Radioactivity: Principles and Radioactivity: Principles and Applications (11 October 2007) Applications (11 October 2007) Background Background Radioactivity and natural background exposure Radioactivity and natural background exposure Quantitative analysis of radioactivity Quantitative analysis of radioactivity Principles of radioactivity and human health Principles of radioactivity and human health Application of the principles of radioactivity Application of the principles of radioactivity
Transcript
Radioactivity: Principles and Radioactivity: Principles and Applications (11 October 2007)Applications (11 October 2007)
BackgroundBackgroundRadioactivity and natural background exposureRadioactivity and natural background exposureQuantitative analysis of radioactivityQuantitative analysis of radioactivityPrinciples of radioactivity and human healthPrinciples of radioactivity and human healthApplication of the principles of radioactivityApplication of the principles of radioactivity
IsotopesIsotopes
Atoms have specific number of protons, neutrons and Atoms have specific number of protons, neutrons and electronselectrons
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OO1616
If the number of protons is unchanged but the number If the number of protons is unchanged but the number of of neutronsneutrons goes up or down, what happens to the goes up or down, what happens to the properties of that element?properties of that element?
88
OO1717
Isotopes ContinuedIsotopes Continued
AtomsAtoms of an element with same number of of an element with same number of protonsprotons but different number of but different number of neutronsneutrons are are isotopesisotopesKey is the number of neutronsKey is the number of neutronsConventional notation (Conventional notation (AA
ZZX)X)Not all elements have isotopes, but many do:Not all elements have isotopes, but many do:
1 1 H or tritium)H or tritium)Sulfur (common isotope: Sulfur (common isotope: 1616
3232S; S; 33331616SS
Isotopes of HydrogenIsotopes of Hydrogen
Hydrogen11H
Stable
Deuterium21H
Stable
Tritium31H
Unstable
Proton Neutron
Stable Versus Unstable IsotopesStable Versus Unstable Isotopes
Stable over time Stable over time 1212
66CC1313
66CC
Unstable: “transmutated” by releasing mass Unstable: “transmutated” by releasing mass and/or energy (i.e., radioactive)and/or energy (i.e., radioactive)
141466C C
3311HH
Discovery of RadioactivityDiscovery of Radioactivity
Rutherford (as in the nucleus) and three forms Rutherford (as in the nucleus) and three forms of “transmutated” activity (“radioactivity”)of “transmutated” activity (“radioactivity”)
Alpha Alpha (α):(α): nucleus of the helium atom (nucleus of the helium atom (4422He)He)
Beta (Beta (ββ): high energy electron): high energy electronGamma (Gamma (γγ): electromagnetic radiation with very ): electromagnetic radiation with very short wavelengths short wavelengths
Early ObservationsEarly Observations
Atom exhibits “Atom exhibits “spontaneousspontaneous” release of” release ofmass (mass (αα or or ββ) ) energy (energy (γγ))
Nucleus changes identity to simpler atomic Nucleus changes identity to simpler atomic structurestructureNatural processNatural processSpontaneous release of mass or energy is called Spontaneous release of mass or energy is called radioactive decayradioactive decay
SourcesSourcesCosmic rays from outer spaceCosmic rays from outer spaceSoilsSoilsWaterWaterBuilding materialsBuilding materialsNuclear sourcesNuclear sources
ExamplesExamplesRadon gas (Radon gas (222222Ra) Ra) -- soilssoilsBeryllium (Beryllium (77Be) Be) -- atmosphereatmosphere
Radioactive DecayRadioactive Decay
Uranium Uranium –– 238238238238
9292U (92 protons; 238U (92 protons; 238--92 (146) neutrons92 (146) neutronsSpontaneous release of an alpha (Spontaneous release of an alpha (αα) subatomic ) subatomic particle (helium nucleus or particle (helium nucleus or 44
22He) results in an He) results in an atom with 90 protons and mass of 234atom with 90 protons and mass of 234
2342349090? or _____ (periodic table)? or _____ (periodic table)
All isotopes of all elements with > 83 protons All isotopes of all elements with > 83 protons (Bismuth) are unstable and radioactively decay(Bismuth) are unstable and radioactively decay
Radioactive DecayRadioactive Decay
Types of DecayTypes of Decay
Alpha (Alpha (αα))4422HeHe
Travel distance: stopped by sheet of paper (even air)Travel distance: stopped by sheet of paper (even air)
