Particle Energy Loss in Matter

Transcription

1 Particle Energy Loss in Matter Charged particles 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 (not electrons) K/A = 4π N A r e 2 m e c 2 / A = MeV g -1 cm 2 m e c 2 = MeV (electron rest mass) r e = fm (classical electron radius) ze = charge of incident particle T max = kinetic energy δ (βγ) = density effect correction to ionization energy loss

2 Bethe-Bloch Equation

3 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

4 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

5 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)

6 Principle of Operation Particle Track Cathode V Ionization mm Gas Volume (Argon) Drift Electrons E-Field Avalanche Electrons Amplification Readout V 0 V

7 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)

8 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

9 Gas Data Table Gas Prim. ionization de/dx [kev/cm] v [cm/sec] T attach [sec] He 5 / cm O 2 22 / cm Ar 29 / cm Xe 44 / cm 6.76 CO 2 34 / cm e

10 Gas Properties Desired Property Gas Components Purpose High specific mass Xe Efficient detection Low specific mass H 2 He Minimize multiple scattering High drift velocity CH 4 He High rates Low drift velocity CO 2 DME Spatial resolution Small diffusion HC CO 2 DME Spatial resolution Electron capture O 2 H 2 O Efficiency drift dependent (avoid) Photon absorption CO 2 DME Suppress secondary avalanche

11 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 )

12 The STAR Heavy Ion Experiment at RHIC

13 The STAR Detector in the Hall

14 100 GeV Gold on Gold Event

15 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 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

16 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

17 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 for a photomultiplier (PMT)

18 Scintillator with PMT

19 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

20 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