Chapter 29Chapter 29

Nuclear PhysicsNuclear Physics

Milestones in the Development Milestones in the Development of Nuclear Physicsof Nuclear Physics 1896 – Becquerel discovered radioactivity in 1896 – Becquerel discovered radioactivity in

uranium compoundsuranium compounds Rutherford showed the radiation had three typesRutherford showed the radiation had three types

Alpha (He nucleus)Alpha (He nucleus) Beta (electrons)Beta (electrons) Gamma (high-energy photons)Gamma (high-energy photons)

1911 Rutherford, Geiger and Marsden 1911 Rutherford, Geiger and Marsden Established the point mass nature of the nucleusEstablished the point mass nature of the nucleus Nuclear forceNuclear force was a new type of force was a new type of force

1919 Rutherford and coworkers first observed 1919 Rutherford and coworkers first observed nuclear reactions in which naturally occurring alpha nuclear reactions in which naturally occurring alpha particles bombarded nitrogen nuclei to produce particles bombarded nitrogen nuclei to produce oxygenoxygen

MilestonesMilestones

1932 Cockcroft and Walton first used 1932 Cockcroft and Walton first used artificially accelerated protons to produce artificially accelerated protons to produce nuclear reactionsnuclear reactions

1932 Chadwick discovered the neutron1932 Chadwick discovered the neutron 1933 the Curies discovered artificial 1933 the Curies discovered artificial

radioactivityradioactivity 193 Hahn and Strassman discovered 193 Hahn and Strassman discovered

nuclear fissionnuclear fission 1942 Fermi and collaborators achieved 1942 Fermi and collaborators achieved

the first controlled nuclear fission reactorthe first controlled nuclear fission reactor

Some Properties of NucleiSome Properties of Nuclei All nuclei are composed of protons and neutronsAll nuclei are composed of protons and neutrons

Exception is ordinary hydrogen with just a protonException is ordinary hydrogen with just a proton The The atomic numberatomic number, Z, equals the number of , Z, equals the number of

protons in the nucleusprotons in the nucleus The The neutron numberneutron number, N, is the number of , N, is the number of

neutrons in the nucleusneutrons in the nucleus The The mass numbermass number, A, is the number of nucleons , A, is the number of nucleons

in the nucleusin the nucleus A = Z + NA = Z + N Nucleon is a generic term used to refer to either a Nucleon is a generic term used to refer to either a

proton or a neutronproton or a neutron The mass number is not the same as the massThe mass number is not the same as the mass

SymbolismSymbolism

X is the chemical symbol of the elementX is the chemical symbol of the element Example: Example:

Mass number is 27Mass number is 27 Atomic number is 13Atomic number is 13 Contains 13 protonsContains 13 protons Contains 14 (27 – 13) neutronsContains 14 (27 – 13) neutrons

The atomic number, Z, may be omitted The atomic number, Z, may be omitted since the element can be used to determine since the element can be used to determine ZZ

XAZ

Al2713

IsotopesIsotopes

IsotopesIsotopes of an element have the same of an element have the same Z but differing N and A valuesZ but differing N and A values

Example: Example: C116 C146C136C126

ChargeCharge The proton has a single positive charge, The proton has a single positive charge,

+e+e The electron has a single negative The electron has a single negative

charge, -echarge, -e The neutron has no chargeThe neutron has no charge

Makes it difficult to detectMakes it difficult to detect e = 1.60217733 x 10e = 1.60217733 x 10-19-19 C C

MassMass It is convenient to use It is convenient to use atomic mass atomic mass

units,units, u, to express masses u, to express masses 1 u = 1.660559 x 101 u = 1.660559 x 10-27-27 kg kg Based on definition that the mass of Based on definition that the mass of

one atom of C-12 is exactly 12 uone atom of C-12 is exactly 12 u Mass can also be expressed in Mass can also be expressed in

