1 / 37

Principles of Detection for Particle Physics Part 1: Passage of Radiation Through Matter

Maria Laach Summer School Maria Laach Abbey 9-18 September, 2015. Principles of Detection for Particle Physics Part 1: Passage of Radiation Through Matter. Bruce A. Schumm Santa Cruz Institute for Particle Physics and the University of California, Santa Cruz.

brendanj
Download Presentation

Principles of Detection for Particle Physics Part 1: Passage of Radiation Through Matter

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Maria Laach Summer School Maria Laach Abbey 9-18 September, 2015 Principles of Detection for Particle PhysicsPart 1: Passage of Radiation Through Matter Bruce A. Schumm Santa Cruz Institute for Particle Physics and the University of California, Santa Cruz http://scipp.ucsc.edu/~schumm/talks/public_talks/Maria_Laach

  2. Introduction These lectures are divided into three (unequal) sections… Passage of Radiation Through Matter: The different ways in which energetic particles deposit energy in materials sets the stage for understanding detection techniques Charged Particle Tracking: Precise determination of individual particle kinematics (charged particles only!) High-Energy Calorimetry: Measurement of the energy and approximate trajectory of individual particles (charge and neutral) and, more and more, jets of particles arising from the production of strongly-charged partons (energy-flow) Some mention also of transition radiation and Cerenkov detection… Maria Laach 2015, Part 1: Passage of Particles through Matter

  3. Nomenclature: Incident and Target Target Material (metal, gas, semiconductor) Incident particle (electron, photon, hadron) Characterized by: Z = atomic number A = atomic mass N = density of scatterers (cm-3)  = density (g/cm3) Individual scatterers (atoms, nuclei, orbital electrons) Maria Laach 2015, Part 1: Passage of Particles through Matter

  4. Processes: Incremental vs. Catastrophic Let’s begin with the qualitative description of the relevant modes of energy loss when high-energy particles traverse matter-filled regions Somewhat arbitrarily, I divide energy-loss processes into two categories… Incremental: The particle emerges intact, usually with only very slightly degraded energy or slightly altered trajectory Catastrophic: The particle is completely absorbed or highly degraded, with energy Maria Laach 2015, Part 1: Passage of Particles through Matter

  5. Incremental Processes I: MCS Multiple Coulomb Scattering • Charged particle suffers slight deflection off of nucleus of atom in traversed material • Will do this many times in traversing a macroscopic material  statistical treatment • Scattering is electromagnetic, off coherent charge of nucleus; one of several such processes, all characterized by one empirical parameter LRAD (stay tuned…) Z • Slight deflection • Essentially no energy loss Maria Laach 2015, Part 1: Passage of Particles through Matter

  6. Incr. Proc. II: Cerenkov and Transition Radiation Cerenkov Radiation “Shock wave” photon front associated with faster-than light traversal of particle through transparent medium (remember: effective speed of light in materials is c/n, where n is the index of refraction). Think of sonic boom (sounds waves) or bow wave (water waves). Transition Radiation Burst of photons associated with transition from one material to another (“impedance mismatch”) Maria Laach 2015, Part 1: Passage of Particles through Matter

  7. Incremental Processes III: Atomic Excitation • Ionization and Atomic Excitation • Traversing charged particle ionizes or excites the orbital electrons of the atoms in the material • Ionization: • Leads to incremental energy loss, but not much directional change (see MCS) • “dE/dX”, typically in MeV per cm or g/cm2 of traversed material • Important concept of “minimum-ionizing particle” will arise • Excitation: • Scintillation • Fluorescence Maria Laach 2015, Part 1: Passage of Particles through Matter

  8. Incremental Processes IV: Brehmsstrahlung Bremsstrahlung • Radiation by charged particle in field of atomic nucleus • Relevant for light particle only (electrons, very energetic muons) • Again, dominated by coherent scatter of charge of nucleus  LRAD again (in fact, is the origin of the term) Z Maria Laach 2015, Part 1: Passage of Particles through Matter

  9. Incr. Proc. V: (Quasi-)Elastic Nuclear Scattering Nuclear Elastic Scattering and Nuclear Excitation Strong-force mediated (large cross section) For example, for charged pion + +N  + + N Fully elastic + +N  + + N* Quasi-elastic + +N  0 + N+ Quasi-elastic (charge exchange) Maria Laach 2015, Part 1: Passage of Particles through Matter

  10. Catastrophic Processes I: Photon Absorption Photon detection plays a large role in particle physics experiments Three main processes, in order of E at which they dominate e+ e  e  ’ e- e Z Photoelectric Effect (sub-MeV) Compton scattering (~MeV) Pair production Ethresh = 2me; fully on for E ~10 MeV Coherent off of nucleus (third such process) Maria Laach 2015, Part 1: Passage of Particles through Matter