Beta (Beta (ββ))high energy electronhigh energy electronTravel distance: ~10 meters; 1 cm aluminum blockTravel distance: ~10 meters; 1 cm aluminum block
Gamma (Gamma (γγ))high energy radiationhigh energy radiationTravel distance: 100’s meters; 5 cm lead brickTravel distance: 100’s meters; 5 cm lead brick
Penetration of RadiationPenetration of Radiation
Radioactivity: Principles and Radioactivity: Principles and ApplicationsApplications
BackgroundBackgroundRadioactivity is naturalRadioactivity is naturalQuantitative analysis of radioactivityQuantitative analysis of radioactivityPrinciples of radioactivity and human healthPrinciples of radioactivity and human healthApplication of the principles of radioactivityApplication of the principles of radioactivity
Radioactive DecayRadioactive Decay
Radioactive DecayRadioactive Decay
Rate of decay to a stable state (no more Rate of decay to a stable state (no more spontaneous decay) is specific for each isotopespontaneous decay) is specific for each isotopeRate has unique terminology called Rate has unique terminology called halfhalf--lifelifeTime for ½ (50%) of the nuclei to decay to the Time for ½ (50%) of the nuclei to decay to the stable state (abbreviated tstable state (abbreviated t1/21/2))Example: M&M’sExample: M&M’s
Radioactive DecayRadioactive Decay
The discovery of The discovery of destroyed Kelvin’s argument for the age of Earthdestroyed Kelvin’s argument for the age of Earthprovided a clock to measure Earth’s ageprovided a clock to measure Earth’s age
RadioactivityRadioactivity is the spontaneous decay is the spontaneous decay of an atom’s nucleus to a more stable formof an atom’s nucleus to a more stable form
The heat from radioactivityThe heat from radioactivityhelps explain why the Earth is still warm insidehelps explain why the Earth is still warm inside
Radioactivity provides geologists Radioactivity provides geologists with a powerful tool to measure with a powerful tool to measure absolute agesabsolute ages of of rocks and past geologic eventsrocks and past geologic events
AbsoluteAbsolute--Dating MethodsDating Methods
Understanding absolute dating requires knowledge Understanding absolute dating requires knowledge of of atomsatoms and and isotopesisotopesAll matterAll matter is made up of atomsis made up of atomsThe The nucleusnucleus of an atom is composed of of an atom is composed of
protonsprotons –– particles with a positive electrical particles with a positive electrical chargechargeneutronsneutrons –– electrically neutral particleselectrically neutral particles
with with electronselectrons –– negatively charged particles negatively charged particles ––outside the nucleusoutside the nucleusThe number of protonsThe number of protons ((atomic numberatomic number) )
helps determine the atom’s chemical properties helps determine the atom’s chemical properties and the element to which it belongsand the element to which it belongs
Atoms: A ReviewAtoms: A Review
Atoms: A ReviewAtoms: A Review
Atomic mass numberAtomic mass number= number of protons + number of neutrons= number of protons + number of neutronsThe different forms of an element’s atoms with The different forms of an element’s atoms with varying numbers of neutrons are called varying numbers of neutrons are called isotopes isotopes Different isotopes of the same element have Different isotopes of the same element have different atomic mass numbersdifferent atomic mass numbers
butbut behave the same chemicallybehave the same chemicallyMost isotopes are but some are Most isotopes are but some are Geologists use decay rates of Geologists use decay rates of unstableunstable isotopes isotopes to determine absolute ages of rocksto determine absolute ages of rocks
Isotopes: A ReviewIsotopes: A Review
Let’s try another one….Let’s try another one….
Atom 1 Atom 2 Atom 3Atom 1 Atom 2 Atom 3Number of protons 6 6 7Number of protons 6 6 7Number of neutrons 6 8 7Number of neutrons 6 8 7Atomic numberAtomic number _ _ __ _ _Atomic massAtomic mass _ _ __ _ _
Radioactive decayRadioactive decay is the process whereby is the process whereby an unstable atomic nucleus spontaneously transforms an unstable atomic nucleus spontaneously transforms into an atomic nucleus of a different elementinto an atomic nucleus of a different element
Three types of radioactive decay:Three types of radioactive decay:In In alpha decayalpha decay, two protons and two neutrons , two protons and two neutrons (alpha particle) are emitted from the nucleus.(alpha particle) are emitted from the nucleus.
