Nuclear Physics Revision

7/28/2019 Nuclear Physics Revision

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Jamie Saunders

Alpha Particle ScatteringExperiment- -source has small aperture to makefine beam.- Gold foil used since it can be made

very thin.- Vacuum to stop -particles beingabsorbed.- Rotating detector to measure differentangles.

(Note: This dispelled the Plum Puddingatom model).

Alpha Particle Scattering Results- Most -particles passed straight

through the foil. Most of the atommust be empty space.- Some were deflected slightly and

occasionally were reflected by anangle ofmore than 90There mustbe nucleus with a +ve charge to repelthe +ve -particles. The nucleus mustbe very dense to hold all the mass.

Why the Nucleus Stays TogetherUsing Newtons law of gravitationF=GMm/r2 and Coulombs law

F=Qq/40r2 you can determine thatthe electrostatic force of repulsion

(90N) is far 106 greater than thegravitational attraction between them

(7.3x10-35N). This means that theStrong Nuclear Forcemust beattractive over very short distances.

Estimating Density

We can use =m/V and V=4r/3 to

estimate the density of a proton, and soof the nucleus, to be roughly 7.8 x1017kgm-3.

R = (1.2 x10-15) x A1/3

Nuclear DecayIn any nuclear process, nucleonnumber and proton number are both

conserved.

(Note: Proton number conservationrepresents the conservation ofcharge).

Words and Notation for Atoms- Protons and neutrons collectively arenucleons.- Nucleon/Mass number is A, and

proton/atomic number is Z.- A specific group of nucleons is called a

nuclide, represented by .- Isotopes of the same element have thesame no. of protons but a different no.of neutrons.

QuarksQuarks make up hadrons. There are 6flavours; up, down, strange, charm,top & bottom. They are elementary

particles (cannot be broken downfurther). Neutrons (dud) and Protons(uud) are made up from quarks.

(Note: antiquarks have opposite signs).


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Hadrons & LeptonsHadrons: Particles affected by theStrong Nuclear Force (e.g.protons/neutrons). (There are 2 types ofhadron; baryons 3 quarks and mesons

1 quark & 1 antiquark).

Leptons: Elementary particles notaffected by the strong force (e.g.electrons/neutrinos). Affected by theweak interaction.

Conserved PropertiesIn any interaction between hadrons,the following things are conserved:

Charge

Baryon number

Strangeness

Charm

Topness

Bottomness.

Beta DecayThe weak force between quarks isresponsible for:

decay: (d

u)+ decay: (u d)

(nu) is a neutrino and-bar is an

antineutrino is a positron.-particles are emitted with a range of speeds.

(Note: neutrinos must take away some of the

energy and momentum to slow some -particles).

RadiationRadiation is the spontaneousdisintegrationof the nucleus of anatom where it emits an , particle or ray. (Spontaneous no change inthe amount of radiation emitted).Radiation is a random process in whichan unstable nucleus attempts tobecome more stable by losingmass/energy. (Random non-continuous, irregular decay pattern).

Basic Properties of DifferentRadiation

-particles: , a +vely charged ion.(Heaviest, most ionising and slowestradiation). (~1mm paper)-particles: -e, electrons from nucleus.(Very light (0.00055u), have a charge of-1 and fast-moving). (~2mm Al)

-rays: , an EM wave ( 10-12). (Nomass & no charge but travel at speed oflight (c)). (~2cm Pb)

Radiation in Fields Electric Fields

Radiation in Fields MagneticFields

Activity & Decay ConstantThe activity A of a sample is the rate

at which nuclei decay or disintegrate.(Measured in Becquerel; 1Bq = 1s-1)The probability that an individualnucleus will decay per unit timeinterval is called the decay constant. (Unit s-1 )For a sample of N un-decayed nuclei:A = N = N/t.


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Measuring Radioactive DecayRadioactive decay is measured usingGeiger-Muller (GM) tubes. Radiationfrom a sample will be emitted in alldirections but only a fraction will be

detected and counted. To find a countrate R, you must account forbackground radiation:Corrected count rate = count rate background rate.

