Outline, measurement of position Proportional and drift chambers: issues: field shape, geometry of the chamber, gas mixture, gas gain, resolution, drift velocity and drift time, Lorentz angle. Cathode readoutcenter of gravity method. Large planar detectorswire tension and stability. TPC: issues: gas mixture, drift in electric-magnetic fields, field distortions, calibration using lasers, resolution, double track resolution, drift time. Cathode readout- center of gravity method Microstrip gas chambers: high rate operation, resolution Bubble chambers, streamer chambers, flash chambers, spark chambers, nuclear emulsions, Silicon detectors and CCD.
Proportional and drift chambers, shapes Proportional chambers Drift chambers - - - - - - + -8-2 0 The price to pay is left-right ambiguity, and longer drift time of electrons-> slower detectors Field shaping wires Tracking detectors contain as little material as possible, to not to "disturb" particles Measure drift time of electrons to anode wire to reconstruct particle passage
Field shape, parameters
Secondary coordinate
Drift chambers
Some field designs
Resolution, geometry
What geometry? Anodes Cathode CDF outer tracker Potential wires mixed with sense wires allow tuning of voltage Cathode planes define cell Shaper wires at ends allow fine tuning E,B fields perpendicular
What geometry: II CDF outer tracker 8 superlayers, each with 12 wires 30,000 sense wires, 3.1m long Max drift distance 9mm, 100ns Resolution 180µm
Drift chamber-jet chamber The Inner Detector- tracking drift chamber detector surrounds the Silicon Vertex detector Inner detector, computer track reconstruction The beam pipe It consists of 24 sectors with, each of them has 36 anode wires in the middle. To reconstruct particle tracks one has to reconstruct drift times to wires - left-right ambiguity beam Spatial resolution ~ 200 microns pipe C02 + isobuthane used
Avalanche shape Use amplification near wire Field grows as 1/r At some point electrons cause ionisation Gas gain 10 4 10 6 possible +ve ions drift back to cathode
What is this gas? Several mixtures possible 95/5 Argon/C 4 H 10 90/10 Argon/CO 2 50/50 He/Ethane Many 'cooking' mixtures: Must allow electron drift yet damp any shower Speed may well be an issue (5cm/µs) Gas gain important Sometimes stability (c/f pressure, T, V) vital
How electrons drift Multiple collisions with gas molecules limit speed If B field applied, try to spiral round B field lines and move along E field Move at Lorentz angle: tan θ = v D B/E ev D B θ ee
Muon Detectors Wire chambers very appropriate technology 'Filtering' has been done by steel beforehand. ATLAS uses more magnets to measure muon momenta
A gentler world: ALEPH TPC measured in meters Structure of wires & pads gives arrival point Time gives z coordinate Fully 3D Resolution ~ 160µm
The TPC Aligned E, B fields make for uniform drift Essential when drifts are of of metres Any irregularities must be very carefully monitored Lasers used to create straight reference tracks 2m drift path implies 20µs+ drift times Only possible with comparable bunch crossing All the wires etc. are outside the tracking volume: minimum material in particles' path
TPC pads Electrons still drift to wires But charge is induced on pads next to the wires Provide extra dimension Both are readout. Might be 150 wires crossed, less pads.
