Accelerator Mass Spectroscopy
Accelerator Mass Spectroscopy
Accelerator
Mass Spectroscopy (AMS) is a highly sensitive technique that is useful
in isotopic analysis of specific elements in small samples (1mg or less
of sample containing 106 atoms or less of the isotope of interest).[1]
Accelerator Mass Spectroscopy
AMS
requires a particle accelerator, originally used in nuclear physics
research, which limits its widespread use due to high costs and
technical complexity. Fortunately, UC Davis researchers have access to
the Lawrence Livermore National Laboratory Center for Accelerator Mass
Spectrometry (CAMS LLNL), one of over 180 AMS research facilities in the
world. AMS is distinct from conventional Mass Spectrometry (MS)
because it accelerates ions to extremely high energies (millions of
electron volts) compared to the thousands of electron volts in MS
(1keV=1.6×10-16 J). This allows AMS to resolve ambiguities
that arise in MS due to atomic and molecular ions of the same mass. AMS
is most widely used for isotope studies of 14C, which has applications in a variety of fields such as radiocarbon dating, climate studies, and biomedical analysis.[2] Some of the most fascinating applications of AMS range from exposure dating of surface rocks, 14C labeled drug tracer studies, and even radiocarbon dating of artifacts such as the Shroud of Turin and the Dead Sea Scrolls.[3]
Theory
In
conventional atomic mass spectrometry, samples are atomized and
ionized, separated by their mass-to-charge ratio, then measured and/or
counted by a detector. Rare isotopes such as 14C present a
challenge to conventional MS due to their low natural abundance and high
background levels. Researchers were challenged by isobaric
interference (interference from equal mass isotopes of different
elements exemplified by 14N in 14C analysis),
isotopic interference (interference from equal mass to charge isotopes
of different elements), and molecular interference (interference from
equal mass to charge molecules, such as 12CH2-, 12CD, or 13CH- in 14C
analysis). Most AMS systems employ an electrostatic tandem
accelerator that has a direct improvement in background rejection,
resulting in a 108 time increase in the sensitivity of isotope ratio measurements. As the natural abundance of 14C in modern carbon is 10-12 (isotopic ratio of 14C:12C), a sensitivity of 10-15 is a prerequisite for 14C analysis.
Figure 1. A schematic of the AMS system at Lawrence Livermore National Laboratory Center for Accelerator Mass Spectrometry.
Figure 1, above, starts with a negative ion sputter source, which commonly consists of a stream of Cesium ions (Cs+)
with energies of 2-3 keV focused on the surface of a solid sample in
order to transfer enough energy to the target material to produce free
atoms and ions of the sample material. This process, called sputtering,
separates neutral, as well as positive and negative ions from the sample
surface. The sample is held at a negative potential, and negatively
charged ions are accelerated away from the sample, resulting in a beam
of negative ions (Figure 2, below). Cs+ is particularly useful in 14C studies because it does not form a negative ion from 14N, thereby eliminating isobar interference.[4]
It is important to have a beam of negative ions entering the
accelerator because the negative ions are attacted to the high -voltage
terminal which results in their net acceleration.
Figure 2. Cs sputter ion source.
The
low energy (~5-10 keV) diverging beam that leaves the ion source is
accelerated, focused and transported to the accelerator by the injector
system.[2] CAMS LLNL employs a low-energy mass spectrometer that selects for the desired atomic mass[5] that separates ions by their mass to charge ratio (12C, 13C, and 14C
ions pass through separately). Most AMS systems use sequential
injection, a process that switches between stable and rare isotopes via
the application of varying voltages to the electrically insulated vacuum
chamber of the analyzer magnet. In sequential injection, typical
injection repetition rates are 10 sec-1 to minimize variations in the electrical load.[2] This process allows the development of more versatile systems, allowing for analysis of a wide range of isotopes.[1] The alternative to sequential injection is simultaneous injection, a process adopted in accelerators dedicated to 14C
analysis. A recombinator is used following sequential injection, which
is a sequence of magnetic analyzers and quadrupole lenses that focus
the stable and rare isotopes so they recombine and enter the accelerator
together.
