01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09|
Summary and conclusions
The beauty of bubble chamber pictures is that they provide detailed pictures
of the motion of any charged particles that have appeared in a bubble chamber
as a result of an interaction between an incoming beam particle and a nucleus
of an atom in the liquid. The tracks we see are trails of bubbles caused
The momenta of these charged particles are obtained by measuring
the curvatures of the tracks. If the particles stop in the bubble chamber,
(distance travelled) give their kinetic energies.
- In most bubble chamber pictures there will be some
small spirals caused by knock-on electrons. These can be used to tell the
direction of the magnetic
field. (Remember: the current is in the opposite direction to the direction
of motion of the electron)
- Dark tracks: these are usually slow protons that
have received only a small amount of momentum during the collision.
In the tutorial we have concentrated mainly on the features that tell us
about the decays of strange particles (link to ptleprop1). Since these are
weak-interaction decays (like beta-decay), the particles have a long enough
lifetime to travel a measurable distance before they decay. Visual analysis of
bubble chamber pictures is mainly one of looking for.
in hydrogen, a kink, for example, is a signature for the decay of a charged particle
into another charged particle of the same sign but with a
lower momentum. (Some momentum will be carried off by one or more neutral
The chances of noticing a low angle kink are improved if you
print off the
event and examine the picture with a low angle of viewing.
In liquids other
than hydrogen, we cannot be so sure that kinks signal decays because a charged
particle bouncing off a neutron might look like a kink.
- Vees: a vee, for example, signifies the
decay of a neutral particle. An important part of the pattern recognition
is to check
where the vee is coming from; there
are three possibilities:
- From the main collision
- From a kink (for charged
- Somewhere downstream of a kink (neutral
Another source of useful information:
Electron-positron pairs: these tell us that a high energy photon has
the electric field of a nucleus. Most pairs
come from the decay of a to
This is an electromagnetic decay, with a very short lifetime,
and so the pairs
appear to come directly from collision points.
one finds two pairs.
Measurement can show if they could be from a decay or are just a random pair of photons.
It is very satisfying to be able
to recognize particles as an exercise in
pattern recognition; however, it is not always possible – for example,
particles might leave the bubble chamber before decaying.
(It must also be remembered
that, in a real experiment, tracks are then measured – on
more than one view if we are to be able to reconstruct the event in 3-dimensions
- with the aim of determining the energies and momenta of as many particles
We end with three exercise events:
- Exercise 1: click here
Exercise 2: click here
- Exercise 3: click here
< step back