the nucleus of the atom · 2009. 1. 9. · the nucleus of the atom in 1919, rutherford starts to...

33
The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus- the proton. Two years later, James Chadwick and E.S. Bieler conclude that some strong force holds the nucleus together. Why? If protons in nucleus (and not electrons) then what keeps the repulsive force from driving the nucleus apart? Nucleus confined to a very small volume (<10 -14 m in diameter), 0.01% of diameter of atom density is 10 17 kg/m 3

Upload: others

Post on 19-Aug-2020

6 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

The nucleus of the atom In 1919, Rutherford starts to

collect the first data indicatingthat there is another structurewithin the nucleus- the proton.

Two years later, JamesChadwick and E.S. Bielerconclude that some strongforce holds the nucleustogether.◆ Why? If protons in nucleus

(and not electrons) thenwhat keeps the repulsiveforce from driving thenucleus apart?

◆ Nucleus confined to a verysmall volume (<10-14 m indiameter), 0.01% ofdiameter of atom

▲ density is 1017 kg/m3

Page 2: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

So here we are in 1930 There were three fundamental

particles- electron, proton, andphoton.

There were three fundamentalforces- gravity,electromagnetic, and thestrong nuclear force.

Gravity we know is muchweaker than the other forces◆ strong force is about 100X

stronger than EM force▲ naturally occuring

elements only have up to100 protons

▲ strong force betweenprotons saturates but EMrepulsive force doesn’t

▲ for more than 100protons repulsive forcewins

There were still some“details” to work out butmany felt like Max Born,who said, "Physics as weknow it will be over in sixmonths."

◆ Who is Max Bornsgranddaughter ?

Page 3: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Olivia Newton-John◆ as far as I know, John

Travolta is not relatedto any famousphysicist

Page 4: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

The nucleus

The nucleus hadceased to befundamental. It wascomposed ofpositively chargedprotons and neutrallycharged neutrons(not actuallydiscovered until 1931by Chadwick).

Page 5: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Characterizing atoms

Nuclei of atoms consistof protons and neutrons

Surrounding the nucleusare the electrons

Characterize nuclei byfollowing characteristics◆ Atomic number Z: # of

protons in nucleus (alsoequal to # electrons fornormal atom)

◆ Neutron number N: # ofneutrons in nucleus

◆ Mass number A: # ofnucleons (protons +neutrons) in nucleus

AZX : X is symbol for

element, A is atomicmass and Z is atomicnumber

Isotopes of an elementhave the same atomicnumber but differentatomic weight◆ same number of protons

and electrons▲ same chemical properties

◆ different number ofneutrons

▲ different nuclearproperties

Page 6: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Masses

note the factor of 2000difference

unified mass unit: 12C has a mass of 12 amu1 u = 1.660559 X 10-27 kg

ER = mc2

Page 7: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Radius of nucleus

Rutherford found anexpression for how closean alpha particle cancome to a nucleus

For a head-on collisionall of kinetic energy ischanged to potentialenergy◆ 1 2 (2e)(Ze)

2 mv = k d◆ d is the distance of closest

approach

Page 8: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Solve for d 4kZe2

d = mv2

…or a distance of about 3.2 X10-14 m for alpha particles ona gold nucleus

From these results, Rutherfordconcluded that the positivecharge in a nucleus isconcentrated in a spherewhose radius is no greaterthan ~10-14 m or about 10fermis (1 fermi = 1 X 10-15 m)

Nuclear radii go as:◆ r=roA1/3

◆ A is the # of nucleons andro=1.2X10-15 m

◆ all nuclei have roughly thesame density

Page 9: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Stable nuclei The nucleus can exist only

because the strong nuclearforce between nucleons in anucleus is stronger than theelectrostatic repulsive forcebetween the protons

Light elements have roughlythe same number of protonsand neutrons

Heavier elements have moreneutrons than protons(neutrons contribute to thestrong binding force inside thenucleus but not to therepulsive EM force)

Isotopes within the shadedregion are stable

Page 10: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Unstable nuclei The heavier elements in the

universe were all made insupernova explosions

Unstable isotopes areproduced (too many neutrons)which then decay to the stableisotopes

One of areas of concentrationof NSCL (and of RIA)

