How it works:
An accelerator mass spectrometer with professor lan Clark
The AMS itself,an accelerator mass spectrometer,has real three
components:one is called the low-energy end.Let's take the
example of carbon anr radiocarbon is present in the environment
at one millon millonth of an atom,so it's very,very trace isotope,so
what we do is we take any sample,we convert it to grahite,they're
like a little aluminium button with the graphite in the center of it
and we can mount up to 200 of these graphite targets into a cartridge
tye...we call an ion source,because what we're going to do is taken
the atoms of carbon in the graphite and convert them to ion.We're
going to ionize them.Once you get things ionized,you can move
them because we use electrical fields and voltage fields,voltage
gradients magnetic fields,to move these ions around.So we're
going to craete an ion beam.It's a beam of carbon,racing out
of our source,we bend that beam around with magnets an
electrostatic devices and aim it into a big tank.So we're looking at
a tank right here,it's a T-shaped tank,filled with an insolent-sulfur
hexafluoride because we're going to create a huge voltage
in that tank.So it's lke a big transformer that you see in hydro
systems,except ours is three millon volts.So we're gonna race those ions
towards that positive pole and then we're gonna switch them;we're
gonna strip tthose electrons off the carbon and turn into a positive ion
just as it hits that positive pole and it's gonna shoot them out the
other end.Now we're going up to a few percent of the speed of
light so our ion beam is really rocketing as we're walking over here now
to the west side of the machine,we're at the high energy end.
So our ion beam is racing out and the flight path goes into a magnet.
And we've got a few control centers along it;these are electronic lenses so we can
change the shape with that beam.So now we're looking at a magnet,it's actually our
big,what we call the analyzer magnet.It'a 18 tons,a whack
of energy,because we have to bend that beam ninety degrees.
And so we've got just pure carbon coming down there but carbon has these
isotopes is carbon-12 and carbon-13 with an extra neutron
and carbon-17 with an extra two neutrons and that's the guy we're
looking for.So as we bend that beam,the lighter isotopes,carbon-12
bends the most,carbon-13 likewise,but carbon-14 continues on
straight on down that pipe.So we're gonna now count,we're going to measure the strength
of the carbon-12 and the carbon-13 beams.But carbon-14 is a very weak beam and we can't measure its
strength with current.We have to actually count the atoms of carbon in that beam.So at the
very end,we have a detector filled with the gas isobutane and the
carbon-14 atoms come down and collide with the isobutane and they
give off thier energy.So this is a high-speed collision.Aall that energy
is dissipated and each time there's a collision,we measure that energy.
So now we come up with a count and the number of carbon atoms is compared
to the carbon-12 that we measured,and that tells us the concentration.