The
traditional accelerator was first developed in the early 1930s for
nuclear physics research. In 1939, UC Berkeley scientists Luis Alvarez
and Robert Cornog were the first to use AMS in the detection of 3He in nature using the 88-inch Berkeley cylclotron.[5]
Now, over 70 years later, cyclotrons have been replaced by an
accelerator type with greater energy stability: the tandem electrostatic
accelerator. An electrostatic accelerator works by accellerating
particles though a magnetic field generated by high voltages using a
mechanic transport system that continuously transports charges from
ground to the insulated high-voltage terminal. All tandem accelerators
with a maximum terminal voltage above 5 MV use such a mechanical system.[2]
The negative ions that enter the accelerator are attracted to the
high-voltage terminal, which is what accellerates theCAMS LLNL employs a
tandem Van de Graaff accelerator, in which a second acceleration of
millions of volts is applied. In all tandem accelerators, atoms are
stripped at the high-voltage terminal using either a thin Carbon foil or
Argon gas. Stripping is the process in which two or more electrons are
removed. The Van de Graaff accelerator removes at least four
electrons. It is preferrable to remove at least three electrons because
by this process that molecular isobars of 14C (such as 12CH2-, 12CD, or 13CH-) are destroyed due to the high instability of their positively charged forms, and atomic C+ ions such as 12C+, 13C+, and 14C+ are separated due to their different mass to charge ratios.[4]
The negative ions are changed to positively charged ions and are thus
accelerated back to the ground potential in the high-energy part of the
accelerator. Transmission through a foil changes with time due to
radiation damage and foil thickening, thus gas strippers are used in all
modern analyzers due to their increased transmission stability.[2]
Magnetic
lenses focus the high energy particles leaving the accelerator into a
magnetic dipole, (the high energy analyzing magnet). Stable isotopes
can be collected at off-axis beam stops where secondary focusing lenses
and additional analyzing equipment remove unwanted ions and molecular
fragments to eliminate background. At CAMS LLNL, a magnetic quadrupole
lens focuses the desired isotope and charge state to a high-energy mass
spectrometer which passes 12C+ and 13C+ into Faraday cups and further focuses and stabilizes 14C in a quadrupole/electrostatic cylindrical analyzer that leads to a gas ionization detector.[5]
The magnetic quadrupole and electrostatic selectors coupled together
ensure high selectivity and sensitivity, respectively. Other detectors
commonly found in AMS systems include surface barrier, time-of-flight,
gas filled magnets, and x-ray detectors.
Interpretation
Rare
isotopes analyzed by AMS are always measured as a ratio of a stable,
more abundant (but not too abundant) isotope. For example, the ratio in
14C studies is generally shown as 14C/13C.
Less abundant isotopes are preferable in AMS because the decreased flux
of ions reduces background and wear on the instrument, which is of
particular concern due to the quick deterioration of particle detectors
(performance deteriorates at rates higher than a few thousand particles
per second[1]).
Applications
Common radioisotope elements measured with AMS and their applications are shown in Table 1[4], below. Because 14C analysis is by far the most popular application of AMS, the methods discussed below are all techniques used involving 14C.
Table 1. Radioisotope elements generally measured with AMS and their applications.
