Presentation on theme: "Chapter 8. Radioactive isotopes and Their Applications"— Presentation transcript:
1 Chapter 8. Radioactive isotopes and Their Applications IntroductionProduction of RadioisotopesSome Commonly Used RadionuclidesTracer ApplicationsThickness GaugingRadioisotope DatingRadioisotope Applications in Space Exploration
3 4.1 Radioisotopes are ideal tracers(示踪） The use of some easily detected material to tag or label some bulk material allows the bulk material to be followed as it moves through some complex process.Fluorescent dyes, stable isotopes, radioisotopes …Why radioisotopes?
4 The amount of tagging material needed If a sample contains N atoms of the radionuclide, the observed count rate (CR) isε: detection efficiencyTo detect the presence of the radionuclide tag, this count rate must be greater than some minimum count rate CRmin which is above the background count rate.Then the minimum number of radioactive atoms in the sample needed to detect thepresence of the radionuclide isIf the atomic weight of the radionuclide is A, the minimum mass of radionuclidesin the sample is
5 A typically gamma-ray detector efficiency is ε ~ 0 A typically gamma-ray detector efficiency is ε ~ 0.1 and a minimum count rate is CRmin ~ 30 min-1 = 0.5 s-1 Thus, for 14C (T1/2 = 5730 y = 1.18 x 1011 s), the minimum detectable mass of 14C in a sample is:How about 32P (T1/2 = d) ?P is often used in plant studies to follow the uptake of phosphorus by plants.few atoms are needed!
6 4.2 Medical ApplicationsRadioisotopes with short half-lives are used in nuclear medicine becausethey have the same chemistry in the body as the nonradioactive atoms.in the organs of the body, they give off radiation that exposes a photographic plate (scan) giving an image of an organ.Thyroid scan
7 4.3 Leak DetectionThis use of radionuclide tracers to find leaks or flow paths has wide applications:finding the location of leaks in oil-well casings,determining the tightness of abandoned slate quarries for the temporary storage of oil,Locating the positions of freon leaks in refrigeration coils,finding leaks in heat exchanger piping,locating leaks in engine seals.To find the location of a leak in a shallowly buriedpipe without excavation
8 Tracer will be added to the liquid in the pipe Underground pipe leaksTracer will be added to the liquid in the pipeDetector is moved along the pipeThe count rate will increase as there is large amount of waterThe radioactive source will be a short half-life γ emitter
9 4.4 Other applications Pipeline Interfaces Flow Patterns Flow Rate MeasurementsSurface Temperature MeasurementsOil from different producers is often carried in the same pipeline.measuring the spatial distribution of the activity concentration(1) ocean current movements, (2) atmospheric dispersion of airborne pollutants, (3) flow of glass lubricants in the hot extrusion of stainless steel, (4) dispersion of sand along beaches, (5) mixing of pollutant discharges into receiving bodies or water, and (6) gas flow through a complex filtration system.measurements of the activity concentrations of a radioactive tag in the fluid mediumThe time required for the radionuclide (and the flowing material) to travel to a downstream location is given by the time for the activity to reach a maximum at the downstream location.kryptonated surfacekrypton atoms are only released at certain high temperatures. remaining 85Kr
11 Thickness control The manufacture of aluminium foil β emitter is placed above the foil and a detector below itSome β particle will penetrate the foil and the amount of radiation is monitored by the computerThe computer will send a signal to the roller to make the gap smaller or bigger based on the count rate
12 Chapter 8. Radioactive isotopes and Their Applications IntroductionProduction of RadioisotopesSome Commonly Used RadionuclidesTracer ApplicationsThickness gaugingRadioisotope DatingRadioisotope Applications in Space Exploration
13 6.1 Radiocarbon dating principles By observing how much of a long-lived naturally occurring radionuclide in a samplehas decayed, it is possible to infer the age of the sample.Carbon datingCarbon has 3 isotopes:12C – stable13C – stable12C:13C = : 1.1114C – radioactiveAbundance:
14 t ½ = 5730 yr. Radiocarbon Forms: in the upper atmosphere Decays: Living Tissue 14C/12C, Tissue ratio same as atmospheric ratioDead Tissue 14C/12C< 14C/12Ctissue atmosphere
15 Clock starts when one dies CalculatedMeasured???ConstantClock starts when one dies
16 This ratio also decays with the same radioactive decay N ( t ) = N(0)exp(-λt)we never know N(0).the initial ratio N(0)/NS of the radionuclide and some stable isotope of the same element can be estimated with reliabilityThis ratio also decays with the same radioactive decaylaw as the radionuclideIt is usually easier to measurethe specific activity of 14C in a sample, i.e., A14 per gram of carbon
17 As a consequence, 14C is no longer in equilibrium with its atmospheric production rate. However, before humans upset this ancient equilibrium, the ratio of 14C to all carbon atoms in the environment was about , a value that has remained constant for the last several tens of thousand years. It is usually easier to measurethe specific activity of 14C in a sample, i.e., A14 per gram of carbon. This specific activity is proportional to the N14/NC ratio,
18 What is the age of an archaeological sample of charcoal from an ancient fire that has a A14(t)/g(C) ratio of 1.2 pCi/g of carbon?
