Transcript 2004-WilsonCloudChamber

Melanie Carter and Jamie Hegarty
CLOUD CHAMBER OUTLINE
• Introduction: Cloud
Chamber Principles
• All About Ionizing
Radiation
– Types of Radiation,
why it’s “ionizing”
– Where it comes from
– How it interacts with
vapor in the chamber
• Vapor, Condensation,
and …Polarity?
• The Wilson Chamber
–
–
–
–
A Brief History
Experimental Setup
Pictures and Video!
Discussion/Conclusions
• The Diffusion Chamber
– Experimental Setup
– Pictures
• Summary
• References
CLOUD CHAMBER PRINCIPLES
1. Liquid is placed into a
chamber and allowed to
reach an equilibrium state
of evaporation and
condensation. (saturation)
2. Chamber is adiabatically
cooled or expanded,
resulting in supersaturation
of the air inside.
(disruptions will cause
quick & easy
condensation)
PV = nRT
CLOUD CHAMBER PRINCIPLES
3.
Ionizing radiation
passing through the vapor
ionize nearby atoms and
molecules.
4.
Condensation occurs
around ionized molecules,
revealing the path of the
ionizing particles.
IONIZING RADIATION
Comes in 3 tasty flavors: alpha (a), beta (b),
and gamma (g).
Must be at least 12.4 eV to be considered
“ionizing”.
Alphas (helium nuclei) are the most
massive, slowest moving, and least
dangerous of the three.
Gammas (photons) are the least massive,
fastest moving, and most dangerous.
Betas (electrons) are in between.
CAUTION:
Ionizing Radiation is
Harmful to Animals!
ALPHA DECAY
Alpha Particles (2p + 2n) are primarily
produced during the decay of heavy
elements as part of a lengthy decay
chain.
Due to the Coulomb barrier, alpha
particles must tunnel out of the
nucleus! For this reason, half-lives of
heavy elements are directly related to
the decay energy of the alpha particle
involved.
Alpha decay energies typically range from 2-10MeV. In the cloud chamber,
their tracks are thick due to the high ionization from the +2e charge. A
5.5MeV alpha will travel 4cm in air at STP.
Alpha particles can be stopped by very thin physical barriers, such as skin,
aluminum, or even thin plastic wrap.
BETA DECAY
Beta Decay can occur in a number of ways, the most
frequent being a neutron decay after an alpha decay:
n ---> p + e + ne
The decay p ---> e+ + n + ne may occur for an excited
atom.
Beta particles travel faster than alpha particles, are
less ionizing, and are not stopped as easily. Beta
particles typically have energies around 175keV.
In a cloud chamber, they produce very light streaks.
GAMMA DECAY
Gamma Decay refers to any radioactive decay where
photons are emitted.
They can be either X-rays or Gamma rays.
Gamma decay occurs either as a by-product of another
decay process, or for excited atoms to release excess
energy.
Gamma decay may be observed through secondary pairproduction interactions, via Compton scattering, or via the
Photoelectric Effect. Auger electrons may also show up.
Gamma energies may range from a few keV to several
hundred MeV.
CLOUDS FORM ON IONS
• Water is a polar molecule, so
it’s attracted to the ions.
Ethanol Stream
• Many molecules begin to
stick together, thus
condensing into tiny droplets.
• Alcohols like Ethanol and
Methanol are also polar, and
behave similarly.
• Alcohols are less dense, and
more readily form vapors,
making them more suited to
use in cloud chambers.
WILSON CLOUD CHAMBER
• Invented by John Aitken,
1880
• Couldn’t create clouds
without dust particles
• Adapted by C.T.R. Wilson,
between 1897 and 1912.
• Noticed droplets without
dust at expansions over
125%
• Hypothesized clouds were
forming on microscopic
nuclei or ions
• Tested extensively with
newly-discovered X-Rays
Images: Cambridge Physics
WILSON CLOUD CHAMBER
1
WILSON CLOUD CHAMBER
ADIABATIC
EXPANSION
2
WILSON CLOUD CHAMBER
ADIABATIC
EXPANSION
3
WILSON CLOUD CHAMBER
…in action!!
Diffusion Cloud Chamber
• Operates on smooth temperature
gradient
• Continuous Supersaturation in the
sensitive region
• More sensitive than expansion
chamber
•
Tested beta, alpha, and x-ray sources
•
Compton Effect, Photoelectric Effect
Diffusion Cloud Chamber
Freezing point for…
70/30 Ethanol:
-55.5 C
90/10 Ethanol:
Below -80 C
Dry Ice: -78.5 C
First Run: 70% ethanol, 30% water … terrible results
Conclusion: Requires 90% ethanol to prevent freezing
Chamber must be filled before cooling in order for
equilibrium to be established.
Diffusion Cloud Chamber
Natural Thorium Rod Alpha Source
Diffusion Cloud Chamber
C-14 Beta Source
Diffusion Cloud Chamber
Fe-55 X-Ray Source
CLOUD CHAMBER SUMMARY
More interesting experiments could be done with a
thermometer inside the Diffusion Chamber.
Helmholtz Coils above and below the chamber
could be used for quantitative measurement of
particle charge/mass ratios. Tear them off the
broken e/m device?
Non-polar liquids should be tested.
A career in video editing would suck.
REFERENCES
Diffusion Cloud Chamber Owner’s Guide, v. 2.3
Tipler Modern Physics
Whitten, Davis, Peck General Chemistry
Cambridge Physics Wilson Chamber Photos:
http://www.phy.cam.ac.uk/camphy/cloudchamber/cloudchamber1_1.htm
Ethanol Images:
http://jchemed.chem.wisc.edu
A
All other artwork and photos © 2004 Melanie and Jamie :)