Microstrip gas chambers
Microstrip gas Field geometry
Silicon Detectors 0V Depletion area close to implantsthickness depends on U and resistivity n bulk. p+ implants Primary particle Depleted diode - no charge carriers Charged particle creates electron-hole pairs (depletion zone) Electrons drift towards backplane Holes drift towards implants - mobility 1/3 electrons +100V Typical voltage 20
Silicon Detectors Detector typically 300µm thick gives 23000 e-h pairs (calculate!) Readout in 10-20ns, fast! Position resolution normally limited by readout pitch Charge division improves it (capacitance coupling) delta rays degrade it Shift due to Lorentz angle (calculate! ) B-Field typicaly parallel to strips, thus perpendicular to E-field (why?) 5µm reasonable aim. Silicon quite high 'z'; radiation length important (1-2%X 0 per layer) 100 layer silicon tracker has TOO MUCH material 21
Microstrips and Pixels Like wire-chambers, geometry is flexible. Can create 'pixel' or 'microstrip' arrangements Pixels maybe 50µm by 200µm have higher granularity, good but more expensive (more electronics) Microstrips typically 50-100µm by several cm. Often readout from end, no extra material in tracker. Pixels may be individually read (fast but Millions of channels) or CCD-style R/O at end All may have analogue or digital electronics analogue records signal size (de/dx) digital just records hit present/absent
CCD and pixel Charge-Coupled Devices (CCDs) Charge-Coupled devices are more commonly used as optical detectors (in solid state cameras and video recorders) but can also be used as particle detectors. They typically consist of a rectangular array of some 30,000 to 200,000 potential wells (pixels) about 30 microns square covering the surface of the semiconductor. The potential wells are maintained by a set of phased electrodes insulated from the bulk of the semiconductor. When a charged particle passes through the detector, the charge is trapped in one of the wells. Subsequently, if the electrode potentials are "clocked" in an appropriate manner, the entire image of charges is moved coherently across the detector. One side of the detector is adapted to form a shift-register which can be clocked to move the charge in a perpendicular direction. The readout sequence thus consists of moving the image across by one column, then reading a column out pixel by pixel from the shift-register port, before clocking the image across another column, etc. In this way, the 100,000 or so channels of the whole CCD can be read out through a single preamplifier and readout channel. The price paid for this is that the readout is very slow (of the order of milliseconds), with the device integrating the signal over a long period. Typically, the particle source must be vetoed during this period, and the detector cooled with liquid nitrogen to reduce thermal noise. CCDs have very good spatial resolution, typically of the order of 5 microns in each direction.
CCD and pixel detectors CCD (used in digital cameras) are silicon pixel detectors with simplified single channel read-out)
Which to use? Pixels and CCD's are 3D Strip detectors give essentially 2D information Crossed strips gives 3D...with 'ghosts' One hit unambiguous Two hits gives ambiguous results Made worse if inefficient If particle densities low enough, crossed strips are cheaper In more difficult cases, use pixels.
ATLAS Strip detector 6 ASIC's 128 channel each Two sides 1536 channels 12cm by 6cm 4000 of these required for ATLAS RAL building 600 major excercise
Measuring tracks Precise measurement of the particle tracks - curvature - in the magnetic field - is the basis of the momentum reconstruction of the particle. It also allows us to "notice" longer lived particles, like K_0 and Lambda_0 which decay few cm after they were produced. Precise tracking detectors- silicon tracker- allow to "notice" particles containing the b-quark, which lives of the order of picoseconds! and travels in the detector a fraction of a milimeter. This is very important when searching for the Higgs boson. Silicon tracker surrounds the beam pipe It consists of planks covered with 20 micron wide semiconductor strips. Each strip is read-out- million of channels! Passing particle creates minority carriers in a semiconductor p-n junction which give the signal. Precision of position reconstruction ~10 microns DELPHI silicon
silicon and pixel detectors Are a part of every major detector in high energy physics: CMS, Atlas, all LEP detectors, Aleph, DELPHI, OPAL, L3 had silicon detector. Tevatron experiments D0 and CDF have silicon detectors.
Material in a tracker ATLAS ALEPH Total in ATLAS tracker exceeds 1 X 0 at worst photon conversion seen in ALEPH Material causes photon conversions, brehmstahlung, multiple scattering etc.
ATLAS TRT Each anode enclosed in a 2mm straw, 40ns drift time 300K straws 36 hits per tracks Resolution 180µm BX: 25ns
Bubble chambers, spark chambers, streamer chambers - Bubble chambers. First large scale particle detectors. Liquidified gas such as H2, D2, Ne, C3H3 is kept close to the boiling point. The chamber volume is expanded after passing of the particle. Gas bubbles form in the liquid after passing of the particle. Can be photographed. Reconstruction of track in magnetic field, moreover bubble density is proportional to ionization energy loss de/dx. - Spark chambers. First fast large scale detector allowing for electronic read-out. Sparks between plates, passage of particle perpendicular to plates. 20 kv/cm. Filled with noble gases- quick discharge - streamer chambers. Gas-filled detectors with two planar electrodes. Particles pass parallel to electrodes. Has to be externally triggered. After passage of the particle high E-field (40 kv/cm) is applied for a short while. Small contained sparks (streamers) are formed, which can be photographed.
Picture from a bubble chamber Interactions of kaon beam in H2. Note that tracks spiral (why?) and that the track thickness increases at the end of the track. Why?? Typically two or more cameras to reconstruct stereo view after
A famous bubble chamber, Gargamelle One of the largest, filled with heavy gases - freon. Constructed to record neutrino interactionsdiscovery of neutral currents.
