Download Inside A Particle Physicist`s Toolbox

A Particle Physicist’s Toolbox
Particle physicists do
experiments in the
following way:
1. Bang bits of matter together
2. Look at the little bits given off
3. Observe their properties & interactions
4. Describe underlying structure & fundamental principles
5. Go to step 1 but bang them together harder
Particle Physicist’s Experiments are a bit like this:
How do we “naturally” observe our world?
Source
Probe
Target
Scattered Rays Detector
The Particle Physics Scattering Model
Source
Probe
Target
Emerging
Rays
Detector
Ancient Atomic Theory
Around 400 BC two ancient Greek Philosophers
called Leucippus and Democritus first suggested
that all matter is made up of atoms.
Democritus
Democritus wrote that matter is "composed of
minute, invisible, indestructible particles of pure
matter which move about eternally in infinite
empty."
According to the ancient Greeks, atoms were all made of the
same basic material, but atoms of different elements had
different sizes and shapes. The sizes, shapes, and
arrangements of a material’s atoms determined the material’s
properties. It was believed that there were four elements that
all thing were made from; Earth, Air, Fire and Water.
Sir. J. J. Thomson
In 1897 Cathode rays were investigated by JJ
Thomson.
This is a really famous experiment because it
changed the course of history!
Anode
Cathode
Positive
Negative
Cathode Ray Tube
JJ Thomson showed that a stream of
particles were emitted from the negative
cathode.
• He called these particles Electrons.
• Electrons had a negative charge.
•They had an extremely small mass.
As atoms were
neutral he proposed
the “plum pudding”
model of electrons
in a positive
surround.
The Plum Pudding Model of the Atom
In order to explain how atoms were neutral in charge JJ
Thomson came up with the Plum Pudding Model of the
Atom. The negative electrons were like the plums, randomly
scattered in a positive “dough”.
Positive charge surround the
electrons, like the dough of the
pudding.
Negative electrons (like the plums)
Note the absence of protons and neutrons, they were not yet discovered!
So How do we test to see whether the
Plum Pudding model is correct?
Remember what particle physicists do?
Rutherford's Experiment
In 1911 British scientist
Ernest Rutherford set out to
test J.J.Thomson’s Plum
Pudding proposal.
Ernest Rutherford
He decided to fire a beam of positively charged
particles at Gold atoms.
The positive particles were called alpha
particles.
Probe: Alpha
(He++)
Source:
Radium
Lead
Collimator
Target:
gold foil
Detector: Zinc
Sulphide Viewing
Screen
The predicted result was that the vast majority of
alpha particles would pass straight through with
very little scattering.
Proposed Atomic Plum Pudding Model.
Alpha particle’s path curves due to
the electrostatic force of
attraction between positive alpha
and negative electron
++ Alpha Particle
Very little
bending should
happen!
++
A Plum Pudding
Atom
Positively charged mass
Negatively charged
embedded electrons
Rutherford expected to see small deflections of the alpha
particles as they punched through the ultra-thin Gold foil.
Source of
Alpha particles
Ultra thin Gold
foil
What Rutherford actually observes are many large angle
scatters or ricochets.
Source of
Alpha particles
Ultra thin Gold
foil
Some alpha particles are even bounced
back along their original path!
From the scattering angles
Rutherford calculates:
Positive charged mass
cannot be distributed
over whole atomic
volume.
The observed scattering
can only occur when the
positive charge is
concentrated at the center
in a volume of 0.0001
atomic diameter.
Very small positive Nucleus
Rutherford’s Experiment meant that the
Plum Pudding Model of the Atom was
finished!
The Nuclear Model of the Atom was
born.
Source of
Alpha particles