03/21/2005Michael Spitznagel (MIT)Page 1 The Babar Drift Chamber Michael Spitznagel Massachusetts Institute of Technology

03/21/2005Michael Spitznagel (MIT)Page 2 Interaction of Charged Particles with Matter Some important processes for relativistic particles traveling in a material medium: Excitation of atoms/molecules Ionization of atoms/molecules Cherenkov radiation Bremsstrahlung

03/21/2005Michael Spitznagel (MIT)Page 3 Purpose of a Drift Chamber The basic idea of a drift chamber is to measure the track of a charged particle by collecting electrons from ionization. This is particularly useful for measuring: Momentum Energy loss rate Timing information Number of tracks Sign of charge Multiple scattering and inelastic collisions are to be avoided!

03/21/2005Michael Spitznagel (MIT)Page 4 Some Basic Characteristics of Drift Chambers (Not necessarily a definitive list…) Many wires in a gas-filled volume Both anode (“sense”) and cathode (“field”) wires Usually operated in a magnetic field

03/21/2005Michael Spitznagel (MIT)Page 5 Drift Chamber Cell Wires are parallel (more or less…) A cell is a region in which electrons are drawn to some particular sense wire. In the Babar DCH, the cells are hexagonal.

03/21/2005Michael Spitznagel (MIT)Page 6 The Babar Detector

03/21/2005Michael Spitznagel (MIT)Page 7 Babar DCH (during assembly)

03/21/2005Michael Spitznagel (MIT)Page 8 Dimensions of Babar DCH Dimensions of the DCH (in mm) Inner cylinder is 1 mm beryllium near IP, and aluminum elsewhere. Outer cylinder is a carbon fiber composite.

03/21/2005Michael Spitznagel (MIT)Page 9 Babar cell layout 7104 cells Cells are hexagonal 40 layers 10 superlayers (the innermost 4 are shown)

03/21/2005Michael Spitznagel (MIT)Page 10 Charge Multiplication Electrons drift from site of primary ionization to a sense wire. The accelerating gradient causes an avalanche of secondary ionization. This amplifies the ionization signal received by the anode. Sense wire

03/21/2005Michael Spitznagel (MIT)Page 11 Wires A small sense wire diameter is good, since E ~ 1/r. The high-field region is critical to charge multiplication. For Babar DCH, Sense wires are 20  m Au-plated W-Rh. Field wires are 80 or 120  m Au-plated Al. Potential difference is 1930V. Sense wire

03/21/2005Michael Spitznagel (MIT)Page 12 Gas Drift chambers generally use a noble gas as the main part of the gas mixture. The main reason is to minimize energy loss due to molecule excitations. Tradeoffs: Argon and neon have more ionization Helium has less multiple scattering Babar DCH uses 80% He in its gas mixture.

03/21/2005Michael Spitznagel (MIT)Page 13 Beware the Photoelectric Effect Excited atoms will release photons. Many have energies exceeding the work function for the field wires. Electrons liberated from the field wires will start a new avalanche. This positive feedback will lead to chamber breakdown, which is bad.

03/21/2005Michael Spitznagel (MIT)Page 14 Quenching Gas To avoid chamber breakdown, a polyatomic gas is included in the gas mixture. The wide absorption band will accept the photons. Their energy can then be thermalized. The Babar DCH gas mixture includes 20% isobutane for this purpose.

03/21/2005Michael Spitznagel (MIT)Page 15 Electronegative gas Electronegative gases such as O2 can form negative ions. This effectively soaks up electrons. It can be used to absorb electrons emitted from field wires and thus avoid breakdown. The Babar DCH does not use O2 in this way. It’s designed to minimize O2 conc.

03/21/2005Michael Spitznagel (MIT)Page 16 Gas Mixture Additives (1/2) As a drift chamber ages, it can develop current spikes leading to additional damage and eventual breakdown. (the Malter effect) This may be due to a polymer buildup on field wires (cathodes) acting as a dielectric. When the chamber is turned on, positive ions drift to the dielectric surface. Electrons tunnel through the potential and are emitted from the cathode.

03/21/2005Michael Spitznagel (MIT)Page 17 Gas Mixture Additives (2/2) Adding water vapor or an alcohol to the gas mixtures greatly reduces the risk of “Maltering.” The Babar DCH uses 3500 ppm H2O. The mechanism is not understood for certain. It is hypothesized that these additives may penetrate the polymer layer and provide conductivity. (?)

03/21/2005Michael Spitznagel (MIT)Page 18 Babar DCH Hitmap HV has been turned off in a region due to some damage incurred before H2O was added.

03/21/2005Michael Spitznagel (MIT)Page 19 Bethe-Bloch model Bethe-Bloch formula models energy loss due to ionization and excitation. Minimum Ionizing Particle (mip) Relativistic Rise Fermi Plateau

03/21/2005Michael Spitznagel (MIT)Page 20 dE/dx (Babar data) Truncated mean is used due to long tail of Landau distribution of integrated dE/dx. dE/dx resolution is about 7%.

03/21/2005Michael Spitznagel (MIT)Page 21 Measurement of momentum In a solenoidal B field, a charged particle travels along a helical track. Its radius of curvature determines p T (momentum transverse to B field) Note that tracks below a minimum p T cannot go from the IP to the DCH. Z is determined by the slight skew of some layers.

03/21/2005Michael Spitznagel (MIT)Page 22 Axial and Stereo Superlayers A = axial superlayer U, V = stereo superlayers Aligning track segments describes the z position of that part of the track.

03/21/2005Michael Spitznagel (MIT)Page 23 Axial and Stereo Superlayers A = axial superlayer U, V = stereo superlayers Aligning track segments describes the z position of that part of the track. (Note: I may have switched the orientation of U vs. V in this diagram…)

03/21/2005Michael Spitznagel (MIT)Page 24 Lorentz Angle Crossed E and B fields cause charged particles to drift in the E x B direction. The Lorentz Angle is the angle between drift velocity and the E field. For a 1.5 T field, the Babar DCH has a Lorentz angle of about 32 degrees.

03/21/2005Michael Spitznagel (MIT)Page 25 Time-to-Distance 50 ns isochrones of a typical cell

03/21/2005Michael Spitznagel (MIT)Page 26 Babar DCH readout system

03/21/2005Michael Spitznagel (MIT)Page 27 Reading / References R. Gilmore, Single Particle Detection and Measurement, Taylor & Francis (1992). W. Blum and L. Rolandi, Particle Detection with Drift Chambers, Springer-Verlag (1994). S. Eidelman et al., Phys. Lett. B592, (aka PDG04. see especially sec. 28) The BABAR detector, B. Aubert et al., Nucl. Inst. Meth. A479 (2002) 1.

03/21/2005Michael Spitznagel (MIT)Page 28 The End! There are only a few backup slides which I have left in after this point.

03/21/2005Michael Spitznagel (MIT)Page 29 Babar DCH, Crosstalk Space charge can induce a signal on adjacent sense wires.

03/21/2005Michael Spitznagel (MIT)Page 30 Bethe-Bloch eqn (alternate slide) Note: the fonts keep getting messed up on this one. Even when I embed them or whatever. So I include this slide only as an appendix… d E d x = 0 : 3071 µ M e V g = cm ¶ Z A ½ 1 ¯ 2 z 2