Particles and Matter

1. Particles and Matter Sarah Eno MD Quarknet 9 July 2003 Selection and Comments Jim Linnemann MSU Sarah Eno

2. Detectors • Goal: produce some sort of detectable signal that depends on the things we want to measure (energy, position, particle type) • current in a wire • charge on a capacitor • light (detectable with photomultiplier, for example) • The physics processes we care about will be the ones that lead to this kind of signal. References: The Physics of Particle Detectors, Dan Green,Cambridge University Press (2000) Techniques for Nuclear and Particle Physics Experiments, William Leo, Springer-Verlag (1987) Radiation Detection and Measurement, Glenn Knoll, John Wiley and Sons (1979) Sarah Eno

3. A minor Miracle • Detectors must find 1 particle among 1023 • How??? • The particle is very energetic • So it behaves differently than the others • The detector is in a special meta-stable state • So the particle disturbs it and causes a physical change • This amplifies the effect • Then an electronic device can amplify it further Sarah Eno

4. Particles What particles do we detect? But those that are stable on the time scale of a few microseconds when moving fast Not the fundamental particles Mostly… electrons, muons, neutrinos, photons (light), charged pions (bound state of ud quarks) Also occasionally proton, neutron, alpha particle (ionized hydrogen), charged kaon (bound state of su quark) Sarah Eno

5. Bulk Matter • What happens when a high speed particle passes through bulk matter? • Non-destructive • elastic scattering (+ionization) • scintillation • Cherenkov radiation • transition radiation • destructive • bremsstrahlung • pair-production • Nuclear interactions Sarah Eno

6. Units eV is the energy an electron gains when it is accelerated through 1 Volt. Light from the sun keV → atomic energies X rays MeV → nuclear energies Gamma Rays GeV → high energy physics 1eV = 1.6x10-19 J Sarah Eno

7. Cross section Measure of the probability for an interaction to occur. Units of area. 1 barn = 10-28 m2 L is the luminosity, which is a measure of the flux of the incident particles (number per area per time) Sarah Eno

8. Elastic energy • When a charged particle passes through a bulk material (say gas, Silicon, etc), it will scatter elastically off the nuclei in the material (change direction) and inelastically off the electrons (lose energy) (we’ll discuss inelastic scattering of nucleii later) • transferred energy can do 2 things: atomic excitation or ionization. Atomic excitation can lead to the production of photons (talk about later), ionization of the bulk material -> freed electrons can be collected to give a current or charge • tell us if the particle is charged or neutral • energy loss/length depends on the particles velocity Sarah Eno

9. Bethe-Bloch Formula Tells you the total energy lost to both elastic and inelastic collisions Units MeV-cm2/g Because, for the same thickness, you’ll put more energy into a dense material than a non-dense one. Multiply by the density to get MeV/length. Lead: p=18 g/cm3 MeV/cm=36 Argon: p=0.0017 g/cm3 MeV/cm=0.0034 Sarah Eno

10. Bethe-Bloch • Why that shape? • At low energy-> slower. Spends more time near each nucleus -> more interactions. Higher energy… can’t go faster than c, so doesn’t keep getting smaller once near c • At high energy->relativity • Imagine a heavy particle with mass mp and momentum P incident on an electron at rest. Maximum kinetic energy that the electron receives is: If ignore g, KE approaches a fixed value as v->c This g2 factor accounts for the relativistic rise Sarah Eno

11. To keep in mind • The constancy of energy loss at high speed • That means the total loss proportional to the length of the path traversed Sarah Eno

12. Electron-Ion Pairs When energy is transferred from the high energy particle to the bulk material, some of it causes excitations of the atom, and some causes ionization (delta rays). Typically 1 ion-electron pair per 30 eV of energy lost. Leo Sarah Eno

13. Uses Most commonly used in “tracking” chambers. High energy particle going through gas or silicon. Freed electrons are collected to make a signal. Tells you where the particle is. Can tell you its velocity as well. And maybe radio signals? Sarah Eno

14. Scintillation As we discussed, when a charged particle goes through bulk matter, it can excite the atoms of this matter. Some materials, when they deexcite, emit photons with visible light wavelengths (usually blue, around 400 nm), and these (rather than electrons from ionization) can be collected as the signal, using a photomultiplier tube for example (which converts a photon to a current). Leo Fluorescent materials: emission of photons with a decay constant of 10-8 s Sarah Eno

