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keep it simple science®

1

HSC Physics Option Topic

From QUANTA to QUARKSWhat is this topic about?To keep it as simple as possible, (K.I.S.S.) this topic involves the study of:1. RUTHERFORD & BOHR MODELS OF THE ATOM

2. DE BROGLIE & MATTER WAVES3. INTO THE NUCLEUS

4. APPLICATIONS OF NUCLEAR PHYSICS...all in the context of the history, nature and practice of Physics.

1. RUTHERFORD & BOHR MODELS OF THE ATOMWhat Has Gone Before...The entire Science of Chemistry and much ofPhysics is built on the foundation of AtomicTheory... the concept that all matter iscomposed of atoms.

Initially conceived as tiny, unbreakable particlesof matter, by the beginning of the 20th century itbecame apparent that the atom was composedof smaller parts.

In 1900, Max Plank had proposed the QuantumTheory to explain the details of the “Black BodyRadiation Curves”.

In 1905, Einstein then explained the strangephenomenon of the Photoelectric Effect byusing Plank’s quantum idea. He proposed thatlight is not just a wave, nor a stream of particles,but made up of “wave packets”.

Einstein also proposed his “Theory ofRelativity” in 1905. Classical Physics was beingturned upside-down by this sequence of new,fundamental discoveries.

The Rutherford Model of the AtomIn 1911, Ernest Rutherford carried out anexperiment which indicated that the positivelycharged part of an atom must be concentratedinto a tiny “nucleus”, with the electrons orbitingaround it.

Rutherford’s modelproposed that:• At the centre is a tiny, dense nucleus with a

positive electrical charge.• The negatively charged electrons orbit around

the nucleus.• The distance from nucleus to the electron

orbits is very large compared to the size of theparticles, so the atom is mostly empty space.

Since negative charge was carried by particles(the electrons) Rutherford thought it likely thatthe nucleus was made of positive particles.These were soon called “protons” and theirexistence was confirmed a few years later.

The electrons were too light to account for muchof the mass of an atom, so he thought theprotons must be relatively heavy.

Even at this early stage there was speculationthat there might be another massive particle inthe nucleus as well, but its discovery had to wait20 years.

In his famous experiment withcathode rays, J.J.Thomson had

discovered the (negatively charged)electrons in all atoms.

This meant that there also had tobe a positive part of each atom.

LLiigghhtt iiss NNOOTT aa ssttrreeaamm ooff ppaarrttiicclleess......

LLiigghhtt iiss NNOOTT aa wwaavvee......

LLiigghhtt iiss aa ssttrreeaamm ooff ““wwaavvee ppaacckkeettss””...... ““PPHHOOTTOONNSS””

EEaacchh pphhoottoonn iiss bbootthh aa ppaarrttiiccllee AANNDD aa wwaavvee!!

Rutherford’sATOM

Electrons in orbitaround central

nucleus

Atom mostlyempty space

Nucleus of positivelycharged matter,

possibly made up ofof particles

+++

+

--

-

-




2

Problems with Rutherford’s AtomEven as he proposed his atomic model, Rutherfordknew there was a problem with it.

The existing theory of Electromagnetic Radiation(EMR) contained the concept that if an electricallycharged particle was accelerating, then it must emitEMR, in the form of light waves.

Since Rutherford’s electrons were imagined to be incircular orbits around the nucleus, and since circularmotion involves constant (centripital) acceleration,then it follows that each electron should beconstantly emitting light. Trouble is... they obviouslydon’t!Existing accepted theory required that an orbiting electron should emit light energy continuously.

Obviously they don’t, or all matter would constantly glow with light.

However, atoms DO emit light if stimulated with energy, such as in a high-vvoltage discharge tube.

Emission SpectraYou should be familiar with the idea of a“spectrum” of light. For example, if “white” lightis passed through a prism, the differentwavelengths are separated, and the familiarrainbow colours appear.

(use yourimagination...

we can’t print colours)

If the light emitted by atoms of aparticular element is put through a prism, thespectrum shows very narrow bright lines on adark background because only certainwavelengths are given out. The pattern of linesis characteristic for each element.

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Practical WorkEmission Spectrum of Hydrogen

You will have observed the emission spectrumfor hydrogen by using a spectrometer to view thelight from a discharge tube filled with low-pressure hydrogen gas.

You will have seen that the light from a hydrogendischarge tube is composed of 4 visible brightlines of light.

Each line is one single wavelength of light.

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“Telescope” can berotated to view thedifferent “lines” of theemission spectrum

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The Balmer Series & Rhydberg Equation

The lines in the emission spectrum of hydrogenhad been discovered some 20 years beforeRutherford’s work, and were known as the“Balmer Series”.

Each line was given a name (Hαα, Hββ, Hχχ & Hδδ) andthe precise wavelength of each had beenmeasured. Other similar series of lines wereknown to exist in the invisible infra-red and ultraviolet parts of the EMR spectrum.

No-one could explain them, but mathematiciansBalmer and (later) Rhydberg had worked out thatthe exact wavelengths of the hydrogen spectrumlines could be calculated from an empiricalequation:

The fact that the Rhydberg equation worked was strongevidence that there was an underlying “law” controllingthe hydrogen spectral lines. The fact that a series ofinteger numbers were involved was a clue thatconnected the whole thing to Plank’s Quantum Theory...

The Rhydberg Equation 1 = RH( 1/nf

2 - 1/ni2 )

λλ

λλ = wavelength of the spectral line (in metres)RH = the “Rhydberg constant” = 1.097 x 107

nf = an integer number. For the Balmer series nf = 2ni = an integer number. To calculate the wavelengths

of the 4 lines of the Balmer series, ni takes the values 3, 4, 5 or 6.

Each line islight of oneexact wave-

length.

Light is onlyemitted at

certainprecise

wavelengths

Eachelement

has its ownunique setof spectral

lines

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3

Plank’s Quantum TheoryA quick revision of what you learned previously...

In 1900, Max Plank proposed a radical new theoryto explain the black body radiation. He found thatthe only way to explain the exact details comingfrom the experiments, was that the energy wasquantised: emitted or absorbed in “little packets”called “quanta” (singular “quantum”).

The existing theories of “classical” Physicsassumed that the amount of energy carried(say) by a light wave could have any value, on acontinuous scale. Plank’s theory was that theenergy could only take certain values, based on“units” or quanta of energy.

Plank proposed that the amount of energy carriedby a “quantum” of light is related to the frequencyof the light, and can be calculated as follows:

Neils Bohr Puts It All TogetherBohr used Plank’s Quantum Theory to modify theRutherford model of the atom in such a way that:

• the problem of radiation that should be emittedconstantly from accelerating electrons wasovercome.

• the underlying reasons for emission spectra wereexplained.

• the empirical nature of the Rhydberg Equation was given theoretical backing and mathematical validity.

• the reasons for the “valency” of different atoms, and how and why they combine in fixed ratios became clearer.

Not bad for an afternoon’s work!

(The last point above is fundamental to Chemistry andunderstanding chemical bonding and formulas. It willnot be pursued any further in this topic)

Bohr’s Postulates• Electrons revolve only in certain “allowed” orbits.Bohr theorised that there are a series of orbits, atfixed distances from the nucleus, in which an electronwill not constantly emit radiation as demanded byclassical theory.(Why was explained later by de Broglie)

• Electrons gain or lose energy to “jump” betweenorbits. To jump up to a higher orbit, an electron mustgain a certain quantity of energy. If it drops back tolower orbit, it must emit that exact same amount ofenergy.

These quantities of energy are “quantised”, so eachorbit is really a “quantum energy level” within theatom.

The amount of energy absorbed or emitted during a“jump” is defined by Plank’s Equation E = hf, and thecorresponding wavelengths of light are defined bythe Rhydberg Equation. The integer numbers nf and niturn out to be the “quantum numbers” of the orbits,counting outwards from the nucleus.

• Electrons in “allowed orbits” have quantised amounts of angular momentum too.

Bohr figured out that the amount of angular momentumpossessed by an electron must always be a multiple ofh/2ππ. The significance of this will be dealt with in a latersection.




E = h.fE = energy of a quantum, in joules ( J)h = “Plank’s constant”, value 6.63x10-34

f = frequency of the wave, in hertz (Hz)

You are reminded also, of the wave equation:

V = λλ.f (or, for light) c = λλ.f

c = velocity of light (in vacuum) = 3.00x108ms-1.λλ = wavelength, in metres (m).f = frequency, in hertz (Hz)

Example Calculationa) Use the Rhydberg Equation to find thewavelength of the Hδδ line of the hydrogenspectrum, given that nf= 2 and ni = 6.

1 = RH( 1/nf2 - 1/ni

2 )λλ

= 1.097x107( 1/22 - 1/62 )1/λλ = 2.438 x 106

∴∴ λλ = 4.10x10-7 m (410 nm nanometres)

b) Use the “Wave Equation” to find thefrequency.

c = λλ.f3.00x108 = 4.10 x10-7x f

∴∴ f = 3.00x108/4.10x10-7

= 7.32x1014Hz.

c) Use Plank’s Equation to calculate theenergy carried by one photon of light in the Hδδspectral line.

E = h.f= 6.63x10-34 x 7.32x1014

= 4.85x10-19 J.

“Allowed” orbitpositions.

Electrons cannot orbitanywhere else.

Electrons can “jump”from one orbit toanother, but must

absorb energy to jumphigher, or emit energy

to drop lower.

Quantum numbers ofthe orbits.

1

23




4

Bohr & the Balmer SeriesLet’s see how Bohr’s ideas work with regard to theBalmer Series of hydrogen emission lines.

Bohr suggested that the Hαα emission line was due toan electron dropping from the 3rd orbit down to the2nd orbit. It must lose a precise quantum of energy,so it emits a photon of light at a precise frequencygiven by E = hf.

In the Rhydberg Equation, ni = 3 and nf = 2. Thecalculated wavelength (λλ) agrees perfectly with theobserved spectral line. Plank’s Quantum Equationcalculates the energy of that photon of light.