  11. Catastrophic Proc. II: Inelastic Nuclear Scattering Inelastic Nuclear Scattering High-energy particle from collision process shatters nucleus in detector material For example, for charged pion + +N  n+ + m0 + Y1 + Y2 + Y3 + ... where the Yn are nuclear fragments (typically including several neutrons) Next: How can we characterize these processes? Maria Laach 2015, Part 1: Passage of Particles through Matter

  12. Nomenclature: Incident and Target Target Material (metal, gas, semiconductor) Incident particle (electron, photon, hadron) Characterized by: Z = atomic number A = atomic mass N = density of scatterers (cm-3)  = density (g/cm3) Individual scatterers (atoms, nuclei, orbital electrons) Maria Laach 2015, Part 1: Passage of Particles through Matter

  13. Reminder: Mean Free Path Consider a particle entering a medium composed of scattering centers (atoms, nuclei, bound electrons…) Let  (cm2) be the scattering cross section. In traversing a distance x (cm), with a number density N of scatterers (cm-3), the interaction probability I will be I = N .  . x leading to a probability of survival of P(x) = exp(-x/) with  = “mean free path” = 1/N.  treated as empirical parameter characterizing the target material (limited dependence on incident species) Maria Laach 2015, Part 1: Passage of Particles through Matter

  14. Nuclear Collision and Interaction Lengths • Strictly speaking, defined for neutrons between 60 and 375 GeV • Good to ~10-20% down to 500 MeV, and for other particle species • For strongly-charged incident particles only (mesons, baryons) Nuclear Interaction Length: • Mean free path I for inelastic collisions Nuclear Collision Length: • Mean free path T for any nuclear interaction • Includes elastic, quasi-elastic scatters • So always, T < I Maria Laach 2015, Part 1: Passage of Particles through Matter

  15. Example: I for Iron, Hydrogen It will be helpful if we work up an estimate of I for Iron (  0.70 b) N = N0/( . A) = (6.02x1023/mole) . (7.87 g/cm3 / 55.9 g/mole) = 8.5x1022cm-3 and then, since 1b = 10-24 cm2, I = 1/(N . I) = 1/(8.5x1022cm-3 . 0.70 b) = 16.8 cm • It takes a few meters of iron (10-20 I) to shield hadronic radiation On the other hand, for H gas I  10m. But of course! H is much less dense than iron. Hypothesis: if you scale by the density, I is a universal, material-independent quantity (each nucleus in target has same cross section!) I (cm)  I.  (g/cm2) of material traversed Maria Laach 2015, Part 1: Passage of Particles through Matter

  16. Table of Interaction Lengths (cm and g/cm2) Hypothesis fails! Why?  Nucleon shielding (important principle) Gram per gram, Hydrogen is best for absorbing nuclear radiation (kinematics too…)  Hydrocarbon absorbers. Concrete not bad. N.B.: Lengths (interaction, radiation) often expressed in terms of g/cm2 rather than cm! Become familiar… Maria Laach 2015, Part 1: Passage of Particles through Matter

  17. Ionization Energy Loss: dE/dX Hans Bethe and others… Incident charged particle b = impact parameter impulse p; energy transferred is p2/2me Orbital e- • Leading order dependencies of impulse p: • Time spent in vicinity of orbital election  1/ • Set by energy of incident particle • 1/2 effect on deposited energy • Impact parameter b • Random luck… but just area-average over multiple ionizations Maria Laach 2015, Part 1: Passage of Particles through Matter

  18. Mean (Impact-Parameter Averaged) dE/dX • As a function of incident particle energy, primary effect is 1/2 • Relativistic contraction leads to density-dependent modification for relativistic particles Radiative losses (brehmsstrahlung); Important for electrons (see later) 1/2 regime Density effect Relativistic rise; “minimum ionizing” (min-i) regime http://pdg.lbl.gov/ Maria Laach 2015, Part 1: Passage of Particles through Matter

  19. Application: Proton Therapy 1/2 behavior leads to concentrated energy transfer in relatively localized region (Bragg Peak) Increasing focus for tumor therapy 1/2 deposition region Maria Laach 2015, Part 1: Passage of Particles through Matter

  20. Distribution of Energy Loss (Landau, Vavilov) Consider particle incident upon given scatterer at random location Small impact parameter (b) suppressed by 2D geometry, but can happen occasionally  large energy loss! b Tail so long that Central Limit Theorem doesn’t even apply, leading to Landau Distribution Which has a finite mean/median but an infinite RMS (hold that thought…) (Practically, 100% energy loss not possible  Vavilov distribution. But conceptually similar) Landau Distribution Maria Laach 2015, Part 1: Passage of Particles through Matter

  21. Brehmsstrahlung I h E0 E0 - h Reminder: Coherent off of nucleus Z Maria Laach 2015, Part 1: Passage of Particles through Matter

