LECTURE                     RADIOACTIVE DATING

In this lecture you will learn:

RADIOACTIVE DECAY - HALF LIFE
"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.[3][4][5]" 

"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.[6] "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 half life............

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. 

  1000 yrs ago now  in 1000 yrs in 2K yrs in 3K yrs
dogs 200 100  50 25 12.5
cats 0 100 150 175 187.5
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
Any naturally formed rock material has the potential of containing varying amounts of naturally occurring radiation. Natural radioactive elements like uranium, radium, and thorium can be present in a wide number of minerals that appear as crystals in granite from around the world.  So, it is not unusual for materials such as granite to have some amount of radioactivity (emissions of alpha or beta particles or gamma rays). Depending on the composition of the molten rock from which they formed, some pieces of granite can exhibit more radioactivity than others.

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.  Its 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


 
DATING THE EARTH
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. 
 
It isnt until the rock is solid that the products of decay accumulates in situ and can be aged.  When rock  has been remelted (metamorphic) it RESETS the time clock.  So on Earth there is rock of all different ages (like lava) so it is important to sample many rocks to find the oldest. 

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.

half life.
           235U> 208Pb -- (710 million years)
           87Rb > 87Sr -- (47 billion years)

Lead has two forms,  206 protons and neutrons,   204.
The ratio of 206 to 204 in naturally occurring lead is a constant.
Uranium decays to    206 protons and neutrons.

Measure the amount of 204 in the sample, calculate  the amount of 206 there should be
Measure the 206, the excess 206 is produced by Uranium decay.
 


 
CARBON 14 DATING
Use of half-life decay in radioactive dating

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!). 

Hypothesis
1.  C-14 is made at a steady rate

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. 
 

The half-life for C-14 is 5,730 years.

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)

REVIEW QUESTIONS:

Half life
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.