General Information. Today s Agenda. Review Photon Interactions with Matter Gaseous Ionization Detectors

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1 General Information Today s Agenda Review Photon Interactions with Matter Gaseous Ionization Detectors

2 Interaction of Photons Thomson and Rayleigh Scattering No energy transfer (just change in photon direction) Low energies Rayleigh scattering off the atom as a whole (coherent effect) Photo Effect Low energy (~ binding energy of electrons in atoms) Higher cross section for high Z material (~ Z 4-5 ) Compton Scattering Medium energies Klein Nishina formula d 2 r d 2 e Compton edge (maximum recoil energy) T max E Pair Production E > MeV cos 2 1 cos cos E with 2 1 cos m e c

3 Analyzing a Spectrum Spectrum of a radioactive source taken with a scintillator. Energy resolution ~ 3% The peak on the right corresponds to an energy of 662 kev 1. What is the FWHM of the peak at 2.0? 2. Is this spectrum predominantly caused by,, or emission? 3. What causes this feature of the spectrum? 4. What is the energy of the edge? 5. What causes this peak? 6. What is its energy What effect leads to counts in this range? 5,6 8. What is the source of the counts in this range? 9. What is the source of this peak? 3,4 10.What radio-isotope is used to record this spectrum? 8 7 Energy

4 Analyzing a Spectrum Spectrum of a radioactive source taken with a scintillator. Energy resolution ~ 3% The peak on the right, a.k.a as photo-peak, corresponds to an energy of 662 kev 1. What is the FWHM of the peak at 2.0? E = 0.03 x 662 kev = 20 kev FWHM = 2.35 x E = 47 kev 2. Is this spectrum predominantly caused by,, or emission? This is (mostly) a source What causes this feature of the spectrum? 4. What is the energy of the edge? Compton Edge ( escapes) 1 Emax E 478keV mec / E 5. What causes this peak? 6. What is its energy Back-scattering of photon: E = E E max = 184 kev 5,6 3,4 7. What effect leads to counts in this range? Compton Scattering (e detected, escapes) 8 8. What is the source of the counts in this range? -decay, continuous e spectrum, low rate 9. What is the source of this peak? K-shell X-rays, E ~35 kev 7 Energy 10.What radio-isotope is used to record this spectrum? Cs -137

5 Photon Interactions In a senior lab experiment you are asked to measure the energy spectrum from the radiation of a 137 Cs source forward scattered electron backscatter e - decay gives off electrons with a range of energies Emax = 512 kev, 1174 kev decay gives off a monchromatic photon E = 662 kev

6 Detectors based on Registration of Ionization: Gas and Solid State Tracking Detectors Charged particles leave a trail of ions (and excited atoms) along their path: Electron-Ion pairs in gases and liquids, electron hole pairs in solids. The these charges can be registered Position measurement Tracking Detectors. Old(er) Instruments: Cloud Chamber: Charges create drops photography Bubble Chamber: Charges create bubbles photography (CCD camera) Emulsion: Charges blacked the film New(er) Instruments: Gas and Solid State Detectors: Moving Charges (electric fields) induce electronic signals that can be read by dedicated electronics.

7 fgas Detector Solid State Detector The induced signals are readout out by dedicated electronics. Particle 1 cm 200 m The noise of an amplifier determines whether the signal can be registered. Signal/Noise >>1 Argon Gas Germanium The noise is characterized by the Equivalent Noise Charge (ENC) = Charge signal at the input that produces an output signal equal to the noise. de/dx = MeV/cm de/dx = MeV/cm I = 26 ev -> ~ 80 e/ion pairs/cm I = 2.9 ev -> e/hole pairs/cm I=2.9eV 2.5 x 10 6 e/h pairs/cm ENC of very good amplifiers can be as low as 50e-, typical numbers are ~ 1000e-. In order to register a signal, the registered charge must be q >> ENC i.e. typically q>>1000e-. Gas Detector: q=80e- /cm too small. Solid state detectors have 1000x more density and factor 5-10 less ionization energy. Primary charge is times larger than in gases. 1/ Gas detectors need internal amplification in order to be sensitive to single particle tracks. ßγ Without internal amplification they can only be used for a large number of particles that arrive at the same time (ionization chamber).

