Tracking Detectors Basic Tracking Concepts Footprints Charged Particles Common Detector Technologies

Tracking Detectors Basic Tracking Concepts Footprints Charged Particles Common Detector Technologies

Tracking Detectors Basic Tracking Concepts Footprints Charged Particles Common Detector Technologies

Bolinger, Brenda, Features Reporter has reference to this Academic Journal, PHwiki organized this Journal Tracking Detectors Masahiro Morii Harvard University NEPPSR-V August 14-18, 2006 Craigville, Cape Cod Basic Tracking Concepts Moving object (animal) disturbs the material A track Keen observers can learn Identity What made the track Position Where did it go through Direction Which way did it go Velocity How fast was it moving Footprints A track is made of footprints Each footprint is a point where “it” passed through Reading a track requires: Looking at individual footprints = Single-point measurements Position, spatial resolution, energy deposit Connecting them = Pattern recognition in addition to fitting Direction, curvature, multiple scattering To as long as m a good track, footprints must require minimal ef as long as t It cannot be zero — or the footprint won’t be visible It should not affect the animal’s progress too severely

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Charged Particles Charged particles leave tracks as they penetrate material “Footprint” in this case is excitation/ionization of the detector material by the incoming particle’s electric charge Discovery of the positron Anderson, 1932 16 GeV – beam entering a liquid-H2 bubble chamber at CERN, circa 1970 Common Detector Technologies Modern detectors are not necessarily more accurate, but much faster than bubble chambers or nuclear emulsion Limited by electronics From PDG (R. Kadel) Coulomb Scattering Incoming particle scatters off an electron in the detector Trans as long as m variable to T Integrate above minimum energy ( as long as ionization/excitation) in addition to multiply by the electron density See P. Fisher’s lecture from NEPPSR’03 charge Ze energy E charge e mass me recoil energy T = dE energy E – dE Ruther as long as d

Bethe-Bloch Formula Average rate of energy loss [in MeV g–1cm2] I = mean ionization/excitation energy [MeV] = density effect correction (material dependent) What’s the funny unit E E +dE How much material is traversed dx = thickness [cm] density [g/cm3] How much energy is lossed –dE [MeV] Bethe-Bloch Formula dE/dx depends only on ( in addition to z) of the particle At low , dE/dx 1/2 Just kinematics Minimum at ~ 4 At high , dE/dx grows slowly Relavistic enhancement of the transverse E field At very high , dE/dx saturates Shielding effect dE/dx vs Momentum Measurement of dE/dx as a function of momentum can identify particle species

Minimum Ionizing Particles Particles with ~ 4 are called minimum-ionizing particles (mips) A mip loses 1–2 MeV as long as each g/cm2 of material Except Hydrogen Density of ionization is Determines minimal detector thickness Primary in addition to Secondary Ionization An electron scattered by a charged particle may have enough energy to ionize more atoms Signal amplitude is (usually) determined by the total ionization Detection efficiency is (often) determined by the primary ionization 3 primary + 4 secondary ionizations Ex: 1 cm of helium produce on average 5 primary electrons per mip. A realistic detector needs to be thicker. Multiple Scattering Particles passing material also change direction 1/p as long as relativistic particles Good tracking detector should be light (small x/X0) to minimize multiple scattering x is r in addition to om in addition to almost Gaussian

Optimizing Detector Material A good detector must be thick enough to produce sufficient signal thin enough to keep the multiple scattering small Optimization depends on many factors: How many electrons do we need to detect signal over noise It may be 1, or 10000, depending on the technology What is the momentum of the particle we want to measure LHC detectors can be thicker than BABAR How far is the detector from the interaction point Readout Electronics Noise of a well-designed detector is calculable Increases with Cd Increases with the b in addition to width (speed) of the readout Equivalent noise charge Qn = size of the signal that would give S/N = 1 Typically 1000–2000 electrons as long as fast readout (drift chambers) Slow readout (liguid Ar detectors) can reach 150 electrons More about electronics by John later today Shot noise, feedback resistor Silicon Detectors Imagine a piece of pure silicon in a capacitor-like structure Realistic silicon detector is a reverse-biased p-n diode +V dE/dxmin = 1.664 MeVg–1cm2 Density = 2.33 g/cm3 Excitation energy = 3.6 eV 106 electron-hole pair/cm Assume Qn = 2000 electron in addition to require S/N > 10 Thickness > 200 m +V Lightly-doped n layer becomes depleted Heavily-doped p layer Typical bias voltage of 100–200 V makes ~300 m layer fully depleted

