In this lecture you will learn:
|"Radioactive decay is the process in which
an unstable atomic nucleus loses energy by emitting ionizing particles
and radiation. This decay, or loss of energy, results in an atom of one
type, called the parent nuclide transforming to an atom of a different
type, called the daughter nuclide. For example: a carbon-14 atom
(the "parent") emits radiation and transforms to a nitrogen-14 atom (the
"daughter"). This is a random process on the atomic level, in that it is
impossible to predict when a given atom will decay, but given a large number
of similar atoms the decay rate, on average, is predictable." http://en.wikipedia.org/wiki/Radioactive
" This is very much like popping popcorn. When we pour our popcorn kernels into a popcorn popper, the is no way to know which will pop first. And once that first kernel pops, it will never be a kernel again...it is forever changed!"http://serc.carleton.edu/quantskills/methods/quantlit/RadDecay.html
Great applet on half life click HERE
|"While the moment in time at which a particular
nucleus decays is unpredictable, a collection of atoms of a radioactive
nuclide decays exponentially at a rate described by a parameter known as
the half-life, usually given in units of years when discussing dating techniques.
After one half-life has elapsed, one half of the atoms of the nuclide in
question will have decayed into a "daughter" nuclide or decay product.
In many cases, the daughter nuclide itself is radioactive, resulting in
a decay chain, eventually ending with the formation of a stable (nonradioactive)
daughter nuclide; each step in such a chain is characterized by a distinct
half-life. In these cases, usually the half-life of interest in radiometric
dating is the longest one in the chain, which is the rate-limiting factor
in the ultimate transformation of the radioactive nuclide into its stable
daughter. Isotopic systems that have been exploited for radiometric dating
have half-lives ranging from only about 10 years (e.g., tritium) to over
100 billion years (e.g., Samarium-147).
In general, the half-life of a nuclide depends solely on its nuclear properties; it is not affected by external factors such as temperature, pressure, chemical environment, or presence of a magnetic or electric field."
"The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation. The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created. It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration. "http://en.wikipedia.org/wiki/Radiometric_dating
"Rubidium-strontium dating method
This is based on the beta decay of rubidium-87 to strontium-87, with a half-life of 50 billion years. This scheme is used to date old igneous and metamorphic rocks, and has also been used to date lunar samples." . http://en.wikipedia.org/wiki/Radiometric_dating
Mass spectrometer used in radiometric dating
|A dog/cat model system for radioactive
Lets say you start with dogs and they decay into cats over time. The "half life of dogs is 1000 years. That means that after 1000 years 1/2 the dogs will decay into cats.
If you have 100 cats, that means you STARTED with 200 dogs a thousand years ago. And it means that in another 1000 years you will have 150 cats. At right is the time table. Of course, after 2K years it may be hard to find those cats!!
All you need to do is be able to identify a dog or cat when you see them and be able to count them. You also have to make sure the cats havent slinked out of the counting area.
|Granite Countertops and Radiation:
A real life example
"In geological terms, granite is an igneous rock, meaning it was formed when magma (molten rock) cooled very slowly until it solidified in a process that can take many of thousands, or even millions of years.
Radiation From Granite
EPA has not conducted studies on radioactivity in granite countertops. However, based on the limited information available, EPA believes that most types of granite used in countertops and other aspects of home construction are probably not major contributors of radiation and radon in the home.
When present, certain radioactive elements in granite will decay into radon, a colorless, odorless, radioactive gas which may be released from the granite over time. You can see in the diagram below how the decay of Uranium-238 (a radioactive element) produces Radon-222 gas.
However, since granite is generally not very porous, less radon is likely to escape from it than from a more porous stone such as sandstone. It’s important to know that radon originating in the soil beneath homes is a more common problem and a far larger public health risk than radon from a granite countertop or other building materials. Also, any radon from granite in kitchens or bathrooms is likely to be somewhat diluted in the typical home since those rooms are among the most ventilated. http://www.epa.gov/rpdweb00/tenorm/granite-countertops.html
|Radioactive dating is limited to when
the rock was last molten. That is, when molten the various
isotopes like 206 and 204 are uniformly mixed together.
The age of the Earth is 4.6 billion years?
How to double check the age of the solar system? The oldest rocks in the Solar System are the ones that HAVE NOT been remelted... the meteorites which fall to Earth from interplanetary space. And the meteroites are all uniformly 4.6 billion years old.
Lead has two forms, 206 protons and
Measure the amount of 204 in the
sample, calculate the amount of 206 there should be
|Use of half-life decay in radioactive
Fusion in the sun results in a constant stream of nuclei and electrons (called the solar wind) that streams outward toward us. The high energy particles interact with our high atmosphere and produce isotopes of elements. Nitrogen is hit and makes C-14 (by losing a proton, remember the element is a function of the number of protons!).
2. As it reaches ground level, it is incorporated into plants by carbon fixation (plants and some bacteria). Everything else eats the plants and gets their dose of C-14 at a constant rate. So the level of C-14 in living things is
3. However, the minute a plant or
animal dies, the incorporation ends and C-14 begins to decay at a predictable
rate back into NITROGEN.
After 5 half-lives (or 30,000 years) decay is not detectable above background levels.
C-14 can be used to date anything that
has been alive, bones, rope, seashells, etc.
|In this case all you need to count is how much C-14 is left. The proportion of radioactive to non-radioactive carbon is constant: ca. 1 part per trillion.||1/1,000,000,000
600 billion atoms/6x1023 atoms (mole)
discuss carbon 14 dating, where does the C14 come from, how does it end up in living things, how can organics be dated?
discuss uranium dating. what is dated with it, what cannot be dated with it.