PHYS 3446 – Lecture #10Slide 2Slide 3Parity ViolationSlide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Monday, Oct. 11, 2010 PHYS 3446, Fall 2010 Andrew Brandt 1PHYS 3446 – Lecture #10Monday, Oct. 11, 2010Dr. Andrew Brandt•Weak Interactions•Energy Deposition in Media- Charged Particle Detection- Ionization Process- Photon Energy Loss HW4 Due today; HW5 due 10/18; Review 10/18; Test will be Weds 10/20 on Ch 1-4; will discuss project on Weds.Monday, Oct. 11, 2010 PHYS 3446, Fall 2010 Andrew Brandt 2HW5End of the chapter problems: 2.2, 3.2,3.3, 4.1,4.4,4.5Due Monday 10/18Monday, Oct. 4, 2010 PHYS 3446, Fall 2010 Andrew Brandt 3• -decay can be written at the nucleon level as:•Fermi postulated a new weak force responsible for -decay, due to long lifetimes. Strong:EM:Weak:Gravity 1:10-2:10-5:10-40 •Weak force is relativistic and is also known as four fermion interaction•Parity:•A system is parity invariant if it does not change under inversion of spatial coordinates: (this was generally assumed to be true)•The handedness, helicity s·p, should change upon spatial inversion (parity operation) since the direction of motion changes while the spin direction does not•Only left-handed neutrinos and right-handed anti-neutrinos are observed •Since there are no right handed neutrinos, parity must be violated in weak interactionsWeak Interactionsn �p �p e-+ �ep e n-+ +en e n++ +en n+,r r� -r r p p� - �r r( ) ( ) L r p r p L= � = - �- =r rr r r r)()( rrMonday, Oct. 11, 2010 PHYS 3446, Fall 2010 Andrew Brandt 4Parity Violation •Lee+Yang, Wu et al•observed which direction the electrons were emitted•inverting coordinate gives same spin but now electrons would be emitted in opposite direction (not observed)Monday, Oct. 11, 2010 PHYS 3446, Fall 2010 Andrew Brandt 5•When a heavy nuclei undergo alpha and beta decays, the daughter nuclei are often in an excited state–Must either break apart–Or emit another particle–To bring the daughter into its ground state•Typical energies of photons in -decays are a few MeV’s–These decays are EM interactions thus the life time is on the order of 10-16 sec.•Photons carry one unit of angular momentum–Parity is conserved in this decayGamma DecaysMonday, Oct. 11, 2010 PHYS 3446, Fall 2010 Andrew Brandt 6•Prior to Nuclear Physics two forces were known:–Gravitational force: formulated through Newton’s laws–Electromagnetic force: formulated through Maxwell’s equations•We have learned about two additional forces–Strong nuclear force: Discovered through studies of nuclei and their structure–Weak force: Discovered and postulated through nuclear -decayFundamental ForcesMonday, Oct. 11, 2010 PHYS 3446, Fall 2010 Andrew Brandt 7•Physics is an experimental science–Understand nature through experiments•In nuclear and particle physics, experiments are performed through scattering of particles•In order for a particle to be detected:–Must leave a trace of its presence generally through deposition of energy in detectorParticle PhysicsMonday, Oct. 11, 2010 PHYS 3446, Fall 2010 Andrew Brandt 8•An ideal detector would–Detect particles without affecting them•Real detectors–Use electromagnetic interactions of particles with matter•Ionization of matter by energetic, charged particles•Ionized electrons can then be accelerated within an electric field to produce a detectable electric current–Sometime catastrophic nuclear collisions but rare–Particles like neutrinos, which do not interact through EM force and have low cross sections, require special detection methodsDetecting ParticlesMonday, Oct. 11, 2010 PHYS 3446, Fall 2010 Andrew Brandt 9•What do you think is the primary interaction when a charged particle is traversing through a medium?–Interactions with the atomic electrons in the medium•If the energy of the charged particle is sufficiently high–It deposits its energy (or loses its energy in matter) by ionizing the atoms along its path –Or by exciting atoms or molecules to higher states–What are the differences between the above two methods?•In the former case you get electrons, for the latter photons•If the charged particle is massive, its interactions with atomic electrons will not affect the particle’s trajectory•Sometimes, the particle undergoes nuclear collisions Charged Particle DetectionelectronsphotonsMonday, Oct. 11, 2010 PHYS 3446, Fall 2010 Andrew Brandt 10•Ionization properties can be described by the stopping power variable, S(T)–Definition: amount of kinetic energy lost by any incident object per unit length of the path traversed in the medium–Referred to as ionization energy loss or energy loss•T: Kinetic energy of the incident particle•nion: Number of electron-ion pair formed per unit path length• I : The average energy needed to ionize an atom in the medium; for large atomic numbers ~10Z eV.Ionization Process( )S T =Why negative sign?The particle’s energy decreases.dTdx- =ionn IMonday, Oct. 11, 2010 PHYS 3446, Fall 2010 Andrew Brandt 11•What do you think the stopping power of the given medium depends on?–Energy of the incident particle–Electric charge of the incident particle•Since ionization is an EM process, easily calculable–Bethe-Bloch formula for relativistic particle–z: Incident particle atomic number–Z: medium atomic number–n: number of atoms in unit volume (=A0/A)–m: electron massIonization Process( )S T =( )222 2 222 242lnze e nZmcIm cpg bbb� �� �-� �� �� �� �� �Monday, Oct. 11, 2010 PHYS 3446, Fall 2010 Andrew Brandt 12•In natural -decay, the formula becomes–Due to its low kinetic energy (a few MeV) and large mass, relativistic corrections can be ignored•For energetic particles in accelerator experiments or beta emission, the relativistic corrections are substantial•Bethe-Bloch formula can be used in many media, for various incident particles over a wide range of energiesIonization Process( )S T =( )222 22 242lnze e nZmcIm cpbb� �� �� �( )222 2 222 242lnze e nZmcIm cpg bbb� �� �- �� �� �� �� �� �10Monday, Oct. 11, 2010 PHYS 3446, Fall 2010 Andrew Brandt 13•Why does the interaction with atomic electrons dominate the energy loss of the incident particle?–Interactions with heavy
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