RADIATION, ROCKS AND THE WORLD AROUND US.

P. BENHAM, 2003 © for the Yoho-Burgess Shale Foundation

RADIATION ARE YOU EXPOSED?

Only 19% of one’s annual exposure to radiation is man made. The majority comes from colourless and odourless radon gas a decay product of uranium in soil or rock. Radon gas enters homes through cracks in concrete walls and floors, floor drains, and sumps. When radon becomes trapped in buildings and concentrations build up indoors, exposure to radon becomes a concern.

“NATURAL” RADIATION

Numerous hot springs through the Rockies have low levels of radiation. These include Miette, Canyon, Ainsworth and Radium Hot Springs (which in its early days was promoted for its healing radioactive properties!). Rainwater seeps through surface fractures to depths of several kilometres Here it gets heated and picks up dissolved minerals.

Hundred to thousands of years later it cycles back to the surface as hot springs

THE ATOM and RADIATION

In 1899, Ernest Rutherford discovered that uranium compounds produce three different kinds of radiation. He separated the radiations according to their penetrating abilities and named them alpha, beta and gamma radiation, after the first three letters of the Greek alphabet. The alpha radiation can be stopped by a sheet of paper. Rutherford later showed that an alpha particle is the nucleus of a He atom, 4 He. Beta particles were later identified as high speed electrons. Six millimeters of aluminum are needed to stop most beta particles. Several millimeters of lead are needed to stop gamma rays , which proved to be high energy photons. Alpha particles and gamma rays are emitted with a specific energy that depends on the radioactive isotope. Beta particles, however, are emitted with a continuous range of energies from zero up to the maximum allowed for by the particular isotope.

Text from the ABC’s of Nuclear Science http://www.lbl.gov/abc/Basic.html#Nuclearstructure

THE ATOM and RADIATION

ALPHA DECAY

: Emission of 4 He nucleus containing protons and neutrons results in change in nuclear charge and transformation of one element to another. Illustrated is decay of seaborgium, 263 Sg to Rutherfordium 259 Rf. Atomic number changes from 106 to 104.

BETA DECAY

: Beta particles are negatively charged electrons emitted by the nucleus. Beta decay occurs when a neutron is changed into a proton within the nucleus, increasing the atomic number. In diagram, the isotope 14 C is unstable and emits a beta particle, becoming the stable isotope 14 N.

Text and images adapted from the ABC’s of Nuclear Science http://www.lbl.gov/abc/Basic.html#Nuclearstructure

THE ATOM and RADIATION

GAMMA DECAY

: Gamma rays are a type of electromagnetic radiation that results from a redistribution of electric charge within a nucleus. A gamma ray is a photon with a shorter wavelength than visible light. Gamma rays can be emitted when a nucleus undergoes a transition from one such configuration to another. For example, this can occur when the shape of the nucleus undergoes a change. Neither the mass number nor the atomic number is changed.

Text adapted from the ABC’s of Nuclear Science http://www.lbl.gov/abc/Basic.html#Nuclearstructure Image from: http://www.lbl.gov/abc/experiments/Experiment3.html

Half-life: The time required for parent atoms to radioactively decay to half of their starting amount, leaving an equivalent amount of daughter atoms

US Geological Survey technician using a mass spectrometer to determine isotope ratios in an igneous rock

rockhttp://pubs.usgs.gov/gip/geotime/radiometric.html

Parent Isotope Uranium-238 Uranium-235 Thorium-232 Rubidium-87 Potassium-40 Samarium-157

ROCKS AND RADIOACTIVE DECAY

Stable Daughter Product Lead-206 Lead-207 Lead-208 Strontium-87 Argon-40 Neodymium-143 Half Life Values (Billion Years) 4.5

0.7

14 48.8

1.25

106

By contrast C-14 half life is 5730 years. The age equation relates radioactive decay to geologic time

rockhttp://pubs.usgs.gov/gip/geotime/radiometric.html

Example of radation damage in rocks

. Gamma radiation from zircon crystals (clear crystals above) in biotite mica (brown mineral above). The zircons are surrounded by a black rim caused when emission of gamma rays alters the crystal structure / composition of the mica.

Sample from PreCambrian age rocks, Foxe Fold Belt, Baffin Island, Microscope photo by Clinton Tippett.

Carbon-14 Dating

In the 1940's Dr. Willard F. Libby invented carbon dating for which he received the Nobel Prize in chemistry in 1960. Carbon dating has given geologists and archaeologists a more accurate method by which they can determine the age of ancient artifacts. The halflife of carbon 14 is 5730 ± 30 years, and the method of dating lies in trying to determine how much carbon 14 (the radioactive isotope of carbon) is present in the artifact and comparing it to levels currently present in the atmosphere. Above is a graph that illustrates the relationship between how much Carbon 14 is left in a sample and how old it is.