MeV/cMeV/c22

From EFrom ERR = m c = m c22

1 u = 931.494 MeV/c1 u = 931.494 MeV/c22

Summary of MassesSummary of Masses

MassesMasses

ParticleParticle kgkg uu MeV/MeV/cc22

ProtonProton 1.6726 x 101.6726 x 10--

2727

1.0072761.007276 938.28938.28

NeutronNeutron 1.6750 x 101.6750 x 10--

2727

1.0086651.008665 939.57939.57

ElectronElectron 9.101 x 109.101 x 10-31-31 5.486x105.486x10-4-4

0.5110.511

The Size of the NucleusThe Size of the Nucleus First investigated by First investigated by

Rutherford in Rutherford in scattering experimentsscattering experiments

The KE of the particle The KE of the particle must be completely must be completely converted to PEconverted to PE

2e

1 (2e)(Ze)mv =k2 d

2e2

4k Zed=

mv

d gives an upper limit for the d gives an upper limit for the size of the nucleussize of the nucleus

For gold, d = 3.2 x 10For gold, d = 3.2 x 10-14-14 m mFor silver, d = 2 x 10For silver, d = 2 x 10-14-14 m m

d is often expressed in d is often expressed in femtometersfemtometers; 1 fm = 10; 1 fm = 10-15-15 m (also m (also called a fermi)called a fermi)

Size of Nucleus, CurrentSize of Nucleus, Current

Since the time of Since the time of Rutherford, many Rutherford, many other experiments other experiments have concluded the have concluded the followingfollowing Most nuclei are Most nuclei are

approximately approximately sphericalspherical

Average radius isAverage radius is

rroo = 1.2 x 10 = 1.2 x 10-15-15 m m

31

oArr

Density of NucleiDensity of Nuclei

The volume of the nucleus The volume of the nucleus (assumed to be spherical) is (assumed to be spherical) is directly proportional to the total directly proportional to the total number of nucleonsnumber of nucleons

This suggests that This suggests that all nuclei haveall nuclei have nearly the same densitynearly the same density

Nucleons combine to form a Nucleons combine to form a nucleus as though they were nucleus as though they were tightly packed spherestightly packed spheres

Nuclear StabilityNuclear Stability There are very large repulsive electrostatic There are very large repulsive electrostatic

forces between protonsforces between protons These forces should cause the nucleus to fly These forces should cause the nucleus to fly

apartapart The nuclei are stable because of the The nuclei are stable because of the

presence of another, short-range force, presence of another, short-range force, called the called the nuclear forcenuclear force This is an attractive force that acts between all This is an attractive force that acts between all

nuclear particlesnuclear particles The nuclear attractive force is stronger than the The nuclear attractive force is stronger than the

Coulomb repulsive force at the short ranges Coulomb repulsive force at the short ranges within the nucleuswithin the nucleus

Nuclear Stability, contNuclear Stability, cont Light nuclei are most Light nuclei are most

stable if N = Zstable if N = Z Heavy nuclei are most Heavy nuclei are most

stable when N > Zstable when N > Z As the number of As the number of

protons increase, the protons increase, the Coulomb force Coulomb force increases and so more increases and so more nucleons are needed to nucleons are needed to keep the nucleus stablekeep the nucleus stable

No nuclei are stable No nuclei are stable when Z > 83when Z > 83

Binding EnergyBinding Energy

The total energy of the bound system The total energy of the bound system (the nucleus) is less than the (the nucleus) is less than the combined energy of the separated combined energy of the separated nucleonsnucleons This difference in energy is called the This difference in energy is called the

binding energybinding energy of the nucleus of the nucleus It can be thought of as the amount of energy It can be thought of as the amount of energy

you need to add to the nucleus to break it you need to add to the nucleus to break it apart into separated protons and neutronsapart into separated protons and neutrons