Bohr argued that this amount of energy mustrepresent the difference in energy level from orbit 2to orbit 3.

The other lines of the Balmer Series representelectrons dropping from higher orbits into the 2ndorbit:

It all worked! Bohr’s idea gave a theoreticalexplanation for the Rhydberg Equation, which hadbeen empirically derived to explain the observedspectral lines.

Limitations of the Rutherford-Bohr Model

Despite the way that Bohr’s Postulates seem to solvethe problem with Rutherford’s brilliant new concept ofthe atom, there were still unexplained difficulties.

Bohr Model worked only for HydrogenHydrogen is the simplest atom, with only oneelectron and one proton.

Attempts to apply the model to larger atomsfailed, because multiple, orbiting electronsinteract with each other as well as the nucleus,and the situation becomes too complex todescribe in a simple mathematical way.

Different Intensities of Spectral LinesThe different spectral lines showed differentintensities or brightness. This means that someorbital “jumps” by electrons always occur moreoften than others. Bohr’s model had noexplanation as to why.

“Hyperfine” Spectral LinesWhen the spectral lines were examined moreclosely, each one was found to be made up of anumber of very fine lines close together.

The Zeeman EffectWhen a discharge tube is operated within a magneticfield, each spectral line is split up into severalseparate lines.

This, and the presence of the “hyperfine lines”,suggested that the energy levels or orbits weredivided into a number of “sub-orbits” of slightlydifferent energy. Bohr’s model had no explanation forthis.

Like all scientific models, the Rutherford-Bohr atomis a human attempt to explain the observed facts ofnature. In its day, this model was the best explanationavailable, but it was recognised that certain factsremained unexplained.

This doesn’t make the model wrong... simplyincomplete. It was a “work-in-progress”, to be addedto and refined by later scientists. This is the wayScience works.

If further evidence had proven it totally wrong (as canhappen) you would not be studying it!

1234

56

Hδδ line. nii = 6Hχχ line. nii = 5

Hββ line. nii = 4

Hαα line. nii = 3

lliigghhtt pphhoottoonn eemmiitttteedd

+Quantum energylevels or “allowedorbits” around thehydrogen atom

Nucleus

nff = 2 in each case

The Hydrogen Spectrum &Development of Bohr’s Model

Without a knowledge of the emission spectrum ofhydrogen, it seems very unlikely that Bohr could havecome up with his idea.

The fact that the spectrum shows distinct lines, andthat integer numbers are involved in the RhydbergEquation, all pointed to some kind of discrete,quantised atomic arrangement, rather than the more-or-less random orbits of Rutherford. Withoutknowledge of the hydrogen spectrum, (and Plank’sQuantum Theory) Bohr could not have made the(literally) quantum leap to his idea.

Like all great scientists, Bohr built on the knowledgediscovered by others. His genius was to put it alltogether in a new synthesis, that helped establishRutherford’s new structure of the atom.

However, there were still some problems...

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Spectral lines are ofdifferent brightness

Spectral lineis made up ofa number ofseparate,finer lines

Hαα line. nii = 3 )Hββ line. nii = 4 )Hχχ line. nii = 5 )Hδδ line. nii = 6 )


1. Sketch a labelled diagram to show the mainfeatures of Rutherford’s atomic model.

2. Outline the major problem with Rutherford’satomic model, based on the accepted theory ofthat time.

3. a) What is the “Balmer Series”?

b) Calculate the wavelength of the Hββ spectralline for hydrogen, given that ni = 4 and nf = 2.

c) Use the wave equation, and Plank’s equationto find the amount of energy carried by onephoton of the Hββ line.

d) According to Bohr, what does this amount ofenergy represent within a hydrogen atom?

4. Analyse the significance of the hydrogenspectrum in the development of Bohr’s atomicmodel.

5. The Hχχ spectral line for hydrogen is due to anelectron dropping from the 5th to the 2nd orbit.Compared to the Hββ line (in Q3):a) would a photon of the Hχχ line carry more, less,or the same amount of energy? Explain.

b) would the Hχχ line have a higher, lower, or thesame frequency? Explain.

c) would the Hχχ line have a longer, shorter, orthe same wavelength? Explain.

6. a) List, in brief form, 3 of “Bohr’s Postulates”.

b) List, in brief form, 4 limitations of the Bohrmodel.

7. It is known that other spectral lines for hydrogenare present in the infra-red and ultra-violet partsof the spectrum. One line, for example, is due toelectrons dropping from the 8th to the 1st orbit.

Calculate the wavelength of this spectral lineand state if it is infra-red or ultra violet.

5



Summary Worksheet for Section 1 is at the end of the next section

Worksheet 1 Test Questions section 1 Student Name...............................

de Broglie’s Quantum ProposalRemember that in 1905 Einstein had explainedthe Photoelectric Effect by suggesting that lighthas both wave and particle properties. (For thishe was awarded the Nobel Prize)

Einstein had used Plank’s Quantum Theory toexplain a phenomenon that “classical” Physicswas unable to explain.

In 1924, a young graduate student Louis deBroglie turned this concept around...

If light waves can have particle-likeproperties, why can’t particles have

wave-like properties?

Using Quantum Theory and Bohr’s atomicmodel, de Broglie developed a mathematicalmodel for an electron in orbit around thenucleus acting as a particle with waveproperties.

De Broglie began from Bohr’s equations whichshowed that (as a particle) the angularmomentum of the electron would be a multipleof h/2ππ.

From this he was able to show that (whenshowing its wave properties) the electron wouldhave a wavelength related to its mass andvelocity:

Impact of de Broglie’s HypothesisDe Broglie’s proposals had almost no impact onthe scientific community at first. Hismathematics were checked and found to betotally correct. His hypothesis was totallyconsistent with the Quantum Theory, and withthe Bohr model.

The physicists of the day, including Plank,Einstein, Rutherford and Bohr were all veryinterested by his work, but it was just a neatmathematical exercise, without any evidencebased in experiment or observation.

Usually, scientists observe a phenomenon andthen try to explain it by theory. de Broglie wasputting theory first, without any facts to explain!

Eventually, (as happens in Science) anexperiment was done to test the hypothesis.Before learning about that, you need tounderstand an important wave phenomenon...

DiffractionWaves can undergo various “wave phenomena”such as reflection, refraction and interference.In fact, it is these things which can identifywaves. For example, it was interference whichallowed Hertz to prove the existence of invisibleradio waves back in the 1880’s.

Diffraction is something that only waves do.

You can see diffraction occur if you watch waterwaves enter a harbour or similar.

At this point you might think “so what?”The “so what” is what happens AFTERdiffraction occurs...

6

2. DE BROGLIE & MATTER WAVESkeep it simple science

®



LLiigghhtt iiss aa ssttrreeaamm ooff ““wwaavvee ppaacckkeettss””...... ““PPHHOOTTOONNSS””

EEaacchh pphhoottoonn iiss bbootthh aa ppaarrttiiccllee AANNDD aa wwaavvee!!

λλ = h mv

λλ = wavelength (metres) of the electron.h = Plank’s constant (= 6.63x10-34)m = mass of the electron (= 9.11x10-31 kg)v = velocity of the electron, in ms-1.

Example CalculationFind the wavelength of an electron which istravelling at a velocity of 4.35x105 ms-1.

Solutionλλ = h

mv= 6.63x10-34/(9.11x10-31 x 4.35x105)= 1.67x10-9 m (1.67 nanometres)

Barrier wwiitthh ggaappss iinn iitt

The part of thewave which getsthrough a gapwill act like a

point source ofwaves. A semi-circular wavepattern forms

from each gap.

This is Diffraction

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7

Diffraction Forms Interference Patterns

Once a set of waves have been diffracted, the 2(or more) sets of spreading waves now meeteach other and wave interference occurs:

If light waves are diffracted, then projected ontoa screen, or captured on photographic film, aninterference pattern appears... perhaps a line oflight spots (where waves add togetherconstructively) and dark zones (where wavesare cancelling). The exact appearance of thepattern depends on the geometry of the “slits”and the wavelength of the waves.

Davisson & Germer’s ExperimentDavisson and Germer used a modified cathode raytube to test de Broglie’s hypothesis.

A beam of electrons travelling through a vacuum wasallowed to strike a crystal of nickel, speciallyprepared so that electrons would reflect from parts ofit. Different parts of the beam could then overlap theirpathways as they travelled into a detection devicewhich could measure the intensity of the beam.

Result?An interference pattern was detected! This provedthat electrons have wave properties, and confirmedthe de Broglie hypothesis.

Why Are the Bohr Orbits Stable?A quick review of some important points:

Rutherford’s atomic model places electrons in orbit,but classical theory predicts they should constantlybe emitting light because they are accelerating.

However, this isn’t happening, so Bohr proposes thatthere are “allowed”, stable orbits where electronsdon’t constantly give off light. (They only radiatewhen they “jump” orbits)

What makes these “allowed orbits” stable?de Broglie’s particle-wave theory of the electronexplains:

An allowed orbit is where the wavelength of theelectron exactly fits to form a “standing wave”around the nucleus.

“Standing waves” are a well-known wavephenomenon in which an exact number of fullwavelengths can “resonate” or reverberate in astable way. For example, all musical instrumentsinvolve standing waves of sound energy in astring or air space.

The “allowed orbits” around an atom are locatedat distances from the nucleus which allow thequantum energy of the electron to fit in an exactnumber of wavelengths to form a standing wave.

At any other distance, the orbit cannot fit astanding wave with an exact number ofwavelengths, so the electron cannot exist there.

The electron is a particle, with mass andmomentum. It is also a wave, with a wavelength(λλ = h/mv) and capable of diffraction,interference and standing wave behaviour.

Welcome to the world of Quantum Physics!




+

+ =

If the waves are “in phase” (crest matches crest) thewaves add together for double the amplitude

If the waves are “out of phase” (crest matchestrough) the waves cancel for zero amplitude

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Diffracting waves formInterference Patterns

Can you guess what’s coming?

de Broglie has proposed an hypothesis thatelectrons may have wave properties.