Text and image: http://www.chepd.mq.edu.au/boomerang/teaching.www/java/carbdate.htm

website also has a C-14 half life calculator

HOW CARBON-14 DATING WORKS

As soon as a living organism dies, it stops taking in new carbon. The ratio of carbon-12 to carbon-14 at the moment of death is the same as every other living thing, but the carbon-14 decays and is not replaced. The carbon-14 decays with its half-life of 5,700 years, while the amount of carbon-12 remains constant in the sample. By looking at the ratio of carbon-12 to carbon-14 in the sample and comparing it to the ratio in a living organism, it is possible to determine the age of a formerly living thing fairly precisely. A formula to calculate how old a sample is by carbon-14 dating is: t = [ ln (Nf/No) / (-0.693) ] x t1/2 where ln is the natural logarithm, Nf/No is the percent of carbon-14 in the sample compared to the amount in living tissue, and t1/2 is the half-life of carbon-14 (5,700 years). So, if you had a fossil that had 10 percent carbon-14 compared to a living sample, then that fossil would be: t = [ ln (0.10) / (-0.693) ] x 5,700 years t = [ (-2.303) / (-0.693) ] x 5,700 years t = [ 3.323 ] x 5,700 years t = 18,940 years old Continued….

HOW CARBON-14 DATING WORKS, Part II

Because the half-life of carbon-14 is 5,700 years, it is only reliable for dating objects up to about 60,000 years old. However, the principle of carbon-14 dating applies to other isotopes as well.

Potassium-40 is another radioactive element naturally found in your body and has a half-life of 1.3

billion years. Other useful radioisotopes for radioactive dating include Uranium -235 (half-life = 704 million years), Uranium -238 (half-life = 4.5 billion years), Thorium-232 (half-life = 14 billion years) and Rubidium-87 (half-life = 49 billion years). The use of various radioisotopes allows the dating of biological and geological samples with a high degree of accuracy. However, radioisotope dating may not work so well in the future. Anything that dies after the 1940s, when Nuclear bombs, nuclear reactors and open-air nuclear tests started changing things, will be harder to date precisely.

Text from : http://science.howstuffworks.com/carbon-142.htm

Another good site: http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/carbondating.htm

C-14 has been used to date mammoth tusks, human and animal bones, wood artifacts, pottery, shells.

COIN FLIPPING EXERCISE: THE RANDOMNESS OF RADIOACTIVE DECAY

Random events, like the flipping of a coin, mimic the process of radioactive decay.

1) Gather 100 coins (need not be the same denomination) 2) Create table with three columns (Flips, heads, tails) 3) Start them all as tails ( record flip=0, heads=0, tails=100) and then flip them 4) Set aside all coins that are now heads. Record number of heads and tails at flip 1 5) Repeat process until no tails are left 6) The flips represent time, plot on a graph the number of tails versus flips Equipment required: 100 coins, graph paper If possible perform this experiment in 3 or 4 groups so the data can be compared continued….

COIN FLIPPING EXERCISE: THE RANDOMNESS OF RADIOACTIVE DECAY

QUESTIONS 1) If each flip is equivalent to 1 million years, what is the half life of this isotope?

2) How long did it take for all the tails “isotope” to convert to the heads “isotope” ?

3) What is the limit of dating of a radioactive isotope with this kind of half-life?

4) What kind of variation was there in the other groups’ data?

5) Does this mean the method is inaccurate? If not, how do you explain the different answers?

COIN FLIPPING EXERCISE: THE RANDOMNESS OF RADIOACTIVE DECAY

ANSWERS 1) As about half the tails will disappear on the first flip the answer is 1 million years. 2) Answers will obviously vary. Most will have an answer of 7-8 million years but could range from 6-10 million years. 3) Somewhat less than 7-8 million years because if there are no tails isotopes left you have no idea how old the sample is. 4) Last tails isotopes will disappear in 6-10 flips. More or less than 50% of the tails will disappear at each flip - groups will observe half life variation at each flip. 5) The method is not inaccurate but there will be statistical variation, or what scientist would call margin of error. For example, the half life of Carbon-14 is 5730 years plus or minus 30 years.

