Aside: Why is high energy physics so expensive

1
Aside Why is high energy physics so expensive?

• Rule of thumb from optics

To resolve detail on a distance scale of ?x
requires a wavelength satisfying
1/kV
7B/LHC
2
Nuclear Phenomenology
Read Das and Ferbel Chap. 2. Homework 2 post
Friday, Jan. 30.

• Phenomenology???
• Not Theory! Usually an intermediate step in the
  development of theory.
• Data-driven. Models, computational framework for
  organizing empirical information.
• Can have predictive power, but it is the
  predictive power of patterns, rather than
  fundamental understanding.
• By bringing order, identifying the patterns that
  may be deep clues, phenomenology can stimulate
  development of a theory.
• Its just phenomenology a bit like Its only a
  theory.
• Nuclear Physics Its just phenomenology.
• There was no theory predicting nuclei.
  Rutherford and successors gradually accumulated
  information about nuclear properties that
  revealed the nature of the Strong Nuclear Force.
• Totally alien to prior experience. Nuclear force
  is so strong that it defies the usual techniques
  of quantum mechanics. Even now we have no theory
  of the strong force that allows us to calculate
  the full range of nuclear phenomena.

3
How it all began.

• Particle Physics J.J. Thomson electron discovery
  (1897)
• Nuclear physics Rutherford Scattering (1911)
• Mass of atom is concentrated in hard nucleus,
  diameter 10-14 m within an atom of diameter
  10-10 m.
• Nuclei of heavier atoms have nuclei of hydrogen
  within them!
• Rutherfords discovery of the proton in 1918
• ? 14N ? 17O 1H
• Heavier nuclei contain H nuclei ? elementary
  particle, the proton
• Rutherfords first model of the nucleus

• A protons to get the mass right
• A-Z electrons to get the charge right without
  messing up the mass.

4
Nice try, but all wrong.

• Other problems (not all immediately recognized)
  spin statistics, magnetic moments, ?-decay all
  demonstrated that theres more inside the nucleus
  than es and ps. By the end of the 20s the
  time was ripe for.

5
Discovery of the Neutron Chadwick (1932)

• Specific clue
• ?-decay of nuclei (X ? Y e- something else)
• Continuous electron-energy spectrum ? not 2-body
  decay. Unseen third particle must be produced,
  and there and must be more than es and ps in
  nucleus!

• Chadwick observed neutrons indirectly, producing
  them in
• and then bouncing them off of and ionizing
  atoms, and detecting the ions.
• Neutron was difficult to study. Scattering
  revealed mn ? mp. Later it was demonstrated that
  neutrons participated in the (strong) nuclear
  force and, except for charge, were very like
  protons .
• Chadwicks discovery led to the modern view of
  the nucleus, protons and neutrons bound by a
  nuclear force strong enough to overcome the
  electromagnetic repulsion of the protons.
• Next steps identify, measure properties, look
  for patterns in as many different nuclei as
  possible.

? 9Be ? 12C n
6
Terminology
Nucleons protons and neutrons
Nuclide Z - of protons and NA-Z - of
neutrons
Radionuclide unstable nuclide, undergoes
radioactive decay
Phenomenology of nuclei developed from
observations of patterns of properties and
behavior masses, stability, etc.

• Isotopes Same Z, different A
• Identical chemistry ? tracing, dating, isotope
  separation
• Isobars Same A, different Z
• Different chemistry same nuclear physics
• Isotones Same N, different Z
• Isomers Same Z and A, different energy states
  (excited states tend to decay by emission of a
  photon isomeric transitions)

7
Masses of Atoms
Z protons
A-Z neutrons
Z electrons

• First guess

mp 938.27 MeV/c2 mn 939.57 MeV/c2 me 0.511
MeV/c2
Precise measurements (next slide) show this to be
wrong. There is a nonzero binding energy that is
given up when all of the constituents are
assembled into a nucleus, which appears as a mass
deficit of the atom compared to its parts.
at rest
deuteron
8

• Mass Spectrometer
• Precise determination of ion (and therefore)
  nuclear masses.
• Developed by J.J. Thomson and others, refined in
  this building by A.O.C. Nier 235U isolation.
• Focuses ions of a given mass independent of KE
  and entrance angle.

Electrostaticdeflection
Accelerated by 40 kV
Magnetic focusing
V
Picture of Nier spectrometer from Nuclei and
Particles by E. Segrè
Electron multiplier
9
Stability of Nuclei

• Nuclei are stable because the composite state is
  energetically advantageous compared to the
  disassembled parts.
• Relative stability is measured by the binding
  energy per nucleon.

Average energy needed to release a nucleon from
the nucleus

• Low-mass nuclei, A?20
• B/A bounces up and down, overall increasing
  steeply with A.
• A?60
• B/A maximal at 8.7 MeV per nucleon for 56Fe.
• Agt60
• B/A falls slowly to 7.6 MeV per nucleon for
  238U.