What should a good scientist do?Test the hypothesis by experiment, of course!

How do you test for wave properties?Test electrons to see if they show

Diffraction & Interference Patterns, of course!

BBeeaamm ooff lliigghhtt ssttrriikkiinngg aabbaarrrriieerr wwiitthh sslliittss iinn iitt An electron

forms a“StandingWave”around thenucleus

+




8

The Contributions of Heisenburg & PauliBefore you leave the electron orbits and dive into the atomic nucleus,

the syllabus asks you to assess the contributions of 2 other great scientists.

Werner Heisenberg (1901-76)was a German physicist who is best

remembered for the “Heisenberg Uncertainty Principle”,

for which he was awarded the Nobel Prize in 1932.

Heisenberg developed the mathematicalframework for Quantum Mechanics. He showedthat the dual nature of the “particle-wave” which

describes the electron (and the light photon),makes it impossible to know everything aboutany particle at any moment. Either you know

where it is, or you know how much momentumit has, but you cannot know both things at once

with any certainty.

This “uncertainty” about things at the atomicscale was described by Heisenberg as

mathematical probabilities. Thus an electronorbit becomes a “region of probability” in which

there is a good chance (but not a certainty) that the electron exists.

This all sounds very airy-fairy, but its validityhas been spectacularly confirmed by manyexperiments and phenomena such as the“quantum tunnelling” effect, involved insemiconductor operation and electrical

superconductivity.

Wolfgang Pauli (1900-58)was born in Austria, but became an American

citizen. He is best remembered for the“Pauli Exclusion Principle”, (Nobel Prize 1945)

which states that 2 electrons in the sameatom cannot have exactly the

same quantum state.

His mathematical analysis established theidea that the Bohr-de Broglie orbits are justone of several different types of quantum

properties that electrons can have.

This gives rise to the idea of “sub-orbits”within an atom (this explains the “hyperfinelines” in emission spectra) and shows why 2

electrons with almost the same quantumstate, but opposite “spin”

will tend to pair up. (Hence “Cooper Pairs”, and electron pairs in chemical bonding.)

Later in this topic you will see that Pauli also made an important contribution

to understanding nuclear processes as well.

An AssessmentIn the 1920’s, Quantum Theory was being accepted as a “necessary evil” to

satisfactorily describe the structure of an atom, and account for all the known observations.

However, the explanations being used were a mixture of new “quantum” ideas overlaid on a framework of “classical” Physics, so it was all rather

artificial or contrived.

It was the theoretical work of Heisenberg & Pauli that built QuantumMechanics into a complete, new branch of Physics without the

need for any reference to the “old” Physics.

Therefore, their contributions must be seen as being very important. Although the details of their work are beyond the scope of this course, theyallowed Physics to become a fully modern study with a complete theoreticalbase which can explain atoms, super-conductivity, semi-conductors, nuclear

processes and even the creation of the Universe itself.

“If you think you understandQuantum Theory...

then you really don’tunderstand Quantum Theory!”


9

COMPLETED WORKSHEETSBECOME SECTION SUMMARIES



Worksheet 2 Rutherford-Bohr Model of the AtomFill in the blank spaces Student Name..........................................

Worksheet 3 de Broglie & Matter WavesFill in the blank spaces Student Name..........................................

• Electrons can q).............................. from oneorbit to another. When they do so they mustr)............................ or ..................................... anamount of energy. This energy differencerelates to the s)................................ of a spectralline in accord with t)...........................’s QuantumTheory and the u).................................. equation.• Electrons in “v)............................... orbits” havea quantity of w)...................................... which isalways a multiple of h/2ππ.

Bohr was able to link his idea to the BalmerSeries of hydrogen spectral lines. In fact, it ishighly unlikely he could have developed hisidea without this evidence.

However, the Bohr model had a number oflimitations:• It worked only for x).............................................• It could not explain the differenty)...................................... of the spectral lines.• There was evidence from the “z).........................Effect”, and the observed “aa).............................”spectral lines, that each orbit was actuallyab)......................... ..................................................The model could not explain theseobservations.

Rutherford’s model of the atom:• in the centre is a tiny, dense a).............................• Electrons (discovered by b)................................)

are in c)................................ around the outside.The model had a major problem: theoretically,electrons which are d)................................ shouldconstantly emit e)...................................., causingall matter to constantly f)....................... with light.

The “g).......................................................” of anelement refers to the precise set ofh).................................... of light emitted if theelement is energised, for example, in ai).............................................................. The linesare visible if the light is viewed through aj)...................................................

The visible lines in the spectrum ofk)................................. had been named the“l)................................. Series”, and them)........................................ equation had beenformulated to calculate the n).................................of each of the lines in the series.

Bohr used the evidence of the Balmer Series to refineRutherford’s atomic model. He suggested that:• Electrons o).........................................................,in which they will not p).........................................

Louis de Broglie argued that if Einstein’sphotons of light are waves with a)........................properties, then electrons could beb)....................... with c)....................... properties.

He extended Bohr’s model to derive an equationfor the d).............................. (wave measurement)of the electron. Bohr’s “allowed orbits” wereexplained as e)....................................... waves,with an integer number of f)..................................fitting exactly around that orbit.

De Broglie’s hypothesis had g)..............................impact on the scientific community. It seemedan interesting idea, but there was noh)............................... from observations ori)................................... to connect it to.

Two scientists, j)........................... &............................ carried out an experiment inwhich a beam of k)............................... was aimedat a crystal.

They detected an l).............................. patternwhich proved that the electrons wereundergoing m)................................. This provedthat electrons do have n)..................... properties,and confirmed de Broglie’s hypothesis.

o).............................. is a wave phenomenon inwhich waves which penetrate a small aperture,then act like a point source of waves andp)........................ in a q)..........................................pattern. When waves from 2 (or more) aperturesoverlap, they r).................................... with eachother. Where crest meets crest the wavess)................... ........................... creating a highert).................................... wave. Where crest meetstrough, the waves u)........................ each other.With light, this results in a pattern ofv)......................... and ................................. spots.

Following the confirmation of de Broglie’stheory, the science of Quantum Mechanics wasgiven a complete theoretical framework by thework of Werner w)....................................... andWolfgang x)...............................


10



Worksheet 4 Test Questions section 2 Student Name...............................1. Use de Broglie’s equation to calculate:a) the wavelength of an electron with velocity2.25x106 ms-1 (mass of electron = 9.11x10-31kg)

b) the velocity of an electron if its quantumwavelength is 4.75x10-9m.

c) Use the wave equation to find the quantumfrequency of the electron in (b).

d) Use Plank’s equation to calculate thequantum energy of the electron in (b).

2. Describe the impact of de Broglie’s proposalthat particles could have wave properties.Account for this reaction by the scientificcommunity.

3. Outline the experiment of Davisson & Germer.State the result of the experiment and explainthe significance of this result.

4. Explain how de Broglie would describe Bohr’s“allowed orbits” around the nucleus.

5. a) What is diffraction?

b) The diagram shows a breakwall with parallelwater waves approaching. There are 3 boatchannels through the wall. Complete thediagram showing the pattern of the waves whichgo through the boat channels.

6. Assess the contribution of Heisenberg & Paulito the development of atomic theory.

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NucleonsA “nucleon” means any particle located in thenucleus of an atom. We now know that there are 2types of nucleon:

ProtonsThe existence of protons was considered likelyalmost as soon as the electron was discovered. Bythe 1920’s the proton had been positively identified,and its properties measured.

NeutronsAs early as 1907 it had been suggested that protonsalone were not sufficient to account for the mass ofmost atoms. It was suspected that there must beanother nucleon, with considerable mass, but noelectric charge. However, it was 25 years before theneutron’s existence was proven.

Contrasting the Properties of the Nucleons

Proton NeutronElectricalCharge +1.602x10-19C 0 (neutral)

Mass 1.673x10-27kg 1.675x10-27kg

Note that:• The charge on a proton is exactly the samemagnitude, but of opposite sign to that carried by anelectron.

• In a normal atom:No. of protons = No. of electrons = “Atomic No.”

• Protons and neutrons have almost identical masses. (The neutron is slightly heavier)

Both are almost 2,000 times heavier than an electron,so virtually all the mass of an atom is in the nucleus.

No.protons + No.neutrons = “Atomic Mass Number”

For example: Sodium atom

electrons = 11protons = 11

neutrons = 12

Total nucleons = 23(protons + neutrons)

Atomic Mass Number = 23

Atomic Number = 11

Discovery of the NeutronThe existence of the neutron was proven in 1932by James Chadwick (1891-1974).

It was impossible then to detect and measureneutrons directly. The method Chadwick usedrelied upon neutrons colliding with otherparticles, then applying the scientific principlesof Conservation of Energy and Conservation ofMomentum to measure the properties of theneutron.

• The alpha (αα) particles emitted by a radioactivesubstance were used to bombard a berylliumtarget.

• The beryllium emitted neutrons, which (havingno electrical charge) are very penetrating andare unaffected by electric or magnetic fields, socould not be measured or studied directly. Otherscientists had thought the radiation was gamma( γγ ) waves of extreme high energy.

• Some of the neutrons then hit a second targetof paraffin wax, which has a lot of hydrogen in it.Occasionally a neutron collision would dislodgea proton.

• Chadwick was able to study some of theseprotons and measure the energy they carried.

• Chadwick could then apply the principles ofConservation of Momentum and Energy tocalculate the mass and velocity of whatever hadhit the protons and dislodged them.

The results indicated the presence of a particle(not γγ-rays) with a mass almost the same as aproton, and no electric charge. This matchedperfectly with the (then hypothetical) neutron,so the existence of the “missing” nucleon wasconfirmed.