Isotopes and Reconstructing the Ancient Environment

Fossils as a Proxy for Climate

The ratio of oxygen isotopes in the shells of forams has allowed scientists to reconstruct climate well back into the Cretaceous

Everything you ever wanted to know on Climate Change:

http://www.aip.org/history/climate/in dex.html#contents A shell of the microscopic foraminifer Globigerinoides

sacculifer, a key indicator of past temperatures

.

http://www.aip.org/history/climate/xforam.htm

Isotopes and Reconstructing the Ancient Environment

Water is made of hydrogen and oxygen atoms arranged in molecules of H 2 O, but you may not be aware that there are different types of oxygen atoms. Different atoms of the same element are called isotopes. All oxygen atoms have 16 protons and 16 electrons, but some oxygen atoms have 16, 17, or 18 neutrons in the nucleus. The most abundant isotopes of oxygen in seawater are oxygen sixteen ( 16 O) and oxygen eighteen ( 18 O). Water molecules with 16 O atoms evaporate more easily than water molecules with 18 O atoms, so the relative numbers of 16 O and 18 O atoms that remain in the water change as evaporation occurs. Water from which 16 O atoms have preferentially evaporated has a higher ratio of 18 O to 16 O atoms than water that has experienced less evaporation. As salinity also increases as evaporation occurs, we can generalize the relationship to state that water with an increased 18 O to 16 O ratio is saltier and warmer than water with a lower 18 O to 16 O ratio.

Text from: Exploring Earth website:http://earthsci.terc.edu/content/investigations/es2307/es2307page01.cfm

Image from: http://geoweb.tamu.edu/courses/geol101/grossman/Quat.18O.strat.jpg

Isotopes and Reconstructing the Ancient Environment

Lighter isotopes become concentrated in the atmosphere through evaporation. It is easier to vaporize a light molecule. Rain further concentrates lighter isotopes because heavier isotopes preferentially precipitate The process of separating lighter isotopes from heavier isotopes is called

fractionation

. The lower the temperature of evaporation the greater the fractionation.

Text and image from http://www.gly.uga.edu/schroeder/geol1121H/isotope.html

Isotopes and Reconstructing the Ancient Environment

Snow precipitated from the water vapor is also slightly richer in 16 O relative to 18 O than the original ocean water, as are the glaciers that eventually form from this snow. The oxygen isotope ratio of ocean water remains relatively constant as long as there is no net change in the global amount of glaciers or groundwater. However, as the Earth's climate cools and glaciers expand, more 16 O-rich precipitation is stored on land as glacial ice, and the 16 O to 18 O ratio of the remaining sea water drops slightly. The change in oxygen isotope ratios is subtle, but measurable. The shells of foraminifera that grew during glacial intervals have slightly lower 16 O to 18 O ratios than shells that grew during interglacial intervals. During warmer intervals, glaciers melt and release their stored water back to the ocean - raising the ratio of 16 O to 18 O in ocean water. Hence the oxygen isotope composition of plankton shells indirectly reflects the amount of glacial ice stored on the land. Oxygen isotope changes also occur in ice cores and the ratio of 16 O to 18 O in preserved ice appears to reflect the size of continental ice sheets. The oldest ice recovered from glaciers dates back to 160,000 years ago, so the temporal record of ice cores is somewhat limited compared to deep Modified text and image from: http://jesse.usra.edu/articles/iceagemodule /iceagemodule-15.html

sea cores.

How can we get this Information?

To find shells that formed at different times, geologists drill into the ocean floor and bring layers of sediments to the surface encased in tubes. These are called cores. Sedimentologists and paleontologists observe samples from the cores under high-powered microscopes to detect microfossils. The deeper the microfossils are found, the older they are.

By extracting the Oxygen isotope data from these shells, and knowing the order of deposition of the fossils, geologists can then reconstruct the paleo climate.

Text above adapted from Exploring Earth website: http://earthsci.terc.edu/content/investigations/es2307/ es2307page01.cfm

Photos to right show core sampling and microfossil study in the global scientific project Ocean Drilling Program http://www.oceandrilling.org/ODP/Photos.html

Foraminifera called

Cibicides

http://www.gns.cri.nz/what/e arthhist/fossils/forams.html

When Were the Pacific and Atlantic Oceans Connected?

With data extracted from a coring program run by the Deep Sea Drilling Project (an oceanic exploration project run over decades with the participation of many countries), scientists have reconstructed changes in salinity of the oceans east and west of the Isthmus of Panama. By recording the ratio of oxygen isotopes captured in the shells of single celled organisms called foraminifera (see picture) we now have continuous record of salinity decline over the last 6 million years in the Caribbean Map: "From Glaciation to Gateways", Ocean Drilling Program; Microfossils: Top left: Ronny G. Thomas Research Associates Department of Chemistry Texas A&M University - Kingsville; Bottom right: The Astrobiology Institute, Marine Biological Laboratory, Woods Hole From: Exploring Earth website: http://earthsci.terc.edu/content/investigations/es2307/es2307page01.cfm

When Were the Pacific and Atlantic Oceans Connected?

