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”