Monday’s quiz

1. If five samples containingdifferent radioactivematerials have the half-lives listed below, whichone has the mostradioactive decays persecond:

(a) 1 second(b) 1 minute(c 1 year(d) 106 years(e) 109 years

2. If a sample originallycontains 240 atoms att=0, how many atomsare left after 3 half-lifes have gone by?

(a) 240(b) 120(c) 3(d) 60(e) 30

Solar neutrinosl An experiment set up by Ray Davis to

look for neutrinos from the Sun (in theHomestake Mine in South Dakota)found only ~1/2 of the numberexpected (running from 1970 throughthe present)

l Tank was filled with perchloroethylene(a good source of chlorine and auseful dry cleaning fluid); he waslooking for neutrinos interacting withchlorine atoms producing aradioactive isotope of argon (which hecould detect).

l He joked that if the experiment was afailure, he could always go into thedry cleaning business

l When his early results showed thisdeficit of neutrino interactions, one ofthe miners tried to cheer him up bysaying that it had been very cloudy forthe last few months and that mayhave been the problem

Nuclear fissionl Nuclear fission occurs when a

heavy nucleus such as 235Usplits or fissions into 2 smallernuclei

l In such a reaction the totalmass of the products is lessthan the original mass of theheavy nucleus

u where does the extra massgo?

u into kinetic energy of decayproducts, about 200 MeV (notjust of academic interest)

l Consider a slow neutroninteracting with a 235U nucleus(one possible reaction)

1 235 141 92 1 0n + 92U 56Ba + 36Kr + 30n

l 235U captures slow-movingneutron

l Capture results in formation of236U*; excess energy of thisnucleus causes it to undergoviolent oscillations

l 236U nucleus becomeselongated

l Nucleus splits into 2fragments, emitting severalneutrons in the process

Chain reactionAverage number of neutrons emitted is ~2.5; when chain reactionoccurs fast, it’s a bomb; when reaction occurs slowly and controlledit’s a nuclear reactor

First chain reaction

l In 1942, Enrico Fermiand collaborators built anuclear pile underneaththe football stadium andobtained the firstcontrolled nuclear chainreaction

l Their measurementsconfirmed that theconstruction of an atomicbomb was a possibility

Enrico Fermi Physicist 1901 - 1954

Nuclear Reactor• in a reactor have control

rods which can absorbneutrons to slow thereaction down

Operating principles

Nuclear Fusion

l Remember when welooked at the bindingenergy curve shown onthe right

l Heavier elements have alarger binding energythan lighter elements

l So if two light elementsfuse together to form aheavier element, energywill be released

l This is known as fusion

Nuclear Fusion in the Sun

l Fusion powers the sunl Fusion reactions take place in the

center of the sun where thetemperature and density are largeenough

l In hydrogen-rich stars like the sun(relatively young), the proton-proton cycle shown below isimportant

u 1 1 2 + 1H + 1H 1H + e + n

1

2

3 1H + 1H 2He + g

1 3 4 + 1H + 2He 2He + n

u resulting in the release of 25 MeVof energy

u carried by gamma rays,positrons, neutrinos

u neutrinos escape at speed of light

Fusion in reactors

l Lot of energy released infusion; fuel is cheap(water); few radioactivebyproducts

l Proton-proton fusionrequires very hightemperatures andpressures

u Why?

l Another possibility isdeuterium-deuteriumburning

2 2 3 1 1H + 1H 2He +

0n

releasing 3.27 MeV

Tokamak: device for confiningdeuterium plasma at high temperatures

Laser fusion is also a possibilitybut neither technique has yet reached the break-even point.

Back to particle physics

l From 1927-1934,theoretical work by PaulDirac on the existence ofantimatter was provenexperimentally.

l Charged particles suchas electrons and protonshave their oppositelycharged counterparts,the antielectron (akapositron)and theantiproton.

u the positron wasdiscovered by CarlAnderson using a cloudchamber of the type that Ishowed you

Forcesl In addition, the neutrino must

have its own antiparticle, theantineutrino.

l It is currently believed thatmost particles have their ownantiparticle pair (the photon isa major exception).

l By mid 1930s, physiciststhought they were close tofiguring out the fundamentalforces acting betweenparticles.

l In some ways, it was similar toa basic concept fromChemistry:

“two atoms can form a covalentchemical bond by theexchange of electrons.”

Electromagnetic forces

lWith the photon, apicture had arisen ofthe ElectroMagneticForce as

“charged particlesinteracting throughthe exchange ofphotons.”

Feynman diagram: more on this later

Strong forcesl Hideki Yukawa suggested a

similar model to explain thestrong nuclear force that washolding the protons andneutrons together inside thenucleus.

l So, in his model, a newparticle whose exchangebetween nucleons producesthe strong force.

l Theoretically, the new particlewould have a mass betweenthat of an electron and anucleon, (~200 me) thus it gotthe nameMeson, Greek for “middle.”

I should have called it the Yukawa

Back to cosmic raysl The Earth is constantly

bombarded by cosmic rays(mainly high energy protons)from beyond the earth.

l Looking at Cosmic Rays,physicists found in 1937 aparticle that was similar topredictions, but did not matchexactly. It did not interactstrongly with matter.

l Then World War II happenedand most work on particlephysics was put on hold.

l After WWII, physicists startedto realize that the mesondiscovered in 1937 could notbe the same as the onepredicted by Yukawa.

The 1937 meson receives the name

of mu meson or muon for short.Realizing this was an “extra” particle, made I.I. Rabi comment "who ordered that?" The word Lepton is introduced to describe particles that do not interact strongly, electrons & muon become part of this family.

The right order is delivered in 1946

l A meson that doesinteract strongly is foundin cosmic rays, and isnamed the pi meson orpion for short.

l The pion comes in 3charge states: p+,p-,po

and has a mass of about140 MeV/c2 (which isabout what theyexpected the mass tobe)

u the muon has a mass ofabout 108 MeV/c2

u hence the confusion

Feynman and his diagrams

l 1947: Feynman developsQuantumElectrodynamics (QED)and his Feynmandiagrams.

l Physicists now haveprocedures to calculateelectromagneticproperties of electrons,positrons, and photons.

l Pictorial way ofrepresenting interactionsbetween particles andsimplifying calculations

u virtual photon exchangesenergy and momentumbetween electrons

Strong Interactions

l For example, the stronginteraction between aproton and a neutron canbe represented by aFeynman diagraminvolving the exchangeof a virtual pion

l Creation of virtual pionrequires an energy ofDE=mpc2

l Where does this energycome from? From thevacuum, or in otherwords from nowhere.

u from the uncertaintyprinciple then, it can’t hangaround for very long.

lDEDt ~ huDt~h/DE=h/mpc2

ud=cDt~hc/mpc2

umeasured range of nuclear force is 1.5 X 10-15 musolve for mp: ~130 MeV

Example of Feynman diagrams from a talk I’m givingon Friday

One of the most interesting people of the 20th Century

• Most of the public knows himfrom the investigation of theChallenger disaster

• Also the father of nanotechnology

• Some quotes:

• “For those who want some proofthat physicists are human, theproof is in the idiocy of all thedifferent units which they use formeasuring energy.”

• “I love only nature, and I hatemathematicians.”

• “Physics is like sex: sure, it maygive some practical results, butthat's not why we do it.”