• Roughly 4 eV per pair in a silicon diode
at room temperature
• Temperature dependent
• Alpha, beta – determined by the material
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Setting Things Up
• A p-n junction (a diode
essentially)
• Doping to increase the
number of charge carriers
• The interface region is
depleted of charge
carriers
• Forward bias:
• Push electrons to the
right, holes to the left,
depleted region small, E
can’t hold electrons from • Equilibrium
moving to the left, holes
• Zero bias (no voltage)
to the right
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Building a Detector
• Apply reverse bias:
• The depleted region broadened
• That’s where you want
ionization to happen
• No current except thermal
• Thermal excitations grow fast
with temperature and reduction
in bandgap
• Want it cold or have larger
bandgap to avoid noise current
• A typical MIP leaves tens of
keVs in a 300mm of silicon
• Tens of thousands of electronhole pairs
• Move in electric field creating •
current
• Enough to detect with low noise
electronics & low noise current
Equilibrium
• Zero bias (no voltage)
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Why Silicon Detectors?
• Advantages:
• Fast signals ~ 10’s of ns
(small distance to travel)
• High spatial resolution
• Can make small strips or
pixels of silicon (tens to
100 microns)
• In special conditions can get a few micron precision, 20-50
microns would be more typical
• Disadvantages:
• “Heavy”: Particles interact more than you want them
• Complex infrastructure:
• Cooling to keep noise low, Tilting to offset drift of carriers in
magnetic field
• Detectors deteriorate with the radiation doze
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LHC Trackers
• From top left
clockwise:
• CMS Tracker layout:
pixel & strip detectors
• CMS Strip Detector
• ATLAS pixel detector
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Silicon Versus Gas
• Cost versus performance is important:
• Silicon detectors are incredibly expensive
• Gaseous detectors are much less expensive
• But don’t take high rate well
• One area where it can still work is muon
chambers
• Muons get through a lot of material without much
energy loss
• Only ionization, but it’s heavy enough to make those small, it
doesn’t radiate and weak interactions don’t happen often
• Muon chambers are usually positioned on the far
periphery of the detector beyond a lot of material
• Not much gets there except muons, so rates are pretty low
making them a good use case for gaseous detectors
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Strip Cathode Chambers
• CMS endcap muon
system uses CSCs
• Small chambers so
easier to operate
• Position resolution of
~100 microns
• Using center of gravity of
the avalanche
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Drift Tubes
• A single unit is a wire
in enclosure
• Another way to avoid
difficulties with one
wire goes wrong, the
whole chamber is gone
• Used in the central
part of the CMS muon
system
• Good choice for the
same reasons as CSC
• Rates are low enough,
spatial precision is
sufficiently good
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CMS DT Muon System
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Put Everything Together
• Have we missed anything?
• Calorimeters! – next time (and more)
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