11

3. INTO THE NUCLEUSkeep it simple science

®



Thus we get the familiar atomic model, withelectrons (in Bohr’s allowed orbits) around a

nucleus of protons and neutrons.MMAAKKEE SSUURREE YYOOUU UUNNDDEERRSSTTAANNDD TTHHEE SSHHOORRTTHHAANNDD DDEESSCCRRIIPPTTIIOONN

Na2311

αα n0 p+

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PPaarraaffffiinn WWaaxxttaarrggeett

RRaaddiiooaaccttiivveessuubbssttaanncceeeemmiittttiinngg

αα-ppaarrttiicclleess

DDeetteeccttiinnggeeqquuiippmmeenntt

Background InformationRadioactivity had been discovered in 1896.

Although it was not fully understood, the use ofαα-pparticles as “atomic bullets” in experiments

had become quite routine.

After Chadwick’s experiment, the neutronbecame the next “bullet” of choice.

12

TransmutationTo “transmute” something means to changeit into a different form or substance.

In Nuclear Physics, “Transmutation” refers to anatom changing into an atom of a different element, by undergoing a nuclear reaction.

Transmutations occur during:• radioactive decay of natural or artificial radioisotopes.• nuclear fission in a nuclear power station, or “atom bomb”.• nuclear fusion in the stars, and in a “hydrogen bomb”.

Beta ( ββ ) DecaySome atomic nuclei, of any size, have anunstable mix of protons and neutrons. If there isan excess of neutrons, a neutron can be turnedinto a proton plus an electron.

How can this happen? It seems like magic, but itshows what a strange place the quantum worldis. Some detail on how such things can happenwill be covered later; for now you must acceptthat it actually happens.

The result is that:• Number of neutrons decreases by 1.• Number of protons increases by 1.

(This means Atomic Number goes up by 1but Atomic Mass Number does not change)

• The electron is ejected from the nucleus athigh speed. This is the Beta particle... a highspeed electron.

ExampleCarbon-14 is a well-known radioisotope whichdecays:

Once again Transmutation has occurred.

In many cases of beta-decay there is a gammaray emitted as well.

Note that the Mass Numbers and AtomicNumbers ALWAYS BALANCE across the

equation.




It would be wise to revise...

See Preliminary Topic “Cosmic Engine” to revise the properties of αα,, ββ & γγ radiation.

92238 U 2+ +

4 He 90

234 Th

+ nn +

UUrraanniiuumm-223388 TThhoorriiuumm-223344 AAllpphhaa ppaarrttiiccllee

GGaammmmaa rraayy aallssoo

eemmiitttteedd iinnmmoossttccaasseess

Note that the Mass No.always decreases by 4,

and the Atomic No. by 2

The αα-pparticleconsists of 2 protons & 2 neutrons.

It is the nucleusof a Helium atom

The Uranium atom hasTRANSMUTED

into a different element

RadioactivitySome naturally-occurring atoms have a nucleus which is unstable and willspontaneously undergo transmutation to change into a more stable form. During the reaction, a variety of radiations are emitted from the nucleus.

There are several different reactions which can occur; knowledge of only the 2 most common reactions is required by the syllabus.

Alpha ( αα ) DecayAlpha decay occurs in atoms which have a verylarge nucleus and are unstable. To achievegreater stability, the nucleus mayspontaneously eject an alpha particle to carryaway excess mass and energy.

Example:Uranium is well known as a radioactivesubstance, and “nuclear fuel” for nuclearreactors and bombs. Its most common isotopeis U-238, meaning it has a mass number of 238.It decays as follows:

Example 2Radium-226 transmutes by alpha decay:

Hint: Use the Periodic Table to find AtomicNumbers and identify names and symbols.

614 C -11+ +

0e-

714 N

01 n0

11 p+

-110 e-+

Neutron

Carbon Nitrogen Gamma rayββ-pparticle

Proton Electron

γγ

88226 Ra 2+ +

4 He 86

222 Rn γγ

γγ




13

It was known that the electrons ejectedduring Beta decay varied considerablyin their velocity, and the amount ofenergy they carried. This was puzzling,because it was thought that the processinvolved was the same in every ββ-decay,so why did the energy vary?

In 1931, Wolfgang Pauli suggested aquantum explanation.

What if there was another particle beingproduced, that no-one had detected?This “missing” particle could carryaway some of the energy in varyingamounts.

nnnneeuuttrroonn

pprroottoonnaannttiinneeuuttrriinnoo

ββ-ppaarrttiiccllee((eelleeccttrroonn)) -

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ppaarrttiiccllee aannddnneeuuttrriinnoo aallwwaayyssaaddddss uupp ttoo tthheessaammee aammoouunntt..

What Holds the Nucleus Together?This question had been asked as soon asRutherford had proposed that atoms have anucleus. There were just 2 forces then understood,which could be operating in the nucleus:

GravityAll masses attract all other masses by gravity.This would attract all nucleons to each other.

Electrostatic ForcesAll charged particles exert a force on othercharged particles. This force would not act onneutrons, but should cause protons to berepelled by other protons.

Calculations showed that theelectrostatic repulsion would be much,

much stronger than gravity. Thenucleus should instantly fly apart!

Since the nucleus does exist, and doesn’tinstantly explode, it was realized that there mustbe another force operating. It was called simplythe “Strong Nuclear Force”.

Its properties could be inferred and calculated:• It must be much stronger than the proton-proton electrostatic repulsion. (it’s over 100Xstronger)

• It must be independent of charge and attract all nucleons... protons & neutrons.

• It must be extremely short-ranged, operatingonly across the tiny distances of the nucleus.(Otherwise it might cause neighbouring atomicnuclei to fuse together, and eventually pull allmatter into one lump!) Even before its existencewas proven, the Strong Nuclear Force wasknown to exist, and scientists beganspeculating on how to tap into its enormousenergy potential...

To have avoided detection, this hypotheticalparticle must have no mass (or so little that itwas not measurable) and no electric charge.However, it could carry quantum energy. Pauli’sidea was that there was a certain total energyinvolved in b-decay; some was carried off by thebeta particle, the rest by the mystery particle.

Enrico Fermi did the mathematics andthe whole scenario worked so well intheory that the scientific communityaccepted the new particle, even thoughit was not positively detected andidentified until 1956.

This new particle was eventuallychristened the “neutrino” (little neutralone) and is now a totally accepted factof the sub-atomic quantum world. Infact, there are a whole family ofneutrinos; to keep it simple (KISSPrinciple!) the one released in betadecay is an “anti-neutrino”.

The symbol used for the anti-neutrino isνν. The full equation for a beta decay istherefore:

Pauli and the Neutrino

614 C -11+ + +

0e-

714 N

Carbon Nitrogen anti-neutrino

Gammaββparticle

γγνν




14

Mass Defect in the NucleusIt was realized that incredibly powerful forceswere operating within the atomic nucleus. Howcould such forces arise?

The answer lies in the fact that the mass ofevery atomic nucleus (except hydrogen ) DOESNOT ADD UP.

This difference is called the “Mass Defect”. It’sas if a little bit of mass “went missing” when theprotons and neutrons joined together to formthe nucleus.

Where is the missing mass?

It has converted to energy...

(you should have knownthat Einstein would be

involved sooner or later!)

...to provide the “Binding Energy” of the StrongNuclear Force which holds the nucleustogether.

Einstein had developed his most famousequation as part of his Theory of Relativity. Henever anticipated that it would find anotheruse...

Measuring Mass & Energy in the NucleusBefore going any further, you need to know about the commonly used methods of

measuring mass and energy at the atomic level.Mass in Atomic Mass UnitsThe “atomic mass unit” (u) is a measure of massdevised for convenience in Chemistry. Roughlyspeaking, both a proton and a neutron have amass of 1 u, although in the calculationsfollowing, you need to be much more precise.Obviously, 1 u is a very small mass:

1 u = 1.661x10-27 kg

You need to be able carry out calculations usingeither unit, so the following data may be useful.

Proton NeutronMass (in kg) 1.673x10-27 1.675x10-27

Mass (in u) 1.0073 1.0087

Energy in Electron-VoltsThe “electron-volt” (eV) is an energy unit that isconvenient because the energy of sub-atomicparticles has traditionally been measured by theirbehaviour within electric fields.

1 eV is the energy gained by an electron acceleratingin an electric field with a potential difference of 1 volt. 1 eV is an extremely small amount of energy:

1 eV = 1.602 x 10-19 joules of energy

so the unit often used is the mega-electron-volt(MeV)

1 MeV = 1x106 (one million) eV

This is convenient when dealing with individualatoms or particles.

If you add up the mass of all theprotons+neutrons in any nucleus,

the total is always more than the actualmeasured mass of the whole nucleus.

Mass of Mass ofProtons + Neutrons > Whole Nucleus

E = mc2

Example CalculationA normal carbon atom contains 6 protons and 6neutrons. (also 6 electrons, but mass is negligible)The nucleus is known to have a mass = 11.9967 u

= 1.993x10-26 kgCalculate the Mass Defect,

and total Binding Energy.

SolutionIn kg and joules In u and MeV

Mass of 6 protons Mass of 6 protons= 6 x 1.673x10-27 = 6 x 1.0073= 1.004x10-26 kg = 6.0438 u

Mass of 6 neutrons Mass of 6 neutrons= 6 x 1.675x10-27 = 6 x 1.0087= 1.005x10-26 kg = 6.0522 u

Total particle mass Total particle mass= 2.009x10-26kg = 12.0960 u

∴∴ Mass defect ∴∴ Mass defect= 2.009x10-26- 1.993x10-26 = 12.0960 -11.9967= 1.600x10-28kg = 0.0993 u

This missing mass has Each 1 u converts to converted to binding 931.5 MeV of energyenergy according to (This value is in your

Physics Data Table)E = mc2 So, binding energy

= 1.6x10-28 x (3.00x108)2 = 0.0993 x 931.5= 1.44 x10-11J = 92.50 MeV

From here on, all calculations will be done inatomic mass units (u) and MeV.

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TThheessee aarree tthhee ssaammee,, jjuussttddiiffffeerreenntt uunniittss

15

Nuclear FissionIn the 1930’s, it was discovered that bombarding“target” atoms with alpha particles couldoccasionally produce a transmutation to a newradioactive isotope.

