Radioactive Dating

Nature’s Clock

When sedimentary rock is deposited in layers it is deposited horizontally.

Scientists use this “Principle of Original Horizontality” to help them determine the age of the layers.

The older layers of rock are found below the younger layers of rock.

Law of SuperpositionThe Law of Superposition doesn’t tell scientists how old a rock layer is, it only tells them that one layer is

older than another.

Another way to say this is that the Law of Superposition tells scientists the order the layers

were deposited.

Any method of dating that only orders events only measures:

Relative Age

If the Law of Superposition only tells scientists relative age, how can we determine the numerical age of a rock layer, a fossil or an artifact?

To determine the numerical age of a geologic object (rock, fossil, stone tool, …) scientists use a method called Radioactive Dating.

In radioactive dating, scientists find traces of a radioactive material in the object and determine the number of times that material has decayed to determine the age of the object.

Determining Geologic Time

Any method that determines the numerical age of an object measures:

Absolute AgeSo radioactive dating measures absolute age.

To understand radioactive dating you first need to

understand radioactive decay and the concept of half-life.

To understand radioactive decay and the concept of half-life you

need to understand the structure of an atom.

An atom consists of a nucleus containing positively charged protons and neutral neutrons.

The negative electrons surround the nucleus in an electron cloud.

The Atom

ProtonElectron

Neutron

The number of protons determines the type of atom.

On the periodic table, the number of protons is shown as the atomic number.

Since the protons are all positively charged they want to repel (push away from) each other.

So what keeps the nucleus of an atom together?

Enter the neutrons.

ProtonsWhat element has 1 proton?

H - HydrogenWhat element has 2 protons?

He - HeliumWhat element has 6 protons?

C - CarbonWhat element has 92 protons?What element has 7 protons?

You Decide!!!!!!!

Neutrons have a very important job in the nucleus of an atom.

They supply a strong force that glues the protons together in the nucleus.

Neutrons are unstable when they are isolated from protons.

When a neutron is by itself or just very far away from a proton they break apart (decay) into a proton and an electron.

Neutrons

If I have two atoms of the same element, they must have the same number of protons.

But these same two atoms of the same element can have different numbers of neutrons.

Atoms of the same element that have different numbers of neutrons are called isotopes.

Different isotopes of an atom are shown as:

Isotopes

or

Carbon has 6 protons in its nucleus.

A stable isotope of carbon also has 6 neutrons in its nucleus.

The symbol for this isotope of carbon is either:

or

When a nucleus is unstable it can emit (spit out) particles or energy.

This is called radiation. When the nucleus emits particles it

changes the type of atom. This is called radioactive decay. There are three types of radiation we will

look at here:

Radiation & Radioactive Decay

Alpha Decay Beta Decay Gamma Radiation

When the number of protons in the nucleus of an atom is very large compared to the number of neutrons, the protons repel each other with more force than the neutrons can provide to glue the protons together.

This causes protons and neutrons to be expelled (pushed out) of the nucleus.

An Alpha Particle (α) is what is expelled from the nucleus.

An Alpha particle consists of 2 protons and two neutrons – it is the nucleus of a helium atom!!!

Alpha DecayAn Alpha Decay changes the atom because during the decay the atom

loses 2 protons.

In the example shown below, the parent nucleus (the nucleus before the decay), Uranium, has 92 protons before it decays.

After it decays and emits an alpha particle, the daughter nucleus (what’s left after the decay) now has 90 protons because it lost 2.

This means the uranium nucleus with 92 protons has changed into thorium with 90 protons!

When the number of neutrons in the nucleus of an atom becomes much larger than the number of protons the neutrons begin to decay into a proton and an electron.

The proton is added to the nucleus of the atom.

This changes the type of atom. The electron is expelled (pushed out)

from the nucleus. This electron is the beta(β) particle.

Beta DecayA beta decay changes the atom because it adds a proton to its

nucleus.

In the beta decay shown below, the parent nucleus is potassium which has 19 protons in its nucleus before the decay.

After the decay, a beta particle is expelled adding one proton to the nucleus of the daughter nucleus giving it 20 protons. This makes the

daughter nucleus calcium!!!

After an alpha or beta decay, the daughter nucleus has a large amount of stored energy in it.

We say it is excited. When the nucleus “shakes out” this

energy it releases a high energy gamma (γ).

The release of the gamma ray does not change the type of atom, although the atom changed due to the alpha or beta decay.

Gamma Radiation

When a radioactive element decays it decays gradually over time, not all at once.