Binding Energy per Binding Energy per NucleonNucleon

Binding Energy NotesBinding Energy Notes

Except for light nuclei, the binding Except for light nuclei, the binding energy is about 8 MeV per nucleonenergy is about 8 MeV per nucleon

The curve peaks in the vicinity of A = 60The curve peaks in the vicinity of A = 60 Nuclei with mass numbers greater than or Nuclei with mass numbers greater than or

less than 60 are not as strongly bound as less than 60 are not as strongly bound as those near the middle of the periodic tablethose near the middle of the periodic table

The curve is slowly varying at A > 40 The curve is slowly varying at A > 40 This suggests that the nuclear force saturatesThis suggests that the nuclear force saturates A particular nucleon can interact with only a A particular nucleon can interact with only a

limited number of other nucleonslimited number of other nucleons

RadioactivityRadioactivity

RadioactivityRadioactivity is the spontaneous is the spontaneous emission of radiation emission of radiation

Experiments suggested that Experiments suggested that radioactivity was the result of the radioactivity was the result of the decay, or disintegration, of decay, or disintegration, of unstable nucleiunstable nuclei

Radioactivity – Types Radioactivity – Types

Three types of radiation can be Three types of radiation can be emittedemitted Alpha particlesAlpha particles

The particles are The particles are 44He nucleiHe nuclei Beta particlesBeta particles

The particles are either electrons or positronsThe particles are either electrons or positrons– A positron is the A positron is the antiparticleantiparticle of the electron of the electron– It is similar to the electron except its charge is +eIt is similar to the electron except its charge is +e

Gamma raysGamma rays The “rays” are high energy photonsThe “rays” are high energy photons

Distinguishing Types of Distinguishing Types of RadiationRadiation

The gamma The gamma particles carry no particles carry no chargecharge

The alpha The alpha particles are particles are deflected upwarddeflected upward

The beta particles The beta particles are deflected are deflected downwarddownward

Penetrating Ability of Penetrating Ability of ParticlesParticles

Alpha particlesAlpha particles Barely penetrate a piece of paperBarely penetrate a piece of paper

Beta particlesBeta particles Can penetrate a few mm of aluminumCan penetrate a few mm of aluminum

Gamma raysGamma rays Can penetrate several cm of leadCan penetrate several cm of lead

The Decay ConstantThe Decay Constant

The number of particles that decay in a The number of particles that decay in a given time is proportional to the total given time is proportional to the total number of particles in a radioactive number of particles in a radioactive samplesample ΔN = -λ N ΔtΔN = -λ N Δt

λ is called the λ is called the decay constantdecay constant and determines and determines the rate at which the material will decaythe rate at which the material will decay

The The decay ratedecay rate or or activityactivity, R, of a , R, of a sample is defined as the number of sample is defined as the number of decays per seconddecays per secondN

t

NR

Decay CurveDecay Curve The decay curve The decay curve

follows the equationfollows the equation N = NN = Noo e e- - λtλt

The The half-lifehalf-life is also a is also a useful parameteruseful parameter The half-life is defined The half-life is defined

as the time it takes as the time it takes for half of any given for half of any given number of radioactive number of radioactive nuclei to decaynuclei to decay

693.02lnT 21

UnitsUnits The unit of activity, R, is the The unit of activity, R, is the Curie, Curie,

CiCi 1 Ci = 3.7 x 101 Ci = 3.7 x 101010 decays/second decays/second

The SI unit of activity is the The SI unit of activity is the Becquerel, BqBecquerel, Bq 1 Bq = 1 decay / second1 Bq = 1 decay / second

1 Ci = 3.7 x 101 Ci = 3.7 x 101010 Bq Bq The most commonly used units of The most commonly used units of

activity are the mCi and the µCiactivity are the mCi and the µCi

QUICK QUIZ 29.1What fraction of a radioactive sample has decayed after two half-lives have elapsed?