Aluminium αα-particle new isotope neutronof phosphorus

In Italy, brilliant young physicist Enrico Fermi (1901-54) decided that using neutrons as “atomic bullets”would be even more productive.

In 1934 he began bombarding every possible element,in turn, with neutrons and studying the resultingradioactivity to detect any new radioisotopes. Over 40were discovered very quickly. For example:

In one experiment he bombarded uranium atoms withneutrons, confidently expecting to produce atoms of“transuranic” elements. The radiation “signatures”detected were unexpected and puzzling, but he wasfocused on other things and failed to investigatefurther.

Fermi had “split” the nucleus, but it was another 4years before other scientists in Germany confirmedwhat had happened. In his sample of uranium wereatoms of U-235 which had absorbed a neutron, thentotally disintegrated:

This is Nuclear Fission; the splitting of the nucleus.,with enormous energy release, due to a mass defectand E=mc2.

Meanwhile, Fermi had continued on with his work,and was awarded the Nobel Prize of 1938 for hisproduction of new radioactive materials.

With war looming in Europe and a Fascist regime inItaly, Fermi and his Jewish wife used attendance atthe Nobel Prize ceremony in Sweden to flee to theUSA, where Fermi was immediately accepted into thescientific community.

By then he was aware of nuclear fission and its hugeenergy potential, and that the experiments confirmingfission had been done in Nazi Germany. On the eve ofWorld War II, it seemed that the knowledge to developan “atom bomb” was in the hands of the enemy.

The Manhattan ProjectFollowing a “letter of concern” (outlining the dangerof nuclear research in Nazi Germany) from Einstein tothe President of the USA , the top secret “ManhattanProject” was set up in 1942. Its objective was toresearch nuclear fission and develop an “atomicbomb” if this was possible.

The first step was to discover if a self-sustainingchain-reaction of fissions was possible. Enrico Fermiwas appointed the leader of the scientific team. Hedesigned the reactor or “nuclear pile”, which wasbuilt in a squash court at the University of Chicago.

In December 1942 the reactor achieved the first self-sustaining, controlled chain reaction.

The Fission Chain ReactionSince fission is set off by a neutron, and since itreleases more neutrons, it follows that a chainreaction can occur, in which each atom which splitscan set off more.

In a critical mass of “fissile” atoms, if every fissionsets off (say) 2 more, then the chain reaction growsexponentially within a fraction of a second. This isuncontrolled fission, and results in a nuclearexplosion of devastating power... an “atomic bomb”.

If a neutron-absorbing material (such as cadmium) ispresent, it is possible to absorb many of the neutronsso that each fission sets off exactly one other. This iscontrolled fission and is what Fermi achieved in his“pile” in 1942, and what occurs in every nuclearpower station.

There are only 2 nuclei which will readily undergofission:




1327 Al

24

He 1530 P

01 n + +

919 F

01

n 9

20 F +FFlluuoorriinnee

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92235

U 0 01 1

n 3 n

3692

Kr

56141

Ba

+33 eexxttrraanneeuuttrroonnssrreelleeaasseedd..

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92235

U 94239

Pu

Uranium-235 whichoccurs naturally inuranium ores, but invery small amounts.

Plutonium-239 which canbe made from U-238 byneutron bombardment ina nuclear reactor.

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cchhaaiinn rreeaaccttiioonn iiss nnoott sseellff-ssuussttaaiinniinngg aanndd ddiieess ddoowwnn..

Start




16

Mass Defect During Nuclear FissionThe enormous energy released by nuclear fission is due to a “mass defect”

between the starting nucleus and the product nuclei.

For example, in the fission of Uranium-235:(Note: fission products can vary)

UU-223355mmaassss

223355..00443399 uu

nneeuuttrroonnmmaassss

11..00008877 uu

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114477..88111144 uu

BBrr-8855mmaassss

8844..88991177 uu

33 nneeuuttrroonnssmmaassss

33..00226611 uu

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223366..00552266 uu

TToottaall MMaassss aafftteerrFFiissssiioonn

223355..77229922

Mass Defect = (Mass Reactants - Mass Products) = 236.0526 - 235.7292 = 0.3234 u

Energy yield per fission: Remember that 1 u 931.5 MeV

of mass of energy

So, energy released = 0.3234 x 931.5= 301.2 MeV

(This equates to about 5 x 10-11 joules of energy)

The energy releasedmight seem a very

small amount, but thisis from just one atom.

In (say) 10kg ofuranium there are

about 2.5x1025 atoms.If all of these were toundergo fission, the

total energy releasedwould be about 1x1015

joules, all released in a split second, in the case of an atom bomb.

This is the amount of energy generated by anaverage size power station in about 30 years.

Photo by Daron Cooke

SimulatedNuclearExplosion

92235

U 0 01 1n 3 n 35

85Br 57

148La + ++

Practical WorkObserving Nuclear Radiations

You may have done practical work with one or moremethods of detecting and observing radiation from aradioactive isotope.

The Wilson Cloud Chamberis a simple device which allows the “trails” of alphaparticles to be seen.

The chamber is cooled with “dry ice” so that thevapours within are on the point of condensation.

If a source of alpha particles is placed inside thechamber, tiny “tracks” can be seen. An alpha particlecollides with air molecules and ionises millions ofthem along its path. The ionised molecules serve assites of condensation, so a visible “condensationtrail” briefly shows the path of each alpha particle.

Simple SchoolCloud Chamber

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TThhee ttrraacckkss ooffaallpphhaa ppaarrttiicclleessaappppeeaarr aass tthhiinn““ccoonnddeennssaattiioonnttrraaiillss””

This is the plutoniumfission bomb,

nicknamed “Fat Boy”,which destroyed the city

of Nagasaki in 1945.

Enrico Fermiin 1943

working onthe

“ManhattanProject”

When you add up the totalmass of all the products of a

fission reaction, it is lessthan the starting mass.

This “mass defect” has beenconverted to energy.

E = mc22

A “nucleon” refers to all the particlesa)......................................., and includesb)........................ & ..................................These are different in their properties inthat c).......................... are slightlyheavier, and d)........................... carrye)................ electric charge.

The existence of the neutron had beensuspected, and was finally proven byf)...................................... in 1932. Wheng)...............-particles were smashed intoa beryllium target a penetratingradiation was produced. Others hadthought it was h)........................ rays.Chadwick allowed this radiation tostrike a second target ofi).......................... This dislodgedj)............................ which he could detectand measure their energy. By applyingthe principles of k)............................................................... he could calculatethe properties of the “mysteryradiation”. His results indicated al).............................. with mass similar tom).................... but without n)..................................................

“Transmutation” refers to an atomo)..................................................... whenit undergoes a p)...............................reaction. This can occur duringq)....................................... decay, orduring nuclear r).......................... or............................. (opposite processes).

Alpha decay occurs in a nucleus whichis unstable because s).................................................... It ejects an alphaparticle (which is made up of t)......................................................) so that the MassNumber u)...................................... andthe Atomic Number v)............................................ There is usually emissionof w)............................. as well.

Beta decay occurs when a neutronconverts to a x)............................... Any).......................... is created as well, andit is ejected from the nucleus at highspeed... the beta particle. The AtomicNumber z)..........................................while the Mass Number aa)................................ ..................

It was discovered that the beta particlesfrom different isotopes carriedab).................................... .......................Pauli suggested this was becauseac)...........................................................which shared the energy with theelectron. This particle is an ad)...........................................

The nucleus is held together by the“ae)................................................” whichhas to be much more powerful than theaf)....................................... betweenprotons. It acts only overag)........................ distances, andattracts all ah)......................... to eachother. The force arises from the “Massai).........................” of the nucleus. Asmall amount of the mass has beenaj)........................ ................... accordingto ak).................................(equation)

Nuclear al).......................... occurs whena nucleus is struck by aam)............................., and thenan).................. ................. It also releases2 or 3 more ao)................................ whichcan cause a ap)............................Reaction to occur. During each fissionthere is a large energy release due toaq).............................................................

The first controlled fission reaction wasachieved in 1942 as part the secret“ar).................................... Project”. Thereactor was designed byas)..................................................

17





Worksheet 5 Into the NucleusFill in the blank spaces Student Name..........................................




Alpha Decay EquationsWork out the missing nuclide, identifying

• Mass Number & Atomic Number• Symbol & name

6. Write the equation for the alpha decay ofActinium-227

7. Write the equation for the alpha decay ofPlutonium-244

18

86222 Rn 2+ +

4 He γγ

95241 Am 2+ +

4 He γγ

84210 Po 2+ +

4 He γγ

91233 Pa 2+ +

4 He γγ

84210 Po 2+ +

4 He γγ

4.

3.

2.

5.

1.

Worksheet 6 Practice ProblemsNuclear Reactions Student Name..........................................

Beta Decay Equations1. If each of the following nuclides underwentbeta decay, write the symbol, Mass Number &Atomic Number of the new nuclide.

a) Iodine-131

b) Thorium-234

c) Hydrogen-3

d) Sodium-24

e) Uranium-239

f) Cobalt-60

2. Write complete decay equations for the betadecay of:a) Lithium-8

b) Xenon-135

c) Phosphorus-31

d) Chlorine-38

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(02) 6583 4333 FAX (02) 6583 9467www.keepitsimplescience.com.au [email protected]

kkeeeepp iitt ssiimmppllee sscciieennccee

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19



Worksheet 7 Practice ProblemsMass Defect Student Name..........................................

Data for CalculationsNuclide Nuclear Mass Nuclide Nuclear Mass

1.0087 1.0073

4.0026 7.0160

24.9575

140.8167

91.8776

21.9780

91.8804

144.8115

235.0439 239.0446

11.99679.01226

12 C

24 He

49

Be

01

n

92235

U

3692

Kr

56141

Ba

37 Li

11

H

94239

Pu

3892

Sr

56145

Ba

1225 Mg 11

22Na

(u) (u)

612 C

24 He

37 Li

49

Be 01

n + +

1225 Mg

24 He11

22Na 1

1H + +

4.