BABAR Silicon Detector Double-sided detector with AC-coupled readout Aluminum strips run X/Y directions on both surfaces n- bulk Al Al n- bulk n+ implant SiO2 p stop Al Al p+ implant X view Y view 300 m BABAR Silicon Detector Bias ring p+ Implant Al p+ strip side P-stop n+ Implant Polysilicon bias resistor Polysilicon bias resistor Edge guard ring Edge guard ring n+ strip side 50 mm 55 mm Wire Chambers Gas-based detectors are better suited in covering large volume Smaller cost + less multiple scattering Ionization < 100 electrons/cm Too small as long as detection Need some as long as m of amplification be as long as e electronics From PDG A. Cattai in addition to G. Rol in addition to i Gas Amplification String a thin wire (anode) in the middle of a cylinder (cathode) Apply high voltage Electrons drift toward the anode, bumping into gas molecules Near the anode, E becomes large enough to cause secondary ionization Number of electrons doubles at every collision Avalanche Formation Avalanche as long as ms within a few wire radii Electrons arrive at the anode quickly (< 1ns spread) Positive ions drift slowly outward Current seen by the amplifier is dominated by this movement Signal Current Assuming that positive ion velocity is proportional to the E field, one can calculate the signal current that flows between the anode in addition to the cathode This “1/t” signal has a very long tail Only a small fraction (~1/5) of the total charge is available within useful time window (~100 ns) Electronics must contain differentiation to remove the tail Gas Gain Gas gain increases with HV up to 104–105 With Qn = 2000 electrons in addition to a factor 1/5 loss due to the 1/t tail, gain = 105 can detect a single-electron signal What limits the gas gain Recombination of electron-ion produces photons, which hit the cathode walls in addition to kick out photo-electrons Continuous discharge Hydrocarbon is often added to suppress this effect Drift Chambers Track-anode distance can be measured by the drift time Need to know the x-vs-t relation Time of the first electron is most useful Drift time t Drift velocity Depends on the local E field Drift Velocity Simple stop- in addition to -go model predicts = mobility (constant) This works only if the collision cross section is a constant For most gases, is strongly dependent on the energy vD tends to saturate It must be measured as long as each gas c.f. is constant as long as drift of positive ions = mean time between collisions Bolinger, Brenda Claremont Courier Features Reporter

Drift Velocity Example of vD as long as Ar-CF4-CH4 mixtures “Fast” gas Typical gas mixtures have vD ~ 5 cm/s e.g. Ar(50)-C2H6(50) Saturation makes the x-t relation linear “Slow” gas mixtures have vD E e.g. CO2(92)-C2H6(8) T. Yamashita et al., NIM A317 (1992) 213 Lorentz Angle Tracking detectors operate in a magnetic field Lorentz as long as ce deflects the direction of electron drift Early cell design of the BABAR drift chamber Spatial Resolution Typical resolution is 50–200 m Diffusion: r in addition to om fluctuation of the electron drift path Smaller cells help “Slow gas” has small D Primary ionization statistics Where is the first-arriving electron Electronics How many electrons are needed to register a hit Time resolution (analog in addition to digital) Calibration of the x-t relation Alignment D = diffusion coefficient Micro vertex chambers (e.g. Mark-II)

Other Per as long as mance Issues dE/dx resolution – particle identification Total ionization statistics, of sampling per track, noise 4% as long as OPAL jet chamber (159 samples) 7% as long as BABAR drift chamber (40 samples) Deadtime – how quickly it can respond to the next event Maximum drift time, pulse shaping, readout time Typically a few 100 ns to several microseconds Rate tolerance – how many hits/cell/second it can h in addition to le Ion drift time, signal pile up, HV power supply Typically 1–100 kHz per anode Also related: radiation damage of the detector Design Exercise Let’s see how a real drift chamber has been designed Example: BABAR drift chamber Requirements Cover as much solid angle as possible around the beams Cylindrical geometry Inner in addition to outer radii limited by other elements Inner radius ~20 cm: support pipe as long as the beam magnets Out radius ~80 cm: calorimeter (very expensive to make larger) Particles come from decays of B mesons Maximum pt ~2.6 GeV/c Resolution goal: (pt)/pt = 0.3% as long as 1 GeV/c Soft particles important Minimize multiple scattering! Separating in addition to K important dE/dx resolution 7% Good (not extreme) rate tolerance Expect 500 k tracks/sec to enter the chamber

One Wedge of Electronics Per as long as mance Average resolution = 125 m Further Reading F. Sauli, Principles of Operation of Multiwire Proportional in addition to Drift Chambers, CERN 77-09 C. Joram, Particle Detectors, 2001 CERN Summer Student Lectures U. Becker, Large Tracking Detectors, NEPPSR-I, 2002 A. Fol in addition to , From Hits to Four-Vectors, NEPPSR-IV, 2005

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