Radioactive DecayRadioactive Decay
In In beta decaybeta decay, a neutron emits a fast moving electron , a neutron emits a fast moving electron (beta particle) and becomes a proton.(beta particle) and becomes a proton.
In In electron capture decayelectron capture decay, a proton captures an , a proton captures an electron and converts to a neutron.electron and converts to a neutron.
Radioactive DecayRadioactive Decay
Some isotopes undergo only one Some isotopes undergo only one decay step before they become decay step before they become stable.stable.
Examples:Examples:rubidium 87 decays to strontium 87 by rubidium 87 decays to strontium 87 by a single beta emissiona single beta emissionpotassium 40 decays to argon 40 by a potassium 40 decays to argon 40 by a single electron capturesingle electron capture
Radioactive DecayRadioactive Decay
But other isotopes undergo several decay stepsExamples:
uranium 235 decays to lead 207 by 7 alpha steps and 6 beta stepsuranium 238 decays to lead 206 by 8 alpha steps and 6 beta steps
Uranium 238 decayUranium 238 decay
The The halfhalf--lifelife of a radioactive isotope of a radioactive isotope is the time it takes for one half of the atoms of the is the time it takes for one half of the atoms of the original unstable original unstable parent isotopeparent isotope
to decay to atoms of a new more stable to decay to atoms of a new more stable daughter daughter isotopeisotope
The halfThe half--life of a specific radioactive isotope life of a specific radioactive isotope is constant and can be precisely measuredis constant and can be precisely measured
HalfHalf--LivesLives
The length of halfThe length of half--lives for different isotopes lives for different isotopes of different elements of different elements can vary from can vary from less than one billionth of a second less than one billionth of a second to 49 billion years!to 49 billion years!
Radioactive decay Radioactive decay is geometric, NOT linear, is geometric, NOT linear, and produces a curved graphand produces a curved graph
HalfHalf--LivesLives
In this example In this example of uniform of uniform linear change, linear change, water is water is dripping into a dripping into a glass glass at a constant at a constant raterate
Uniform Linear ChangeUniform Linear Change
In radioactive In radioactive decay, decay, during each equal during each equal time unit time unit
halfhalf--life life the proportion the proportion of parent atoms of parent atoms decreases by 1/2decreases by 1/2
By measuring the parent/daughter ratio By measuring the parent/daughter ratio and knowing the halfand knowing the half--life of the parentlife of the parent
which has been determined in the laboratorywhich has been determined in the laboratory
geologists can calculate the age of a sample geologists can calculate the age of a sample containing the radioactive elementcontaining the radioactive element
The parent/daughter ratio The parent/daughter ratio is usually determined by a mass spectrometeris usually determined by a mass spectrometer
an instrument that measures the proportions an instrument that measures the proportions of atoms with different massesof atoms with different masses
Determining AgeDetermining Age
Example: Example: If a rock has a parent/daughter ratio of 1:3If a rock has a parent/daughter ratio of 1:3or a ratio of (parent)/(parent + daughter) = 1:4 or a ratio of (parent)/(parent + daughter) = 1:4 or 25%, or 25%, and the halfand the half--live is 57 million years, live is 57 million years,
how old is the rock?how old is the rock?
25% means it is 2 half25% means it is 2 half--lives lives old.old.
Determining AgeDetermining Age
the rock is 57my x 2 =114 the rock is 57my x 2 =114 million years old.million years old.
Let’s try another one…..Let’s try another one…..
A radioactive A radioactive isotope has a isotope has a half life of 4 half life of 4 million million years. years. How How old is a old is a specimen in specimen in which 6.25% which 6.25% of the parent of the parent isotope isotope remains?remains?
Because they are the right sizeBecause they are the right sizesome radioactive parents some radioactive parents are included in the crystal are included in the crystal structure of cooling mineralsstructure of cooling minerals
What Materials Can Be Dated?What Materials Can Be Dated?
Most radiometric dates are obtained from igneous rocks. Why?
