INTERPRETATION OF BUBBLE CHAMBER IMAGES

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INTERPRETATION OF BUBBLE CHAMBER IMAGES
Bubble Chamber images look very beautiful, but at
the same time very complicated to try to
interpret!
The picture on the left shows the tracks left by
charged particles and antiparticles

• Fermilab bubble chamber 4.6 m in diameter in a 3
  T magnetic field

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HOW ARE TRACKS FORMED?
A moving charged particle interacts with the
liquid along its path via the Coulomb force it
exerts on atomic electrons. In this way it
transfers enough energy to start the liquid
boiling and thus leaves behind it a trail of
small bubbles.
However
A photon has no charge and therefore does not
interact via the Coulomb force. Therefore it
cannot leave a trail of bubbles
Q. What other particles would not leave a
track in a bubble chamber ?
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With so many complicated tracks to look at, how
can the paths of individual particles be
interpreted?
We can do one of the following -
Relate the movement of electrons produced by
Compton Scattering with the direction of the
magnetic field
1
Relate the movement of the deflected particles
with their charge and the direction of the
magnetic field
2
Relate the production of particle / antiparticle
pairs (matter antimatter) to the energy of the
photon which produced them
3
Apply conservation of momentum to a head-on
collision between an electron and a positron
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Relate the movement of electrons produced by
Compton Scattering with the direction of the
magnetic field applied to the bubble chamber
1
Notice the trajectory of the spiralling lone
electron indicated by the arrow - this electron
was knocked out of the atom that originally held
it by a high energy photon.
Questions
1. Identify the other examples of this
interaction in the picture above 2. What is
the direction of the magnetic field? 3. Why
doesnt the photon leave a track ?
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Answers
1. Other Compton electrons are indicated by
the arrows (above) 2. From the rule for the
force exerted by a magnetic field on a current -
the field is going into the page 3. The
photons path is not visible because it does not
carry any charge and so does not cause the
formation of bubbles in the chamber
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Relate the movement of deflected particles with
their charge and the direction of the magnetic
field
2
Remember that the direction of the force on a
moving charged particle is found using the rule
for the force exerted by a magnetic field on a
current . So the electrons all turn clockwise in
the above diagram
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B
A
An electron ( e - ) and a positron (e ) leave
the tracks shown in the picture above
Question Which is which?
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e-
e
Answer
The particle which deflects to the right (track
B) is an electron, e- The particle which
deflects to the left (track A) is a positron, e
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Relate the production of particle / antiparticle
pairs (matter antimatter) to the energy of the
photon which produced them
3
Two photons are produced at C where a positron
annihilates with an electron. Just one travels
to D where it interacts with a nucleus in the
liquid and materializes into an electron/positron
(e- e) pair.
To a good approximation, all of this photons
energy is shared by the e-/e pair.
Question
Calculate the kinetic energy of the e-/e pair,
knowing that the momentum of the photon is 265 ?
31 MeV/c . ( me 0.511 MeV/c2 )
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Answer
E? pc If the photon momentum is 265 MeV/c then
E? 265 MeV Converting to Joules, 1MeV 1.6 x
10-13 J E? 265 x 1.6 x 10-13 4.24 x 10-11 J
K(e-,e)
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Apply the Conservation of Momentum to a head-on
collision between an electron and a positron
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E
e
e-
For a snooker example of a head on collision,
click below
At point E a rare event took place. The positive
track (e) seems to change into a negative track
(e- ). What happened was that the positron made
a head-on collision with an electron.
Question
What is the linear momentum of the electron if
the incoming positrons linear momentum was 54 ?
15 MeV/c? . ( me 0,511 Mev/c2 )
(melecton mpositron)
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E
e
e-
Answer
Visually, just before and just after E, the
tracks seem to be more or less equally curved
thus having the same momentum. Since the track of
the positron appears to have stopped at the point
of collision, all of its momentum must have been
transferred to the electron. Therefore the linear
momentum of the electron must be about 54 ? 15
MeV/c since the mass of the electron is the same
as that of the positron