Hi, I'm Chris Prior, I'm the team leader for the rafter radiocarbon laboratory here at GNS science.
This is the sample preparation laboratory for radiocarbon dating
and today we're going to show you a little bit about how radiocarbon dating is done.
So how does radiocarbon dating work? It's actually pretty simple.
All living things contain carbon, and there are three naturally occurring isotopes of carbon,
carbon 12, 13 and 14.
Carbon 14 is naturally radioactive.
It's formed in the upper atmosphere through the interaction of cosmic rays with nitrogen atoms,
and that carbon 14 that has been formed is rapidly oxidised as carbon dioxide
and gets incorporated into the atmosphere.
Plants take it up from the atmosphere as CO2, so the C14 is rapidly mixed throughout the environment.
Actually, there's a lot more carbon 12 and carbon 13 in living organisms than carbon 14,
but all living things are naturally radioactive because they contain carbon 14.
As long as an organism is living and breathing and taking in food,
then it's in equilibrium with its environment, and it has a constant ratio of 12 to 13 to 14 .
When an organism dies it's no longer taking in any new carbon,
so at that point the carbon 14 in the organism begins to decay away.
That's kind of like the start of the stopwatch for radiocarbon dating.
If can accurately and precisely measure how much c14 is in our sample now,
we know how much should have been in the sample when it was alive,
we can then use an equation to calculate how much time has passed since that organism was alive.
and that's radiocarbon dating.
Of course the tricky part is being able to accurately and precisely measure how much c14 is left,
and that's where all the effort in the laboratory comes in.
So the types of materials that we normally radiocarbon date include
things like wood, plant remains, charcoal from charred wood,
animal products like bone or leather or sinew,
or horn or antler or skin -you know, parchment.
also archaeological artefacts that are made from plants and animals.
The types of things we can't radiocarbon date are geological materials,
so we can't date stone or glass,
and we also can't date some things that are beyond the dating range of radiocarbon.
We can only radiocarbon date back to about 50,000 years ago,
because of the half-life of radiocarbon which is 5,730 years,
after about ten half-lives, which would be some 57,000 years,
the amount of C14 that was originally in that organism is so tiny
that we can't really accurately and precisely measure it with most of the radiocarbon dating
equipment and laboratories around the world.
A typical example of the type of thing we date in our laboratories this textile.
The client sent it to us hoping that it is 1,500 years old or possibly older,
but what he really wants to do is make sure that it's not a modern fake.
We look at it first under the microscope, and what we're looking for is identification,
to see if the sample is what it's supposed to be.
We also look under the microscope in order to look for any
contaminants that have been added to the outside of the sample that we want to remove.
The second step in the process is a chemical cleaning of the sample,
because what we want to do is isolate carbon that is just from the sample
and not from any external contaminating materials.
So what you see here is people doing a lot of chemical processes to clean up the sample.
In the textile example that we've been talking about,
we're going to give it a series of washes with organic solvents,
and those solvents will dissolve any greases or fats that have been attached to the sample
by people handling it with their fingers.
The next step is to convert the solid sample to carbon dioxide,
and then to convert the carbon dioxide to graphite
for measurement in the accelerator.
We load the sample into a quartz combustion tube
with copper oxide and a little bit of silver wire,
then we put that combustion tube on vacuum line,
and we pump all of the air out of the tube.
When all of the air has been sucked out of the tube,
we can flame seal it, and that leaves a vacuum in the inside.
Once the sample has been sealed into an evacuated combustion tube,
it's put into a muffle furnace and heated to 900 degrees.
The copper oxide will cause the solid sample to combust, to burn,
and we'll end up at the end of the process, after it's been combusting overnight,
with carbon dioxide and a little bit of water inside the combustion tube.
The combustion tube is cracked and the co2 is transferred through these traps into a small bottle.
The first trap is a water trap, and the second trap is where we collect the co2.
The volume of the co2 is then measured very exactly so that we know how much we got,
and then it's transferred into this small bottle here.
The final step in the process is making graphite.
what we do is attach this little bottle containing the co2 to the vacuum line over here,
and that co2 is mixed with hydrogen inside a graphite reaction vessel.
There's a small amount of iron powder inside the reaction vessel, which is what we call the catalyst.
When the hydrogen and the co2 are heated to a high temperature,
the hydrogen will take the oxygens off the co2
and deposit elemental carbon in the form of graphite on the Iron catalyst.
A tiny amount of graphite is the end product from the sample preparation laboratory,
and now we're taking the graphite from the sample preparation laboratory
into the accelerator room for measurement.
Albert Zondervan, the accelerator operations team leader and his team,
take over the measurement from this point on.
the graphite is pressed into an aluminium target holder,
and that target holder is placed into this wheel which has 40 positions on it.
In the wheel, 25 of the positions are unknown samples,
and the remaining positions are graphites that are made from standards and blanks,
materials that allow us to determine our quality assurance and how to calculate the age.
at this point the wheel is placed into the ion source.
The source is a caesium beam, that blasts the carbon
out of its target holder and into the accelerator.
The stream of carbon ions goes through the accelerator,
being bent at certain places by magnets
which serve to separate out the carbon 12, 13 and 14 into separate streams.
The carbon 12 and 13 are separated out at this point
and the carbon-14 continues on through the accelerator
to this point where it is detected.
Once the data has been analysed on the accelerator, then we have to calculate a radiocarbon age.
The information that comes off the accelerator is actually a ratio of the carbon 14 to carbon 13,
and we have to use an equation to calculate a radiocarbon age.
A radio carbon age is always reported with a plus or minus error,
And that plus or minus error means that the radiocarbon age
can fall anywhere within that full age span.
So because the radiocarbon age is not the same as a calendar year,
we have to use calibration to convert the radiocarbon age to a calendar age span.
In this particular case, the radiocarbon age intercepts the calibration curve at two different locations.
The majority of the intercept falls within this area here
which is about 400 to 350 BC,
but because the outer bounds of the radiocarbon age range
intercepts the part of the curve that comes up here,
there is a small but significant possibility
that the calendar age of the sample could be in the 280 to 230 bc age range as well.
So while we're not able to determine an exact specific calendar year for this textile,
we know that the radiocarbon age range converts to a calendar age range
which shows that it is indeed an antique textile and not a modern forgery.
I hope our client is happy with the result.