As magma cools and crystallizes, radioactive parent atoms separate from previously formed daughter atoms
What Materials Can Be Dated?What Materials Can Be Dated?The daughter atoms are different elements
with different sizes
and, therefore, do not generally fit
into the same minerals as the parents
Geologists can use the crystals containing
the parent atoms
to date the time of crystallization
The daughter atoms are different The daughter atoms are different elements elements
with different sizes with different sizes
and, therefore, do not generally fit and, therefore, do not generally fit
into the same minerals as the parentsinto the same minerals as the parents
Geologists can use the crystals Geologists can use the crystals containing containing
the parent atoms the parent atoms
to date the to date the time of crystallizationtime of crystallization
CrystallizationCrystallization of magma separates parent atoms of magma separates parent atoms from previously formed daughtersfrom previously formed daughters
This resets the radiometric clock to zero.This resets the radiometric clock to zero.Then the parents gradually decay.Then the parents gradually decay.
Igneous Crystallization Igneous Crystallization
Generally, sedimentary rocks can NOT be Generally, sedimentary rocks can NOT be radiometrically datedradiometrically dated
The date obtained would correspond to the time of The date obtained would correspond to the time of crystallization of the mineral when it formed in an igneous or crystallization of the mineral when it formed in an igneous or metamorphic rock,metamorphic rock,NOT the time that it was deposited as a sedimentary particleNOT the time that it was deposited as a sedimentary particle
Exception: The mineral Exception: The mineral glauconiteglauconite can be datedcan be datedbecause it forms in certain marine environments as a reaction because it forms in certain marine environments as a reaction with clay mineralswith clay mineralsduring the formation of the sedimentary rockduring the formation of the sedimentary rock
Sedimentary RocksSedimentary Rocks
In glauconite, potassium 40 decays to argon 40In glauconite, potassium 40 decays to argon 40Because argon is a gas, Because argon is a gas, it can easily escape from a mineralit can easily escape from a mineral
A closed system is needed for an accurate date!A closed system is needed for an accurate date!Neither parent nor daughter atoms Neither parent nor daughter atoms can have been added or removed can have been added or removed from the sample since crystallizationfrom the sample since crystallization
If leakage of daughters has occurred, If leakage of daughters has occurred, this partially resets the radiometric clock this partially resets the radiometric clock and the age of the rock will show to be too youngand the age of the rock will show to be too young
If parents escape, the date obtained will be too old.If parents escape, the date obtained will be too old.The most reliable dates use multiple methods.The most reliable dates use multiple methods.
Sources of UncertaintySources of Uncertainty
During metamorphism, some During metamorphism, some of the daughter or parent of the daughter or parent atoms may escapeatoms may escape
leading to a date that is leading to a date that is inaccurate.inaccurate.However, if all of the daughters However, if all of the daughters are forced out during are forced out during metamorphism, metamorphism, then the date obtained would then the date obtained would be the time of be the time of metamorphismmetamorphism——a useful piece a useful piece of information.of information.
Sources of UncertaintySources of Uncertainty
a. a. A mineral has just A mineral has just crystallized from magma.crystallized from magma.
Dating MetamorphismDating Metamorphism
b. As time passes, parent atoms b. As time passes, parent atoms decay to daughters.decay to daughters.
c. Metamorphism drives the c. Metamorphism drives the daughters out of the daughters out of the mineral as it recrystallizes.mineral as it recrystallizes.
d. Dating the mineral today d. Dating the mineral today yields a date of 350 million yields a date of 350 million years = time of years = time of metamorphism, provided the metamorphism, provided the system remains closed system remains closed during that time.during that time.
Dating the whole rock Dating the whole rock yields a date of 700 yields a date of 700 million years = time of million years = time of crystallization.crystallization.
