Subatomic Particles and Quantum Theory

Atomic Physics

Subatomic Particles A cloud chamber is a confined space super

saturated with water or alcohol vapour

high energy particles moving though them condense the droplets and produce a vapour trail

Only charged particles and ionizing radiation or photons create tracks in the cloud chamber

http://www.youtube.com/watch?v=mLDASjzbxlA&feature=related

http://www.youtube.com/watch?v=kuzWNOUqLmQ&feature=related

Charged Particles

One of the earliest ways of detecting the path of charged particles.

Could be placed in magnetic fields to study path.

Principle tool of particle physics for 50 years.

Uses

Contains a liquified gas (ie. H2, He, Xe)

Lowering the pressure reduces the boiling pt to just below actual temperature.

Charged particles passing through cause liquid to boil, leaving a trail.

Bubble Chamber

http://www.youtube.com/watch?v=qcUwLH8L5AU

Interpreting

Cloud and bubble chambers do not detect neutral particles (neutral particles do not form ions).

In a bubble chamber, these particles create a series of bubbles in the trail

Particle tracks are used to determine the radius of the path of a charged particle or q/m ratio

-q

+q

. . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Right hand rule

B directed out of the plane of the page

The spiral results from the charged particle slowing in

the bubble chamber.

Spiral Tracks11

11

1.8 10 /

1.76 10 /e

e

q C kgmq

C kgm

2

c mF F

mv qv BrmvrqB

r decreases as v decreases

Eg) A charged particle traveling at 2.50 x 107

m/s enters a magnetic field of 2.44 x 10-3T and travels in a circular path with a radius of 5.82cm. Determine the q/m ratio for the particle.

2

7

3

11

2.50 10 /(2.44 10 / )(0.0582 )

1.76 10 /

c mF F

mv qvBrq vm Brq m sm Ns Cm mq C kgm

If initial v of this particle is 0.10c, initial r of spiral is 5.7 mm and B = 30 mT (into page), find q/m for the particle.

Determine the particle’s charge and its identity.

Analyzing

B: into page v: to top of page F: toward centre

Left or right hand?

Left. Charge is negative.

Charge

q/m

2

8

11

0.10 3.00 10 /0.030 0.0057

1.8 10 /

c mF F

mv qv Brq vm Br

m sT mC kg

Particle appears to be an e¯ (evidence consistent with of charge and q/m for an e¯)

Identify11

11

1.8 10 /

1.76 10 /e

e

q C kgmq

C kgm

Collisions can be seen when two tracks emerge from one.

Collisions with a neutral particle (that leave no track) appear to emerge from nowhere.

This is something different.

Collisions

• early 1900’s, scientists thought there were 3 fundamental particles: electrons, protons, and neutrons

• fundamental particle – particle that cannot be split or divided into smaller pieces

• Carl Anderson, 1923, discovered antimatter where a particle track in a cloud chamber had the mass of an electron but the opposite charge

• the collision of a particle (e-) and antiparticle (e+) can annihilate both particles and create a pair of high energy gamma rays traveling in opposite directions

Neutral particles – neutron n antineutron - place a bar over the symbol for

matter to represent antimatter

Interaction of Electron and Positron

2e e

e e

n

Electron - positron

fluorine-18 emits positrons as it decays, these collide with electrons in body cells and the gamma rays are detected showing the concentration of the radioactive tracer element injected into the body

Eg) PET (positron emission topography)

annihilation produces gamma rays

Quantum theory describes mediating particles that exist to indicate that fundamental forces act on particles over some distance

Particles that mediate forces for a brief time are virtual particles, energy, mass, momentum, etc. are not the same as real particles

4 Fundamental Forces

Range Relative Strength for Protons in a Nucleus

Mediating Particle

Electromag-netic

Infinite 10-2 Photon

Weak Nuclear

< 0.003 fm (x10-15)

10-6 W+,W-, Z0

Strong Nuclear

<1 fm 1 Gluons ?

Gravitational Infinite 10-39 Gravitron ?

Physicists believe (hope?) they will eventually show that these are all different manifestations of single fundamental force (aka Grand Unified Theory or GUT).

strong nuclear force is very large over a short distance (x10-8m) which is the size of a nucleus

- to separate the parts of the nucleus, scientists need very large energies

cosmic rays – energies of 102 – 1014 MeV - are EM photons protons,

- antiprotons, electrons positrons, alpha particles

Muons – unstable particles having similar properties to electrons but 207 x the mass of the electron

Pion – unstable particle – 270 x mass of the electron

Subatomic Particles ( ~ 300)

The Large Hadron Collider at CERN can accelerate protons to an energy of about 7 TeV (7 x 1012 eV) each, or lead nuclei to

about 574 TeV per nucleus.

Beams of protons, moving in opposite directions, will be collided (each beam moving very close to c).

http://www.youtube.com/watch?v=j50ZssEojtM CERN on Wikipedia

Leptons – subatomic particles that do not interact by means of a strong nuclear force eg) electrons, neutrinos, muons

Hadrons – subatomic particles that do interact by means of a strong nuclear force eg) protons and neutrons

Two Separate Families

1. Mesons – hadron with an integer spin eg(0, 1, or 2) eg) pion

2. Baryons- hadron with a half integer spin eg (1/2, 3/2) eg) proton or neutron

Text: p. 842 – 843 tables

*Fermion – a particle with a half integer spin – all leptons and baryons

Boson – a particle with an integer spin

Hadrons are divided into 2 groups based upon spin

, , ,e e v v

Eg) electron

Masses of subatomic particles are expressed in MeV/c2

2

2 2

2 2 2 2 2 2 2

362

// / /

1.7827 10

E mc

E J Nm kgm sm kgc m s m s m seVm kgc

31 4 29.11 10 5.4858 10 0.5110 /m kg u MeV c

Proton27 21.6726 10 1.0073 938.23 /m kg u MeV c