The time it takes half (50%) of the atoms in a substance to decay is called the half-life of the substance.

For example, if you now have 200 atoms of a radioactive substance, then in one half-life of the substance you will have 100 atoms.

That is half of 200. The half-life for different substances can be less

than a second to millions of years. But the half-life of the same substance (isotope)

will remain the same.

Half-LifeBelow is a link to an animation of half-life. While you work with this animation notice:

1)What effect does changing the half-life have on how quickly the dots disappear?

2)How much time does it take for half the dots to disappear?

Click on the picture to go to the animation!

A very important radioactive isotope for radioactive dating is carbon-14.

C-14 undergoes a beta decay and decays into nitrogen-14.

C-14 has a half-life of 5730-years. This means that on average ½ (50%) of the C-14 will

decay every 5730-years into N-14. Let’s assume we started with 256 C-14 atoms, this

means that after 5730-years we should have 128 atoms (1/2 or 50%).

After another 5730-years we should have 64 atoms (1/4 or 25%).

This will continue until there are no more C-14 atoms left.

The Half-Life of Carbon-14Number of Half-Lives

Number of Years

Number of C-14 Atoms

Left

Fraction of C-14 Atoms

Left

Percent of C-14 Atoms

Left

0 5730 256 1 100%

1 11,430 128 1/2 50%

2 17,190 64 1/4 25%

3 22,920 32 1/8 12.5%

4 28,650 16 1/16 6.25%

5 34,380 8 1/32 3.125%

6 40,110 4 1/64 1.5625%

7 45,840 2 1/128 0.78125%

8 51,570 1 1/256 0.390625%

9 57,300 0 0 0%

While an animal or plant is living it takes in two different isotopes of carbon.

Carbon 12 which is stable and Carbon-14 which is unstable.

After the animal or plant dies, it no longer takes in C-12 or C-14.

At this point the C-14 in the animal or plant will decay and not be replaced but the amount of C-12 will remain the same.

Radioactive Carbon Dating

Scientists can use the amount of C-12 in the animal or plant to determine the amount of C-14 in the animal or plant at its death.

Once the original amount of C-14 is known scientists can compare this to what is left and figure out the number of half-lives that have occurred since the animal or plant died.

Since the half-life of C-14 is 5730 years, multiplying the number of half-lives that have occurred by 5730 years gives a very good estimate of absolute age of the plant or animal.

Radioactive Carbon Dating

A clay pot was found buried near an ancient temple . Based on an analysis of the amount of C-12 in the clay pot, it has been determined that the pot originally had 0.034 grams of C-14. The same analysis has determined that the clay pot now contains 0.0010625 grams of C-14. How old is the clay pot?

Radioactive Carbon Dating

Mass 0f C-14

Number of Half-Lives

   

   

   

   

   

0

2

4

3

5

1

The best way to solve this

problem is to use a table to figure out the number

of half-lives:

0.017-g

0.034-g

0.00425-g

0.0085-g

0.0002125-g

0.0010625-g

5 half-lives have occurred since the materials that contained carbon in the

clay pot “died”.

Each half-life takes 5730-years.

This means that the pot is

5 X 5730 = 28,650 years old.

Using this technique, almost any sample of organic material can be ABSOLUTELY dated. There are a number of limitations, however:1) First, the size of the archaeological sample is

important. Larger samples are better, because purification and distillation remove some matter.

2) Second, great care must be taken in collecting and packing samples to avoid contamination by more recent carbon. Also, the sample should be carefully examined to determine that a carbon sample location was not contaminated by carbon from a later or an earlier period.

Limitations to Radioactive Carbon Dating

Using this technique, almost any sample of organic material can be ABSOLUTELY dated. There are a number of limitations, however:3) Third radiocarbon dating has significant

upper and lower limits. It is not very accurate for fairly recent deposits. In recent deposits so little decay has occurred that the error factor may be larger than the date obtained. The practical upper limit is about 50,000 years, because so little C-14 remains after almost 9 half-lives that it may be hard to detect and obtain an accurate reading, regardless of the size of the sample.

Limitations to Radioactive Carbon Dating

Using this technique, almost any sample of organic material can be ABSOLUTELY dated. There are a number of limitations, however:4) Fourth, the ratio of C-14 to C-12 in the

atmosphere is not constant, although it was originally thought that it was. To compensate for this variation, dates obtained from radiocarbon laboratories are now corrected using standard calibration tables developed in the past 15-20 years.

Limitations to Radioactive Carbon Dating