Lecture 13: Detectors

Transcription

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 3 sheet 4 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. At rest: dm/dr = κ In motion: dm/dr = γκ R γ = (1-β 2 ) -½ = [1-(r/R) 2 ] -½ Thus, M = 2κ [1-(r/R) 2 ] -½ dr = κrπ

3 Similarly, J = 2κ vr [1-(r/R) 2 ] -½ dr R 0 R 0 = (2κ/R) r 2 [1-(r/R) 2 ] -½ dr In natural units v = β = (r/r) = κr 2 π/2 but M = κrπ thus, J = M 2 /(2πκ) From experimental measurements of J versus M ( Regge trajectories ) it is found that κ 0.18GeV 2 when 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: E c = ρ c /ε c Gaussian surface E c A = q c /ε c Flux tube E c = q c /(Aε c ) Assume A ~ 1 fm 2 κ = energy/length = (energy density) x A q c2 /(4πε c ħc) = κa/(2πħc) = ½ ε c E c 2 A = q c2 /(2Aε c ) α s (14.4x10 4 kg m/s 2 )(10-15 m) 2 2π (10-34 J s)(3x10 8 m/s) = 0.76

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:

7 Antimatter Anderson 1933

8 Evaporation-type Cloud Chamber:

9 Photographic Emulsions ν µ Discovery of the Pion (Powell et al., 1947) µ - e - π - ν e ν µ

10 DONUT (Direct Observation of NU Tau) July, 2000

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

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

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

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

15 Acts as both target & detector Difficult to trigger Track digitization cumbersome High beam intensities swamp film Spatial resolution µm Mechanically Complex Slow repetition rate

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

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

18 Proportional Counter Typical Parameters r in = µm E = 10 4 V Amplification = 10 5 E(r) = V 0 r log(r out /r in ) Multiwire Proportional Counter (MWPC) George Charpak Typical wire spacing ~ 2mm

19 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/µs) Timing measurement compared with prompt external trigger can thus yield an accurate position determination (~200µm)

20 Time Projection Chamber (TPC)

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

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

23 Spark Chamber

24 Silicon Strip Detector etched 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 10µm

25 CDF Silicon Tracking Detector

26 Cerenkov Radiation

27 Cerenkov Radiation cosθ C = ct/(nvt) = 1/(nβ) θ vt (c/n)t ( ) d 2 N γ αz 2 1 = 1 - dxde c β 2 n 2 # photons de dλ/λ 2 blue light

28 Threshold Cerenkov Counter: discriminates between particles of similar momentum but different mass (provided things aren t too relativistic!) m 1, β 1 m 2, β 2 just below threshold 1/(nβ 1 ) = 1 1/n 2 = β 1 2 ( 1-1/(β 22 n 2 ) ) = ( 1 - β 12 /β 22 ) = (β β 12 )/β 2 2 [(1-m 22 /E 22 ) - (1-m 12 /E 12 )] = (1-m 22 /E 22 ) β 2 = 1-1/γ 2 (m 2 1 /E m 22 /E 22 ) = = 1 - m 2 /E 2 (1 - m 22 /E 22 ) length of radiator needed increases as the square of the momentum! (m m 22 ) (E 2 - m 22 ) = (m m 22 )/p 2

29 Medium n-1 γ (thresh) helium 3.3x CO2 4.3x pentane 1.7x aerogel H2O glass Ring Imaging CHrenkov detector Muon Rings liquid radiator light detectors on inner surface gaseous radiator

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

31 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/ E 0 Typically, for an electromagnetic calorimeter: Scale is set by radiation length: X 0 37 gm/cm 2 ΔE 0.05 E E GeV For hadronic calorimeter, scale set by nuclear absorption length iron Λ nuc = 130 gm/cm 2 lead Λ nuc = 210 gm/cm 2 ~ 30% of incident energy is lost by nuclear excitations and the production of ''invisible" particles ΔE 0.5 E E GeV

32 Examples of Calorimeter Construction:

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

34 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 potential energy excited state ground state interatomic spacing

35 Some Commonly Used Scintillators: Scintillator Relative Decay λmax Density light yield time (ns) (nm) (gm/cm 3 ) { anthacene organic toluene polystyrene p-terphenyl { NaI (Tl) inorganic CsI (Tl) BGO (Bi 4 Ge 3 O 12 ) some ways of coupling plastic scintillator to phototubes to provide fast timing signal :

36 Time Of Flight (TOF): An Application of Promt Timing (used to discriminate particle masses) t = Lc/β Δt = Lc (1/β 1-1/β 2 ) β 2 = 1-1/γ 2 1/β = ( 1-1/γ 2 ) -1/2 1-1/(2γ 2 ) Δt Lc/2 (1/γ 2 2-1/γ 12 ) = Lc/2 ( m 22 /E m 12 /E 1 2 ) Lc/2 ( m m 1 2 )/E 2

37 High Energy Particle Detectors in a Nutshell: