Tracking Detectors: Proportional and Drift Chambers How we measure particle momenta & ID (in some cases): 2 types of tracking detectors Prof. Robin D. Erbacher University of California, Davis References: R. Fernow, Introduction to Experimental Particle Physics, Ch. 9, 10 D. Green, The Physics of Particle Detectors, Ch. 8 K. Kleinknecht, Ch. 3 http://pdg.lbl.gov/2004/reviews/pardetrpp.pdf

Radiative Energy Loss Optical behavior of medium: characterized by complex dielectric constant Sometimes instead of ionizing an atom or exciting matter, the photon can escape the medium: (transition, C, etc).

Nuclear Interactions We have seen that the electromagnetic interaction is responsible for: Energy loss of charged particles Small angle scattering of charged particles Production and interaction of photons In some processes, the nuclear interaction dominates: Particle creation reactions Interactions at high energy / large momentum transfer Interactions of neutral particles other than photons After Beryllium, Nuclear cross section: Goes as A2/3 times Compton wavelength2 of proton Brem. cross section: Goes as Z2 due to coherence, 3 penalty due to 3 vertices For A~3(Li), they’re comparable, for A>3, X0(Brem) dominates

Strong Interactions abs = T - el - q Hadrons are subject to the strong interaction in matter. The neutron: ideal probe since no real em interactions. For attenuation in matter, the absorption cross section is: abs = T - el - q Here el is coherent elastic scattering off a nucleus, and q is quasielastic scattering from individual nucleons. Absorption lengths of particles: For particle production, T(pp, pn)~ln(s), T(n-nucl)~A2/3. The e+e- or e-nucleon initial state can couple to hadronic final state via a virtual , and apart from resonances, the e+e- hadronic production cross section falls off as s-1. For elastic and quasielastic scattering, the hadron retains its identity and its momentum is slightly perturbed.

Nuclear interaction length: Strong Interactions What is λn? The mean distance (in cm, or g/cm2) a particle travels in a material before encountering a nucleus Nuclei act as hard-sphere scattering centers Cross section is proportional to nuclear radius squared, or Z2/3 material X0 (g/cm2) λn (g/cm2) H2 63 52.4 Al 24 106 Fe 13.8 132 Pb 6.3 193 Nuclear interaction length: lead ~ steel: 17 cm

Weak Interactions The 2nd fundamental nuclear interaction: weak interaction. Important for  interactions, decays of quasistable particles. Neutrinos are direct probes of the weak interaction, but do not interact with matter very easily. Weak interaction can play role in charged lepton interactions: does not preserve parity. Thus, signatures separating weak from em interaction are parity-violating. Asymmetries: polarized e-N scattering (SLAC) and muon pair production in e+e- collisions at PETRA. Two major classes of weak interaction known: Charged current: e-nucleon via W e (lepton # cons.) Neutral current: e-nucleon via Z0, final state lepton neutral: e Charged current: final state lepton is charged

Proportional Counters Tracking: Proportional Counters and Drift Chambers Charged current: final state lepton is charged

Charged Particle Tracking Two main types: gas wire chambers, silicon (We heard about silicon last time) Innermost detectors: want precise trackingSi! Outer detectors: silicon too expensive! Basic design: ionization chamber with HV sense wire: amplification of 103 - 105 in high field near wire

Ionization Wire Chambers Wire Chambers: Probably the most commonly used detection devices in high energy physics experiments. The Basics of Wire Chambers: Charged particles travels through a volume of gas Gas is ionized by the passage of the particle Ionization drifts & diffuses in an electric (and magnetic) field toward an electrode Collection and amplification of anode signal charge, and charge induced on cathode creates detectable signals Measurement of points on trajectory determines p

Processes in Gases Ionization potential When a charged particle passes through gases subject to an E field, it loses energy by: Elastic scattering (small) Excitation: gas atoms/molecules excite then deexcite by  emission Ionization (most important) Ionization: One or more electrons are liberated from atoms of the medium, leaving positive ions and electrons. Energy imparted to atom exceeds ionization potential of gas. Shown in 1920 in an electron gas tube, the current that passes from a cathode to an anode in a given E field follows Langmuir’s equation. At some point, as the voltage is increased, though, the current rises faster than that equation. The point of deviation is the ionization potential Of the gas… here ions are created and the current becomes larger. Ionization Potentials of Vapors -Clifton G. Found, G.E. Phys. Rev. 16, 41-53 (1920) ©1920 The American Physical Society

Primary Ionization Potential Charged current: final state lepton is charged Ionization Potentials of Vapors -Clifton G. Found, G.E. Phys. Rev. 16, 41-53 (1920) ©1920 The American Physical Society

Secondary and Total Specific Ionization Average energy lost in creating ion pair ~30 eV. Primary ionization: number of ionizing collisions per unit length for the incident particle. (Poisson distribution) Secondary ionization: some of the electrons ( rays) have energy to cause more ionization. Total specific ionization: total # ions actually created per unit length. (Landau distribution: long tails) Deexcitation: Further ionization from deexcitation of meta-stable atoms (Penning effect). Example: Ne* + Ar  Ne + Ar+ + e- Peak fraction of E lost can double by adding 0.1% Ar gas. production of delta rays (and large 2ndary ionization) can create long tails in the ionization frequency distribution

Number of Ions v. Voltage Simplest case: Parallel plate capacitor Charged current: final state lepton is charged Ionization chamber: Voltage increased such that the charge arriving on plates = charge formed Proportional region: Initial electrons accelerated enough to ionize more; avalanche pulse proportional to primary ionization; reaches ~108

After Ionization, What Next? Once ion pairs are created, many processes can occur: Recombination, charge exchange, attachment, absorption. Want electrons/ions to recover after signals have been read out. Recombination of positive/negative ions to neutrals: X+ + Y-  X + Y +  Electrons can be removed by recombination as well: X+ + e-  X +  Add gas w/ large electron affinity: energy difference of lowest (ground) state of neutral and lowest state of corresponding negative ion. X + e-  X- +  Water vapor, O2, ethanol, SF6, CCl4, freon Electron affinity: in some cases the electrons can attach themselves to neutrals esp if it closes an outer shell

Charge Exchange Another way to eliminate positive ions is through Ionization potential of the ion is greater than some molecule mixed with the gas, usually polyatomic gas like ethanol or methylal. Gas quenches the ion multiplication by neutralizing ions of the main chamber gas: dissipates ionization energy by dissociating into smaller fragments and absorbs s emitted in radiative deexcitation process. This is called a quenching gas.

Proportional Chambers V0 V=0 Consider a simple cylindrical proportional tube of outer radius b at voltage V0 and inner (wire) of radius a at voltage zero. Electric field inside the chamber: Charged particle -> ionization. e- move toward anode. High fields near wire->mulitiplication of e-s by collisions: at small r the energy gain can exceed ionization potential. Runaway process, like avalanche in PMTs. Gas gain: Change dN(r) in # electrons at r in multiplication process, & depends on first Townsend coefficient : inverse distance b/w collisions Mean free path for ionization: Typical Gas gain~105. For > r~20 pairs, or 108, Geiger region: runaway!

Pulse Formation In the ionization chamber region, we have a gain of unity. For these field strengths, the dominant signals are due to electrons. For proportional chambers, primary ionization electrons are multiplied due to high fields near wire, E(r)~1/r. Electrons collect on anode, ions remain, ‘sheathing” wire, moving slowly toward cathode. Moving chargesenergy changescapacitively-induced signal, dV. Suppose multiplication takes place at N radii away r~Na. Voltage on anode due to e- and ions: V- and V+. Let qs be source charge. The induced voltage due to e- and ion motion is: Signal due to ions dominates, as they travel all the way to the cathode.

Drift Velocity and Mobility A gas with molecules of mass M has a thermal velocity vT: For N2, vT~0.1 cm/s (Earth escape v~1.1 cm/s). For electrons, vT~4-40 cm/s, below the binding energy of atoms. Wire chambers operate in an electric field: consider the drift velocity for electrons and ions in a uniform electric field. Let the mean time between collisions, , be , and the acceleration be a~eE/M. Then Lighter gases have thermal velocities in the range 0.1 to 1.0 cm/microsecond, so escape the atmosphere. Using that the mean free path is proportional to the collision cross section: we express the drift velocity in terms of mobility. Mobility: drift velocity per unit reduced electric field E/. Here

Multiwire Proportional Chambers The MWPC was invented by Charpak at CERN. Principle of proportional counter is extended to large areas. Stack several wire planes up in different direction to get position location. 1992 Prize Avalanche developing

Precision Drift Tracking Next idea: stack up proportional wire drift tubes, measure time of arrival of the ionization pulse Find track from tangents to circles Can get about 150 μm position resolution But: too much material! Charged current: final state lepton is charged

Large Area Drift Chambers The “open cell” drift chamber uses field and sense wires: field wires create shape of electric field, sense wires detect time of arrival of pulse. This is the design of the CDF drift chamber - the cells are tilted to take into account ExB drift! Charged current: final state lepton is charged

Drift Chambers There is an unequivocal correlation of the time difference t between the particle passage and the rising edge of the anode signal with the distance between the point of primary ionization and the anode wire in a proportional chamber. t0=initial ionization, t1=avalanche near wire begins If the drift velocity is vD a constant, it becomes linear. Example: With constant vD~55 mm/s, a time measurement of 4 ns can give spatial resolution ~200 m! (Typical resolutions for MWPC~0.5 mm) To keep vD constant, keep field along direction of path constant. Impossible to do with MWPC layout! Zero field between anode wires. Change geometry: add field wire -HV between two anode wires +HV: -HV1 +HV2 Negative potential Drift region

Chamber Geometries Planar: Large planar detectors resolution limited by spatial positioning of wires, and gravity sagging. ~100-300 m. Smaller limited by TDC resolution (>40 m) and e- diffusion. Cylindrical: Four main types are proportional, cylindrical, jet, and TPC (time projection chamber). Used in colliding beam experiments, in conjunction with solenoidal fields. The regular cylindrical is open celled, whereas the jet chamber has radial partitions. The TPC is unique in that the electrons drift along the E and B field lines, and z coordinate is more accurate than the rest. Proportional: These are the familiar (mainly rectangular) counters with anode wires sandwiched between cathode sheets. Simple, but not as good resolution without cathode strip readout.

Chamber Geometries E E Proportional: wires between planes Jet Drift Chamber +HV + + +HV -HV E E B Cylindrical Drift Chamber Time Projection Chamber

Drift Gases

Performance The.

Performance

Particle Identification The. Charged current: final state lepton is charged