(a) 1/4 (b) 1/2 (c) 3/4 (d) not enough information to say

QUICK QUIZ 29.2

The activity of a newly discovered radioactive isotope reduces to 96% of its original value in an interval of 2 hours. What is its half-life?

(a) 10.2 h (b) 34.0 h(c) 44.0 h (d) 68.6 h

Alpha DecayAlpha Decay When a nucleus emits an alpha particle it When a nucleus emits an alpha particle it

loses two protons and two neutronsloses two protons and two neutrons N decreases by 2N decreases by 2 Z decreases by 2Z decreases by 2 A decreases by 4A decreases by 4

SymbolicallySymbolically X is called the X is called the parent nucleusparent nucleus Y is called the Y is called the daughter nucleusdaughter nucleus

When one element changes into another When one element changes into another element, the process is called element, the process is called spontaneous spontaneous decaydecay or or transmutationtransmutation

HeYX 42

4A2Z

AZ

Alpha Decay -- ExampleAlpha Decay -- Example Decay of Decay of 226226 Ra Ra

Half life for this Half life for this decay is 1600 yearsdecay is 1600 years

Excess mass is Excess mass is converted into converted into kinetic energykinetic energy

Momentum of the Momentum of the two particles is equal two particles is equal and oppositeand opposite

HeRnRa 42

22286

22688

QUICK QUIZ 29.3If a nucleus such as 226Ra that is initially at rest undergoes alpha decay, which of the following statements is true? (a) The alpha particle has more kinetic energy than the daughter nucleus. (b) The daughter nucleus has more kinetic energy than the alpha particle. (c) The daughter nucleus and the alpha particle have the same kinetic energy.

Beta DecayBeta Decay During beta decay, the daughter During beta decay, the daughter

nucleus has the same number of nucleus has the same number of nucleons as the parent, but the atomic nucleons as the parent, but the atomic number is one lessnumber is one less

SymbolicallySymbolically

eYX

eYXA1Z

AZ

A1Z

AZ

The emission of the electron is from the The emission of the electron is from the nucleusnucleus

The nucleus contains protons and neutronsThe nucleus contains protons and neutrons

The process occurs when a neutron is The process occurs when a neutron is transformed into a proton and an electrontransformed into a proton and an electron

Energy must be conservedEnergy must be conserved

Beta Decay – Electron Beta Decay – Electron EnergyEnergy

The energy released The energy released in the decay process in the decay process should almost all go should almost all go to kinetic energy of to kinetic energy of the electronthe electron

Experiments showed Experiments showed that few electrons that few electrons had this amount of had this amount of kinetic energykinetic energy

NeutrinoNeutrino To account for this “missing” energy, in To account for this “missing” energy, in

1930 Pauli proposed the existence of 1930 Pauli proposed the existence of another particleanother particle

Enrico Fermi later named this particle the Enrico Fermi later named this particle the neutrinoneutrino

Properties of the neutrinoProperties of the neutrino Zero electrical chargeZero electrical charge Mass much smaller than the electron, probably Mass much smaller than the electron, probably

not zeronot zero Spin of Spin of ½½ Very weak interaction with matterVery weak interaction with matter

Beta Decay – Completed Beta Decay – Completed

SymbolicallySymbolically

is the symbol for the neutrinois the symbol for the neutrino is the symbol for the antineutrinois the symbol for the antineutrino

To summarize, in beta decay, the following To summarize, in beta decay, the following pairs of particles are emittedpairs of particles are emitted An electron and an antineutrinoAn electron and an antineutrino A positron and a neutrinoA positron and a neutrino

eYX

eYXA1Z

AZ

A1Z

AZ

Gamma DecayGamma Decay Gamma rays are given off when an Gamma rays are given off when an

excited nucleus “falls” to a lower energy excited nucleus “falls” to a lower energy statestate Similar to the process of electron “jumps” to Similar to the process of electron “jumps” to

lower energy states and giving off photonslower energy states and giving off photons The excited nuclear states result from The excited nuclear states result from