19 Radiocarbon Measurements and Reporting Radiocarbon dates are determined by measuring the ratio of 14C to 12C in a sample, relative to a standard, usually in an accelerator mass spectrometer.standard = oxalic acid that represents activity of 1890 wood14C ages are reported as “14C years BP”, where BP is 1950
20 First 14C date: wood from tomb of Zoser (Djoser), 3rd Dynasty Egyptian king (July 12, 1948). Historic age: 4650±75 BPRadiocarbon age:3979±350 BPSecond 14C date: wood from Hellenistic coffinHistoric age: 2300±200 BPRadiocarbon age: (C-?) Modern! Fake!First “Curve of Knowns”:6 data points (using seven samples) spanning AD 600 to 2700 BC.Half life used: 5720± 47 yearsCarbon-14 dating lends itself to age determination of carbon-containing objects that are between 1,000 and 40,000 years old
21 The Shroud of TurinReputed as the burial cloth of Jesus Christ. C-14 dating by 3 independent labs report the Cloth originated during the Medieval times, between A.DCredit: The Image Works
22 Mummified remains found frozen in the Italian Alps In 1991,hikers discovered the body of a prehistoric hunter that had been entombed in glacial ice until the ice recently moved and melted.pathologists also examined his well-preserved remains, he died from a fatal wound in the back—most likely delivered during his prolonged struggle with at least two other prehistoric hunters.At least 5000 years oldBy carbon-14 dating
23 The Radiocarbon Calibration Curve (atmospheric 14C history) Principle: compare radiocarbon dates with independent datesExamples of independent dating: tree-ring counting, coral dates, varve counting,correlation of climate signals in varves with ice coredata from:corals (bright red)lake varves (green)marine varves (blue)speleothems (orange)tree rings (black)Observation:radiocarbon datesare consistently younger than calendar agesequilinetimeHughen et al., 2004
24 Source of Error in 14C dating Variations in geomagnetic flux. Geomagnetic field strength partly controls 14C production in the atmosphere because of attenuation affects on the cosmic flux with increasing magnetic field strength.Modulation of the cosmic-ray flux by increased solar activity (e.g., solar flares) leads to attenuation of the cosmic-ray flux.Influence of the ocean reservoir. Any change in exchange rate between ocean reservoir and atmospheric reservoir will affect the level of 14C in the atmosphere.Industrial revolution (ratio of 14C to stable carbon decreased because of burning fossil fuels) and bomb effects (14C to stable carbon increased because of increased neutron production from detonation of nuclear bombs in the atmosphere) have made modern organic samples unsuitable for as reference samples.
25 Radioactive elementsNot all elements are radioactive. Those are the most useful for geologic dating are:U-238 Half-life = 4.5 By the age of the earthK-40 Half-life = 1.25 By rocksAlso, Sm-147, Rb 87, Th-232, U-235
26 Blocking temperatures for some common minerals and decay series. The blocking temperature is the temperature above which a mineral or rock no longer behaves as a closed system and the parent/daughter ratios may be altered from that due to pure radioactive disintegration.This can result in resetting the isotopic clock and/or give what are called discordant dates.These types of problems have given opponents of the radiometric dating of the Earth ammunition to attack the 4.5 By age geologists cite.
27 Fission tracks in an apatite crystal. Fig. 5.9Fission tracks in an apatite crystal.They are produced when an atom of U-238 disintegrates emitting an alpha particle, a Helium nucleus (He-4). This massive atomic particle causes massive structural damage in the crystal that can be revealed by etching.The number of tracks in a given area is proportional to the age of the mineral.(Why not just use the U-238 to Pb-206 method directly in such cases?)
28 7. Radioisotope Applications in Space Exploration Radioisotope Thermoelectric Generator (RTG)if two dissimilar metals were joined at two locations that were maintained at different temperatures, an electric current would flow in a loopIn an RTG, the decay of a radioisotope fuel provides heat to the “hot” junction, while the other junction uses radiation heat transfer to outer space to maintain itself as the “cold” junctionhigh degree of reliability
29 an RTG loaded with 1 kilogram of plutonium (238) dioxide fuel would generate between 21 and 29 watts of electric power for the spacecraft. After five years of travel through space, this plutonium-fueled RTG would still have approximately 96 percent of its original thermal power level available for the generation to electric power
34 Chapter 8. Radioactive isotopes and Their Applications IntroductionProduction of RadioisotopesSome Commonly Used RadionuclidesTracer ApplicationsThickness GaugingRadioisotope DatingRadioisotope Applications in Space Exploration