8 Gas Detectors with internal Electron Multiplication Principle: At sufficiently high electric fields (100kV/cm) the electrons gain energy in excess of the ionization energy secondary ionization dn = N α dx N(x) = N 0 exp (αx) α = 1/ Townsend Coefficient N/ N 0 = A (Amplification, Gas Gain) Avalanche in a homogeneous field: Problem: High field on electrode surface breakdown E Ions Electrons In an inhomogeneous Field: α(e) N(x) = N 0 exp [α(e(x ))dx ] W. Riegler/CERN 8

9 Wire Chamber: Electron Avalanche Wire with radius (10-25m) in a tube of radius b (1-3cm): Electric field close to a thin wire ( kV/cm). Example: V 0 =1000V, a=10m, b=10mm E(a)=150kV/cm This electric field is sufficient to accelerate electrons to energies large enough to produce secondary ionization electron avalanche signal. a b b Wire W. Riegler/CERN 9

10 Principle of Signal Induction by Moving Charges A point charge q at a distance z 0 Above a grounded metal plate induces a surface charge. The total induced charge on the surface is q. Different positions of the charge result in different charge distributions. The total induced charge stays q. q q -q The electric field of the charge must be calculated with the boundary condition that the potential φ=0 at z=0. For this specific geometry the method of images can be used. A point charge q at distance z 0 satisfies the boundary condition electric field. The resulting charge density is (x,y) = 0 E z (x,y) -q (x,y)dxdy = -q I=0 W. Riegler/CERN 10

11 Principle of Signal Induction by Moving Charges If we segment the grounded metal plate and if we ground the individual strips the surface charge density doesn t change with respect to the continuous metal plate. q V The charge induced on the individual strips is now depending on the position z 0 of the charge. If the charge is moving there are currents flowing between the strips and ground. -q The movement of the charge induces a current. -q I 1 (t) I 2 (t) I 3 (t) I 4 (t) W. Riegler/CERN 11

12 Pulse Formation in a Cylindrical Wire Chamber In a cylindrical chamber the electric potential, E-field and capacitance are given by: CV0 CV0 1 ( r) ln( r / a) E( r) 2L 2L r C 2L ln( b / a) wire radius= a, tube radius=b, length of tube= L 1. The potential energy stored in the electric field is W=1/2CV Assume a charged particle goes through the cylinder and ionizes the gas. 3. As a charge, q, moves a distance dr there is a change in the potential energy (dw): d( r) q d( r) dw q dr and dw CV0dV dv dr dr CV0 dr 4. The total induced voltage from electrons produced at r is: q a d r q a ( ) ( CV0 ) dr ( q) a r V dr ln CV0 ar dr CV0 ar 2L r 2L a 5. The total induced voltage from positive ions produced at r is: q b d r q b ( ) ( CV0 ) dr q b V dr ln CV0 ar dr CV0 ar 2L r 2L a r Note: the total induced voltage is: V=V + +V - = -q/c Richard Kass

13 Pulse formation in a cylindrical wire chamber Note: the positive ions and electrons do not contribute equally to the V if there is multiplication in the gas. Since the avalanche takes place near the wire (r=1-2m) and the electrons are attracted to the wire the positive ions travel a much greater distance. For typical values of a (m) and b (1cm) we find: b ln 3 V ln10 75 a a r r V ln ln(11/10) a Time dependence of wire chamber signal We can find the voltage vs time by looking at V(t) for the positive ions: r( t) dv ( r) q r( t) V ( t) V ( t) dr ln r( 0) a dr 2L a The problem now is to find r(t). By definition, the mobility,, of a gas is the ratio of its drift velocity to electric field. v / E( r) For cylindrical geometry we have: dr dt E t) 1 E( r) dr dt CV0 1 CV rdr dt 2L r 2L ( 0

14 Pulse Formation in a Cylindrical Wire Chamber From previous page we had: r( t) rdr r(0) a CV t 0 dt 2L q r( t) V ( t) ln 2L a 0 r( t) a q CV ln(1 4L La CV0 rdr dt 2 L 2 CV L 0 2 t) 0 t 1/ 2 q ln(1 4L t0 2 2 b The total drift time is: T ( b a ) t a a Typical gas mobilities are =1-2 cm 2 s -1 V -1. Example: Let =1.5 cm 2 s -1 V -1, V=1500V, a=10m, b=1cm then: t 0 =1.5x10-9 s and T=1.5x10-3 s. t ln[1+t/t 0 ] 0 0 t t t t T t t 0 ) With: t 0 a 2 ln( b / a) 2V pulse shape determined by electronics 0 Time development of voltage pulse Richard Kass

