The Hidden Beauty of Bubble Chambers

Activity 1:What do you see?What questions would you like answered about this picture?

Activity 2:If the target is a proton, what is the charge of the incoming beam particle?

All the tracks are curving in the same direction in the magnetic field. Since one of the outgoing tracks is made by the struck proton, the beam particle must also be positive.

Knock on electrons

Electrons spiral because they are much lighter than all other charged particles and lose energy quickly by another process called bremsstrahlung.

Slow moving protons

Low energy protons leave short, thick tracks

How would you use this picture to teach students to apply charge conservation?

Activity 3:Apply your knowledge• Analyze the next photographs to determine

the charge of each particle that made a track in the interaction.

Summary points for teaching charge conservation

Activity 4:

For the interaction, which track is least curved?

A

C

B

Activity 4:

What property did the particle that made track A have that caused this track to be the least curved?

A

C

B

Relating Radius to Momentum

It is harder to curve a high momentum projectile than a low momentum one.

p α r

Full derivation

What would we have to measure to determine if momentum was conserved in this collision?

A

C

B

Measuring Radius Using a Transparency Overlay

Look from the end to match

the appropriate radius

Establish an axis and measure the relevant angles

Calculations for checking if momentum is conserved

The sum of the momenta in the x-axis should be the same, within uncertainty, before and after the collision. To determine x-components after the collision:

Since there is no initial momentum in the y-axis, 0 =

For collisions that appear to not conserve momentum

Momentum may appear not to be conserved. For example, tracks that appear to be straight may actually be curving into or out of the plane of the photograph.

Another possibility is that a neutral particle, that does not leave tracks, was produced and escaped undetected.

Exercises in Pattern Recognition

A) Electrons spiralling due to bremsstrahlungB) The Compton effectC) Pair production

D→E) Positron-electron collision

P →Q) Annihilation

More information

Pair production

bremsstrahlung

Questions:1)Find a Compton electron

2) Find an example of pair production

Compton electron

3) Find examples of bremsstrahlung

Link to Bubble Chamber Website

ATLAS and Bubble Chamber

ATLAS data compared with bubble chamber picture

Bubble Chamber Art

Background Information

CERN High School Teachers’ Bubble Chamber WebsiteSuggested reading:1. Introduction to the BC site2. What is particle physics?3. How does a bubble chamber work?4. How does one “read” bubble chamber

pictures? A step-by-step tutorial (Advice on key sections to read.)

Key sections to read from How does one “read” bubble chamber pictures?

For activities involving only charge and momentum conservation, you need only read: steps 1-6; and from step 7 you just need to know that electrons spiral due to a process called bremsstrahlung, and that slow protons often leave short, dark tracks; steps 8 and 9 can be skipped; as can step 10 except for 3 exercises (with solutions) at the end: These are stand alone exercises appropriate for a workshop.

Summary points for teaching charge conservation

1. The beams are made of particles of a specific charge.

2. We assume the liquid in the bubble chamber is hydrogen; so the collision is always with a proton.

3. The direction that charges move can be determined by using knock on electrons and slow protons.

4. A magnetic field will curve the trajectory of charged particles.

5. Charge is conserved in bubble chamber interactions.

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Relating Radius to Momentum

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