Element (Common Isotope)
|
Radioisotope with AMS
|
Natural abundance
|
Half-life (yr)
|
Study application
|
Hydrogen (1H)
|
3H
|
trace
|
12.33
|
Biological/biomedical
Nutritional trace
|
Beryllium (9Be)
|
10Be
|
trace
|
1,510,000
|
Geochronology
Hydrogeological study Exposure dating |
Carbon (12C)
|
14C
|
1 x10-10%
|
5730
|
Biological/biomedical
Nutritional trace |
Aluminum (27Al)
|
26Al
|
trace, synthetic
|
720,000
|
Biological/biomedical
Exposure dating
|
Chlorine (35Cl)
|
36Cl
|
7x10−11%
|
301,000
|
Earth Science
Hydrogeological study
Exposure dating
Migration of nuclear waste
|
Calcium (40Ca)
|
41Ca
|
trace, synthetic
|
116,000
|
Biological/biomedical
Nuclear weapon testing |
Nickel (58Ni)
|
59Ni
|
trace, synthetic
|
112,000
|
Nutritional trace
|
Iodine (127I)
|
129I
|
trace, synthetic
|
15,700,000
|
Biological/biomedical
Migration of nuclear waste Environmental study |
Radiocarbon dating is an analytical method based on the rate of decay of 14C, a radioactive carbon isotope formed in the atmosphere by the reaction between neutrons from cosmic rays and 14N (neutron + 14N = 14C + proton).[2] Resultant 14C atoms are taken up by plants in the form of 14CO2, then transferred to animals though the food chain. When animals and plants die, they cease to uptake 14C, and a steady decay of 14C continues in their tissues over time. 14C atoms decay via electron emission (β radiation) to form 14N, a process which has a half life of 5,730 years.[5]
Radiocarbon levels in the atmosphere change according to complex
patterns which are affected by a variety of fluctuations ranging from
the sun’s solar activity and the earth’s magnetic field, to ocean
ventilation rate and climate. 14C analysis of tree rings, corals, lake sediments, ice cores, and other sources has led to a detailed record of 14C
variations through time, allowing researchers to establish an official
radiocarbon calibration curve (also referred to as a radiocarbon clock)
dating back 26,000 calendar years. In the 1960s, nuclear weapons testing
released large amounts of neutrons into the atmosphere, nearly doubling
14C activity.[2] Samples taken after this time period can be radiocarbon dated using a 14C
bomb curve like the peak shown below in Figure 3, can retrieve very
precise dates (within 1 year at the steepest part of the curve).
Figure 3. The New Zealand curve (red) is representative of atmospheric 14C in the Southern Hemisphere, and the Austrian curve is representative of the Northern Hemisphere.
14C
analysis provides valuable information in the radiocarbon dating of the
world’s most priceless artifacts. One such example of the monumental
impact of 14C AMS is the radiocarbon dating of the Dead Sea
Scrolls to dates from 300 BC to AD 61 by labs in Zurich and Arizona.
AMS has also contributed greatly to environmental and atmospheric
studies by providing information regarding particle composition and
origin. In the biochemical field, synthesized 14C labeled
compounds can be administered as a tracer dose for in-vivo human
metabolic and drug studies which require AMS analysis of graphitized
biological samples.
AMS
is a highly sensitive method for isotopic analysis that has numerous
key applications that are only growing with advances in technology.
High costs and technical complexities that arise with the use of a
particle accelerator are the only limits to the widespread use of AMS.
Recent times have seen the emergence of commercially available compact
accelerators that use as low as 200 kV for radiocarbon dating and
biomedical applications, and as particle accelerators become more
commonplace, modifications to the instrument have also broadened the
number of isotopes the instrument can measure.
- [1]. Tuniz, C., Bird, J. R., Fink, D. and G. F. Herzog, Accelerator Mass Spectrometry. CRC PRess: Boca Raton, 1998; p 371.
- [2]. Hellborg, R.; Skog, G., Accelerator mass spectrometry. Mass Spectrometry Reviews 2008, 27 (5), 398-427.
- [3]. Gove, H. E., From Heroshima to the Iceman. Institute of Physics Publishing: London, 1999; p 225.
- [4]. Kim, S.. Graphitization for biological, biomedical, and environmental carbon-14-accelerator mass spectrometry applications: Optimization and characterization. Ph.D. dissertation, University of California, Davis, United States -- California. Retrieved February 28, 2011, from Dissertations & Theses @ University of California.(Publication No. AAT 3379582).