3'5' Accelerator Mass Spectroscopy AMS

1
3.5. Accelerator Mass Spectroscopy - AMS -
AMS is a method which counts radioactive
particles (14C) rather than measuring the
characteristic decay activity.
2
What is Accelerator Mass spectrometry (AMS)
The determination of the concentration of a given
radionuclide in a sample can be done in 2
ways a) measure the radiation emitted during
the decay
In many cases where concentrations and/or small
or long t1/2 this becomes impractical
1mg carbon 6 x 107 at 14C ? 1
decay/hour
b) count the number of atoms themselves
In a Mass Spectrometer a sample material is
converted to an ion beam that is then
magnetically (and electrostatically) analysed
MS separates ions by their mass only
3
Comparison Traditional 14C dating and AMS
In 14C dating you count 14C activity
With AMS you count 14C number
Decay constant ? versus efficiency ? of device
including ionization in sources and transmission
in accelerator.
4
Comparison with traditional technique
AMS is technically more demanding than a
radiocarbon dating experiment with LSC, but it is
more accurate, and requires smaller samples!
approximately 4 orders of magnitude
improvement!!!
5
MS vs. AMS
6
Typical AMS setup
7
Method
8
A typical AMS facility
9
Summary C-beam production
The carbon in the sample is converted to nearly
pure carbon in the laboratory. The prepared
sample is placed in an evacuated chamber, where
it is bombarded with positive cesium ions (Cs).
Cesium lowers the work function of the material,
allowing the release of negative carbon ions
(C-). Because the N- ion is unstable, 14N does
not interfere with 14C measurements. However, the
molecular ions 12CH2- and 13CH- are produced, and
are accelerated with the 14C-. The accelerated
ions encounter a position defining slit, which
causes only a fine beam of ions to pass through
entering the accelerator.
10
sample preparation
Mechanical methods to pulverize material to form
a carbon pellet suitable for use in sputter
source.
Alternative method is chemically separating and
oxydizing carbon to use CO2 with subsequent Cs
charge exchange in ion source.
sample preparation needs experience!
11
Cs sputter ion source
Bombardment of sample pellet with Cs beam causes
carbon atom or molecule release with charge
exchange by pick up of electrons from cesium
atoms which can easily be ionized.
12
simulation1 sputter source
13
magnetic separation system
14
Injector magnet
The ion beam from the source enters an injector
magnet, which bends the beam. Heavier ions are
bent less than lighter ones, because of higher
momentum. The second slit is calibrated to only
allow ions of a certain mass to pass.
15
simulation 2 injector magnet
16
The accelerator
The ions then enter the accelerator and are
attracted to the high voltage in the terminal
(gt2 MV). The ions are accelerated to a sufficient
velocity. As they traverse a gas canal they are
stripped of some of their electrons. If the ion
is a molecule, it breaks apart, eliminating
a background interference. If it is an atom, it
becomes positively charged and is accelerated
towards the ground potential.
17
Inside the tank
The charging and acceleration system
18
The stripper
gas stripper or foil stripper ?
most likely charge state between q2 and
3. Total energy E(q1)V
19
simulation 3 tandem accelerator
20
detection system
The ion beam, now positively charged (3)
passes through a position- defining slit to
obtain a concentrated beam, containing minimal
impurity interference. The beam then is
subjected to a final magnet, separating the
isotopes of carbon from the previously uniform
beam. Two Faraday Cups and one 14C detector then
measure the current of each of the separate
beams. This provides information which can be
utilized to obtain the amount of 14C and its
ratio in comparison to 12C and 13C.
21
simulation 4 analyzing system
r(14C)
r(12C)
r(13C)
22
Example Magnetic Separation
Assume a V2 MV tandem! What is the separation of
12C, 13C and 14C in the charge state q3
expressed in terms of radius r for a fixed
magnetic field of B1 Tesla?
with mA1.6710-27kg qq1.610-19C 1 eV
1.610-19 J A mass number q3 charge
V2 terminal voltage in MV
23
Separation in magnetic field
V terminal voltage in MV B magnetic field in
T q electrical charge A mass number
for 12C r124.71 m for 13C r134.90 m for
14C r145.09 m
24
?E-E gas-counter system
is based on measurement of energy loss and total
energy of incoming ions in gas.
U500V
Ug300V
25
separation and identification
With good separation and particle identification
a nearly background free spectrum can be
achieved. Potential background sources are room
background radiation, cosmic rays, leakage of
molecules.

r(14C)
r(13C)
r(12C)
Two-dimensional gas counter spectrum for
radiocarbon 14C analysis
26
Counting efficiency and sample size
counting efficiency is the fraction of 14C ions
detected in the final detector from a sample put
in the ion source. For 14C ?c1
Assume the previous 1 g piece of wood with
1.51010 14C atoms, this translates into a total
number of counts Ndet1.5108 of 14C. (It takes
about a week to sputter the sample completely
away.)
Minimum sample size with 10 statistics
Ndet10210 cts of 14C Nsample(14C) 104 atoms
of 14C, Nsample(12C)Nsample(14C)/1.310-127.71
018 atoms of 12C 12g has 6.0231023 part, min.
sample needs to be 0.15 mg.
27
Comparison again!
How long does the traditional LSC technique take
to analyze the same sample size with equally
good statistics of 10?
to accumulate 10 statistics with the same sample
size requires 104 times longer counting time
180 years. As claimed before 4 orders of
magnitude improvement! (for 2 orders of magnitude
increase in costs 100k?10M)
28
Applications of AMS
There is a rich field of applications for AMS due
to the increased efficiency and accuracy of
radiocarbon dating. It ranges from geology,
hydrology, oceanology, climatology and
environmental studies to history and archaeology.
AMS is now also being used for a number of other
radioisotopes to enhance the sensitivity of
corresponding dating methods. The limitations are
the possible background counts from isotopes in
the same mass range which cannot be separated.