Decay Graphs & EquationsActivity, un-decayed nuclei and countrate (X to represent all) all decayexponentially with a constant half-life:

X = X0e-t

Smoke AlarmsSmoke alarms use -radiation to detectsmoke. The radiation passes through anair-filled chamber to complete thecircuit; if smoke blocks it, the alarmgoes off. An -source is used as it is:- Highly ionising; so sufficient airparticles will be ionized to produce acurrent.- Bad at penetrating; so easily stoppedby smoke and cant get past the plasticcasing.

Carbon DatingMeasuring the amount ofcarbon-14(half-life 5730 yrs) in materials.Roughly constant in living tissue fromintake of CO2 but decays after death.Mass spectrometry is used. Problems:-Less 14C now since burnt fossil fuelshave little 14C.- Doesnt work under 100 years- Smalluncertainty in t(Note: C-14 is formed when cosmicrays strike 14N nuclei).

Decay of Radioactive Nuclei vs.CapacitorCharge

Similarities: Both decay

exponentially. Differences: Q can be determined

exactly whereas N is subject tostatistical variation and so has smallfluctuations and produces a wigglygraph:

Half-life

The half-life (t) of a radioisotope isthe mean time taken for halfof theactive nuclei in a sample to decay.From the equation for exponentialdecay, it can be derived that:

t = ln2


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Einsteins Mass-Energy EquationE = mc2Any reaction involving a change inenergy will have a correspondingchange in mass (and vice versa). (m =

mass defect).This is the conservation of mass-energy.

Note: Mass is often measured in termsof the unified atomic mass unit: u(1.6605 x10-27kg)

Mass Defect & Binding EnergyMass of nucleus < Mass of individualNucleonsThe mass defect of a nucleus is equalto the difference between the total

mass of the separate nucleons & massof the nucleus.The minimum energy required to splita nucleus into its constituent nucleonsis known as the binding energy of thenucleus.Binding energy per nucleon = Bindingenergy / Nucleon no.

Binding Energy per Nucleon Graph-The most stable is 56Fe- Un-expectedly stable nuclides are 16O,12C & especially 4He.- Any process going up the gradienttowards 56Fe will release energy sincebinding energy per nucleon increases somass is lost and energy is released.

Nuclear FissionFissile materials can be used as fuel orin nuclear bombs.Induced nuclear fission is whenneutrons are fired at large unstablenuclei to break the nuclei apart. Slow,thermalneutrons are bombarded at 235Uwhich breaks up and releases fastneutrons which go on to cause morefission. The process is self-sustaining soit is a chain reaction.

Nuclear FusionFusion is when two smaller nuclei jointo make a larger nucleus. It involvesovercoming the electrostatic repulsionbetween positive nuclei and makingthem close enough for the strong forceto pull them together. This can only bedone by using lots of energy. Stars(e.g. the Sun) have a temperature of1.5x107 Kand a density of1.5x105Kgm-3 to allow nuclei to fuse.

Nuclear Fission ReactorsThey all contain these basic parts:- Fuel rods: These contain the 235U whichprovides energy.- Moderator: Graphite for Advanced gas-cooled reactor (AGR), water inPressurized water reactor (PWR) whichslows the fast neutronsinto thermalneutrons, ready for fission.- Control rods: Material like Boron whichabsorbs neutrons to control the rate of

reaction.- Coolant: Used to remove heat which is

Nuclear WasteFuel rods decay to the point wherethey can no longer sustain a chainreaction. Useful, remaining 235U isextracted and then cooled in pools formonths and then moved to long termstorage sites because many of thematerials have a very long half-life.This greatly adds to the cost of nuclearfission.

n3BaKrUUn1

0

141

56

92

36

236

92

235

92

1

0+++ 17.6MeVnHeHH 10

4

2

3

1

2

1 +++


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rotating

microscope

source

thin

metal foil

ZnS screen

vacuum

fine beam

of -particles

lead sheets

with small

hole

Jamie Saunders

produced. This then goes to a heatexchanger which makes steam forturbines.

Nuclear Physics Revision

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