Particle Energy Loss in Matter Charged particles, except electrons, loose energy when passing through material via atomic excitation and ionization These are protons, pions, muons, The energy loss can be described for moderately relativistic particles by the Bethe-Bloch equation K/A = 4π N A r e 2 m e c 2 / A = 0.307 MeV g -1 cm 2 m e c 2 = 0.511 MeV (electron rest mass) r e = 2.818 fm (classical electron radius) ze = charge of incident particle T max = kinetic energy δ (βγ) = density effect correction to ionization energy loss
Bethe-Bloch Equation
Electron Energy Loss in Matter Electrons loose energy when passing through material mainly via ionization and Bremsstrahlung The dominant process depends on the energy of the electron
Photon Energy Loss in Matter Depending on the energy, photons loose energy when passing through material mainly via The Photoelectric Effect Rayleigh scattering Compton scattering Pair production
An Example: Time Projection Chambers The world before the invention of the Time Projection Chamber Particle tracks are usually measured by many planes of wire chambers (Multi-Wire Proportional Chambers, Drift Chambers, ) Particle identification usually via energy deposit in a Calorimeter or Cherenkov Counter Need to develop central detector for 3-D particle tracking and identification in collider experiments Time Projection Chamber provides 3-D charged particle tracking by combining position and drift time information Momentum measurement when detector placed inside magnetic field Particle identification via specific ionization energy loss along the particle track Time Projection Chamber needs Drift volume (high parallel electric field) inside magnetic field (for momentum) Position measurement (2-D) Time measurement via high speed data acquisition (DAQ)
Principle of Operation Particle Track Cathode -5000 V Ionization 5-2200 mm Gas Volume (Argon) Drift Electrons E-Field Avalanche Electrons Amplification Readout -3600 V 0 V
Schematic of Original PEP-4 TPC Time Projection Chamber invented in late 1970 s at Berkeley (D. Nygren) for experiment at the electron-positron collider PEP at SLAC (Stanford) 2 m long 2 m diameter (40 cm inner diameter)
Principle of Operation Particle passing through gas volume creates gas ion / electron pairs Ionizability Chemical stability Collect ions or electrons efficiently at readout (wire, pad, strip) In case of electron readout - low electron attachment probability Absorption of photons from de-excitation of gas Fast and linear drift
Gas Data Table Gas Prim. ionization de/dx [kev/cm] v [cm/sec] T attach [sec] He 5 / cm 0.32 1.4 10 5 O 2 22 / cm 2.26 5.0 10 4 1.9 10-7 Ar 29 / cm 2.44 4.4 10 4 Xe 44 / cm 6.76 CO 2 34 / cm 3.01 7.1 10-9 e - 10 7
Gas Properties Desired Property High specific mass Low specific mass High drift velocity Low drift velocity Small diffusion Electron capture Photon absorption Gas Components Xe H 2 He CH 4 He CO 2 DME HC CO 2 DME O 2 H 2 O CO 2 DME Purpose Efficient detection Minimize multiple scattering High rates Spatial resolution Spatial resolution Efficiency drift dependent (avoid) Suppress secondary avalanche
Gas Mixtures For wire chambers and also TPC gas mixtures are used to combine desired properties Noble gases posses Low electric field for avalanche formation High gain Negative electron affinity High rate capability Problems High excitation energy leading to discharge Add complex molecule to absorb photons Some are expensive Commonly used gases mixtures are Ar / CO 2 ( 90/10 or 80/20 ) He / DME ( 90/10 or similar ) Ar / CH 4 ( 90/10 )
The STAR Heavy Ion Experiment at RHIC
The STAR Detector in the Hall
100 GeV Gold on Gold Event
Conceptual Design of BoNuS RTPC H. Fenker
Gas Electron Multiplier (GEM) Thin polymer foil (Kapton( Kapton,, APICAL) covered on both sides with conductive (copper) layer Double-conical holes by lithography and chemical etching 5 µm F. Sauli, NIM A386, 531 (1997) 50 µm High electric field (kv/cm) through hole when applying 300-500 V potential difference between the two copper layers Gain >100 per GEM 55 µm 70 µm
Amplification with Multi-GEM s Cascading GEM foils Suppress ion feedback Increase gain up to 10 7 with three GEM s Operate GEM s at lower differential voltage B. Ketzer, VIC 2004
Pulse Shape Individual pulse shape given by drift of electrons (or ions) through gas mixture Measure charge (pulse distribution) sliced into time intervals to measure spatial development of charge distribution ( track reconstruction) Integral of charge (when calibrated) leads to particle specific energy loss (de/dx) over the known track length in the detector Gas mixture alters time development of electron drift
Planar Prototype TPC Using initially three standard 10 x 10 cm2 GEM foils from 3M Corp. with gain ~30 Tests with cosmic rays, x-ray sources, and ~100 MeV/c protons at TUNL Readout plane with eight strips Ar (80) / CO2(20) drift gas The planar prototype TPC with two new GEM foils of gain >100 at JLab
Planar Prototype TPC at TUNL Tilted TPC sidewise protons entering TPC under angle Energy deposit mostly on several strips, track reconstruction possible Integrated charge the same as in case of the straight tracks Integrated Charge p + Drift [25 nsec] Drift [25 nsec] Strip Number
Cosmic Ray Detector Cosmic rays detected on Earth consist mainly of high energy muons created by the interaction of high energetic particles in the Earth atmosphere Muon rest mass of 105.66 MeV/c 2 Muon lifetime is about 2.2 µsec Detectable on Earth surface due to muons very large β (relativistic time dilation or length contraction important) Measurable rate at sea level about 1 muon per hand and second
Cosmic Ray Detector Cosmic rays can be detected through their interaction with matter Atoms in scintillator materials get excited by ionizing radiation, like muons, and de-excite by emission of photons, often in the visible spectrum Commonly used are organic crystals dissolved in clear plastic or polystyrene Very fast response time for excitation and emission of photons Emitted photons need to be converted into electrons and amplified, for example by a Photomultiplier
Photomultiplier Photons impinge on photocathode with a low work function, which emits electrons via the Photoelectric Effect (Nobel Prize for A. Einstein in 1921) The electrons are accelerated via an electric field in the range of 100 V/cm towards secondary metal electrodes (dynodes) which will emit more low energy electrons on impact by the accelerated electron Several, often ten to 15, stages lead to an overall electron current amplification, or gain, of 10 5-10 7 for a photomultiplier (PMT)
Scintillator with PMT
Signal Readout The electron current can be measured as a function of time by discharging the collected electrons via a known resistor to ground and measuring the voltage drop across the resistor The integrated signal size is proportional to the collected charge and hence the energy deposited by the muon in the scintillator By setting a voltage threshold for the signal size, only good muon candidates are detected
Signal Logic The discriminator selects the signal sizes Using two or more scintillation counters, a muon can be selected by requiring more than one scintillation counter to have detected a muon within a very short time interval (coincidence) Time intervals are in the nano-second (nsec) range The signals from the individual scintillation counters can be combined in logic units, which give out a logic 1 for the case of more than one input having a logic 1 NIM logic is current based, with a negative current being a logic 1 and logic 0 represented by no current