Not only was the ratio of 18 O to 16 O declining, indicating lowering salinity, but certain species disappeared from the rock record on one side of the ocean. In conjunction with other geological data this showed the gradual isolation of the two oceans by a rising land bridge in Central America. Based on the disappearance of some species and the sharp decline in salinity starting 4.5 Million years ago, this is when scientists theorize the land bridge formed. Map: "From Glaciation to Gateways", Ocean Drilling Program; Microfossils: Top left: Ronny G. Thomas Research Associates Department of Chemistry Texas A&M University - Kingsville; Bottom right: The Astrobiology Institute, Marine Biological Laboratory, Woods Hole From: Exploring Earth website: http://earthsci.terc.edu/content/investigations/es2307/es2307page01.cfm

USING RADIATION TO GET THE MOST OUT OF AN OIL WELL

Vital information on the type of rock drilled and the fluids it contains often needs to be obtained in order to delineate the potential of an oil or gas well.

Specific tools have been designed using gamma radiation to assist determination of rock type (lithology), density and porosity.

The various types of measurement include: (1) electrical resistivity of fluids within the rock; (2) the speed of sound through the rock; (3)

reaction of the rock to gamma ray bombardment

; (4)

production of gamma rays from fluids within the rock due to neutron bombardment

; and (5)

natural gamma radiation

of the rocks. The data obtained give indications of rock type and porosity and the presence of oil or gas.

Other devices measure hole diameter, dip of strata and the direction of the hole. Sidewall corers which punch or drill out small cores of rock, geophones for well velocity surveys and seismic profiling are also lowered into uncased wells. Text and images from : Oil and Gas for Britain Website http://www.ukooa.co.uk/issues/storyofoil/exploration-03.htm

THE GAMMA RAY LOG

•First used commerically in 1939.

•Is the continuous measurement of natural radioactivity along the length of the borehole •Can count either gross radioactivity or applied as a “spectral” gamma to distinguish between Uranium, Thorium and radioactive isotopes of Potassium. •Distinguishes sands, shales, carbonates and anhyrdite and other kinds of rocks.

•Sands and carbonates typically have a lower background radiation than shales because they contain less clay minerals which are rich in potassium. •Can use this to identify where the reservoir is and to correlate layers of rock between different wells.

EXAMPLE OF GAMMA LOG APPLICATION

Lower Tertiary age marine sediments in the North Sea. Cored interval shows that gamma logs distinguish between sand and shales. Each core section is 3 feet (~0.9 m long) http://www.abdn.ac.uk/geology/profiles/turbidites/inje cted/subsurf/substudy.html

EXAMPLE OF GAMMA LOG APPLICATION WELL 1 WELL 2 Gamma Logs

Eocene age Wilcox Delta Sands in Texas. Gamma logs used to correlation a series of shallowing up sandy cycles between wells. Arrows point to same layer in both wells. Radiation decreases to left indicating sands. Source http://www.marcedwards.com/frameset_wilcoxstudy.htm

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The Case of the Dead Guy

Sergeant Potassium calls inspector Argon to the scene of a crime. There has been a murder but no witnesses have come forward and no clues have been found.

“Hey Argon, we have this dead guy, and we need your help to figure out when the deed was done. All we know about the dead guys is that he is a food inspector,” says Sergeant Potassium.

“No problem” says Argon confidently. He strolls to the fridge and opens the door.

He picks up the milk carton and some of the other contents in the fridge.

After some quick mental math he says “at least 2 weeks ago.” “How do you know that?” asks Potassium.

“Well if this guy was a food inspector I am sure he chucked out any food past its expiry date. About half the food in a typical fridge expires within a week of purchase. Half the remaining food expires in the next week and so on. Since about ¾’s of his food has gone bad the dead guy must have been dead for about two weeks. Mmm check out the apples, they are still tasty, wouldn’t touch the ground beef though.”

Natural radioactive decay follows a similar path as the food in the dead guy’s fridge. After an organism dies, Carbon 14 steadily decays into Nitrogen 14. After 5730 years there is half the C14 in the remains, and this interval is known as the “half-life” of C14. Identical processes occur for the decay of Uranium 235 into Lead-207 (704 million-year half-life) or Potassium-40 into Argon-40 (1.25 billion year half-life). These isotopes can be used to date very ancient rocks where-as Carbon-14 is effectively limited to dated materials under 50,000 years because of its short half life.