Physics 736 Experimental Methods in Nuclear Particle and

Physics 736: Experimental Methods in Nuclear, Particle, and Astro Physics Prof. Vandenbroucke, February 4, 2015

Announcements • • Problem Set #1 due Thursday at 5 pm in Ian Wisher’s mailbox Read Tavernier 3. 6 -3. 9 for Mon (Feb 9) Read Tavernier 4. 1 -4. 3 for Wed (Feb 11) Office hour today 3: 45 -4: 45 (Chamberlin 4114)

Compton scattering • Elastic scattering of photon and electron • Typically a high energy photon transfers energy to a low energy electron • “Inverse” Compton scattering: high energy electron transfers energy to a low energy photon • Can be useful for photon detection • Can also be a nuisance: changes photon direction • Cross section given by Klein-Nishina formula: • Low-energy limit is energy independent – Scattering off single electrons: Thomson scattering – Coherent scattering off bound electrons in atom: Rayleigh scattering

Angular distribution of Compton scattering • At high energies, outgoing photon direction similar to incoming photon direction • At low energies, direction is randomized more

Pair production • • • Photon is converted to an electron-positron pair Cross section rises quickly from threshold to a constant value at high energy At high energy, mean free path for pair production is X 0*9/7 Opening angle between electron and positron decreases with photon energy Electron and positron produced preferentially in the polarization plane of the gamma ray

Fermi Large Area Telescope: a pair-production telescope for gamma-ray astronomy

Photo-disintegration / photo-dissociation • • Reverse of fusion Nucleus absorbs gamma ray Excited nucleus decays More likely for heavy than light nuclei

Summary of photon interactions in matter pair production • A single photon interacts with a probability proportional to absorber thickness (for thin absorbers) • A beam of photons is attenuated exponentially with distance

Photoelectric absorption edges • Electron binding energies in lead (ke. V): 88. 0, 15. 9, 15. 2, 13. 0, 3. 9, 3. 6, 3. 1, 2. 6, 2. 5

less absorption Absorption of a photon beam by matter photoelectric Compton pair production • Number (not energy) of photons in a beam is attenuated exponentially • Absorption length inversely proportional to cross section

Strong interactions • So far we have focused on electromagnetic interactions: ionization loss, bremsstrahlung, photoelectric, pair production • High energy hadrons (protons, neutrons, pions, …) can also undergo strong interactions in matter • Inelastic: produces quarks which hadronize to mesons or baryons • Non-hadrons (electrons, muons, neutrinos, …) do not undergo strong interactions • Because strong force has a very short range, strong cross section at high energy (above 1 Ge. V) is same order of magnitude as geometric cross section of nucleus • 1 barn = 10 -24 cm 2 • Proton radius ~ 1 fm, area ~30 millibarn • Nucleus of atomic number A has cross section given approximately by

Strong interaction cross section grows slowly with energy Simple estimate surprisingly accurate

Hadronic (strong) interaction length • Mean free path between hadronic interactions, for protons in matter • Number density of nuclei in matter: N = ρNA/A • For typical solids, between 10 and 100 cm • Typically larger than radiation length X 0 by factor of a few • Example: air (Nitrogen A = 14)

Spallation • Collision with heavy nucleus breaks it into smaller nuclei • Can occur in space, atmosphere, rock, accelerators • Cosmic rays hitting atmosphere produce Carbon-14 (used for carbon dating) and other cosmogenic nuclei • Unstable spallation products in cosmic rays can be used as clocks • In addition to lighter nuclei, extra neutrons typically liberated: spallation neutron sources • Example: 1 Ge. V proton on lead produces 25 neutrons (useful source of neutrons)

Photonuclear interactions • Counter-intuitively, photons can undergo strong interactions with nuclei • Gamma-ray produces virtual quark-antiquark pair that interacts with nucleus • Excitation of nucleus can cause dipole (excess protons on one side) which results in resonant absorption (“giant dipole resonance”, GDR)

(Weak) interactions of high energy neutrinos in matter • In the high-energy regime (above ~30 Ge. V), interactions are dominated by deep inelastic scattering (neutrinos interact with quarks in nuclei) • Charged-current and neutral-current interactions • Cross section increases with energy

The Glashow resonance • An alternative interaction to DIS • Only possible for electron anti-neutrinos, due to presence of atomic electrons in matter • Resonant production of W-, which decays • Resonance is at neutrino energy of 6 Pe. V • W- branching ratios: – 68% quark + antiquark (hadronic shower) – 11% tau + tau antineutrino (tau track) – 11% mu + mu antineutrino (mu track) – 10% electron + electron antineutrino (electromagnetic shower)

Energy loss of muons in ice Photonuclear interactions of muons (or electrons or taus): energy transfer to nucleus via photon, producing energetic hadrons (like a hadronic interaction of a proton)