“jumps” made by a proton or neutron“jumps” made by a proton or neutron The excited nuclear states may be the The excited nuclear states may be the

result of violent collision or more likely of result of violent collision or more likely of an alpha or beta emissionan alpha or beta emission

Gamma Decay – ExampleGamma Decay – Example

Example of a decay sequenceExample of a decay sequence The first decay is a beta emissionThe first decay is a beta emission The second step is a gamma emissionThe second step is a gamma emission

The C* indicates the Carbon nucleus is in an The C* indicates the Carbon nucleus is in an excited stateexcited state

Gamma emission doesn’t change either A or Gamma emission doesn’t change either A or ZZ

C*C

e*CB126

126

126

125

Uses of RadioactivityUses of Radioactivity

Carbon DatingCarbon Dating Beta decay of Beta decay of 1414C is used to date organic C is used to date organic

samplessamples The ratio of The ratio of 1414C to C to 1212C is usedC is used

Smoke detectorsSmoke detectors Ionization type smoke detectors use a Ionization type smoke detectors use a

radioactive source to ionize the air in a chamberradioactive source to ionize the air in a chamber A voltage and current are maintained A voltage and current are maintained When smoke enters the chamber, the current is When smoke enters the chamber, the current is

decreased and the alarm soundsdecreased and the alarm sounds

More Uses of RadioactivityMore Uses of Radioactivity

Radon pollutionRadon pollution Radon is an inert, gaseous element Radon is an inert, gaseous element

associated with the decay of radiumassociated with the decay of radium It is present in uranium mines and in It is present in uranium mines and in

certain types of rocks, bricks, etc that certain types of rocks, bricks, etc that may be used in home buildingmay be used in home building

May also come from the ground itselfMay also come from the ground itself

Natural RadioactivityNatural Radioactivity

Classification of nucleiClassification of nuclei Unstable nuclei found in natureUnstable nuclei found in nature

Give rise to Give rise to natural radioactivitynatural radioactivity Nuclei produced in the laboratory through Nuclei produced in the laboratory through

nuclear reactionsnuclear reactions Exhibit Exhibit artificial radioactivityartificial radioactivity

Three series of natural radioactivity existThree series of natural radioactivity exist UraniumUranium ActiniumActinium ThoriumThorium

Decay SeriesDecay Series of of 232232ThTh

Series starts Series starts with with 232232ThTh

Processes Processes through a through a series of series of alpha and alpha and beta decaysbeta decays

Ends with a Ends with a stable stable isotope of isotope of lead, lead, 208208PbPb

Nuclear ReactionsNuclear Reactions

Structure of nuclei can be changed by Structure of nuclei can be changed by bombarding them with energetic bombarding them with energetic particlesparticles The changes are called The changes are called nuclear reactionsnuclear reactions

As with nuclear decays, the atomic As with nuclear decays, the atomic numbers and mass numbers must numbers and mass numbers must balance on both sides of the equationbalance on both sides of the equation

QUICK QUIZ 29.4

Which of the following are possible reactions?

Q ValuesQ Values Energy must also be conserved in nuclear Energy must also be conserved in nuclear

reactionsreactions The energy required to balance a nuclear The energy required to balance a nuclear

reaction is called the reaction is called the Q valueQ value of the reaction of the reaction An An exothermic reactionexothermic reaction

There is a mass “loss” in the reactionThere is a mass “loss” in the reaction There is a release of energyThere is a release of energy Q is positiveQ is positive

An An endothermic reactionendothermic reaction There is a “gain” of mass in the reactionThere is a “gain” of mass in the reaction Energy is needed, in the form of kinetic energy of the Energy is needed, in the form of kinetic energy of the

incoming particlesincoming particles Q is negative Q is negative

Threshold EnergyThreshold Energy

To conserve both momentum and To conserve both momentum and energy, incoming particles must have a energy, incoming particles must have a minimum amount of kinetic energy, minimum amount of kinetic energy, called the called the threshold energythreshold energy

m is the mass of the incoming particlem is the mass of the incoming particle M is the mass of the target particleM is the mass of the target particle

