Experimental Particle Physics • Day 1 • Particle identification • Detectors and experiment design • Day2 • ATLAS detector components • Physics capabilities • Harold Ogren, Professor of Physics • Indiana University, Bloomington, IN
Outline of lectures for today • HEP experiments • Particle ID • Time of Flight, Cerenkov, • Transition radiation detectors • Gaseous tracking chambers • Small Detector • Designing a mini-detector, how small can you go? • Details of optimization for a small detector.
My Personal history # authors on a typical publication 8 30 42 8 50 120 350 SSC(1000?) 1800 • Major in Physics University of Michigan • PhD Particle physics Cornell- 1970 • Cornell synchrotron- 10 GeV • Post Doc Frascati, Italy- 1973 • Adone e+e- Storage ring • CERN Fellow CERN- 1975 • ISR proton- proton storage ring • Indiana University- 1975 • Fermilab- 1977 • Internal target experiment • Stanford Linear Accelerator Center • PEP e+e- storage ring, HRS 1986 • SLC e+e- Collider Mark 2 1990 • CERN • LEP e+e- storage ring OPAL 1999 • LHC p-p Collider ATLAS 2007 • (US manager for TRT barrel)
Some sources of information • Books: Allison, Wright, Experimental techniques in High Energy Physics, T. Ferbel. • W. Leo, Techniques for Nuclear and Particle Physics. • Online sources: • particle data book (PDG LBL) • DePAC • NLC-detectors • EM showers explained • Particle detector briefbook (CERN) • This is a classroom for the high energy physics.
Time of Flight (TOF) • One of the most common methods of • Particle identification ( at low momenta) is the Time of Flight. If the time of crossing is known for two points on the particle trajectory, then the velocity can be determined. If the momentum (mv) is also known ( by measuring the curvature in a magnetic field), then the mass of the particle can be determined. • The time is usually measured by scintillators using a photomuliplier to detect the scintillation photons. Other methods such as microchannel plates, or drift chambers also can be used.
Time of Flight measurements b1 b2 L • Precision time measurements can determine the mass differences of particles with the same momentum. • I picked 0.5 meters Since that is the scale Of many inner detectors Such as ATLAS ID 1ns= 10-9sec
Measurement Errors t1 t2 L x1 x1 Gluckstern, NIM 24,(1963)381-389
Mass Resolution 30 time measurements, dt=0.8ns For short flight lengths (50 cm) and dt= 3ns/sqrt(13)=0.83 ns, particle ID is only good in the sub GeV/c momentum range.
Mass resolution – dt= 1 ps, L=0.5m ps timing will allow pi/K separation well above 10 GeV! Would require Micro Channel plates, not “normal” scintillators and photomultipliers. Resistive plate tracking chambers can give resolutions as good as 50 ps. (more on this later)
Cerenkov and transition radiation • Describe the radiation by a charged particle as it travel though a dielectric • 1) Energy loss- already discussed • 2) Cerenkov radiation • 3) Transition radiation Use four vector notation Bc=v g=sqrt(1/(1-b2)) P=( p, iE/c) =( gmv, igmc) (P)2= -m2c2
Conservation of 4 momentum requires that: In a vacuum without interaction the radiation of The photon is at an unphysical angle for any velocity of the incoming particle. i.e no radiation
This kinematics constraint of radiation hold for all types of interactions of a moving charged particle. Cerenkov radiation Radiation in the optical region which is produced along the entire path-length of the charged particle in the media Transition radiation can be understood in this manner since it must satisfy the same angular restriction. However, TR is produced only at the boundary. • In a continuous media with an index of refraction, n=sqrt(dielectric constant) the equations are modified: Now, since n >1, there is a range of particle velocities 1>b >1/n that allows physical radiation of photons. The angle Is called the Cerenkov angle.
Cerenkov radiation- intro animation of the radiation from a moving particle.
