Download The Atomic Zoo

List of Experiments
and Exhibits
The
Atomic Zoo
Special exhibition from 7th July 2006 to Spring 2008
Atomic physics
Fundamental unit of charge – the “atom” of electricity.
Is there a smallest quantity of charge – the “atom” of electricity, so to
speak? Using charged droplets of oil, Robert A Millikan succeeded in
identifying the smallest quantity of electrical charge – the elementary
charge – in 1906.
The droplets were observed in the space between the horizontal plates
of a parallel-plate capacitor. By measuring the rate at which they drifted
down and then the electric field strength required to make them hover, he
was able to calculate the electrical charge on any droplet. He found that
the charge was always a whole number multiple of a particular basic
charge, “e”.
For this discovery, Millikan was awarded the Nobel Prize in 1923.
The experiment/exhibit, shows the deflection of charged water droplets
falling between two capacitor plates.
An atom – magnified ten billion times!
Atoms are tiny – very tiny, and the nucleus is very much smaller than the
surrounding electron cloud of the atom. This model illustrates an atom
magnified ten billion times. At this magnification, the nucleus, represented
by a point of laser light, is 1/20th of a millimetre across, while the electron
cloud is 1 metre across. Despite being 50 000 times smaller than the
electron cloud, the nucleus contains 99.95% of the mass of the atom!
Atoms reveal their magnetism (Zeeman effect)
“Excited” Sodium atoms normally emit strictly monochromatic (single
wavelength) light. However, if an external magnetic field is applied, the
energy levels of electrons within the atoms are altered, and the light they
emit is split into a group of wavelengths, very close together and difficult
to distinguish from one another. They can only be observed, as in this
experiment, using an interferometer.
2
Atoms can be felt!
A trip along the mountain range of atoms!
Using a scanning tunneling microscope (STM), the atoms in the surface of
a solid can be outlined using the tip of a platinum-iridium wire, which is
itself only a few atoms wide.
This is done without any actual contact – a tiny current flows between the
surface and the probe-tip, which can be used to keep the tip at a constant
distance from the surface, so that the surface profile may be followed.
Short-lived muons
When muons travel through a block of fluorescent plastic, they produce
tiny flashes of light. If a muon should happen to decay inside the block, it
produces a second flash shortly after the first. The time between these two
flashes is a measure of the muon’s lifetime – around 2.2 microseconds.
When muons are produced in the upper atmosphere, they take about 80
microseconds (at nearly the speed of light) to reach the earth’s surface.
How is it that one can observe these muons down here, when they only
”live” for 2.2 microseconds? The answer lies in the relativity of time experienced by these fast-moving particles.
Low-pressure gases conduct electricity
The strangely beautiful glowing shapes inside this glass vacuum tube occur
when free electrons are accelerated by the electric field, they reach a high
enough speed so that, when they collide with a gas molecule, they can
transfer energy to its electrons. Only at a low gas pressure will the free
electrons have a sufficiently long path to reach these speeds. The gas will
then begin to conduct and glow.
The biggest plasma ball in the world!
Solid, liquid and gas – these are the three states of matter which everybody
knows about. However, 99% of the visible matter in the universe is in a
fourth state – a plasma. In a plasma the atoms are ionized – that is, the
electrons are either partly or completely separated from the atomic nuclei
and move about freely. The Aurorae (Northern Lights), sunlight, sodium
lamps, even ordinary flames, are examples of glowing plasmas. You can
control the discharges in the biggest plasma ball in the world with your
bare hand!
3
Nuclear physics
Misty particle tracks or Particles make cloud trails
Have you ever seen a muon? You can’t see electrically charged particles
directly as they travel through the Diffusion Cloud Chamber, but nevertheless, they all – alpha and beta particles, protons, muons, electrons and
positrons – leave a visible condensation trail behind them. The high-speed
particles ionize gas atoms along their path, and these ions act as condensation centres for droplets of liquid in the super-saturated alcohol vapour
in the chamber. You couldn’t ask for anything more beautiful in the atomic
zoo!
The muon telescope
Muons are unstable elementary particles – similar to electrons, but with a
substantially greater mass (about 200 times). They are produced by
collisions of cosmic ray particles with gas atoms in the upper atmosphere.
As they pass through the two Perspex scintillation blocks of the
“telescope”, they produce a very brief flash of light in each. If flashes occur
in each block within a fraction of a microsecond of each other, one can
assume they were caused by the same particle. The orientation of the
blocks gives a rough estimate of the direction in which the particle was
travelling.