5.3.

2.

1.

24 He

24 He1

1H + +

92235

U 0 01 1n 3 n 36

92Kr 56

141Ba + ++

94239

Pu 0 01 1n 3 n 38

92Sr 56

145Ba + ++

Use the data table at right.

For each of the following nuclear reactionscalculate:a) the Mass Defect (u)b) the energy released (MeV)


4. Discuss why the neutrino was“invented” (and by whom) and itsexistence accepted, many years beforeit was physically detected and proven toexist.

5. a) Explain why a “chain reaction” offissions is possible.

b) Compare the requirements forcontrolled and uncontrolled nuclearfission.

20



Worksheet 8 Test Questions section 3 Student Name...............................

1. Outline Chadwick’s experiment toconfirm the existence of the neutron,and discuss the importance of“conservation laws” in determining theneutron’s mass.

2. Account for the need for the “strongnuclear force” and outline itsproperties.

3. a) What is meant by the “mass defect”of the nucleus?

b) Explain the connections between thestrong nuclear force, the mass defect,and Einstein’s equivalence of mass &energy.




Significance of the Manhattan Project

Fermi’s first controlled fission chain reaction in 1942was just the first step in one of the most significantscientific research projects in human history.

Within 3 years, fission bombs were used to destroythe Japanese cities of Hiroshima and Nagasaki andbring a sudden end to World War II.

The Manhattan Project brought the world into the“Atomic Age”, with the following significant changes:

Technologies Developed• Nuclear power stations, currently meet about 20%of the world’s energy needs. Fission power is“Greenhouse friendly”, but presents the danger ofdevastating accidents such as at Chernobyl (Ukraine)in 1986. There are also great challenges in the safestorage and disposal of radioactive wastes fromfission power stations.

• Nuclear weapons proliferated during the 40 year“Cold War”. On several occasions the world seemedto be on the brink of a nuclear war which potentiallycould have destroyed all human civilization.

• Rockets were developed to deliver the nuclearweapons, but the “spin-off” was their use for spaceexploration and satellite technology. The modernworld relies heavily on satellites for communication,commerce and finance as well as entertainment.

• Nuclear Medicine includes all the ways that nucleartechnology is used for diagnosis and treatment of awide range of health problems, including cancer.

• Even the humble smoke alarm in your home isconnected to nuclear technology. It contains a tinypellet of radioactive material (Am-241) manufacturedin a nuclear reactor.

Later in this section are more examples, and specificdetails, of technologies which are based on NuclearPhysics and are therefore a direct result of theManhattan Project.

Nuclear Technologies have been widely consideredas having more risks and dangers than benefits.However, there have also been many “spin-offs”which have been highly beneficial to society.Whatever your opinion, the Manhattan Project wascertainly one of the most significant scientificresearch events in human history.

As always, the Science (and the technology it leadsto) is neither good nor bad; that is determined by thechoices and decisions made by people.

21

4. APPLICATIONS OF NUCLEAR PHYSICS

This ruinedbuilding inHoroshima,

Japan, has beenpreserved as a

memorial to themany thousandswho died in the

atom bombattacks in 1945

Nuclear Physics is Still Investigating MatterThe Manhattan Project, and the “Nuclear Age”all grew from research by scientists likeChadwick and Fermi who wanted to find outabout the structure of atoms. They used alphaparticles and neutrons as “bullets” to probe thenucleus to try to understand the fundamentalstructure of matter.

Well, guess what? Scientists are still doing exactlythat, and still using (essentially) the same technique.

Neutrons as Nuclear ProbesNeutrons are still used as probes because theirlack of electric charge allows them to penetratethe nucleus more easily than a proton or alphaparticle. A beam of neutrons might be scatteredby a nucleus, or other particles may be ejectedfrom it. This allows scientists to study thestructure of the nucleus.

Particle Acceleratorsare another tool of modern research.

A Particle Accelerator uses powerful electromagnetsto accelerate electrically charged particles throughhuge circular tubes. Other electromagnets “steer”and focus the beam of accelerating particles. At thedesired energy level, the particles are allowed tocollide head-on, or smash into their target. An array ofdetection equipment studies the particle tracks andradiation from the collision.

For example, the accelerator at C.E.R.N.(underground on the French/Swiss border) is 27 km incircumference, and accelerates particles to velocitiesof 99.995% of the speed of light.

At the end of the section is a brief summary of ourunderstanding of matter, as revealed by the “atomsmashers”.

Photo ofHoroshima a

few days afterthe bomb.

Parts of the cityliterally “ceased

to exist”.




22

Nuclear Fission ReactorsThe main peaceful use of nuclear fission technology is to operate controlled chain

reactions in a fission reactor, and use the energy released to make electricity.

There are many different designs. The following schematic diagram shows the main features of all fission power stations

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Photo byLes Powell

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Sizewell Nuclear Power Station, England

The following is background information only...

Australia is a non-nuclear country.We have one small fission reactor in Sydney forresearch, and to produce radio-isotopes for medicineand industry.

Ironically, Australia is also the country with thelargest mineral deposits of uranium ores. Oureconomy benefits greatly by selling uranium to othernations, but our government policy (based on thedemocratic will of the people) has always been NOTto use nuclear power.

Instead, we rely on hydro-electricity and on burningfossil fuels. Most of our electricity is made by burningcoal, which is a major contributor to the “GreenhouseEffect” and Global Warming.

Many people believe that nuclear technologies havebeen improved, and are now safe enough for Australiato look towards nuclear power for our growingenergy demands.

Please have an opinion on this important issue,but make sure it is an informed opinion.

23

Uses of Radio-isotopes in Medicine

One application of Nuclear Physics that is likely toaffect each of us, or our family, is the use of radio-isotopes in health care.

Radio-isotopes are used for:

Imaging and Diagnosis Radio-isotopes have now joined X-rays andultrasound scans for medical imaging and diagnosis.

For example, the artificial isotope thallium-201 isused with a “gamma ray camera” to image heartmuscle and detect any damage from heart disease.

When injected into the bloodstream, thallium tends tocollect in any active muscle because it “mimics”potassium ions. Being radioactive (it gives off a lot oflow-energy gamma rays) it allows a gamma raycamera to make computer-aided images of heartmuscle to identify if any part of it is damaged.

The isotope has an extremely short half-life, so itrapidly disappears and presents little danger to thepatient.

Cancer Treatment“Radiation therapy” relies on the fact that rapidly-dividing cancer cells are more easily killed by gammaradiation than normal healthy cells.

The isotope cobalt-60 (which emits beta and stronggamma radiation) is commonly used as a source ofradiation which is accurately beamed into the tumour.

Radio-isotopes in IndustryThe gamma rays from cobalt-60 are verypenetrating, and very destructive to living cells.

In the manufacture of medical supplies, such asbandages and dressings, it is vital that theproduct is totally sterile (germ-free). This isachieved by irradiating the products with dosesof gamma radiation high enough to destroy anybacteria or fungi spores which might be present.

In paper manufacture, alpha emitting isotopessuch as Americium-241, are used for thicknesscontrol. A radiation detector constantlymeasures the percentage of radiation whichpenetrates the paper as it moves at high speedthrough “thicknessing” rollers. If the radiationlevel drops, this means the paper is too thick, sothe rollers are automatically adjusted.

Radio-isotopes in EngineeringIn aircraft construction, the airplane parts maybe welded together. It is essential that thewelded joints are totally strong and free ofdefects. X-rays are not able to penetrate themetal welds, but gamma rays can.

To “see” inside the weld, gamma rays (again,cobalt-60) are used like X-rays; they are beamedthrough the welded joint and an image capturedby a “gamma-ray camera”. Analysis of theimage allows engineers to be sure of the qualityof the welding.

Radio-isotopes in AgricultureRadio-iosotopes are not used directly infarming, but are very important in Agriculturalresearch, such as that carried out by the CSIRO.

For example, to study and compare the rates ofuptake of fertilisers into crop plants, isotopessuch as nitrogen-15 and phosphorus-32 arecommonly used.

Small concentrations of these isotopes can beincluded in a fertiliser applied to experimentalplants. The uptake of the fertiliser, and where itends up in the plant, can be “traced” by usingradiation detection equipment. This researchultimately helps farmers to produce food cropsmore efficiently and economically.




Another example is the use of iodine-131 in thetreatment of thyroid cancer. The thyroid gland islocated in the throat, and produces a vitalhormone which has iodine atoms in it.

This gland is the only part of the body whichuses iodine, and enzymes in the gland are ableto chemically “recognize” iodine ions and veryefficiently “harvest” iodine from the bloodstream.

Iodine-131 is radioactive and emits beta andgamma rays.

If a small amount of I-131 isinjected into a patient who hasa tumour in the thyroid gland,the radiation level is so low

that there is little risk to theirhealthy tissue.

However, due to the chemistryof the iodine, the thyroidgland rapidly absorbs the

isotope and concentrates it.The radiation is concentratedin the “target organ” and isvery effective in destroying

the tumour.

I-131 has a short half-life andthe radiation disappears

rapidly.

LLooccaattiioonn ooffTThhyyrrooiiddGGllaanndd




24

The “Standard Model” of MatterAfter 100 years of scientific research into the sub-atomic quantum universe,

just what is the latest “picture” we have for the structure of matter?

Our modern understanding is known as the “Standard Model”, and is a description of both matter and energy (since these are inter-changeable)

at its most fundamental level.

The Four Fundamental Forces:Gravity (the weakest of all) acts between all masses, and holds planets,stars & galaxies together and in orbit.

Electomagnetic Force acts only between charged particles. It isresponsible for holding atoms and molecules together (all chemical bondsare basically electrical) as well as causing all electrical and magneticphenomena.

The Nuclear Weak Force is involved in radioactivitysuch as when an electron and an anti-neutrino are producedduring beta decay in the nucleus.