Cerenkov radiation • Half angle of the radiation cone, n=index of refraction, velocity=bc • The energy loss per cm due to the radiation of real photons for a particle passing through a dielectric media is calculated in the literature (Jackson). • The number of photons/cm is:
Integration of the differential intensity In an actual detector the integration over the photon energy (E) must be done taking into account the detector efficiency and the collection efficiency. Threshold Cerenkov counter- simple yes/no decision on whether a charged particle is above or below b = 1/n threshold. Assuming full collection efficiency and a phototube with about 25% efficiency,
Threshold Cerenkov counters • Assume that the index of refraction is n=1.00085, which is characteristic of many Freon gases. Then we can write: The probability of seeing r photons, when the number produced is 0.15*L, is given by the Poisson distribution. The distance necessary to make 1% probability to see 0 photons, P(0)= 0.01 -> L=30 cm ( ~4.5 photons)
Cerenkov Imaging • The Cerenkov cone of photons can be directly imaged in some applications. • Many types of imaging are possible. ( a reflecting mirror or lens will focus all parallel rays to a fixed focal point) RICH- ring imaging Cerenkov counter. So, an imaging counter could separate a pion from a kaon from the threshold momentum of the pion though the momenta where the kaon is also radiating, up to the momenta when the kaon and pion angular separation is comparable to the angular resolution of the detector ( for instance, 5 mrad).
Cerenkov ID schemes • Threshold Cerenkov detectors • A radiator and light detector ( photomultiplier). Particles with a velocity above the threshold for producing Cerenkov light are detected, while others are not. If a gas is used as the radiator, its refractive index, and so the threshold velocity, can be tuned by adjusting the pressure. • Differential Cerenkov detectors • They respond to a range of velocities. This is typically arranged by being sensitive to a certain range of Cerenkov angles, for example by having mirrors and/or baffles between the radiator and the light detector. • Ring-Imaging Cerenkov detectors • In neutrino experiments the photon ring is views directly by an array of photomultipliers. • Mirrors are often used to focus the cone of Cerenkov light into a ring on a position-sensitive light detector or array of detectors. In this way, the centre of the ring indicates the position of the particle and the radius of the ring measures the Cerenkov angle and so the velocity of the particle. • Light detectors can include multiwire proportional chambers, containing a gas mixture sensitive to visible or ultra-violet photons.
Cerenkov calorimeters • the number of Cerenkov photons produced by an electron or photon is proportional to the kinetic energy, so the total energy of a shower is quite accurately proportional to the number of photoelectrons measured. However, absolute energy calibration is still necessary. • Active calorimeters rely on this Cerenkov effect.( Pb-glass, etc) OPAL EM calorimeter
Transition Radiation Detector • Radiation is produced when a charged particle traverses a boundary between two media with different dielectric properties. • We can characterize the media in terms of the index of refraction and the plasma frequency (onset of absorption)which is typically about 20 eV. Ne is the electron density a is the Bohr radius TRT end cap TR detector - tracker
Transition Radiation Calculations • Total rate from one interface • Calculation from foils • Concept for the ATLAS TRT barrel
e Dielectric Constant vs photon energy Im(e) Re(e) 1.0 0 optical Absorptive- Xray 1.001 0 -1.001 hwPlasma Frequency n= (eRe + ieIm)1/2
Transition radiation • The production cross section from one interface is derived (Jackson) in a very similar way as for Cerenkov radiation • (Allison&Wright) • This is the calculation and integration of the TR from one surface, however. • The g dependence is interesting but not really relevant for TR produced from foils for from fibers (as the case for the ATLAS TRT) • Angles typically ~ 1/g • Energy typically ~ gwp • The photons from the front and back faces have opposite phases, so if the two faces are very close, the two waves cancel out!
Production of photons from slice. n • Calculate the time shift between a photon created at the front edge and one created at the back edge, as viewed from infinity. f q b L L tan(q) f
Transition radiation calculations Typical medium for radiators has a value of about 20eV. 1GeV electron has g=2000 Radiation peaks near f ~ 1/g Spectrum peaks ~ 0.3gwp=6 eV*2000 The interference is given by Sin(L/Z), We would like the Physical size of the Radiator foil to be Comparable to Z or Larger. Foils in TRT are about 20 microns. So we expect TR to “saturate” above 300 GeV.
