Lecture 13: Nuclei: Scattering Reveals the Femtoworld

Experiment: measure lifetime of Indium

Indium transparencies

1. Searching for the Atom's Internal Structure
1900: Scientists think atoms were permeable spheres. 1909: Rutherford tests this theory/
Radioactive source shoots stream of collimated, light alpha particles at monomolecular layer of heavy gold atoms
Zinc sulfide screen scintillates when alpha particles (helium nuclei) hit it

2. Rutherford's Scattering Experiment
Expect: high energy (megavolt) charged alpha particles will zip through, lighting up small region at the back of screen/
result: dots mostly at the back of the screen, but to everyone's surprise, some dots backwards!!!

3. Rutherford's Analysis
Most alphas easily pass through outer part of atom, a few alphas bounce off something hard inside atom something small, dense, and positively charged.  Atoms not permeable balls!

new hypothesis: atoms have nuclei, a keystone in the development of modern atomic theory

4. Deflected Probe
Find target hidden by a black cloud shoot beams into the cloud record where the beams come out

5. Targets in the Clouds Revealed
Unfold hidden structure by tracing back to the scattered particles/The first target was a wedge; the second target as a circle

6. Perceiving the World
source/target/detector: the way we see the world
light bulb behind you, tennis ball in front of you, photons travel from light bulb (source) bounce off the tennis ball (target)
hit your eye (detector) different photon wavelengths reveal color, from photon's direction, you "see" a round object

dolphins and bats emit, detect sound waves to "see" their world with acoustic "sonar" /reflected waves reveal surroundings

7. Higher Energy, Better Microscope
using waves to detect the physical world, image quality is limited by wavelength
our eyes attuned to the sun's visible light with wavelengths ~ 500nm, too wide to analyze anything smaller than a cell
for higher magnification, must use waves with smaller wavelengths, e.g. electron microscope for crystals, viruses

But even best microscope only shows fuzzy picture of atom/ see scanning tunneling microscope pictures.

8. Wavelength Determines Resolution
example : swimming pool waves are 1 meter apart (a 1 meter wavelength)
push stick into water - pool's waves just pass around the stick/ 1 meter wavelength pool waves unaffected by a tiny target

E=hc/ =hc/ E      want small wavelength? get big energy

9. The Physicist's Dream... Short Wavelength Particles
Can't use light to explore atomic and sub-atomic structures, wavelength too long
But ALL particles have wave properties - use particles as probes
To see the smallest particles, need particles with the shortest possible wavelength
Most of the particles around us have long wavelengths

10. A Particle Wavelength
A particle's energy and its wavelength are inversely proportional; particle accelerators increase the momentum of a probing particle, thus decreasing its wavelength,=h/p,   P=h/

11. Accelerating Particle
(All text is part of the GIF)

Experiment - Measure decay rate of Indium...second point

12. Scientist's Meterstick of the Universe
For each power of ten / different detector needed to view the world, corresponding to size of the structure viewed

13. Radioactivity
Late 1800's... 1)Röntgen discovers new rays... X-rays electron beam strikes a piece of glass, the first TV tube
...a tube like that of electron diffraction

14. Radioactive Particles
Pierre and Marie Curie discover 3 distinct types of accelerated particles from radioactive decay named after the first three letters of the Greek alphabet: a(alpha), b(beta), and g (gamma) separated by a magnetic field positive alpha particles bend one direction negative beta particles bend opposite neutral gamma rays do not bend at all

15. Curie's Discovery of three types of radioactive particles
Alpha particle: nucleus of helium (2 p, 2 n) 4He22; Beta particle: speedy electron; Gamma radiation: bullets of light/ separated by a magnetic field:  positive (+2) alpha particles bend one direction, negative (-1) beta particles bend opposite, neutral gamma rays do not bend at all

Photons with increasing energies: Radio Waves, Visible Light, X-Rays, Gamma rays

Experiment with Geiger counter and sources
alpha particles: stopped by a sheet of paper
beta particles: sheet of aluminum
gamma radiation: a block of lead... penetrates far into a material, disrupts chemical bonds

neutrons: discovered later, boiled off by a fissioning nucleus

Sadly, many years passed before scientists realized the perils of penetrating radiation

16. Half-Life... Radioactivity Decreases Exponentially with Time
rate of decay measured by how long it takes for half to decay
utterly random when a particular atom will decay / half-life valid only for large number of nuclei

Why will an atom, just sitting there, decay according to some set probability?  Many physicists very unhappy that chance rules physical properties.  Einstein proclaimed, "God doesn't play dice!" Einstein was wrong

17. Alpha Decay: Quantum Mechanical Tunneling
protons and neutrons bounce around inside a nucleus / probability that proton or neutron outside minuscule / but quantum mechanical chance that 2 protons and 2 neutrons appear together outside the nucleus / greater chance for a large nucleus than in a small one / Outside range of strong force the positive alpha particle repelled from positive nucleus
fundamental to quantum mechanics - particle behavior in terms of probabilities

18. Residual Strong Force Holds the Nucleus Together
Nucleus is held together by strong interaction / would blow apart by electrical repulsion between protons if it weren't "glued" together by gluon particles

Nucleus analogy: tightly cocked spring (the electrical repulsion) held in place by very big rope (strong force) / stored-up energy in the spring can't be released because the rope is too strong... and less alpha emitted or neutron absorbed.

19. Fission: Mass Conversion to Energy
many heavy elements...uranium, thorium, radium ...decay into simpler elements spontaneously or by absorption of neutron

Uranium's mass =238.0508 atomic mass units (amu) / decays into thorium (234.0436 amu) plus alpha particle (4.0026 amu) / Uranium's mass minus mass of decay products = 0.0046 amu.  Where did this "missing" mass go?

Einstein said:  lot of energy stored in strong force holding uranium atom together...binding energy / potential energy stored as mass released as kinetic energy (energy of motion)

20. Liquid Drop Model of Nucleons

21. Chart of Nuclear Stability