The Nuclear Strong Force (the strongest of all) actsonly between particles of the “hadron” family. It acts only oververy short range and is what holds protons and neutronstogether in the atomic nucleus.

We now know that protons & neutrons are composed ofsmaller particles called quarks.

The Structure of MatterMany Particles, but Just Two Families.

Once the “atom-smashing” ParticleAccelerators were developed, scientists

began detecting a bewildering assortmentof sub-atomic particles.

This confusion has now been simplifiedwith the realisation that all these particles

belong to just 2 basic types or classes:

Leptons & HadronsLeptons

include the electron, and theneutrino family.(there are several

types of neutrino)

As well as being the particleswhich flow in an electriccurrent, electrons are athome in orbit around a

nucleus. Remember too, thatthey have wave properties

and form (de Broglie’s)“standing waves” within(Bohr’s) allowed orbits.

When formed in the nucleusduring beta decay, theelectron (and an anti-

neutrino) is instantly ejectedat high speed.

Then there are theBosons

These are quantum “particle-waves” and are the means by

which all the particles exert forces on each other.

The best known is the “photon”of electromagnetic radiation,

such as light.

Gravity is thought to involve“gravitons”, but these have not

yet been proven to exist.

The nuclear forces are carriedby gluons (strong force) and

W-particles (weak).

Hadrons are made from QUARKSHadrons include the proton and neutron, and a family ofparticles called mesons.

All the hadrons are composed of combinations of “quarks”.

Each quark has a charge of either +2/3 or -1/3 (compared to the charge of an electron = -1).

Protons contain 3 quarks with charge = +2/3 +2/3 -1/3 = +1Neutrons contain 3 quarks with charge= +2/3 -1/3 -1/3 = 0

Quarks themselves come in a variety of “flavours” whichhave been given whimsical names such as “charm” and“strange”. These names are labels for quantum states andbear no connection to the normal meanings of these words.

Anti-Particles and Anti-MatterIt has been discovered that for every Hadron and Lepton that exists, there is also a corresponding anti-particle.For example, there are electrons, and there are anti-electrons (“positrons”) which have the same mass, butopposite electric charge. There are also anti-protons, anti-neutrons, and so on. As you know, the other particleformed in beta-decay is an anti-neutrino. Theoretically, there could exist “anti-matter” with atoms made entirely of anti-particles.

When any particle and its anti-particle meet, they mutually annihilate each other... all the mass is converted intoenergy (photons of gamma radiation) according to E=mc2.

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25




Worksheet 9 Applications of Nuclear PhysicsFill in the blank spaces Student Name..........................................The Manhattan Project brought theworld into the a).......................... Age andwas one of the b)...............................scientific research projects in history. Itled to technologies such asc)......................................... from whichthe world gets about 20% of itselectricity. d).......................................were a threat to civilization during the“e)............................ War”. Rockets weredeveloped to carry weapons, but nowwe rely on them for f)......................................................... The many uses ofg)............................ substances inMedicine and Industry are also direct“spin-offs”.

Nuclear research is still going on.Neutrons are excellent “probes” or“bullets” because h)................................................... In addition, i)........................................................ are used toaccelerate j)............................... particlesup to near the k).............................................. From the l)....................& .................... from a collision,scientists are able to infer the structureof matter.

A nuclear fission reactor has 3 maincomponents:• Fuel Rods made of a “fissile” materialsuch as m)................................. or...................................• n)........................ Rods (made ofo).......................) These control the rateof fission by absorbing p).........................• The Moderator, which is usuallyq)................................. or “heavy water”.Its job is to r).................................... theneutrons so that fission is more likely tooccur. The energy released by thefission reaction is used to make steam,which then drives a s).............................and ............................. to maket)...........................

Nuclear reactors not only provideelectricity, but are used to make manyartificial u)............................... isotopesthat are useful in Medicine and Industry.

Medical uses include v).............................and diagnosis, as well as treatingw)..................... by irradiation. An isotopeused for imaging is x).............................,while y)........................ radiation forcancer therapy often comes from theisotope z).............................

This same isotope is also used inindustry, for example, toaa)...................................... surgicaldressing and bandages aftermanufacture and packaging. In papermanufacture, the isotope ab)....................is used to control the thickness bymeasuring the penetration ofac).............................. through the paper.

In engineering, gamma rays fromad)......................... are used to check thequality of ae)..........................................,for example in aircraft construction.

In agricultural research, isotopes suchas af)...................... and .............................are used to “trace” the movement ofchemicals into and through a plant.

Our modern “picture” of matter is calledthe ag)................... ..............................There are many sub-atomic particles,but they all belong to 2 classes:• ah)........................, including theelectron and a variety of ai)......................• aj)..........................., including theak)....................... and ............................Each of these is composed of smaller(although more massive) particlescalled al)...........................


26



Worksheet 10 Test Questions section 4 Student Name...............................1. Assess the significance of theManhattan Project to society, includingmention of 2 technologies that weredeveloped from it.

2. Explain the basic principles of a fissionreactor, outlining the composition andfunction of the fuel rods, moderator andcontrol rods.

3. a) What properties of neutrons makethem useful as “probes” to investigatethe nucleus?

b) Identify and briefly describe anothertechnology used in modern nuclearresearch to investigate the structure ofmatter.

4. Using named examples of 4 differentradio-isotopes, describe an applicationof radioactivity ina) medicine.

b) industry.

c) engineering.

d) agriculture.

5. Discuss the the key features of the“Standard Model” of matter includingthe main “classes” of particles,examples of each, and whether each iscomposed of anything smaller.




27

CONCEPT DIAGRAM (“Mind Map”) OF TOPICIn all the Core Topics you were given examples of a “Mind Map”

as a way to summarise the content of the topic. If you have found this a useful way to summarise and learn, then you may want to do it again.

By now you should have developed the skills to do it yourself...

Into the Nucleus

FROM QUANTATO QUARKS

Rutherford & BohrModels of the Atom

de Broglie&

Matter Waves

Applicationsof

NuclearPhysics

28

Answer SectionWorksheet 11.

2.The existing theory for EMR stated that electronsaccelerating in circular motion should constantly emitlight energy, but obviously they don’t.

3.a) Balmer Series is the 4 lines of visible light in theemission spectrum for hydrogen.b) 1 = RH( 1/nf

2 - 1/ni2 )

λλ= 1.097x107( 1/22 - 1/42 )

1/λλ = 2.057 x 106

∴∴ λλ = 4.86x10-7 m c) c = λλ.f, ∴∴ f = c/λλ

= 3.00x108 /4.86 x10-7

= 6.17x1014Hz.E = h.f

= 6.63x10-34 x 6.17x1014

= 4.09x10-19 J.d) The energy difference between the 2nd and 4thquantum levels (or “allowed orbits”).

4.It is very unlikely that Bohr could have developed hisatomic model without the evidence of the hydrogenspectrum. The fact that there were distinct lines atprecise wavelengths all pointed to quanta of energy,rather than variable amounts.

5.a) More energy, because it is the difference between5th-2nd orbits, compared to 4th-2nd.b) Higher frequency, because Plank’s E = hf shows adirect relationship between energy and frequency.c)Shorter, because frequency and wavelength areinversely related by the wave equation , v=lf.

6.a) • electrons revolve only in certain stable, “allowedorbits”• Energy must be absorbed, or emitted, in quantisedamounts when an electron jumps from one orbit toanother.• Within the “allowed orbits” the electron’s angularmomentum is quantised to a multiple of h/2ππ.b) * it applied only to the hydrogen atom.* it could not explain the different intensities of thespectral lines.* it could not explain the “hyperfine” spectral lines.* it could not explain the “Zeeman Effect”.

7. 1 = RH( 1/nf2 - 1/ni

2 )λλ

= 1.097x107( 1/12 - 1/82 )1/λλ = 1.080 x 107

∴∴ λλ = 9.20x10-8 m ( = 92 nm)Visible light has wavelengths from about 400-700 nm.This is much shorter, therefore is in the ultra violet.

Worksheet 2a) nucleus b) J.J.Thomsonc) orbitd) accelerating/in circular motione) (electromag) radiation f) glowg) emission spectrum h) wavelengthsi) discharge tube j) spectroscopek) hydrogen l) Balmerm) Rhydberg n) wavelengtho) revolve only in “allowed” orbitsp) radiate energy/emit lightq) jump r) absorb or emits) wavelength/frequency t) Plank’su) Rhydberg v) allowedw) angular momentum x) hydrogeny) intensities/brightness z) Zeemanaa) hyperfine ab) divided into sub-orbits

Worksheet 3a) particle b) particlesc) wave d) wavelengthe) standing f) wavelengthsg) very little h) evidencei) experiment j) Davisson & Germerk) electrons l) interferencem) diffraction n) waveo) Diffraction p) spread outq) semi-circular r) interferes) add together t) amplitudeu) cancel v) bright and darkw) Heisenberg x) Pauli

Worksheet 41.a) λλ = h = 6.63x10-34/(9.11x10-31 x 2.25x106)

mv = 3.23x10-10 m

b) λλ = h mv so v = h/mλλ

= 6.63x10-34/9.11x10-31x4.75x10-9

= 1.53x105 ms-1.c) c = λλf, so f = c/λλ = 3.00x108/4.75x10-9

= 6.32x1016 Hzd) E = h.f

= 6.63x10-34 x 6.32x1016

= 4.19x10-17 J.




Electrons in orbitaround central

nucleus

Atom mostlyempty space

Nucleus

29

Worksheet 4 (cont)2.His proposal had very little impact at first. It was a“neat” idea, and mathematically valid, but thescientific community took little notice because therewas no evidence from observation or experiment tolink it to. It was not until the hypothesis was tested byDavisson and Germer that the Physics world reallytook notice.

3.Outline: In a vacuum tube, a beam of cathode rays(electrons) were beamed at a specially preparednickel crystal.Result: They detected an interference pattern in thatpart of the beam that reflected from the crystal.Significance: this proved that electrons showed waveproperties (diffraction & interference) and confirmedde Broglie’s hypothesis.

