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Methods of Experimental Particle Physics

Alexei Safonov

Lecture #12

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ISAAC SARVEROn transition radiation

3 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”

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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

• Measure charge, time difference between electron and ion arrival times

• Often stick a lot of sense wires, layers etc.

• 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)

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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 moving to the left, holes to the right

• Equilibrium • Zero bias (no voltage)

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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 electron-

hole 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:

• 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

• 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)

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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)