TR Multiple foils • The total output of TR x-rays will be increased by having multiple interfaces- “stacked foils”. • Three considerations: • Coherence of the photon amplitudes from each interface. • Absorption of x-rays • Multiple scattering Overall coherence is impossible, but it is clear that the foil thickness and the separation should be approximately the same, and that the incoherent mix might approach the sum of the individual intensities ( a photons (peak) per interface) The summed intensity should be similar to a “multiple slit”, with the phase between emitters being from both the foil thickness and the separation of the foils.
Transition radiation- x-ray absorption Spectrum of TR photons Is proportional to 1/w • Although about half of the TR is in the energy region, (0.1-1.0)hw, the spectra is dominated by low energy x-rays. These are absorbed in the radiator and gas (refractive index is imaginary). • This photo-absorption dominates the x-ray spectrum for Kev photons.
X-ray absorption We can expect that in the keV region that the absorption length Will be about 0.001 gm/cm2. For a fiber density of 0.07gm/cm2 This will give an absorption length of 0.013cm! Better information on the web.
ATLAS TR tracker • Particle id is obtained by looking at the energy deposited by E>5 KeV photons in the straws along a particle track. • The evolution is quite complex • Transition radiation produces a spectrum of x-rays • X-rays are attenuated in the material outside the straws. The density of the fiber is about 0.07 gm/cm3. For 5.5 KeV photons the attenuation length is 17mm. • X-rays pass into the straw and deposit all of their energy at one point in the gas. The attenuation length of a 5.5 KeV photon is several centimeters. • Although the probability is only about 25% for deposition in a straw, there are typically about 35 crossed straws for a full track in the barrel. • The PH is triggered if it is above 5.5KeV for every straw and the an electron efficiency for 4, 5, 6, 7 hits can be determined.
Multistraw TRT readout • TRT barrel was designed to have about 6-mm of polyethylene fiber in front of each detector. The detectors were 4 mm diameter straws filled with mainly Xe gas. • Xenon gas was used because the photoabsorption cross-section is large(~z2) • The attenuation length in Xe for a 5KeV x-ray is of the order 0.001 gm/cm2, 4mm of Xe has a thickness of 0.0023 gm/cm2. So the majority of lower energy x-rays are absorbed and deposit their energy in the Xe gas. The
Effects of Magnetic Fields 2T field gives a radius of curvature For the track of: R=p/(0.3*2T)= 1 GeV/0.6=1.6m ( Looping tracks if R<0.5m, p<0.3 GeV.) The deviation of the circular track from The tangent to the circle is given by Y=x2/(2R)= 0.3Bx2/(2p Suppose we pick a maximum deviation Of Y=4mm, then x=11.4 cm sqrt(p), this is the distance A track progresses before the TR Produced at the beginning of the track Becomes separated from the track itself By 4mm. Even for p=0.3 GeV, x = 62mm, which is long compared to the attenuation length of the x-rays. • The 2 T field bends the tracks as they travel outward through the TRT. • If the track is bent too much then the TR will not be deposited in the same straw that the charged particle passes through. • When does this be come serious?
Drift straw tracking • The additional task of the TRT system to track charged particles through the magnetic (2T) in the central region of ATLAS. • Design had to be optimized for both tracking in a high rate regime and for registering TR radiation. ( more later) • Discussion about drift chambers before we look at the ATLAS TRT in more detail.