If the energy is less than this amount, If the energy is less than this amount, the reaction cannot occurthe reaction cannot occur

QM

m1KEmin

QUICK QUIZ 29.5

If the Q value of an endothermic reaction is -2.17 MeV, the minimum kinetic energy needed in the reactant nuclei if the reaction is to occur must be (a) equal to 2.17 MeV, (b) greater than 2.17 MeV, (c) less than 2.17 MeV, or (d) precisely half of 2.17 MeV.

Radiation Damage in Radiation Damage in MatterMatter

Radiation absorbed by matter can cause Radiation absorbed by matter can cause damagedamage

The degree and type of damage depend The degree and type of damage depend on many factorson many factors Type and energy of the radiationType and energy of the radiation Properties of the absorbing matterProperties of the absorbing matter

Radiation damage in biological organisms Radiation damage in biological organisms is primarily due to ionization effects in is primarily due to ionization effects in cellscells Ionization disrupts the normal functioning of Ionization disrupts the normal functioning of

the cellthe cell

Types of DamageTypes of Damage

Somatic damageSomatic damage is radiation damage is radiation damage to any cells except reproductive onesto any cells except reproductive ones Can lead to cancer at high radiation Can lead to cancer at high radiation

levelslevels Can seriously alter the characteristics of Can seriously alter the characteristics of

specific organismsspecific organisms Genetic damageGenetic damage affects only affects only

reproductive cellsreproductive cells Can lead to defective offspringCan lead to defective offspring

Units of Radiation Units of Radiation ExposureExposure

RoentgenRoentgen [R] is defined as [R] is defined as That amount of ionizing radiation that That amount of ionizing radiation that

will produce 2.08 x 10will produce 2.08 x 1099 ion pairs in 1 ion pairs in 1 cmcm33 of air under standard conditions of air under standard conditions

That amount of radiation that deposits That amount of radiation that deposits 8.76 x 108.76 x 10-3-3 J of energy into 1 km J of energy into 1 km33 of air of air

RadRad ( (RRadiation adiation AAbsorbed bsorbed DDose)ose) That amount of radiation that deposits That amount of radiation that deposits

1010-2-2 J of energy into 1 kg of air J of energy into 1 kg of air

More UnitsMore Units

RBERBE ( (RRelative elative BBiological iological EEffectiveness)ffectiveness) The number of rad of x-radiation or gamma The number of rad of x-radiation or gamma

radiation that produces the same biological radiation that produces the same biological damage as 1 rad of the radiation being useddamage as 1 rad of the radiation being used

Accounts for type of particle which the rad Accounts for type of particle which the rad itself does notitself does not

RemRem ( (RRoentgen oentgen EEquivalent in quivalent in MMan)an) Defined as the product of the dose in rad Defined as the product of the dose in rad

and the RBE factorand the RBE factor Dose in rem = dos in rad X RBEDose in rem = dos in rad X RBE

Radiation LevelsRadiation Levels Natural sources – rocks and soil, cosmic raysNatural sources – rocks and soil, cosmic rays

Background radiationBackground radiation About 0.13 rem/yrAbout 0.13 rem/yr

Upper limit suggested by US governmentUpper limit suggested by US government 0.50 rem/yr0.50 rem/yr Excludes background and medical exposuresExcludes background and medical exposures

OccupationalOccupational 5 rem/yr for whole-body radiation5 rem/yr for whole-body radiation Certain body parts can withstand higher levelsCertain body parts can withstand higher levels Ingestion or inhalation is most dangerousIngestion or inhalation is most dangerous