4.The “allowed orbits” are where the the electron canexist as a standing wave around the nucleus. Theorbit circumference is exactly equal to an integralnumber of electron wavelengths.

5.a) When waves pass through a small gap in a barrier,the gap acts like a point source of waves, whichspread out in a semi-circular pattern.b)

6.In the 1920’s, Atomic Physics was using a mixture of“classical” ideas, overlaid with the new quantumideas, but it was artificial and contrived.

It was Heisenberg (“Uncertainty Principle”) and Pauli(“Exclusion Principle”) who developed the theoreticalframework of Quantum Mechanics so it could becomea coherent, modern scientific model of matter.

Worksheet 5a) in the atomic nucleus b) protons & neutronsc) neutrons d) protonse) positive f) Chadwickg) alpha h) gammai) paraffin wax j) protonsk) conservation of momentum and energyl) particle m) a protonn) electric chargeo) changing into a different element p) nuclear q) radioactiver) fission or fusion s) too larget) 2 protons & 2 neutronsu) decreases by 4v) deceases by 2 w) gamma raysx) proton y) electron

z) increases by 1 aa) stays the sameab) different amounts of energyac) another particle was producedad) anti-neutrino ae) Strong Nuclear Forceaf) electrostatic repulsionag) extremely shortah) nucleons ai) defectaj) converted to energy ak) E = mc2

al) fission am) neutronan) splits apart ao) neutronsap) chainaq) mass conversion/defectar) Manhattan as) Enrico Fermi

Worksheet 6

Beta Decay Equations1.

2.




82

206 Pb

Lead

93

237 Np

Neptunium

84

218 Po

Polonium

1. 3.2.

93

237 Np

Neptunium

86

214 Rn

Radon

5.4.

6.

7.

89227 Ac 2+ +

4 He 87

223 Fr γγ

Francium

94244 Pu 2+ +

4 He 92

240 U γγ

Uranium

28

60 Ni93

239 Np12

24 Mg

2

3 He91

234 Pa54

131 Xea)

f)

a)

e)d)

d)

c)

c)

b)

b)

38 Li -11+ + +

0e-

48 Be γγνν

54135 Xe -11+ + +

0e-

55135 Cs γγνν

1531 P -11+ + +

0e-

1631 S γγνν

1738 Cl -11+ + +

0e-

1838 Ar γγνν

30

4.It was noticed that the electrons produced by betadecay varied a lot in the energy they carried, althoughthe process was thought to be the same in each case.Why?Pauli suggested that there was another particleinvolved, which shared the total energy with theelectron... the neutrino (actually an anti-neutrino).This explanation of beta decay was so convincingthat the existence of the neutrino was accepted manyyears before its actual detection.

5.a)A fission reaction is set off by a neutron striking asuitable nucleus. The fission process produces 2 or 3new neutrons, each of which can set off anotherfission. Therefore, once started, it is possible to havea chain reaction of fissions.

b) If 2 or more neutrons are released, and each setsoff another fission, the chain reaction will growexponentially. This is an uncontrolled reaction.If some neutrons are absorbed so that each fissionsets off exactly 1 other fission, then the chainreaction will continue, but at a steady, controlled rate.

Worksheet 9a) Atomic/Nuclear b) most significantc) nuclear power stationsd) Nuclear weaponse) Cold f) launching satellitesg) radioactiveh) their lack of electric charge makes it more likelythey will collide with the nucleusi) particle accelerators j) chargedk) speed of light l) radiation & particlesm) uranium or plutoniumn) Controlo) cadmium/boron p) neutronsq) graphite r) slow downs) turbine & generator t) electricityu) radioactive v) imagingw) cancer x) thallium-201y) gamma z) cobalt-60aa) sterilise ab) americium-241ac) alpha particles ad) cobalt-60ae) welded joints af) nitrogen-15 & phosph-31ag) Standard Model ah) leptonsai) neutrinos aj) Hadronsak) proton & neutron al) quarks

Worksheet 101.This was one of the most significant scientificprojects in history. It led directly to the developmentof nuclear weapons which (during the Cold War)threatened to destroy civilization, and still have thatpotential. It also lead to nuclear technologies such asthe many uses of radioactive isotopes in Medicine (egfor imaging, diagnosis & cancer treatment) Boththese technologies, and others, have had profoundimpacts upon society, both positive and negative.

Worksheet 71.

Mass defect = (mass reactants) - (mass products)= (9.0122+4.0026)-(11.9967+1.0087)= 13.0148-13.0054= 0.0094 u

Energy release = 0.0094 x 931.5 = 8.756 MeV2.

Mass defect = (mass reactants) - (mass products)= (235.0439+1.0087)-(140.8167+91.8804+3.0261)= 236.0526 - 235.7232= 0.3294 u


Mass defect = (mass reactants) - (mass products)=(7.0160+1.0073)-(4.0026 x 2)= 8.0233 - 8.0052= 0.0181 u


Mass defect = (mass reactants) - (mass products)=(239.0446+1.0087) - (144.8115+91.8804+3.0261)= 240.0533 - 239.7180= 0.3353 u


Mass defect = (mass reactants) - (mass products)=(21.9780+4.0026) - (24.9575+1.0073)= 25.9806 - 25.9648 = 0.0158 u

Energy release = 0.0158 x 931.5 = 14.72 MeV

Worksheet 81.Chadwick used a radioactive material to fire alphaparticles at a beryllium target. This produced apenetrating radiation that others thought weregamma rays. Chadwick let this radiation strike aparaffin wax target. From this came streams ofprotons, dislodged by the “mystery” rays. He usedthe laws of conservation of energy and momentum tocalculate the nature of the radiation that haddislodged the protons.This showed it was particles with mass about 1u, andno electric charge... neutrons.

2. Calculations showed that gravity was too weak tohold the nucleons together in the face of electrostaticrepulsion between protons. No other forces wereknown, but there must exist another force in thenucleus.This “Strong Nuclear Force” must attract allnucleons, and must be very powerful. It must beextremely short-ranged, and work only across thedistance of a single nucleus.

3.a) Every nucleus larger than hydrogen has a massslightly less than the sum of the protons andneutrons it contains. The difference is the massdefect.

b) The “missing mass” of the mass defect is massthat has converted to energy according to E=mc2.This energy provides the “binding energy” of thestrong nuclear force.





31

Worksheet 10 (cont)2.The fuel rods are composed of uranium or plutoniumwhich undergoes fission. Each rod is below thecritical mass for a chain reaction, but when manyrods are inserted into the reactor, a chain reactioncan be sustained.

Control rods are made from cadmium or boron andare good neutron absorbers. These control the rate ofthe reaction by adjusting how many neutrons areavailable to continue the chain reaction.

The moderator is graphite or “heavy water” whichslows the neutrons down. This makes collisions morelikely to set off a fission, and allows the reactor to runefficiently at a steady rate.

3.a) Neutrons have no electrical charge. This makesthem more penetrating, and less likely to be deflectedby electrons or protons before they collide with anucleus.

b) Particle Accelerators use powerful electromagnetsto accelerate charged objects up to very high speeds.They are then allowed to collide head-on, or to strike“target” atoms. The radiation and particle tracks fromthe collision are studied to reveal information aboutthe structure of matter.

4. a) Iodine-131 can be used to treat thyroid cancer.Iodine becomes concentrated in the thyroid glandwhere the radiation kills tumour cells with minimaldamage to healthy tissue.

b) Americium-241 is used to monitor the thicknes ofpaper during manufacture. The penetration of alphaparticles through the paper is used as a measure ofthickness, and equipment adjusted automatically.

c) Gamma rays from cobalt-60 can be used to “image”welded joints in aircraft manufacture.

d) Nitrogen-15 is used as a “tracer” in agriculturalresearch. Added to soil or fertilizer, its uptake andtravel through the plant can be traced by radiationdetection equipment.

5.There are many sub-atomic particles, but they allbelong to 2 classes:

Leptons include the electron, and a variety ofneutrinos. These are fundamental particles, notcomposed of anything smaller.

Hadrons include the proton and neutron and others.These are composed of combinations of differentquarks. A proton, for example, is composed of 3quarks, bound together by a huge mass defect.



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Topics Available

Year 11-12 Science CoursesBiologyPreliminary CoreLocal EcosystemPatterns in NatureLife on EarthEvolution Aust. BiotaHSC CoreMaintain. a BalanceBlueprint of LifeSearch for Better HealthOptionsCommunicationGenetics:Code Broken?

ChemistryPreliminary CoreChemical EarthMetalsWaterEnergyHSC CoreProduction of MaterialsAcidic EnvironmentChem.Monit.&MngmentOptionsShipwrecks, Corrosion...Industrial Chemistry

Earth & Envir.Science

Preliminary CorePlanet Earth...Local EnvironmentWater IssuesDynamic EarthHSC CoreTectonic ImpactsEnvirons thru TimeCaring for the CountryOptionIntroduced Species

PhysicsPreliminary CoreWorld CommunicatesElectrical Energy...Moving AboutCosmic EngineHSC CoreSpaceMotors & GeneratorsIdeas to ImplementationOptionsQuanta to QuarksAstrophysics

Year 7-8 General ScienceDisk Filename Topic Name01.Energy Energy02.Forces Forces03.Matter Solids, Liquids & Gases04.Mixtures Separating Mixtures05.Elements Elements & Compounds06.Cells Living Cells07.Life Living Things08.LifeSystems Plant & Animal Systems09.Astronomy Astronomy10.Earth The Earth11.Ecosystems Ecosystems

Year 9-10 General ScienceDisk Filename Topic Name12.Waves Wave Energy (inc. Light)13.Motion Forces & Motion14.Electricity Electricity15.Atoms Atoms & Elements16.Reactions Compounds & Reactions17.DNA Cell Division & DNA18.Evolution Evolution of Life19.Health Health & Reproduction20.Universe The Universe21.EarthScience Earth Science22.Resources Resources & Technology

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