Lecture 13: Detectors

1. Lecture 13: Detectors • Visual Track Detectors • Electronic Ionization Devices • Cerenkov Detectors • Calorimeters • Phototubes & Scintillators • Tricks With Timing • Generic Collider Detector Useful Sections in Martin & Shaw: Sections 4.3, 4.4, 4.5

2. Consider a massless qq pair linked by a rotating string with ends moving at the speed of light. At rest, the string stores energy κ per unit length and we assume no transverse oscillations on the string. This configuration has the maximum angular momentum for a given mass and all of both reside in the string - the quarks have none. Consider one little bit of string at a distance r from the middle, with the quarks located at fixed distances R. Accounting for the varying velocity as a function of radial position, calculate both the mass, M, and angular momentum, J, as a function of κ and R. 3 sheet 4 At rest: dM/dr = k In motion: dM/dr = gk R g = (1-b2)-½ = [1-(r/R)2]-½ R ∫ 0 ∫ Thus, M = 2k [1-(r/R)2]-½ dr = kRp

3. R ∫ 0 In natural units v = b = (r/R) Similarly, J = 2k vr [1-(r/R)2]-½ dr R ∫ 0 ∫ = (2k/R) r2 [1-(r/R)2]-½ dr = kR2p/2 thus, J = M2/(2pk) but M = kRp From experimental measurements of J versus M (“Regge trajectories”) it is found that κ ∼ 0.18GeV2when expressed in natural units. Convert this to an equivalent number of tonnes. ~15

4. Now consider the “colour charge” contained within a Gaussian surface centred around a quarks and cutting through a flux tube of cross sectional area A . By computing an effective “field strength” (in analogy to electromagnetism), derive an expression for the energy density of the string (i.e. κ) in terms of the colour charge and the area A . In analogy with EM: •Ec = rc/ec Gaussian surface Flux tube Ec A = qc/ec Ec = qc/(Aec) Assume A ~ 1 fm2 k = energy/length = (energy density) x A = ½ ec Ec2 A qc2/(4pecħc) = kA/(2pħc) = qc2/(2Aec) (14.4x104 kg m/s2)(10-15m)2 as ≈ = 0.76 2p (10-34 J s)(3x108 m/s)

5. Lecture 13: Detectors • Visual Track Detectors • Electronic Ionization Devices • Cerenkov Detectors • Calorimeters • Phototubes & Scintillators • Tricks With Timing • Generic Collider Detector Useful Sections in Martin & Shaw: Section 3.3, Section 3.4

6. Wilson Cloud Chamber Wilson Cloud Chamber:

7. Antimatter Antimatter Anderson 1933

8. Evaporation Cloud Chamber Evaporation-type Cloud Chamber:

9. Photographic Emulsions  e  m e  Photographic Emulsions Discovery of the Pion (Powell et al., 1947)

10. Emulsions & the Nu-tau DONUT (DirectObservation ofNU Tau) July, 2000

11. Bubble Chamber Idea Bubble Chamber Donald Glazer (1952) Bubbles form at nucleation sites in regions of higher electric fields  ionization tracks

12. Bubble Chamber & Beer Bubble Chamber Donald Glazer (1952) Bubbles form at nucleation sites in regions of higher electric fields  ionization tracks

13. Tips 3 Steve’s Tips for Becoming a Particle Physicist 1) Be Lazy 2) Start Lying 3) Sweat Freely 4) Drink Plenty of Beer

14. Bubble Chamber Liquid superheated by sudden expansion then collapsed during compression stroke Bubbles allowed to grow over 10ms hydrogen, deuterium, propane Freon

15. Bubble Chamber Pros & Cons Difficult to trigger Acts as both target & detector Track digitization cumbersome High beam intensities swamp film Mechanically Complex Spatial resolution 100200 m Slow repetition rate

16. Basics of Ionization Detector Ionization Detectors Electric field imposed to prevent recombination Medium must be chemically inactive (so as not to gobble-up drifting electrons) and have a low ionization threshold (noble gases often work pretty well)

17. Ionization Regimes continuous discharge (insensitive to ionization) acceleration causes avalance of pairs signal reflects total amount of ionization initially free electrons accelerated and further ionize medium signal smaller than initially produced pairs leads to discharge where signal size is independent of initial ionization such that signal is amplified proportional to initial ionization heavily ionizing particle minimum ionizing particle

18. Proportional Counters V0 r log(rout/rin) E(r) = Proportional Counter Typical Parameters rin = 10-50 m E = 104 V Amplification = 105 Multiwire Proportional Counter (MWPC) George Charpak Typical wire spacing ~ 2mm

19. Drift Chamber use of MWPC in determination of particle momenta Drift Chamber Field-shaping wires provide ~constant electric field so charges drift to anode wires with ~constant velocity(~50mm/s) Timing measurement compared with prompt external trigger can thus yield an accurate position determination(~200m)

20. TPC Time Projection Chamber (TPC)

21. TPC & double-beta decay n  p + e + e n  p + e + e but what if e = e ? n  p + e + e e +n  p + e One Application of a TPC:  occurs as a single quantum event  within a nucleus but sometimes... ''double decay" (Majorana particle) then the following would be possible: ''neutrinoless double decay"

