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1 Methods of Experimental Particle Physics Alexei Safonov Lecture #12.

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1 1 Methods of Experimental Particle Physics Alexei Safonov Lecture #12

2 ISAAC SARVER On transition radiation 2

3 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, “Transition Radiation”

4 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 4

5 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 5

6 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. 6 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)

7 What Matters Want charges to be large and come fast Pick gas mixtures with low ionization energy Easy to ionize 7 And with large drift velocities To get signal fast And with small transverse diffusion To better measure position

8 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 8

9 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. 9

10 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 10

11 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) 11

12 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 300  m 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) 12

13 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) 13

14 LHC Trackers From top left clockwise: CMS Tracker layout: pixel & strip detectors CMS Strip Detector ATLAS pixel detector 14

15 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 15

16 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 16

17 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 17

18 CMS DT Muon System 18

19 Put Everything Together Have we missed anything? Calorimeters! – next time (and more) 19

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