  22. Brehmsstrahlung II Maria Laach 2015, Part 1: Passage of Particles through Matter

  23. The Radiation Length I Maria Laach 2015, Part 1: Passage of Particles through Matter

  24. Lrad Scaling Laws 1) From the Koch/Motz formula, rad ~ 1/m2, so Lrad ~ m2 Lrad/Lrade ~ m2/me2 ~ 4x104 2) From the Koch/Motz formula, rad ~ Z2 (coherent scattering). Since the density  is roughly proportional to Z, then Lrad [g/cm2] = /Nrad is roughly proportional to 1/Z Lrad [cm] = 1/Nrad is roughly proportional to 1/Z2 • Lrad drops quickly with Z, especially in terms of physical thickness in cm From Koch/Motz, and considering the nuclear form factor… Maria Laach 2015, Part 1: Passage of Particles through Matter

  25. Lrad and I Comparison N.B. Lrad is always quoted for electron/positron • All hadrons have nuclear interactions; I << Lrad • Electrons/positrons are beset by radiation •  Muons are the only penetrating charged particle! (Particle ID for free…) Maria Laach 2015, Part 1: Passage of Particles through Matter

  26. Critical Energy EC = “critical energy” = energy at which dominant process changes from brehms-strahlung to ionization Important effect for Electromagnetic Calorimetry ( bear in mind for later!) Crude rule of thumb (electron/positron): EC (800 MeV)/(Z + 1.2) (Think ~10-20 MeV for metals) PDG EC Maria Laach 2015, Part 1: Passage of Particles through Matter

  27. Photon Absorption/Attenuation: PE Effect Recall three main processes: photoelectric, Compton, pair production Photoelectric Effect • Photon completely absorbed, electron released • Cross section largest for E = Ebinding, so limited electron KE, dominant in 10-100 keV range • Leads to material-dependent “absorption edges” • Graphical depiction coming up in a few slides… Maria Laach 2015, Part 1: Passage of Particles through Matter

  28. Compton Effect I h h h’ h’ T e In limit that atomic electrons are free, basic QED calculation yields where  = h/mec2, s = T/h, and re = e2/mec2. e Maria Laach 2015, Part 1: Passage of Particles through Matter

  29. Compton Effect II The “Compton edge” is usually quite visible for a calorimetric detector and is often an important control point • Nuclear spectroscopy • High-energy polarimetry “Compton edge” (T) Maria Laach 2015, Part 1: Passage of Particles through Matter

  30. Pair Production Note: Another process dominated by coherent scattering off the nuclear EM charge e+  Mean free path pair for pair production and so e- Z • Lrad characterizes both electron and photon attenuation in matter • Most important parameter for electromagnetic calorimetry Maria Laach 2015, Part 1: Passage of Particles through Matter

  31. Photon Attenuation Summary • High energy photons somewhat penetrating because cross section is relatively low (piar  5mm for Pb) • High-Z materials good shielding for EM radiation (electrons, positrons, photons) • But be aware of opening of “1 MeV gap”: pair production falls off before Compton can take over • But remember: for hadrons (neutrons), dense low-Z materials are best (hydrocarbons) Maria Laach 2015, Part 1: Passage of Particles through Matter

  32. Multiple Coulomb Scattering L • Slight deflection • Essentially no energy loss • Many scatters  Gaussian statistical treatment (Central Limit Theorem) Transverse projection of straggling path traversing a length L Z xplane plane Maria Laach 2015, Part 1: Passage of Particles through Matter

  33. The MCS (Moliere) Distribution 15.7 MeV e- through Al foil A.O. Hanson et al., Phys. Rev. 84, 634 (1951) Maria Laach 2015, Part 1: Passage of Particles through Matter

  34. MCS: Other Technicalities L xplane Correlation coefficient plane Maria Laach 2015, Part 1: Passage of Particles through Matter

  35. Cerenkov Radiation Recall: In a medium, speed of light is cm = c0/n, where n is the index of refraction. • For  > 1/n, v > cm electromagnetic “shock wave” Consider such a particle progressing forward for some time t Wavefront of constant phase c ct • Particle identification from threshold, angle (“ring imaging” detectors) • Background suppression from threshold Maria Laach 2015, Part 1: Passage of Particles through Matter

  36. Transition Radiation • Occurs at transition between different media • Related to plasma frequency p (~ 10 eV) of materials Total energy: E  (2/3)   [ (h/2) p] and peaks around 20 keV •  > 1000 for appreciable effect • Good for discriminating electrons from muons and hadrons in the 1-100 GeV range (ideal for LHC) Maria Laach 2015, Part 1: Passage of Particles through Matter

  37. Next Stop Next stop: Charged particle tracking… Maria Laach 2015, Part 1: Passage of Particles through Matter

More Related