Useful cathode rays
Atomic physics in your living room! For almost 70 years, the cathode ray
tube served as the main TV picture-producing device. In 1883, Thomas
Edison, inventor of the electric light bulb, showed that a glowing metal
filament emitted electrically charged “rays”. At the beginning of the 20th
century, these rays (electrons) were shown to be the same as beta particles
from radioactive substances. You can deflect the electron beam wherever
you want with a permanent magnet.
4
Radioactivity everywhere
We are surrounded all the time by natural radioactive sources. At this
experimental station you can check the radioactivity of various naturally
occurring radioactive materials. It is astonishing that the human body
receives 20% of its yearly radiation dose via potassium atoms, which are
essential for life! The strongest emitter which visitors to this exhibition will
come into contact with is the sample of uranium bearing rock from Ticino
(Switzerland).
How Rutherford discovered the nucleus
This historically important experiment used the weak flashes of light
produced by alpha particles striking a thin zinc sulphide layer. These were
observed through a microscope – though it takes several minutes in the
dark before our eyes are able to see the flashes.
Alpha particles can make sparks fly
Ionising radiation – in this case Alpha particles from a radioactive Americium source – makes the surrounding air conduct electricity. If the alpha
particles reach the high-voltage grid, you can see see it sparking over.
However, a piece of newspaper between the source and the grid is
sufficient to stop the alpha particles, so that there are no longer any
discharges.
Atomic nuclei are hard to hit
After J.J. Thomson had discovered the electron in 1896, people thought
that the negative electrons were equally distributed within the corresponding positive charge in the atom – like currants in a pudding. However, in
1909, E. Rutherford bombarded thin gold foil with a beam of alpha particles (positively charged helium nuclei), and found that about 1 in 8000
particles was deflected back towards the source! This could only be
explained by assuming that all of the positive charge in an atom was
concentrated in a tiny region of the atom – the nucleus. When an alpha
particle (positively charged) comes very close to the nucleus (also positively
charged) of a gold atom, the electrical repulsion deflects the alpha particle
through a large angle – sometimes even backwards!
5
Nuclear physics
Why particle accelerators have to be huge
Alpha particles (helium nuclei) are much more massive than electrons –
7200 times heavier. In this experiment a rather strong magnet bends a
beam of alpha particles into a curve of radius 60 centimetres. Compared
with this experiment, the particles in the accelerator at CERN (Geneva)
have a million times more energy. Using strong superconducting electromagnets, these particles are kept in orbits of radius 4.3 kilometres inside
the accelerator tube.
How can we stop fast electrons?
Some radioactive nuclei decay by emitting electrons (beta particles). These
travel through different materials, are slowed down and eventually absorbed. You can test how different materials (wood, leather, Perspex,
metals, etc) absorb a beta particle beam.
Electrons and anti-electrons in a magnetic field
Twins could not be more different! A radioactive sample emits electrons,
which can be deflected by a magnetic field. Another source emits
positrons, the anti-particle of an electron. Using the same magnet, these
are deflected in the opposite direction.
When matter and anti-matter meet
When an electron (particle of matter) meets its twin, the positron (particle
of antimatter), they both annihilate. The resulting energy converts into a
pair of gamma ray photons, travelling in opposite directions, and whose
total energy is equivalent to the energy of the mass destroyed (E=mc2).
The number of annihilations can be measured by having a pair of gamma
ray counters arranged in opposite directions, and counting only the
gamma rays which pass through each at the same time (coincidence
counting). Positron emission tomography (PET) in medicine uses this
technique.
6
Hard gamma rays
Gamma rays are, like X-rays, very penetrating. They require much thicker
and heavier materials to stop them than alpha and beta particles. Materials like lead, with high atomic number, are suitable for gamma ray
shielding.
Atomic nuclei leave their fingerprint
Germanium crystal semiconductors are used to identify different energies
of gamma rays in the environment. Detectors of this kind were used in
Switzerland to identify the non-naturally occurring radioactivity following
the Chernobyl disaster.
7
Nuclear physics
Half-life: chance, but law-abiding
No-one can predict when a particular radioactive nucleus in a sample will
decay. However, the time required for half of the nuclei in a sample to
decay can be accurately predicted. In this experiment, the decay of a
sample of Radon 220 nuclei (half-life 55 seconds) can be followed. The
column of lights shows how many nuclei have decayed in each 15-second
interval. After about 1 minute, the number will have about halved, after
another minute, it will about one quarter, etc.
