Post on 09-May-2020
transcript
The Electromagnetic Interaction
John Belz (for Carleton Detar)16 April 2010
www.physics.utah.edu/~belz/phys5110/em.pdf
The Electromagnetic Interaction
● Interaction of particles with matter● Hadronic interactions; GZK effect● Resonant absorption; Mössbauer effect
Interaction of Particles with Matter
Energy Deposit or “Stopping Power”
● Important for
– Radioactive shielding
– Medical applications
– Particle physics detector design
● In most interesting energy regime, governed by Bethe-Bloch equation:
Source: http://pdg.lbl.gov
Bethe-Bloch Equation
● Energy loss is proportional to density of electrons
● “Blows up” at small ● Minimum at high
“Minimum Ionization”
● -dT/dx = loss of K.E. per “length” traveled in material
● n = number density of target
● Z = #electrons/atom in target
● z = charge of probe
● I = mean excitation energy of target material
Minimum Ionization
Bremsstrahlung
● Mainly an effect for electrons; Weak for heavier charges.
● At high energies, dominates ionization losses.
● Low-energy photons most likely
(dimensional analysis of Feynman diagram)
Photon Processes
● Raleigh Scattering● Photoelectric Effect● Compton Scattering● Pair production (nuclear and electron fields)
Radiation Length
Defn: Radiation Length L
R
Defn: Pair Production Length L
PP
CAVEAT: The pair-produced electronscan bremsstrahlung!
Hadronic Interactions; GZK Effect
Radiative Decays
● Radiative decays feature 's
● Violate isospin conservation.
● Conserve charge-conjugation (C) and parity (P).
-hadron interactions: Cosmic Ray Spectrum
● Spectrum extends to several Joules per particle.
● Violent processes, e.g. Active Galactic Nuclei are the culprits(?)
“AGN”(Cen A)
-hadron interactions: Cosmic Ray Spectrum
● Complication: space is filled with a sea of low-energy photons: The 2.7 K cosmic microwave background.
● At large Lorentz boosts, CMB looks like hard gamma rays.
● At what energy will this “GZK”* mechanism produce significant energy losses?
● nb: This effect was first observed in Utah.
* Greisen, Zatsepin, Kuz'min, 1966
Resonant absorption;Mössbauer effect
Resonant Absorption
● A photon emitted by a atom as it de-excites can then excite another atom.
● Can this happen on the level of nuclei?● Complications:
– The -ray emitted by a nucleus can have an energy which is a significant fraction of the mass of a nucleon.
– The nucleus will recoil in emitting the .
Radiative Decay of Samarium
Electroncapture
963 keV
151Eu
152Sm
Radiative Decay of Samarium
● The recoil energy being >> the width, another Sm nucleus cannot absorb the gamma ray.
● Actually E is two low
by twice the recoil energy or 6.52 eV. (Why?)
Radiative Decay of Samarium
● We could get 6.52 eV by moving the source (Doppler shift). How fast would we need to move it?
● Impractical!● Alternate solution:
place Sm in crystal lattice...
● Large lattice mass means negligible recoil.
● Now can use resonant absorption to identify Sm in “absorber”
● Process known as “Mössbauer Spectroscopy”