Page 11: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Binding Energy

The total mass of anucleus is less than thesum of the masses of theprotons and neutronsthat comprise it

This difference is calledthe binding energy of thenucleus and can bethought of as the energythat must be added to anucleus to break it apart

iron has the highest binding energy

Page 12: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Radioactivity In 1896, Henri Becquerel, while

investigating fluoresence inuranium salts, accidentallydiscovered radioactivity

Work by Curies and othersshowed that radioactivity was theresult of the decay ordisintegration of unstable nuclei

Up til that point, atoms werebelieved to be indestructible andforever

Cleared up a major question as towhy the interior of the Earth wasstill molten

Shared the 1903 Nobel prize withthe Curies

Work by Curies and othersshowed that radioactivity was theresult of the decay ordisintegration of unstable nuclei

Page 13: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Radiation

Alpha particles arehelium nuclei (2 p, 2n):

Beta particles arespeedy electrons:

Gamma radiation is astream of photons:

these are supposed tobe x’s

Page 14: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Half-life If a radioactive sample

contains N radioactive nucleiat some instant, the number ofnuclei that decay in a time Δtis proportional to N◆ ΔN/Δt α N◆ ΔN = -λNΔt◆ where λ is a decay

constant R = |ΔN/Δt| = λN

◆ rate of which atoms decay N=Noe-λt

T1/2 (half-life) is time it takesfor half of sample to decay

Decay constants vary greatlyfor different radioactivedecays and thus so do half-lives

Page 15: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Alpha decay

A A-4 4ZX Z-2

Y + 2He

X is called the parent nucleusand Y the daughter nucleus

Example is Radium, decayinginto Radon and an alphaparticle

In order for alpha emission tooccur, mass of the parentmust be greater thancombined mass of daughterand alpha particle◆ excess mass converted into

kinetic energy, most of whichis carried by the alpha particle

Page 16: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Beta decay

When a radioactivenucleus undergoes betadecay, the daughternucleus has the samenumber of nucleons asthe parent nucleus buthe atomic number ischanged by 1◆ A A - ZX Z-1

X + e

A neutron turns into aproton emitting anelectron◆

10n 11p + e-

One problem: expect the electronto carry away almost all of the kinetic energy but it doesn’t

It has a range of energies.

Page 17: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Problemo, big problemo Is Momentum Conserved? How can we account for the

discrepancies? Wolfgang Pauli suggested a

bizarre idea, another particle, onenot yet seen, was carrying themissing energy and could beused to solve the momentumissue.

For Conservation of Charge, thenew particle would have to beneutral.

According to other experiments,the particle would have to beincredibly light, perhaps even bemassless!

Page 18: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

A joke name but it stuck

Due to thosecharacteristics, theItalian physicistEnrico Fermisuggested calling it

Neutrino

Italian for “littleneutral one.”

-zero electric charge-very small mass (if any)-a spin of 1/2-very weak interaction with matter,making it difficult to detect

Page 19: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

New interaction force

Since the beta decayhappened to freeneutrons outside thenucleus, the StrongNuclear Force could notbe responsible.

Thus physicists were ledto consider anotherfundamental force- theWeak Nuclear Force.

At this point, there were◆ Five Fundamental

Particles: electron, proton,neutron, electron, photon,and neutrino.

◆ and Four FundamentalForces (or Interactions):Gravity, Electromagnetic,Strong Nuclear Force, andWeak Nuclear Force.

The Weak Force isweaker thanElectromagnetic orStrong, but is strongerthan Gravity.

Page 20: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Mass of neutrinos Not until 1956 was there

convincing experimental evidencethat neutrinos really existed. Anexperiment at a nuclear reactorconducted by Fred Reines wasable to measure interactionsresulting from neutrinos. Whywas this so difficult? Because theneutrinos only interacted via theweak force. The incredibly vastmajority of them passed throughthe detector without leaving anytrace.

Currently there’s an experiment inJapan called Kamland which isdesigned to detect neutrinos fromJapan’s working nuclear powerplants

Page 21: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Solar neutrinos The nuclear reactions in the Sun

produce neutrinos. Trillions of themare passing through your body everysecond. Why don’t you feel them?Because their probability ofinteracting is so small.