TRT Barrel Tracking-Drift cells • Charge particles pass through a gas cell, leaving a trail of ionized atoms and free electron. The electrons move along the electric field lines to the anode signal wire. • Very near the wire the electrons trigger the electrons shower to form an avalanche with typical multiplication of 2x 104. • The time of the arrival of this avalanche is used to determine the radial distance the electron “drifted”. The drift time in a 2mm radius straw detector is about 30 ns. • Multiple measurements along the track allow the trajectory to be determined with a typical accuracy of 100-150 microns/straw. e V=-1500V 30 m dia. signal wire at center
Charge particle interactions • The charged particle loses energy (dE/dX) as it passed through the gas. Many of the interactions (encounters) yield multiple electrons. Multielectron hits are sometimes called clusters. • For Xe there is about 8 electrons/interaction as can be seen in the table, and about 4.6 interactions/mm for an average distance between clusters of about 220 microns.
Effects of Magnetic field -V • The tracks of the electrons as the drift to the signal wire a not radial. The look like a spiral due to the vxB forces. • This increases the time to arrive at the wire but does not alter much else. • For the 4 mm straws this increased the time for max drift from 32ns to 39ns. • Increases the dead time of the straw and increases the pile up from multiple crossings. B out
Wire chamber operation • The electric field near the wire is radial, and is given by • E=CV/2pe0r • C=2pe0/(ln(b/a) • a is wire radii • B is cathode radii • The “Gain” of the wire is as a function of the voltage is exponential. Depends on gas parameters.
Avalanches near the wire • Gains of depend on gas mixtures, Electric fields. Measure signal is a combination of the rapid electron arrival, and the slower motion of the positive ion cloud drifting away from the wire. • For Xe-Co2 with a 31 micron diameter wire, the gain is 2x 104 for a voltage of -1465V applied to the straw and the wire effectively grounded.
Time distributions for Straws The electron drift times for An ensemble of particle tracks Crossing the straws, shows a Roughly uniform distribution from The minimum time, T0, to the Maximum drift time from the Cathode wall.
TRT calibration & alignment • New TRT calibration and alignment constants produced for the run 2102204 (P. Hansen). The following procedure is used: • From TRT standalone tracking, the “V-plot” (drift time vs. track position at a straw) are made for each straw to extract the t0, the drift velocity and the wire position. • R(t) plot is fitted after averaging the two legs of the V-plot and correcting it for t0. A third degree polynomial is used. • The straw displacements with respect to the track are minimized by rotating the modules. • The calibration constants can be read from a text file, the alignment constants are written in the conditions database. P. Hansen
RT dependencies, residuals, straw profiles • RT dependence fit is used for reconstruction of hit coordinate via measured drift-time • Particle track position inside the straw is predicted using information from SCT. • Estimation of coordinate accuracy was made using residual distributions • Two kinds of efficiency were estimated: total and 2.5s • Straw profiles were used to calculate straw total efficiency
Coordinate accuracy and efficiency • Coordinate accuracy and efficiency behave reasonably at different thresholds • Coordinate accuracy and total efficiency are comparable with the previous year values • 2.5s efficiency in this year prototype is higher because of lower noise level
Multiwire proportional chambers • The principle of operation for an MWPC is similar to the previous discussion of drift chambers. One plane of wires with a cathode plane on either side. Near the wire the field is radial, but it linear through out most of the chamber. Wire gain is controlled by running in the proportional region.
TPC • A form of drift chamber is the Time Projection Chamber (TPC). • This contains a large drift volume (~2 m long by ~2 m radius ). • An electric field of the order of 100 kV per meter drifts charge onto MWPCs at the end of the drift volume. • Magnetic field reduces diffusion. • 2-dimensional x-y readout, with the third co-ordinate being supplied by the drift time before the signal arrives at the MWPC at either end of the chamber. Spatial resolution in 3D to 60 microns can be achieved with such a TPC. Measurement of signal size gives dE/dX information about track.
US-Barrel-TRT Type 1-----Type 2----------Type3 M1.32 M2.32 M3.32 Type 1 32 * 329 straws in type 1 =10528 Type 2 32* 520 straws in type2 =16640 Type 3 32* 793 straws in type 3 =25376 52544 total straws • 96 modules and 9 spares.