The isotopes used in radiometric dating The isotopes used in radiometric dating need to be sufficiently longneed to be sufficiently long--lived lived so the amount of parent material left is measurableso the amount of parent material left is measurable
Such isotopes include:Such isotopes include:ParentsParents DaughtersDaughters HalfHalf--Life (years)Life (years)
LongLong--Lived Radioactive Lived Radioactive Isotope Pairs Used in DatingIsotope Pairs Used in Dating
Most of these Most of these are useful for are useful for dating older dating older rocksrocks
Atomic particles in uranium Atomic particles in uranium will damage crystal structure as uranium decayswill damage crystal structure as uranium decays
The damage can be seen as fission tracks The damage can be seen as fission tracks under a microscope after etching the mineralunder a microscope after etching the mineral
The age of the The age of the sample is related to sample is related to
the number of the number of fission tracks fission tracks and the amount and the amount of uraniumof uraniumwith older with older samples having samples having more tracksmore tracks
This method is This method is useful for samples useful for samples between 40,000 between 40,000 years and 1.5 years and 1.5 million years oldmillion years old
Fission Track DatingFission Track Dating
Carbon is found in all forms of lifeCarbon is found in all forms of lifeIt has 3 isotopes It has 3 isotopes
carbon 12 and 13 are stable, but carbon 14 is notcarbon 12 and 13 are stable, but carbon 14 is notCarbon 14 has a halfCarbon 14 has a half--life of 5730 yearslife of 5730 yearsCarbon 14 dating uses the carbon 14/carbon 12 ratio Carbon 14 dating uses the carbon 14/carbon 12 ratio
of material that was once livingof material that was once living
The short halfThe short half--life of carbon 14 life of carbon 14 makes it suitable for dating material makes it suitable for dating material < 70,000 years old< 70,000 years old
It is not useful for most rocks, It is not useful for most rocks, but is useful for archaeology but is useful for archaeology and young geologic materialsand young geologic materials
Radiocarbon Dating MethodRadiocarbon Dating Method
Carbon 14 is constantly forming Carbon 14 is constantly forming in the upper atmosphere in the upper atmosphere
When cosmic raysWhen cosmic raysstrike atoms of upper atmospheric strike atoms of upper atmospheric gases,gases,Splitting nuclei into protons and Splitting nuclei into protons and neutronsneutronsWhen a neutron strikes a nitrogen When a neutron strikes a nitrogen 14 atom14 atomit may be absorbed it may be absorbed by the nucleus and eject a protonby the nucleus and eject a protonchanging it to carbon 14changing it to carbon 14
Carbon 14Carbon 14
The carbon 14 becomes The carbon 14 becomes part of the natural carbon cycle part of the natural carbon cycle and becomes incorporated into and becomes incorporated into organismsorganisms
While the organism lives While the organism lives it continues to take in carbon 14,it continues to take in carbon 14,but when it dies but when it dies the carbon 14 begins to decaythe carbon 14 begins to decaywithout being replenishedwithout being replenished
Thus, carbon 14 dating Thus, carbon 14 dating measures the time of deathmeasures the time of death
Carbon 14Carbon 14
Right now, in our atmosphere, there are countless numbers of nitrogen atoms floating high above the Earth's surface. These atoms are, in a sense, vulnerable.
Cosmic radiation, in the form of neutrons, zips through the atmosphere at a high rate of speed. Occasionally, and purely by chance, some of these neutrons collide with the nuclei of some of the nitrogen atoms.
The nucleus of each nitrogen atom contains seven protons and seven neutrons. That is, it does until a collision happens. The incoming neutron hitting the nucleus causes a proton to shoot out of the nucleus, just as a cue ball on a pool table, hitting one of two balls that are touching, might cause the ball that it hits to stay in place and the other ball to shoot off.
The nitrogen atom now has six protons and eight neutrons. This means two things. One, with this arrangement of protons and neutrons, it's unstable. In other words, it's radioactive. And two, it's no longer nitrogen. The reason is that the number of protons an atom contains determines what that atom is. Since it now contains six protons, it's carbon. Carbon usually has six neutrons as well—in this form it's called carbon-12 (6+6=12). The unstable, eight-neutron version, however, is carbon-14 (6+8=14).
All radioactive atoms will eventually decay, or change, in someway. When a carbon-14 atom decays, one of its eight neutrons turns into a proton, emitting an electron (with a charge of -1) in the process. The atom is now stable. And with seven protons and seven neutrons, it is again nitrogen-14. From PBS website
The carbon-14 created by cosmic radiation makes up only a fraction of the carbon in our atmosphere. But it is there, and just like carbon-12, it can be taken in by a growing plant and become a part of that plant. (As you may know, plants take in carbon dioxide, or CO2, separate the carbon from the oxygen, then release the oxygen back into the atmosphere.)