An experiment set up by Ray Davis tolook for neutrinos from the Sun (in theHomestake Mine in South Dakota)found only ~1/2 of the numberexpected (running from 1970 throughthe present)

Have nuclear reactions in the Sunstopped?

Within the last 2 years, it has beenunderstood that there are 3 types ofneutrinos and that the neutrinosproduced in the Sun metamorphizeinto another type (that can’t bedetected) before they reach the Earth.

Whew!

Page 22: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Radioactive Dating We can often use radioactivity to

measure the age of an object(artifact, fossil, etc)

Consider 14C dating◆ cosmic ray interactions in the

upper atmosphere cause nuclearinteractions that produce 14Cfrom 14N

◆ living organisms breathe incarbon dioxide that has both 12Cand the radioactive 14C

◆ so all living creatures have thesame ratio of 14C to 12C(~1.3X10-12)

◆ when the organism dies,however, it no longer absorbscarbon dioxide from the air, andso the ratio of 14C to 12Cdecreases

Can measure therate for the reactionbelow per unit mass,and from it calculatehow long somethinghas been dead◆ 14 14

12C 7N + e- + ν

◆ the greater the ratethe more recently ithas died

Page 23: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Carbon dating

Works on samples fromabout 1 to 25,000 yearsago◆ why not longer?◆ half-life of 14C is 5730

years, so longer than 5half-lives too small afraction of 14C is left (25=32so <1/32nd left)

Examples:◆ Dead Sea Scrolls date to

1950 years ago◆ Iceman dates to 5300

years ago◆ Shroud of Turin to 700

years ago

Page 24: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Radioactivity

Radioactivity can beclassified into 2groups◆ unstable nuclei found

in nature (naturalradioactivity)

◆ nuclei produced in thelab through nuclearreactions (artificialradioactivity)

Four radioactive series shown below. Uranium, Actinium and Thorium are all found in nature.Neptunium is trans-uranic.

Page 25: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Decay chain for Thorium

Start with 232Th◆ half-life for first decay

is ~14 billions years

End with 208Pb

Page 26: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Radiation damage Radiation damage in biological

organisms is primarily due toionization effects in cells◆ normal function of a cell can

be disrupted when highlyreactive ions or radicals areformed as a result of ionizingradiation

◆ damage to a great number ofcells can lead to death

◆ cells which do survive theradiation may becomedefective and cancerous

Can divide radiation damage into2 types◆ somatic (to non-reproductive

cells)◆ genetic (to reproductive cells)

Units of radiation damage◆ rad: that amount of radiation

that deposits .01 J into 1 kg ofabsorbing material

◆ rem is defined as the productof dose in rads and RBE(relative biologicaleffectiveness) factor

Page 27: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Radiation exposures

Low level radiation fromcosmic rays, radioactiverocks, soil, air deliver adose of about 0.13rem/year

Upper limitrecommended bygovernment is 0.5rem/year

Acute whole-body doseof 500 rem results in amortality rate of ~50%

Natural sources of exposure◆ cosmic rays: 14%◆ radon: 58%◆ food: 12%◆ ground: 16%

Equivalent exposure frommedical X-rays◆ chest/teeth/arms and legs: a

few days▲ also equivalent to the

radiation exposure from a 4-hour plane flight

◆ breast, spine, abdomen: afew months

◆ barium enema: a few years

Page 28: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

What about exposure from microwave ovens?

E=hc/λ

Page 29: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Nuclear fission Nuclear fission occurs when a

heavy nucleus such as 235U splitsor fissions into 2 smaller nuclei

In such a reaction the total massof the products is less than theoriginal mass of the heavynucleus◆ where does the extra mass go?◆ into kinetic energy of decay

products, about 200 MeV (notjust of academic interest)

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

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

235U captures slow-movingneutron

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

236U nucleus becomeselongated

Nucleus splits into 2fragments, emitting severalneutrons in the process

Page 30: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

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

Page 31: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

First chain reaction

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

Their measurementsconfirmed that theconstruction of an atomicbomb was a possibility

Enrico Fermi Physicist 1901 - 1954

Page 32: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Nuclear Reactor• in a reactor have control

rods which can absorbneutrons to slow thereaction down

Page 33: The nucleus of the atom · 2009. 1. 9. · The nucleus of the atom In 1919, Rutherford starts to collect the first data indicating that there is another structure within the nucleus-

Operating principles