15 Typical wire Chamber A wire chamber is just a gas tight container with a wire inside. The gas is the medium that gets ionized by a passing charged particle. The wire helps define an electric field and collects ionization, part of signal path. A typical cylindrical wire chamber has: a thin wire (anode) held at +V outside of cylinder (cathode) held at ground A charged particle passing through cylinder creates ions Movement of ions creates a voltage (or current pulse) Signal pulse travels down wire to outside world usually to preamplifier Location of charged particle is measured relative to wire

16 Operating Characteristics Depending on the applied voltage (E-field) recombination: no signals ionization: proportional: signals, but no gas gain big signals due to gas gain Geiger-Muller: gas gain so large it produces sparks (discharge) W. Riegler/CERN 16

17 Example of a Wire Chamber: BaBar LSTs Recently completed a project where we assembled several hundred wire chambers. Limited Streamer Tubes Ionizing particle produces a streamer a controlled spark-> ~same charge for all tracks Big signal induced on wire Tubes are made out of extruded PVC easy to assemble, low cost. LSTs are used to detect muons basement of Smith Lab 8 cells per tube Extruded PVC sleeve and profile Endcap (HV, Gas Inlet) full size tube ground plane full scale grad student Wire Graphite Wire coating holder

18 LSTs in BaBar August-October 2004: Two sextants of LSTs installed in BaBar August 2006: Remaining 4 sextants cosmic ray e+ e

19 Basic Spark Chamber Operation Chamber is filled with a noble gas (He-Ar) Energetic charged particles enter the chamber and ionize the gas. As the incident particle enters the scintillators above & below, a HV supply is triggered and the plates are taken to 6500V Under the influence of E-field the electron-ion pairs travel towards the plate Discharge occurs along the path of ionization

20 What it looks like...what we did...

21 Add a webcam to acquire images

22 Magic Happens!

23 Multiwire Proportional Chamber (MWPC) In late 1960 s early 1970 s techniques were developed that allowed many sense wires (anodes) to be put in the same gas volume. The MWPC was born! The spatial resolution () of an MWPC is determined by the sense wire spacing (x): x E.g. for x = 2 mm, = 600 m 12 Typical wire spacings are several mm, but MWPC with 1mm spacing have been built. cathode..... anodes sense wires x Gas volume Charpak wins 1992 Nobel Prize for developing MWPCs Advantages of MWPC: can cover large area systems with thousands of wires planar or cylindrical geometry can get pulse height info de/dx easy to get a position measurement (digital) can handle high rates works in magnetic field ease of construction Disadvantages of MWPC: poor spatial resolution elaborate electronics need low noise pre-amps elaborate gas system must understand electrostatics forces on wires Richard Kass

24 Picture of MWPC wire frame (5 layers) With cathodes removed MWPC MWPCs can have very high efficiencies Plateau Gas Mixtures: Noble gases, Methan, Isobutane For example 90% Ar, 10 CH 4

25 The second coordinate Crossed wire planes. Ghost hits. Restricted to low multiplicities. Also stereo planes (crossing under small angle). Charge division (Resistive wires Carbon,2k/m) Q A y track Q B y L Q QB Q A B y L up to 0.4% L Timing difference (DELPHI, OPAL vertex detector) CFD y L T track CFD ( T ) 100 ps ( y) 4cm (OPAL) 1 wire plane + segmented cathode planes

26 Multi Wire Proportional Chamber Cathode strip: Width (1) of the charge distribution distance between Wires and cathode plane. Center of gravity defines the particle trajectory. Avalanche (a) (b) Anode wire 1.07 mm Cathode strips 0.25 mm C 1 C 1 C 1 C 1 C 1 C mm C 2 C 2 C 2 C 2 W. Riegler/CERN 26

27 Drift Chambers Drift Chambers are MWPCs where the time it takes for the primary ionization to reach the sense wire is recorded. This time gives position information. scintillator DELAY Stop TDC Start drift anode Measure arrival time of electrons at sense wire relative to a time t 0. x v ( D t ) dt low field region drift high field region gas amplification What happens during the drift toward the anode wire? Diffusion Drift velocity