So every plant contains a certain percentage of carbon-14. And so do those things that eat plants. And so do those things that eat the things that eat plants.
The percentage of carbon-14 in all of these living things is the same as the percentage of carbon-14 in the atmosphere. At least it's the same while they're living. When a plant or animal dies, no carbon (in any form) can enter its system to become a part of it.
The carbon-14 within every once-living thing will someday turn back into nitrogen-14. If we knew the amount of carbon-14 a once-living thing had while it was alive and the rate at which it changed (i.e., how fast it changed) back into nitrogen, then we could figure out how long ago it lived.
Well, it turns out that we do know. The amount of carbon-14 in the atmosphere (and thereforein living things) has not changed all that much over time. And we do know the rate at which carbon-14 changes back to nitrogen-14, thoughwhat this rate is is not as straightforward as it could be.
From PBS website
Measurement of RadiationMeasurement of Radiation
Number of nuclear disintegrations per time Number of nuclear disintegrations per time called a called a curiecurie (Ci); 3.70 x 10(Ci); 3.70 x 101010 nuclear nuclear disintegrations seconddisintegrations second--11
Radiation at the Radiation at the site of absorptionsite of absorption (living tissues)(living tissues)Radiological dose in units called Radiological dose in units called remrem
Natural dose = 0.001 rem (1 Natural dose = 0.001 rem (1 milliremmillirem)/day)/dayLethal dose = 500 remLethal dose = 500 rem
Radioactivity: Human HealthRadioactivity: Human Health
Radioactivity in biological tissues results in Radioactivity in biological tissues results in atoms being ionizedatoms being ionized
Disrupts bondsDisrupts bondsDNA as primary site of actionDNA as primary site of actionFragments molecules and disrupts biochemistryFragments molecules and disrupts biochemistry
Sensitivity is greatest for actively growing cells Sensitivity is greatest for actively growing cells Blood (StrontiumBlood (Strontium--90)90)Bone marrow (CesiumBone marrow (Cesium--137)137)Thyroid (IodineThyroid (Iodine--131)131)
Application of Atomic Principles of Application of Atomic Principles of RadioactivityRadioactivity
Radiation medicine and radiopharmaceuticalsRadiation medicine and radiopharmaceuticalsNuclear energy (text book)Nuclear energy (text book)Yucca Mountain, NevadaYucca Mountain, NevadaChernobylChernobylDirty bombDirty bomb
Chernobyl Reactor and Chernobyl Reactor and SacophagusSacophagus
A high speed electron emitted from a nucleus A high speed electron emitted from a nucleus during radioactive decay is called a (an) _____.during radioactive decay is called a (an) _____.
A.A. AlphaAlphaB.B. BetaBetaC.C. GammaGammaD.D. All of the aboveAll of the above
A sheet of paper will stop a (an) ____.A sheet of paper will stop a (an) ____.
A.A. AlphaAlphaB.B. BetaBetaC.C. GammaGammaD.D. All of the aboveAll of the above
An isotope’s half life (tAn isotope’s half life (t1/21/2) is affected by ) is affected by changes in ______.changes in ______.
A.A. TemperatureTemperatureB.B. Sample sizeSample sizeC.C. Other radioactive materials nearbyOther radioactive materials nearbyD.D. # of M&M’s# of M&M’sE.E. None of the aboveNone of the above
iClickeriClicker QuestionQuestion
What is meant by background radiation? What is meant by background radiation? AA the radiation in your backyard.the radiation in your backyard.BB The radiation all around you, from all The radiation all around you, from all
directions and sources.directions and sources.CC The radiation from the ground, coming The radiation from the ground, coming
back up to you.back up to you.DD None of the AboveNone of the Above
iClickeriClicker QuestionQuestion
Is the dose of background radiation Is the dose of background radiation the same over the Earth’s surface?the same over the Earth’s surface?AA YesYesBB NoNo
iClickeriClicker QuestionQuestion
For you as an individual, what are the most common For you as an individual, what are the most common sources of natural/background radiation?sources of natural/background radiation?AA Cosmic rays from outer spaceCosmic rays from outer spaceBB SoilsSoilsCC WaterWaterDD Building materialsBuilding materialsEE All of the aboveAll of the above