Bubble chambers -questions Bubble chambers are still used sometimes, they have excellent 3d spatial resolution. Good to look at various decays. - why cannot we make even bigger bubble chambers? - where the dead-time of bubble chamber comes from? Can it be small? - can bubble chambers distinguish high energy muons from other particles, and how? -can we stop a hadronic cascade inside a bubble chamber and measure its energy?
Streamer chambers - Trying to make something with a bubble chamber resolution but a bit faster. - high electric field applied for a short (10ns) while to allow streamers to grow enough to be photographed. + 500 kv, 10ns 0 - used up-to date to investigate short lived particles in fixed target experiments
Famous picture from a streamer chamber pions are produced abundantly in cosmis rays interaction, and they decay to a muon and a neutral, very weakly interacting particle, as shown on a picture below. Picture from streamer chamber shows a pion produced in antiproton -neon interaction. It decays to a µ + and an invisible particle, µ + decays to an e + and some invisible particles again. Particles are in magnetic field -thus spiraling, one can measure their momenta from the radius of the spiral. Big question. Is the neutrino from pion decay the same as neutrino from beta decay, or is it of a different nature? Check it by observing neutrino induced interactions: π + > µ + ν, ν + n -> p µ -? or Experiment of Lederman, X X ν+ n -> p e _ Steinberger,Schwarz (Nobel91)? X
Spark chambers. Cosmic muon registered in a spark chamber. Note that the track is perpendicular to plates of the chamber. High voltage applied to the chamber just after the passage of particle- to create sparks. Then opposite voltage applied to clear out remaining ionization Question: In which gas amplification mode spark chambers operate? proportional?
First and famous experiment to use spark chambers. Trying to observe reactions induced by neutrinos from pion decay an experimenter has to solve several problems: Where to take from high intensity neutrino flux? use intense proton beam to get ν beam How to shield from other particles which interact with much higher probability? use part of an old battle ship as a shielding. How to get rid of other backgrounds? (cosmic, radioactivity) Measure your signal in coincidence with the proton beam spill How to get a detector fast enough to be able to measure in time of a few milisec spill? Use novel at that time spark chambers instead of bubble chambers. Pions are produced in proton interactions in Berylium target. They have average momentum on 0.5 GeV. They are allowed to decay for 21 m. What is the percentage of decaying pions? m=0.14gev,τ=2.6*10-8 Nd=N0[1-exp(-t/(γτ)]=Ν0[1 exp(-βt/(βγτ))] =Ν 0[1-exp(-xm/(pcτ))]0.5N0 the rest of pions and muons are stopped in 13.5 m of steel shield, by nuclear reactions (pions) and ionization energy loss (muons)
neutrino from pion decay Muon neutrino, observation muon or electron Al + - + Neutrinos interact with the material (Al ) of the 10 tonne spark chamber. resulting charged particles leave signs in a form of sparks on their tracks, electric discharges between the charged plates Resulting tracks appeared to be consistent with tracks of muons ( electrons loose energy much faster and give short wiggled tracks) proving that a reaction ν + Αl -> x + µ x gives muons in a final state. Thus neutrinos from pion decay are "muon" neutrinos, not "electron" neutrinos Few interesting numbers: proton beam spill gave 10 11 10 protons/msec. This produced around 10 neutrinos going into direction of the detector with mean energy 0.25 GeV. Interaction xsection per nucleon is 10-40 cm. 2 10tonnes =0.5 10 31 nucleons, total σ10-9 cm. 2 Probability of interaction in a detector of 2 2-13 an area 1m is σ/1m 10. So every 1 in 1000 spills gives an interaction. We must "kill" cosmic background (100 muons per minute), and "surviving" pions and muons from the beam, which will interact in our spark chamber with probability close to 1. This is typicaly done by veto detectors
Muon neutrino discovery, summary First accelerator produced neutrino beam in use. Today neutrino beams are the standard tool to study the nucleon structure, they achieve 100 times higher intensities. Pulsed structure of the beam allows for some supression of the cosmic background. First fast and heavy particle detector in use Prove that neutrinos from pion decay are distinct from neutrinos from beta decay, they produce muons not electrons when interacting with nucleons(quarks) - ν µ + d -> µ + u - ν µ + u -> µ + + d
Position detectors, summary