Applications of RadiationApplications of Radiation

SterilizationSterilization Radiation has been used to sterilize Radiation has been used to sterilize

medical equipmentmedical equipment Used to destroy bacteria, worms and Used to destroy bacteria, worms and

insects in foodinsects in food Bone, cartilage, and skin used in Bone, cartilage, and skin used in

graphs is often irradiated before graphs is often irradiated before graftinggrafting

Applications of Radiation, Applications of Radiation, contcont

TracingTracing Radioactive particles can be used to Radioactive particles can be used to

trace chemicals participating in various trace chemicals participating in various reactionsreactions

Example, Example, 131131I to test thyroid actionI to test thyroid action

CAT scansCAT scans CComputed omputed AAxial xial TTomographyomography Produces pictures with greater clarity Produces pictures with greater clarity

and detail than traditional x-raysand detail than traditional x-rays

Applications of Radiation, Applications of Radiation, finalfinal

MRIMRI MMagnetic agnetic RResonance esonance IImagingmaging When a nucleus having a magnetic When a nucleus having a magnetic

moment is placed in an external moment is placed in an external magnetic field, its moment processes magnetic field, its moment processes about the magnetic field with a about the magnetic field with a frequency that is proportional to the frequency that is proportional to the fieldfield

Transitions between energy states Transitions between energy states can be detected electronicallycan be detected electronically

Radiation DetectorsRadiation Detectors A A Geiger counterGeiger counter is the most is the most

common form of device common form of device used to detect radiationused to detect radiation

It uses the ionization of a It uses the ionization of a medium as the detection medium as the detection processprocess

When a gamma ray or When a gamma ray or particle enters the thin particle enters the thin window, the gas is ionizedwindow, the gas is ionized

The released electrons The released electrons trigger a current pulsetrigger a current pulse

The current is detected and The current is detected and triggers a counter or triggers a counter or speakerspeaker

Detectors, 2Detectors, 2

Semiconductor Diode DetectorSemiconductor Diode Detector A reverse biased p-n junctionA reverse biased p-n junction As a particle passes through the junction, a As a particle passes through the junction, a

brief pulse of current is created and measuredbrief pulse of current is created and measured Scintillation counterScintillation counter

Uses a solid or liquid material whose atoms Uses a solid or liquid material whose atoms are easily excited by radiationare easily excited by radiation

The excited atoms emit visible radiation as The excited atoms emit visible radiation as they return to their ground statethey return to their ground state

With a With a photomultiplierphotomultiplier, the photons can be , the photons can be converted into an electrical signalconverted into an electrical signal

Detectors, 3Detectors, 3 Track detectorsTrack detectors

Various devices used to view the tracks or Various devices used to view the tracks or paths of charged particlespaths of charged particles

Photographic emulsionPhotographic emulsion– Simplest track detectorSimplest track detector– Charged particles ionize the emulsion layerCharged particles ionize the emulsion layer– When the emulsion is developed, the track becomes When the emulsion is developed, the track becomes

visiblevisible Cloud chamberCloud chamber

– Contains a gas cooled to just below its condensation Contains a gas cooled to just below its condensation levellevel

– The ions serve as centers for condensationThe ions serve as centers for condensation– Particles ionize the gas along their pathParticles ionize the gas along their path– Track can be viewed and photographedTrack can be viewed and photographed

Detectors, 4Detectors, 4 Track detectors, contTrack detectors, cont

Bubble ChamberBubble Chamber Contains a liquid near its boiling pointContains a liquid near its boiling point Ions produced by incoming particles leave tracks of Ions produced by incoming particles leave tracks of

bubblesbubbles The tracks can be photographedThe tracks can be photographed

Wire ChamberWire Chamber Contains thousands of closely spaced parallel wiresContains thousands of closely spaced parallel wires The wires collect electrons created by the passing The wires collect electrons created by the passing

ionizing particleionizing particle A second grid allows the position of the particle to be A second grid allows the position of the particle to be

determineddetermined Can provide electronic readout to a computerCan provide electronic readout to a computer