15. Scintillators • Common • Plastics: Polyvinyltoluene, polyphenylbenzene, polystyrene • + fast, cheap, flexible, easy to machine • - suffer radiation damanage • Inorganic Crystals: NaI, CsI • + when need precision measurements • - expensive. • Other • Organic scintillator (aromatic hydrocarbon compounds containing linked or condensed benzene-ring structures. ) Organic Crystals (C14H10 (anthracene), C14H12(trans-stilbene), C10H8(naphthalene)) Organic Liquids (P-Terphenyl, PBD,PPO, and POPO, xylene, toluene, benzene, …) Sarah Eno

16. Quartz bar Active Detector Surface Particle Cherenkov light Total Internal Reflection Sarah Eno

17. Total Internal Reflection Note: can only happen when n1>n2! Sarah Eno

18. Scintillator On display at MoMA in NY until Aug 31 (then on world world tour, “Signatures of the Invisible”, http://www.ps1.org). Scintillator from D0. Sarah Eno

19. Scintillator Sarah Eno

20. Uses • Time of flight counters (measure particle speed) • calorimeters (measure particle energy) • D0 tracking system Sarah Eno

21. Cherenkov Radiation When a particle moves through bulk matter with a speed faster than the speed of light in that medium, it emits radiation (well, not really. Particles moving at constant speed don’t radiate. But, its field interacts with the medium, which emits photons) in an electro-magnetic analog of the “sonic boom” that happens when a jet moves faster than the speed of sound or around the bow of a boat moving faster than the speed of sound in water. (happens for any kind of wave, not just sound!) Sarah Eno

22. Cherenkov Radiation Hygen’s Principal Green Radiation is emitted at a fixed angle to the particle. Cone shape. When photons hit flat surface, make circle. Size of circle depends on particle speed. However, can’t do a good measurement as b->1 Sarah Eno

23. Cherenkov Minimum speed for emission n-=1.33 (water) b>0.752 qmax=41 degrees Sarah Eno

24. Cherenkov Energy loss for a solid around 10-3 MeV cm2 g-1. Photons typically in the visible light frequencies. About 1 part per thousand of the ionization energy loss Sarah Eno

25. Cherenkov Radiation BaBar Sarah Eno

26. Uses Mostly used to get particle type (tell kaons from pions) Cosmic Rays: look at extensive air showers directly Sarah Eno

27. Destructive Measurements Typically happen with the aid of a heavy nucleus Only way to measure neutral particles. Sarah Eno

28. Bremsstrahlung When in the presence of a heavy nuclei, a particle can “bremsstrahlung” off a high energy photon (gamma ray). Because the electron undergoes a large acceleration due to nuclei’s field. Energy lost to this goes as 1/M4, so basically only happens to electrons (happens also for very very high energy muons (100 GeV)) g e Mass of muon is 105 MeV, electron is 0.51 MeV. How does the radiation loss for muons compare to electrons? 40,000 Sarah Eno

29. Bremsstrahlung The photon is not low energy! It is (almost) equally probable for the photon to have any fraction of the electrons energy, from 0% to 100%. On average, thus, will get about ½ the electron’s energy. Sarah Eno

30. Bremsstrahlung (“Brem”) Happens more often in materials with heavy nucleii Material distance an electron loses 63% of its energy to brem Be 35 cm C 19 cm Al 9 cm Pb 6 cm U 0.3 cm Air 30 m (at sea level) Sarah Eno

31. Pair Production When in the presense of a heavy nucleii, a photon with energy above 1.022 MeV can turn into an electron-antielectron (positron) pair. On average, each will have ½ the photons energy. Why 1.022 MeV? Sarah Eno

32. Destructive Calorimeter: use these two processses to measure the energies of electrons and photons. More from Greg later this week. Sarah Eno

33. Nuclear Interactions Hard to predict the cross section, properties of these interactions from first principles, because they involve the strong force. Sarah Eno

34. Nuclear Interactions Rarer than, for example, brem. For example, a high energy electron will lose 64% of its energy to brem every 6.4 g/cm2 in Pb, while a high energy pion will do the same via nuclear interactions only every 194 g/cm2. A puzzle: nuclear interactions are due to the “strong” interaction. So why are they rarer than the electromagnetic interactions? Sarah Eno

35. Nuclear Interactions Messier than Brem. Green Sarah Eno

36. Nuclear Interactions Sometimes the interactions can produce neutral pions, which decay to two photons. These photons will then pair-produce, brem, pair-produce just like photons that came from the interaction point. Sarah Eno

37. Nuclear interactions Study empirically, using accelerator data Green Sarah Eno