X-ray quanta (photons) absorbed or scattered
Here are various materials to test how well they absorb or scatter X-rays.
The higher the atomic number of a chemical element, the better it is at
absorbing X-rays. Calcium (Z=20) is better than Hydrogen (Z=1) and
Carbon (Z=6). This is how X-ray pictures distinguish bones from soft
tissues. Besides absorption via the photo-electric effect, there is another
(but much smaller) effect in which X-rays are scattered by “collision” with
electrons in an atom (the Compton effect). In this, an X-ray photon gives
some of its energy to an electron and is re-radiated as a less energetic
photon in another direction – hence scattered.
Chemical analysis by X-rays
X-rays can be used to identify chemical elements in a sample. The radiation knocks bound electrons out of atoms, and electrons from higher
energy levels can then drop into these empty levels. The X-rays emitted
during these transitions have precise energies, and therefore wavelengths,
depending on the type of atom. The wavelengths and intensities of these
characteristic X-rays can be measured, and the emitting element thus be
identified and the amount of it estimated. The experiment will be demonstrated by Technorama staff.
Neutrons in road-building
Neutrons permit the investigation of thick materials, without damaging
them (non-destructive testing). Neutrons are stopped mainly by hydrogen
nuclei, because these have practically the same mass as the colliding
neutron. The number of neutrons absorbed represents the hydrogen- (and
hence water) content of the sample. Using a neutron probe, a demonstrator will investigate the “moisture-content” of various materials, including
water, soil, wood and paraffin (!). In addition, they will show that silver
becomes radioactive when irradiated with neutrons.
8
In the valley of the isotopes
Different elements have different numbers of protons in the atomic
nucleus. Every element has a number of different isotopes, which differ in
the number of neutrons in the nucleus, and therefore also in the mass of
the nucleus. Around 3000 stable isotopes are known. This computer station is an interactive table of nuclides (isotopes) in which virtual isotopes
can be built and altered. In addition, you can view the results of the decay.
Technical terms – a handy guide
Is your personal atomic zoo populated with muons, mesons, bosons,
neutrons, photons, protons, neutrinos – all the normal particles – or have
some hybrid protrinos, etc, wormed their way in? What, again, was the
difference between alpha and beta rays? No problem – use this computer
station to jog your memory about all the most important definitions and
concept explanations.
Atomic nuclei are mini-magnets
Atomic nuclei are made up of protons and neutrons. Nuclei usually have
angular momentum (spin) which means they also have a magnetic
moment (behave like a tiny magnet). Looked at from the simplistic view of
classical (not quantum) physics, the nucleus may be regarded as a magnetic spinning top. In an external magnetic field, the nucleus precesses –
loops as it spins – in much the same way as a tilted spinning top or
gyroscope. The form and frequency of precession is a characteristic of each
particular nucleus, and is useful in chemical structure analysis and nuclear
spin tomography (MRI). In this experiment, a black and white segmented
ball acts as a model for the spinning and precessing proton.
9
Nuclear physics
Heavy hydrogen sheds light
Not particularly bright, but tritium (Hydrogen-3) light sources keep going
for decades without additional energy supply. Among other things, they
are used for emergency lighting of notices, and the clock dials and hands.
The tritium atoms are weakly radioactive, and the emitted beta particles
make a layer of phosphor scintillate. Nowadays, most wrist watches rely on
a non-radioactive process for their glow.
Here’s how to smash atomic nuclei
Particle accelerators are the tool of basic research. We look for discoveries
and new insights into particles (eg the Higgs boson), energy (perhaps
“dark” energy), or matter (maybe “dark” matter) and also answers to
fundamental questions like “why do particles have mass anyway?” At
CERN in Geneva, the largest particle accelerator in the world – the Large
Hadron Collider – is currently (2007) being built. Our greatly simplified
model of a particle accelerator accelerates a steel ball in several stages
before it smashes into plastic balls.
10
The explanatory texts at the exhibits are at two levels:
• First and foremost, directly beside the exhibit is a brief description of what the exhibit is about, and a point
by point set of instructions, with illustrations, of what to do and what to notice.
• If you want to know more, there is a separate text, with a deeper, but still non-technical and illustrated
explanation.
11
Technoramastrasse 1, CH-8404 Winterthur
Telephone +41 (0)52 244 08 44, Fax +41 (0)52 244 08 45
[email protected], www.technorama.ch
© Technorama, January 2007 / Subject to change
Opening hours:
Tuesday to Sunday, 10 a.m. to 5 p.m.
Open on all public holidays (including Mondays)