28 Drift Velocity For some gas mixtures the drift velocity is ~constant (independent of the electric field): x = v (t-t o ) A gas with almost constant drift velocity is Argon-Ethane, drift velocity 50m/nsec By using the drift time information we can improve our spatial resolution by a factor of 10 over MWPCs (1mm 100 m). Hex-cell drift chamber drift times are circles around the sense wires

29 Spatial Resolution The spatial resolution of a drift chamber is limited by three effects: Statistics of Primary Ionization Location of the primary ionization (a few 100 m apart) Dependent on gas (density -> pressure) Diffusion of electrons as they drift to the wire 1 n 2Dx E N=# of primary ions D=gas diffusion constant =mobility Precision of time measurement (electronics) Must measure time to 1 ns or better Must know start time t o

30 Magnetic Fields Magnetic fields (common in particle detectors for momentum measurement) change the drift path Electrons no longer drift along E field lines Lorentz Angle Also affect drift velocity and diffusion Special case: tan L E B L : Lorentz angle B eb cyclotron frequency m y L v D x E v D E = mean time between collisions CDF Central Tracking Chamber 660 drift cells tilted 45 0 with respect to the particle track.

31 Drift Chamber Configurations Drift chambers come in all sizes, shapes and geometries: planar fixed target cylindrical colliding beam Time information gives a circle of constant distance around the sense wire (more complicated in B field) Wires in different layers are staggered to resolve the left-right ambiguity Typical cylindrical DC: Many wires in same gas volume. Use small angle stereo for z. Usually use single hit electronics. Sense (anode) and field wires. CLEO, CDF, BELLE, BABAR Tube Chamber: Single sense wire in a cylinder Can make out of very thin wall tubes. very little material Small drift cell single hit electronics Good cell isolation broken wire only affects one tube CLEO s PTL detector

32 40 layers total 10 super layers A Real Life Drift Chamber-BaBar 100ns isochromes spatial resolution In B=1.5T the ions do not drift straight to the sense wire (anode) Time to distance relationship complicated! mom. resolution 7104 sense wires (20m diameter) 30g tension in each wire, sag~200m In order to measure z (along wire) some wires are slanted at a slight angle AR/Isobutane gas (80/20%) HV=~1950V

33 Drift Tubes ATLAS MDT R(tube) =15mm Calibrated Radius-Time correlation Primary electrons are drifting to the wire. Electron avalanche at the wire. The measured drift time is converted to a radius by a (calibrated) radius-time correlation. Many of these circles define the particle track. ATLAS Muon Chambers ATLAS MDTs, 80m per tube W. Riegler/CERN 33

34 The Geiger counter reloaded: Drift Tube Atlas Muon Spectrometer, 44m long, from r=5 to11m Chambers 6 layers of 3cm tubes per chamber. Length of the chambers 1-6m! Position resolution: 80 m/tube, <50 m/chamber (3 bar) Maximum drift time 700ns Gas Ar/CO2 93/7 W. Riegler/CERN 34

35 Time Projection Chamber (TPC): Gas volume with parallel E and B Field. B for momentum measurement. Positive effect: Diffusion is strongly reduced by E//B (up to a factor 5). Drift Fields V/cm. Drift times s. Distance up to 2.5m! Full 3-D reconstruction gas volume B drift y x E z charged track Wire Chamber to detect the tracks (x, y) Drift time gives z

36 STAR TPC (BNL) Event display of a Au Au collision at CM energy of 130 GeV/n. Typically around 200 tracks per event. Great advantage of a TPC: The only material that is in the way of the particles is gas very low multiple scattering very good momentum resolution down to low momenta! Drawbacks: Very complicated electric field shaping Long drift times Complicated gas system

37 ALICE TPC: Detector Parameters Gas Ne/ CO 2 90/10% Field 400V/cm Gas gain >10 4 Position resolution = 0.25mm Diffusion: t = 250m cm Pads inside: 4x7.5mm Pads outside: 6x15mm B-field: 0.5T W. Riegler/CERN 37

38 ALICE TPC: Construction Parameters Largest TPC: Length 5m Diameter 5m Volume 88m 3 Detector area 32m 2 Channels ~ High Voltage: Cathode -100kV Material X 0 Cylinder from composite materials from airplane industry (X 0 = ~3%) W. Riegler/CERN 38

39 ALICE TPC: Pictures of the Construction Precision in z: 250m End plates 250m Wire chamber: 40m W. Riegler/CERN 39

40 First Cosmic Muon Event Displays from the ALICE TPC June 2008! 4/23/2012