22. Jet Chamber Reconstruction of 2-jet event in the JADE Jet Chamber at DESY Example of a radial drift chamber (''Jet Chamber") Angular segment of JADE Jet Chamber

23. Spark Chamber Spark Chamber

24. Silicon Strip Detector etched Silicon Strip Detector electron-hole pairs instead of electron-ion pairs 3.6 eV required to form electron-hole pair  thin wafers still give reasonable signals and good timing (10ns) Spatial resolution 10m

25. CDF Silicon Strips CDF Silicon Tracking Detector

26. Cerenkov Radiation Cerenkov Radiation

27. Cerenkov Angle & Photon Yield cosC = ct/(nvt) = 1/(n)  ( ) d2Nz2 1 dxdE ℏc 2n2 = 1  Cerenkov Radiation vt (c/n)t ∝ d/2 # photons ∝ dE blue light

28. Threshold Cerenkov Counter m1 , 1 m2 ,  ( 1 - 1/(22n2) ) = ( 1 - 12/22) just below threshold 1/(n1) = 1 1/n2 = 12 [(1m22/E22)  (1m12/E12)] (1m22/E22) = (m12/E12 m22/E22) (1 m22/E22) = (m12 m22) (E2 m22) ≃ length of radiator needed increases as the square of the momentum! Threshold Cerenkov Counter: discriminates between particles of similar momentum but different mass (provided things aren’t too relativistic!) = (22 )/22 2 = 1  1/2 = 1  m2/E2 = (m12 m22)/p2

29. RICH Detectors light detectors on inner surface Muon Rings Medium n1  (thresh) helium 3.3x105 123 CO2 4.3x104 34 pentane 1.7x103 17.2 aerogel 0.0750.025 2.74.5 H2O 0.33 1.52 glass 0.750.46 1.221.37 liquid radiator Ring Imaging CHrenkov detector gaseous radiator

30. EM Calorimeters Assume each electron with E > EC undergoes bremsstrahlung after travelling 1 radiation length, giving up half it’s energy Assume each photon with E > EC undergoes pair production after travelling 1 radiation length, dividing it’s energy equally t = 0 1 2 3 4 Depth in radiation lengths Neglect ionization loss above EC Assume only collisional loss below EC log(E0/EC) log(2) tmax = Calorimeters Above some ''critical" energy, bremsstrahlung and pair production dominate over ionization EC~ (600 MeV)/Z # after t radiation lengths = 2t E(t) = E0/2t Avg energy/particle: Maximum development will occur when E(t) = EC : Nmax = E0/EC

31. Hadronic vs EM Calorimeters E 0.05 E EGeV ≃ E 0.5 E EGeV ≃  Depth of maximum increases logarithmically with primary energy  Number of particles at maximum is proportional to primary energy  Total track length of particle is proportional to primary energy  Fluctuations vary as ≃ 1/N ≃ 1/E0 Typically, for an electromagnetic calorimeter: Scale is set by radiation length: X0≃ 37 gm/cm2 For hadronic calorimeter, scale set by nuclear absorption length iron nuc = 130 gm/cm2 lead nuc = 210 gm/cm2 ~ 30% of incident energy is lost by nuclear excitations and the production of ''invisible" particles

32. Calorimeter Construction Examples of Calorimeter Construction:

33. Photomultiplier Tubes Photomultiplier Tubes (PMTs) A Typical ''Good" PMT: quantum efficiency30% collection efficiency80% signal risetime2ns

34. Scintillator excited state potential energy ground state interatomic spacing Scintillator Inorganic Usually grown with small admixture of impurity centres. Electrons created by ionization drift through lattice, are captured by these centres and form an excited state. Light is then emitted on return to the ground state. Most important example NaI(doped with thallium) Pros:large light output Cons:relatively slow time response (largely due to electron migration) Organic Excitation of molecular energy levels. Medium is transparent to produced light. Why isn’t light self-absorbed?? Pros: very fast Cons: smaller light output

35. Scintillator Charasteristics { organic { inorganic Some Commonly Used Scintillators: Scintillator Relative Decay max Density light yield time (ns) (nm) (gm/cm3) anthacene 1.0 25 450 1.25 toluene 0.7 3 430 0.9 polystyrene 0.3 3 350 0.9 + p-terphenyl NaI (Tl) 2.2 250 410 3.7 CsI (Tl) 2.4 900 550 4.5 BGO 0.5 300 480 7.1 (Bi4Ge3O12) some ways of coupling plastic scintillator to phototubes to provide fast timing signal :

36. Time Of Flight t = Lc (1/) Time Of Flight (TOF): An Application of Promt Timing (used to discriminate particle masses) t = Lc/  = 1  1/2  = ( 1  1/2 )1/2 ≃ 1  1/(22) t ≃ Lc/2 (1/2) = Lc/2 ( m22/E22 m12/E12 ) ≃ Lc/2 ( m22 m12 )/E2

37. Collider Detector Configuration High Energy Particle Detectors in a Nutshell: