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AbstractAccelerator mass spectrometry (AMS) was developed in the 1970s as an analytical tool for the measurement ofradiocarbon (14C). It is now the most widely used technique for this purpose. This article discusses the design ofa radiocarbon AMS instrument and how it is used to quantify 14C. SampleAU2 preparation protocols for a varietyof common sample types and their many applications are described. Radiocarbon AMS instruments vary indetail, but all contain the following: (1) an ion source; (2) an injection magnet and bouncer; (3) a tandemaccelerator and stripper; (4) a high-energy particle analysis system; and (5) a particle detector. The analyticalbasis for AMS radiocarbon measurements, including measurement uncertainties, is considered in this article.AMS radiocarbon applications are broadly divided between dating studies and 14C isotopic tracer studies. Ineither case, the ubiquitous distribution of carbon on Earth underlies the great utility of the AMS radiocarbontechnique. Applications in archaeology, paleoclimatology, geomorphology, and oceanography are discussed.

KeywordsAuthor and Co-author Contact Information

G.S. Burr

A.J.T. Jull

NSF – Arizona AMS Laboratory Physics Building

1118 E. Fourth St Tucson, AZ

85721 USA

Phone: þ 1-520-621-6816

E-mail: jull@u.arizona.edu

AU12

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A0005 Accelerator Mass Spectrometry for

Radiocarbon Research

S0005 1. IntroductionAU3P0005 Accelerator mass spectrometry (AMS) was developed

in the 1970s as an analytical tool for the measurementof radiocarbon (14C). It is now the most widely usedtechnique for this purpose, with more than 70 activeAMS laboratories worldwide. In the late 1970s, theintroduction of AMS technology moved the horizonfor scientists involved in radiocarbon research byreducing sample size requirements by orders ofmagnitude and sharply reducing analysis times. AMSopened the door to radiochemical 14C tracer studiesand expanded its traditional role in geochronometry.The AMS radiocarbon technique provides a criticaltool for understanding such diverse problems as therise and fall of civilizations, the timing of prehistoricearthquakes and volcanoes, pathways of carbonexchange between the oceans and atmosphere, andthe time elapsed since a meteorite fell from the sky.This article discusses the design of an AMS instru-ment, how it is used to quantify radiocarbon, andhow it is applied in practice.

S0010 2. The AMS Instrument

P0010 The AMS 14C technique was introduced indepen-dently by two groups using tandem electrostaticparticle accelerators (<bib1 bib2>1,2). This workfollowed a paper that demonstrated the feasibility of14C measurements using a cyclotron (<bib3>3).Cyclotron AMS measurements were made success-fully as early as the 1930s, to detect 3He and 3H(<bib4>4). Whether a tandem electrostatic particleaccelerator or a cyclotron is used, the distinguishingfeature of AMS is the combination of particle accel-eration to megaelectron volt energies, and particlediscrimination based on their mass, energy, andcharge. High-energy analysis removes isobaric,atomic, and molecular interferences using techniquesoriginally developed for nuclear and atomic physics(e.g., see this chapter: Instrumentation: Source andDetector Instrumentation (00067)). The use of highenergies to remove molecular interferences distin-guishes AMS from conventional mass spectrometrictechniques, where particles are accelerated to tens ofkiloelectron volt energies. The high-energy suppres-sion of molecular interferences with AMS leads to adetection limit of one part in 1015 or better for carbon(<bib5>5).

P0015 Individual configurations vary, but most AMSinstruments have the following components: (1) ionsource; (2) injection magnet and bouncer; (3) tandemparticle accelerator and stripper; (4) high-energyparticle analyzers; and (5) a particle detector. A

typical AMS instrument is shown in Fig. 1. For anAMS radiocarbon measurement, negative carbonions produced in the ion source are formed into aparticle beam and sent through an injection magnet.After they pass through the magnet, they continue toaccelerate to the AMS terminal. There, they arestripped of electrons and the resulting positive par-ticles are accelerated further, back to ground poten-tial. These high-energy particles are analyzed en routeto the detector using a variety of magnetic and elec-trostatic filters. The particle beam is periodicallyfocused during its journey from the ion source to thedetector, using some combination of Einzel, quad-rupole, and gridded lenses, accompanied by colli-mating slits and electrostatic steerers. The choice oflenses and their placement depends on the particulardesign of each AMS. These are shown for the Ari-zona pelletron in Fig. 1 as an example.

S00152.1 Ion Source

P0020The majority of AMS radiocarbon measurementsutilize cesium sputtering ion sources to producebeams of C ions (<bib6>6) (e.g., see this chapter:Instrumentation: Source and Detector Instrumentation(00067)). Middleton’s original source design is verysimilar to modern ion sources. It produces a Csþ

beam to bombard targets and extraction voltages onthe order of tens of kilovolts to form a beam. Thedesign also features a rotating sample carousel tofacilitate rapid sequential sample analyses. A modernion source of this type is shown in Fig. 2. The targetmaterial for such a source can either be solid or gas.Graphite is the target material of choice for solidtargets for several reasons, including (1) its purity andrelatively inert chemical nature, (2) its characteristicof high C beam currents, and (3) its ease of fabri-cation in the laboratory. Optimized versions of gra-phite sources can routinely produce 120–150 mA C

with source efficiencies above 20% and using as littleas 10 mg of carbon (<bib7>7). Solid targets are cur-rently preferred over gaseous targets because theyproduce higher currents. However, gas sources areadvantageous for very small samples (as little as 1 mgof carbon), and simplify sample processing. They alsocan be fed directly from a gas chromatograph-massspectrometry (GC-MS) unit (see Chapter 2 (Volume8)) to provide radiocarbon results from targetedorganic compounds (<bib8>8), as discussed AU4in thefollowing text. All C sputtering ion sources have anintrinsic advantage in the case of radiocarbon ana-lysis because they sharply reduce isobaric 14N, asnitrogen does not form stable negative ions(<bib9>9) (see this chapter: Instrumentation: Sourceand Detector Instrumentation (00067)).

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F0010 Figure 2Cs sputter ion source at the NSF-Arizona AMS Facility.

Double focusing injection magnet

Collimating slits

Double-focusing analysis magnet

Electrostatic analyzerTerminalMagnet

p.s.

Electrostatic steerer

Einzel lens

Quadrupole lens

Con

trol

con

sole Faraday cups

Surface barrier detector

Switching magnet

Focusingelectrostatic analyzer

MCC

Cs ion source

Scalem

20 1

F0005 Figure 1Schematic view of the Arizona pelletron AMS instrument.

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S0020 2.2 Injection Magnet and Bouncer

P0025 Negative ions produced in the ion source are pre-accelerated to form a particle beam with energies onthe order of tens of kiloelectron volt. This beam isdirected into an injection magnet with an appropriatemagnetic field to select mass 14. Off-axis stationaryFaraday cups are generally used to measure the cur-rents at m/z 12 and 13 after the beam goes throughthe injection magnet. A minimum mass resolvingpower on the order of m/Dm¼ 100 is sufficient formass 14 preacceleration filtering, and modern injec-tion magnets far exceed this value. They are alsotypically double focusing.

P0030 Radiocarbon analysis benefits from the existenceof two stable carbon isotopes – 12C and 13C. The goalof an AMS radiocarbon measurement is to quantify14C/12C, 14C/13C, and 13C/12C ratios. In practice,each isotope is measured separately at high energies(after the accelerator). This objective is complicatedby the fact that both stable carbon isotopes are farmore abundant than 14C. For example, in modernmaterial 14C/12C B1012 and 14C/13C B1010. Whenthe relatively intense 12C beam (B100mA) is sentdirectly through the accelerator, it causes the terminalvoltage to sag and compromises the measurements.Two methods can be used to address this problem: (1)by reducing the 12C beam current and injecting 12C,13C and 14C simultaneously or (2) using short pulsesof 12C (on the order of milliseconds) and injectingeach isotope sequentially.

P0035 Simultaneous injection is an advanced technologyused only with AMS instruments dedicated toradiocarbon. In this case, multiple stages of magneticfiltering are used to divide the 12C, 13C, and 14Ccomponent beams. Once the beams have been iso-lated, the intensity of the 12C beam is reduced with amechanical chopper and the beams are then recom-bined for injection (<bib10>10). This approach hasthe advantage that all three isotopes experience thesame inherent temporal variability associated withacceleration and stripping. With sequential injection,12C, 13C, and 14C are injected separately. In this case,temporal variations are minimized by cycling rapidlybetween isotopes, on a timescale that is faster thanthe voltage variability in the accelerator. An electro-static sequencer, or ‘bouncer,’ is used to select theisotope of interest. In this design, the injection mag-net is electrically isolated so that a voltage can beapplied to it. The injection magnet discriminatesaccording to the (masskinetic energy)/charge2

(mEk/q2) of the particles (discussed in the following

text) and, by applying a voltage to the magnetchamber the kinetic energy of the particles can bechanged. Increasing the energy by an appropriateamount allows mass 12 or mass 13 particles to beselected, while maintaining a static magnetic field. Inthis way, the bouncer can switch very rapidly betweenmasses, with rates as high as 100MHz. In practice,

the electrostatic bouncer is turned off most of thetime, to measure the rare isotope, 14C (see thischapter: Instrumentation: Analyzer Instrumentation(00068)).

S00252.3 Tandem Particle Accelerator and Stripper

P0040A tandem particle accelerator is generally used forAMS radiocarbon analysis. In a tandem configura-tion, singly charged negative ions are accelerated tohigh energies at the terminal of the AMS, where theyare stripped of electrons in a foil or gas cell (Fig. 1).Multiply charged positive ions that result from thisinteraction are further accelerated back to groundpotential. Gas strippers are generally preferred forradiocarbon AMS because they behave more uni-formly than foils and do not change with time. Theyield of different positive charge states that resultfrom gas stripping depends on the kinetic energies ofthe particles that reach the stripper and the density ofthe stripper gas. Under normal operating conditions,the dominant positive carbon ion produced duringstripping, up to 1MeV, is Cþ (<bib11>11), and forions accelerated to 2 or 3MeV, C3þ is dominant(<bib12>12).

P0045Molecular ions, such as 12CH2nþ or 13CHnþ, do

not survive in the 3þ charge state, and this factbecame a standard for AMS instruments designedespecially for radiocarbon (<bib13 bib10>13,10).These AMS instruments are significantly smaller thanthe machines originally used to develop the techni-que, and the latest generation of commercial radio-carbon AMS instruments is even smaller. CompactAMS machines operate with 1-mV terminal voltagesand make 14C measurements with Cþ ions. Relativelyhigher stripper gas pressures are required in theseinstruments to destroy molecular interferences. Thisapproach requires special design considerations, suchas large diameter accelerator tubes and large magnetpole gaps, to overcome beam scattering and beamloss. Even smaller AMS instruments are underdevelopment. The smallest uses a standard 250 kVpower supply at the terminal, and can be used for 14Cstudies that do not require very low backgrounds(<bib14>14).

S00302.4 High-Energy Particle Analysis

P0050High-energy particle analysis, or filtering, is per-formed on the positive carbon ion beam after it exitsthe accelerator. For radiocarbon, this typicallyinvolves some combination of electrostatic andmagnetic filtering. Electrostatic filtering selects par-ticles according to their Ek/q, whereas magnetic fil-tering depends on the particle’s mEk/q

2. Alternatingthese methods leads to the high sensitivity of AMS.Any residual interfering particles with appropriateEk/q values that pass through the electrostatic filter

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are not likely to survive the mEk/q2 filtering through

the analysis magnet. A velocity filter (Wien filter) mayalso be used in some cases. This device offers a thirdtype of particle identification, based on the mEk ofthe particles. When a cyclotron is used, particles aresorted by their momentum/charge. The basic physicsof all of these filtering elements can be understood interms of the Lorentz force.

P0055 For example, it can be shown that, for a magneticfilter with a radius r, a magnetic field B can be chosento select specific particles according to the relation

B2 ¼ 2mEk

rq2; ð1Þ

where B is expressed in tesla, m is the mass in kilo-grams, Ek is the kinetic energy of the particles injoules, r is in meters, and q is the charge in coulombs.One advantage of using higher charge states, such as3þ for radiocarbon, is that it reduces the magneticfield necessary to analyze the beam. An analysismagnet with a 1.2-m radius, filtering 10-MeV 14C3þ

particles, requires a substantial magnetic field(B4700G), usually supplied by a large water-cooledmagnet.

P0060 Electrostatic filtering is mass independent. For anelectrostatic analyzer with an electric field E, particlesare selected by their kinetic energies and charge

according to the relation

E ¼ 2Ek

rq: ð2Þ

P0065The electric field is determined by the voltage appliedto the device and its geometry. An electrostatic ana-lyzer with a 3-m radius, and plate gap of 2 cmrequires an applied voltage of B44 kV to filter10MeV 14C3þ particles, a typical value for radio-carbon AMS (see this chapter: Instrumentation:Analyzer Instrumentation (00068)).

S00352.5 Particle Detection

P0070The filtering described earlier in the text ensures thatparticles will have a specific mass, charge, and energyas they enter the detector. Gas ionization detectorsand surface barrier detectors are used to analyzeparticles in radiocarbon AMS. A schematic view of agas ionization detector is shown in Fig. 3. Thesedetectors measure the energy loss of particles as theyinteract with gas molecules. The gas pressure is cho-sen so that the particles lose all of their energy in thedetector, allowing their total energy to be determined.Ion pairs are produced by the interaction of the beamand the gas, with an average energy of B35 eV per

−+−

30MΩ

10MΩ

Cathode

Anode 2 Anode 1

20 torr isobutane

Beam Silicon nitridewindow

R1

R2

Cathodepower supply

∆Epreamp

Epreamp

F0015 Figure 3Schematic view of a gas ionization detector.

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pair. A 10-MeV C3þ particle will produce B3 105

ion pairs. Positive ions are swept to a cathode, andnegative ions (electrons) to an anode. As the driftvelocity of electrons is orders of magnitude higherthan for positive ions, electrons are used to formpulses. To improve the signal, a Frisch grid, with anintermediate potential between anode and cathode, isused. This serves to minimize potential interferencesfrom slow moving positive ions in the vicinity of theanode (Fig. 3).

P0075 An important feature of gas detectors is that theycan be used to monitor the progressive energy loss ofparticles (DE) with distance through the detector(Dx). This is a significant diagnostic tool for AMS(<bib1 bib2>1,2) (e.g., see this chapter: Instru-mentation: Source and Detector Instrumentation(00067)). In its simplest form, two anodes are used torecord energy loss in the first and second anode (DE1,DE2), which are summed to compute the total energy(ET). Plots of ET–DE1 cleanly resolve isobaric inter-ferences associated with 13C, 14N, and others (Fig. 4).

P0080 The entrance aperture of a gas detector is fittedwith a foil, and the device is filled with a counter gassuch as isobutane or propane. The foil separates thecounter gas from the high vacuum in the beam lineleading up to the detector. When particles strike thefoil they undergo straggling, which affects the energyresolution of the signal. In recent years, silicon nitridehas supplanted mylar as the foil of choice in AMSdetector instrumentation. Silicon nitride is stronger,

allowing thinner foils to be used. This leads to cleanerenergy spectra with striking improvements in energyresolution. Silicon nitride foils are especially impor-tant for relatively low-energy AMS instruments(1MV or less). For these machines, special gas ioni-zation detectors were constructed, some the size of amatchbox (<bib15>15).

P0085Silicon surface barrier detectors are also widelyused for radiocarbon AMS. In a broad sense, theycan be thought of as a kind of solid-state ionizationchamber. Surface barrier detectors are diodes of thep-n or p-i-n type that are reverse biased. Collisionsbetween high-energy C3þ particles and the detectorproduce pairs of positive ‘holes’ and electrons in thesilicon lattice with an average energy of B3.6 eV. Theelectric field across the detector carries the electronsto a preamplifier and amplifier to form pulses. If allof the energy from a 10MeV C3þ particle is convertedinto hole–electron pairs, B3 106 ion pairs will beformed. This number is about an order of magnitudehigher than the number of ion pairs formed in a gasionization detector; however, experiment suggeststhat the resolution of surface barrier detectors forradiocarbon AMS is comparable to gas ionizationdetectors (see this chapter: Instrumentation: Sourceand Detector Instrumentation (00067)).

S00403. Quantifying Radiocarbon Results

S00453.1 Fraction Modern Carbon

P0090The fundamental quantity used in radiocarbon AMScalculations is the fraction modern carbon (F) value(<bib16>16). The fraction modern carbon valueprovides a means of replacing activity units, commonto decay counting, with atomic ratios measured withAMS. This approach preserves long-standing data-reporting conventions for radiocarbon establishedusing decay-counting techniques (<bib17>17).Either 14C/13C or 14C/12C ratios may be used to cal-culate F (<bib18>18) but, for simplicity, the fol-lowing discussion considers only 14C/13C ratios. Inthis case, F is defined as

F ¼ð14C=13CÞS 25½

ð14C=13CÞ1950 25½ ð3Þ

where (14C/13C)S[25] is the measured ratio of thesample, blank-corrected and adjusted tod13C¼ 25%; and (14C/13C)1950[25] is the measuredratio of the standard, blank-corrected and adjusted tod13C¼ 25%, and recalculated to 1950 AD (thezero year or present for radiocarbon dating). AMSradiocarbon dates are standardized using a consensusvalue for the National Institute of Standards andTechnology (NIST) OX I standard (oxalic acid, SRM AU6

4990B) in the year 1950 (<bib17>17). The originalsupply of NIST OX I is largely exhausted, and a

Oxygen

Carbon

Nitrogen

14N6+

14C6+

13C6+

12C5+

12C6+

∆E

E

F0020 Figure 4E–DE plot from a modern charcoal sample. E is the totalenergy and DE is the energy loss for a limited range ofmagnetic rigidity. Adapted with permission fromBennett, C. L.; Beukens, R. P.; Clover, M. R., et al.Radiocarbon Dating Using Electrostatic Accelerators:Negative Ions Provide the Key. Science 1977, 198, 508–510

AU5

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number of secondary standards are now used toobtain equivalent results at most laboratories.

P0095 As noted earlier in the text, there are two correc-tions made in the determination of F: (1) a blankcorrection and (2) a 13C correction. As F is the cor-nerstone of AMS radiocarbon research, it is worth-while considering these corrections in detail, alongwith their uncertainties.

P0100 The blank correction is based on repeated mea-surements of 14C-depleted radiocarbon samples(c50 kyr before present (BP)), processed in the samemanner as the unknowns. Blank values can be con-veniently characterized by their fraction moderncarbonAU7 (f) values, according to the relation

f ¼ ð14C=13CÞblankð14C=13CÞ1950½25

" #ð4Þ

P0105 In this expression, (14C/13C)blank is the measured ratioof the blank, and (14C/13C)1950[25] is the ratio of thestandard at 1950 AD (<bib16>16).

P0110 For small samples, f is inferred from experiment tobe inversely proportional to sample mass(<bib19>19). This observation is consistent with thepresence of a constant amount of contaminant thatbecomes proportionally more significant withdecreasing sample size (especially those less than

B500mg). For these samples, it is important toquantify the mass of carbon (m) to accurately adjustf, as follows (<bib16>16):

f ðmÞ ¼ f 1 mg

m

ð5Þ

P0115This relation is shown graphically in Fig. 5 for thenominal blank value, f¼ 0.00370.001. The uncer-tainty in f (denoted here as df) is determined bytaking the standard deviation of a large group ofblank measurements. As the sample mass decreases,the blank increases sharply and the absolute uncer-tainty increases as well.

P0120Once the value f has been established, the blankcorrection can be applied to the standard (denomi-nator of eqn (3)), using the relation

ð14C=13CÞ1950½25 ¼ð14C=13CÞm;OXIð1þ f Þ½ð1=0:9558Þ þ f ð6Þ

P0125This expression is appropriate for OX I, and includesthe linear coefficient (0.9558) to convert the measuredstandard ratio (14C/13C)m, OXI to the consensus 1950AD value, for d13C¼ 25%.

Bla

nk fr

actio

n (f

)

Sample mass (mg)

0.00

0.05

0.10

0.15

0.20

0.25

100 200 300 400 500

F0025 Figure 5Plot of the variation in the fraction modern carbon in the blank (f) versus sample mass. The nominal blank value of0.00370.001 is plotted, with a one standard deviation error envelope.

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P0130 The blank correction for the sample (numerator ofeqn (3)) is made with the equation (<bib16>16):

Fbc ¼ Fmð1þ f Þ f ð7Þ

where Fbc is the blank-corrected fraction modernvalue, and Fm is the measured fraction modern of thesample (S), as determined by the following equationAU8 :

Fm ¼ð14C=13CÞS½d

ð14C=13CÞ1950½25

" #ð8Þ

P0135 Here the subscript d indicates that the Fm ratioreflects the value appropriate for the measured d13Cvalue of the sample, with no 13C correction.

P0140 The uncertainty in Fm is determined from thenumber of counts collected during the measurementas

dFm ¼Fm

counts1=2ð9Þ

P0145 The uncertainty in Fbc introduced during the blankcorrection propagates the error in Fm and theuncertainty in f:

dFbc ¼ ð1þ f Þ2dF 2m þ ðFm 1Þ2df 2

h i1=2ð10Þ

P0150 The 13C correction completes the calculation of F.This correction removes isotopic fractionation unre-lated to radioactive decay. The isotopic correction formeasured 14C/13C values is (<bib16>16)

F ¼ Fbc0:9750

1þ ½ðd13CÞS=1000

( )ð11Þ

where (d13C)S is the measured d13C value of thesample. This value may be measured with a conven-tional mass spectrometer, or with the AMS. Theuncertainty in F after the 13C correction is

dF ¼ 0:9750

1þ z

1000

264

375

2

dF 2bc þ Fbc

0:9750

1000 1þ z

1000

2

264

375

2

dz2

8><>:

9>=>;

1=2

ð12Þ

where z is the d13C value and dz is the uncertainty inthe d13C measurement. As d13C values are typicallymeasured with much higher precision than F valueswith conventional mass spectrometry, this uncer-tainty is often negligible.

P0155 Up to this point, three sources of uncertainty havebeen accounted in F, including (1) the statistical

uncertainty associated with the number of 14C atomscounted in a given measurement (counting error); (2)the uncertainty in the blank; and (3) the uncertaintyin the 13C correction. An important additional sourceof uncertainty is the random machine errors (rme)associated with temporal variability in the AMSinstrumentation (<bib20 bib21>20,21). This uncer-tainty is propagated together with the counting error,blank error, and 13C error to compute the final AMSresult.

S00503.2 Radiocarbon Dating: 14C Age

P0160The fraction modern carbon value can be used tocalculate a variety of other useful quantities, includ-ing the radiocarbon age of a sample. The age of asample is related to F according to the expression:

14C age ¼ 8033 lnF ð13Þ

where 8033 is the Libby mean life and the calculatedage is expressed in radiocarbon years BP-1950 AD.The uncertainty in the radiocarbon age can be cal-culated by differentiating eqn (13):

d14C age ¼ 8033dF

Fð14Þ

P0165By convention, radiocarbon dates should be 13C-corrected and quoted with one standard deviationuncertainties. They also rely on the assumption thatthe production of radiocarbon in Earth’s atmosphereis constant. This assumption is not true, principallydue to variations in the magnetic fields of the Earthand Sun, and temporal variability in the carbon cycle.In practice, the influence of these factors is removedby calibrating 14C dates. This is accomplished bycomparison of radiocarbon results with knownatmospheric 14C values through time. The knownvalues are from samples that have independent agecontrol, such as tree rings dated by dendrochronol-ogy (<bib22>22).

P0170The age limit of the AMS radiocarbon datingmethod is determined by the minimum value of F thatcan be measured-Flimit. This limiting value is fixed bythe uncertainty in the blank, according to the relation(<bib16>16)

Flimit ¼ Fm f 2df ð15Þ

P0175Hence, a nominal blank uncertainty of 70.001translates into a radiocarbon age limit of B49 900years.

P0180Whether a particular sample is a limit or notdepends on the propagated uncertainty in F. Byanalogy with the convention applied for activity

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measurements, a measured value, F7dF, may beconsidered a limit when Fo2 dF. In this case, thecorresponding radiocarbon age limit should bereported as (<bib17>17)

14C age limit ¼ 8033 lnðF þ 2dFÞ ð16Þ

P0185 It is possible to have negative values of F in thisexpression, after the blank correction. In these cases,F is considered to be zero. For very old samples, theuncertainty in the blank controls the age limit, asdiscussed earlier in the text. The uncertainty in theblank also depends sensitively on the sample mass,especially for samples less than B200 mg, as theabsolute uncertainty of f becomes larger (Fig. 5).Experiments show that 14C contamination is derivedmostly from the pretreatment and sample processingnecessary to convert samples to graphite. An unpro-cessed 14C-free graphite can be measured with AMSat the 1016 level, with an apparent age limit on theorder of 75 kyr BP or more. If the same graphite iscombusted to CO2 and then reduced back to graphitein the laboratory, the subsequent chemical blank willhave a 14C concentration on the order of 1015, witha corresponding age of 50–55 kyr BP.

S0055 3.3 Radiocarbon Tracer Studies: d14C

P0190 For tracer studies, a common unit is d14C, calculatedas

d14C ¼ ½Felt 1 1000% ð17Þ

where l¼ 1/8267 and t is the number of calendaryears that have elapsed since the sample formed,referenced to 1950 AD. This is a measure of thefractional amount of 14C in the sample at the time itwas deposited, compared to amount of 14C in theatmosphere, in 1950. The uncertainty in d14C can bedetermined by propagating the errors in F and t:

dD14C ¼ 1000elt½ðFlÞ2dt2 þ dF21=2 ð18Þ

S0060 3.4 In Situ Studies: Atoms 14C Per Gram SampleAU9P0195 Most of the radiocarbon on Earth is formed in the

upper atmosphere from reactions involving cosmicrays and atmospheric gases. A tiny but measureableamount is formed in situ in rocks and ice by spalla-tion reactions on oxygen. Although the amount issmall, it is also measurable with AMS and can beused to date recent geomorphic surfaces or the ter-restrial ages of meteorites (<bib23>23), or tounderstand in situ production in glacial ice

(<bib24>24). In this case, the quantity of interest isthe 14C concentration, in atoms 14C/gram sample –N14. This can be calculated from the fraction moderncarbon as follows (<bib25>25):

N14 ¼

FA14NAVS

VA B

m

ð19Þ

where A14¼ 1.177 1012 is the fractional abundanceof 14C in modern carbon, NA is Avogadro’s number,VS is the volume of CO2 collected in the sample, VA isthe volume of 1mol of gas at standard temperatureand pressure (STP), B is the number of 14C atomsassociated with the blank, and m is the mass ofsample in grams.

P0200The quantity N14 is a function of F, B, VS, and m.The uncertainties in these four variables are domi-nated by F and B, so that the total uncertainty in N14

can be estimated from the following equation:

dN14 ¼A14NAVS

VAm

2

dF 2 þ 1

m

2

dB2 ð20Þ

S00654. Target Preparation for AMS

P0205AMS target preparation involves physical and che-mical cleaning of samples and conversion of theircarbon to CO2. For gas ion sources, the CO2 is useddirectly as the target material. For solid ion sources,the CO2 extracted is reduced to graphite and thenpressed into target holders for analysis. Specific AMSpretreatment and processing protocols depend on thetype of sample and carbon comes in very manyforms. Typical pretreatment and processing techni-ques are discussed in the following text.

S00704.1 Common AMS Radiocarbon PretreatmentProtocols

P0210The most common AMS target pretreatment techni-ques are (1) acid–alkali–acid (AAA) for charcoal,wood, and other forms of organic carbon; (2) acid-ification for inorganic carbon such as shell, eggshell,and dissolved inorganic carbon (DIC); (3) Soxhletextraction for the removal of curatorial preservationagents sometimes applied to museum specimens; (4)collagen extraction and ultrafiltration for bones; (5)inductive heating/stepped combustion for iron, rocks,and meteorites; and (6) compound-specific radio-carbon analysis (CSRA) to measure the 14C contentin a variety of specific compounds.

P0215AAA pretreatment consists of three steps(<bib26>26). In the first step, inorganic carbon isdissolved and removed by the addition of 0.1N HCl,

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and heated to 50 1C until no gas is evolved. Thesecond step removes soluble humic acids using 0.1NNaOH overnight at 50 1C. The third step consists of afinal acid treatment with 0.1N HCl to removeabsorbed atmospheric CO2 introduced during thesecond step. The sample is rinsed with water untilneutral following each step of the process. Whencomplete, the sample is dried and combusted under avacuum to produce CO2.

P0220 Acidification is used both to pretreat and processcarbonates. In this case, the sample is dissolveddirectly under a vacuum to collect CO2. Studies showthat potential contamination from the outer surfacesof carbonates can be removed by stepwise dissolution(<bib27 bib28>27,28), and that it is generallydesirable to dissolve and discard the outermost B50wt% of the sample before collecting the CO2 foranalysis.

P0225 Samples that were treated with a preservationagent are cleaned with sequential Soxhlet extractions.A combination of increasingly polar organic solvents,such as hexane–acetone–ethanol (<bib29>29), isused in this procedure. In a Soxhlet device, the sol-vent is replenished by a reflux process that produces asteady supply of pure solvent to repeatedly wash thesample. This allows contaminants with relatively lowsolubility to be effectively removed. When thor-oughly cleaned, the sample is rinsed with water toremove the ethanol, dried, and combusted in avacuum to form CO2.

P0230 Bone pretreatment strategies have been activelystudied for many decades. The attention given tobone pretreatment reflects both their critical role inarchaeological studies and the complexities of bonechemistry. The standard pretreatment method forbone is collagen extraction (<bib30>30). Collagen isa fibrous protein and the principal constituent ofbone. Over time, bone collagen degrades as a result ofpostdepositional alteration and eventually breaksdown into individual amino acids. Bone collagen mayalso form hybrid constituents through reactions withexogenous organic compounds in the burial envir-onment. For these reasons, a number of screeningprocedures were developed to characterize the degreeof preservation for individual samples (<bib31>31).The most common screening procedure for AMS isthe determination of percent collagen in a sample.Modern bone consists of B22 wt% collagen and thisvalue decreases with burial time. Samples with as lowas 0.5% collagen may be datable, but these requirespecialized chemical pretreatment, such as the isola-tion and analysis of individual amino acids(<bib31>31). Samples with o0.5% collagen arenormally considered unsuitable for dating. In recentyears, a refinement of the collagen extraction tech-nique has become standard. In this technique, ultra-filtration is used to isolate the 430 kD molecularweight fraction. The method is highly selective and

removes contaminants left from the standard col-lagen extraction technique (<bib32>32).

P0235Iron artifacts are another important archaeologicalarchive, and they contain a significant amount ofcarbon, between B0.1 and 6%, depending on thetype of iron. In ancient times, charcoal or wood wasused to smelt iron, and the 14C content of an ironartifact is often indicative of its age. Iron dates basedon this reasoning were first demonstrated in the late1960s (<bib33>33). Currently, there are a number oftechniques used to extract carbon from iron, includ-ing a method that uses the iron itself as the targetmaterial for AMS (<bib34>34). This novel techni-que produces reliable results, although it is limited torelatively carbon-rich iron samples. Inductive heatingworks on all types of iron samples. This methodfollows several steps: (1) physical removal of oxidecoatings, (2) treatment with acid, (3) heating to500 1C under a vacuum, and (4) combustion at1100 1C or higher using an RF (radio frequency)furnace (<bib35>35). The first heating step of thisprocedure effectively removes organic contaminantsin the sample.

P0240Stepped combustion is used for a variety of sam-ples, including the separation of terrestrial andextraterrestrial radiocarbon in meteorites, and inquantitatively removing organic carbon from in situradiocarbon samples (<bib23>23). In situ 14C isproduced directly in rock by spallation, and extract-ing this carbon from terrestrial rocks poses a sig-nificant analytical challenge. A typical rock containsonly 105 to 106 atoms g1 14C. Experiments show thatCO2 extracted from quartz between 500 and 1100 1Ccontains only in situ carbon (<bib36>36). Steppedcombustion is also useful in discriminating betweenmixed carbon sources contained in charcoals, soils,paleosols, eggshells, and pottery.

P0245CSRA is a relatively new pretreatment and pro-cessing method designed to isolate individual com-pounds for 14C analysis (<bib8>8). The method ispotentially very powerful for both radiocarbon dat-ing and tracer studies. For example, the techniquecan be used to establish reliable chronologies in lakesthat suffer from hard water effects, such as in a lakesituated on a carbonate platform (<bib37>37).Terrestrial n-alkanes are extracted from sedimentcores using a combination of Soxhlet extraction,silica gel chromatography, and a molecular sieve.Following this step, the concentrate is repeatedlyinjected into a preparative capillary GC until 70–170mg of carbon is collected and oxidized to CO2.

P0250CSRA can also be applied to determine radio-carbon ages of diatoms in marine sediments from theSouthern Ocean (<bib38>38). Diatoms contain theprotein sillafin, which can be extracted by dissolutionin HF. The diatoms are first concentrated using acombination of sieving and differential settling.Organic contaminants are then oxidized using CrO3

and H2SO4. Once the sillafin is extracted, it is

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combusted to CO2. In this technique, 10 g of sedimenttypically yields less than 100mg of carbon. Themethod provides a means to date marine sedimentsthat lack foraminifera or other datable carbonate.

S0075 4.2 Graphitization

P0255 Carbon dioxide is easily converted to graphite withan iron or cobalt catalyst (<bib39>39). The catalystis heated to 500–650 1C in the presence of eitherhydrogen gas or zinc to reduce the CO2. CO isformed as an intermediate product in the reduction.In the case of hydrogen, CO is formed along withH2O; and in the case of zinc, CO forms with ZnO.When hydrogen is used, a cryogenic or chemical trapis added to remove water. When zinc is used, it isheated to B325 1C to promote the Zn-ZnO reac-tion. The intermediate CO is further reduced to gra-phite catalytically on the iron or cobalt powder. Afixed ratio of iron (or cobalt) to carbon is chosen toproduce uniform targets, optimized for AMS. Thesereactions occur in sealed vacuum cells with pressure

transducers to monitor their progress. Many labora-tories have developed automated graphitization linesto facilitate sample processing of large numbers ofsamples. One AMS preparation facility, for example,features 20 computer-controlled hydrogen reductionreaction cells (<bib29>29). The graphitization linesat the Arizona AMS facility use a similar design withzinc reduction cells (Fig. 6).

S00805. Applications of the AMS RadiocarbonTechnique

P0260The advent of AMS revolutionized the science ofradiocarbon dating. Objects previously deemedundatable using beta counting techniques, such aspollen grains or single seeds, suddenly became data-ble. At the same time, AMS opened the door for anentirely new 14C application, as an isotopic tracer.Above-ground nuclear weapons testing in the late1950s and early 1960s nearly doubled the globalatmospheric inventory of radiocarbon, which rapidlybecame dispersed into the oceans and on land.

F0030 Figure 6Schematic view of the NSF-Arizona AMS Facility graphitization line. The system contains 24 reaction cells, with twoheated glass tubes. One contains iron and the second contains zinc. During the reaction, zinc is heated toB325 1C andthe iron is heated toB500 1C. Pressure transducers are monitored with a computer to determine when the reaction hasreached completion.

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Twenty years later, the introduction of AMS pro-vided a means to quantify this anthropogenic 14C interrestrial and marine samples. Studies of this ‘bomb’carbon significantly enhanced understanding of thecarbon cycle, quantifying pathways, and exchangerates between global carbon reservoirs.

P0265 A few examples of AMS radiocarbon applicationsare discussed here to illustrate the usefulness andversatility of the technique. A common feature ofthese examples is that none of them would be feasiblewithout AMS. This discussion considers only a smallfraction of existing applications, and many moreexamples can be found in a number of recent reviews(<bib40 bib41 bib42 bib43 bib44 bib45>40–45).

S0085 5.1 Dating Studies

P0270 Radiocarbon dating is a destructive techniquebecause samples must be consumed to determinetheir 14C content. For the beta-counting method, thisimplies grams of carbon per analysis, and suchquantities are impractical or infeasible for datingprecious and irreplaceable samples. The minusculesample size requirement for AMS (o 1mg C) made itpossible to make routine measurements on preciousobjects. The first AMS result on such a sample wasobtained from the Shroud of Turin. The Shroud ofTurin is a famous relic that was believed to be theburial shroud of Jesus Christ. It was independentlydated by three AMS laboratories and yielded anaverage 14C result of 689716 14C years BP (1s), anda 2s calendar age range from 1275 to 1381 AD(<bib46>46). This places the origin of the Shroud inthe Middle Ages.

P0275 AMS radiocarbon dating is a cornerstone forarchaeological research, providing firm temporalconstraints on archaeological finds of all sorts. AMSresults show that early modern humans made musicand painted beautiful colorful frescoes 35 000 yearsago (<bib47 bib48>47,48). Understanding the peo-pling of Eurasia by modern humans over the past50 kyr, and the concomitant demise of the Nean-derthals, relies on AMS 14C results (<bib49 bib44bib45>49,44,45). In the same manner, AMS plays acentral role in ongoing efforts to develop a compre-hensive understanding of the peopling of the NewWorld (<bib50>50).

P0280 The Earth has experienced profound climaticchanges over the past 50 kyr, and AMS radiocarbondating is well suited to studying these events. Paleo-climate studies of natural archives, such as lakedeposits and deep sea sediments in particular, dependon AMS radiocarbon dating. Before AMS, datinglake deposits was problematic because carbonatesfrom lakes often exhibit open system behavior,referred to as ‘hard water’ or ‘reservoir’ effects. Thisproblem can be avoided by dating terrestrial macro-fossils sampled from sediment cores. Terrestrial

macrofossils obtain their 14C directly from theatmosphere and are not susceptible to reservoireffects (<bib41 bib44 bib45>41,44,45). Once a lakesediment chronology has been established, then avariety of physical and chemical sediment propertiescan be used to infer past climate and construct cli-matic time series. The same can be done for themarine environment using deep sea sediments. Theseare typically dated with AMS by measuring the 14Ccontent of foraminifera. AMS radiocarbon datesfrom freshwater and marine sediments played a cen-tral role in determining the site-specific timing ofglobal climatic fluctuations during the late Pleisto-cene and Holocene (<bib45>45). This includes thetiming of the Last Glacial Maximum, Dansgaard-Oeschger events, Heinrich events, the YoungerDryas, the mid-Holocene optimum, and many otherclimatic periods of interest (see this chapter: Appli-cations: Accelerator Mass Spectrometry in Geophysicsand Geochemistry (00079)).

P0285All of the dating applications discussed earlier inthe text measure the gradual decay of radiocarbonoriginally produced in the atmosphere. A much dif-ferent approach is used to date samples that containin situ 14C. In situ radiocarbon forms when a rock isbombarded directly by galactic cosmic rays, either atthe Earth’s surface or in space (in the case ofmeteorites). Meteorites at secular equilibrium areenriched in in situ 14C as compared to terrestrialrocks, due to the higher galactic cosmic ray flux inspace (e.g., see this chapter: Applications: AMS andExtraterrestrial Applications (00070)). After ameteorite falls to Earth, however, it becomes shieldedfrom most cosmic radiation, and its 14C contentbegins to decay. By comparing the measured 14Ccontent in a meteorite with secular equilibrium 14Clevels in space and on Earth, terrestrial ages ofmeteorites are routinely determined (<bib23bib43>23,43) (Fig. 7). In terrestrial rocks, in situ 14Ccan be used to date geomorphic surfaces that devel-oped within about the past 20 kyr. In this case, therock initially contains no radiocarbon, and the date isrelated to the buildup of 14C since the surface wasfirst exposed to galactic cosmic rays. The method wasused to date the ages of the Bonneville shorelines inUtah and can be applied to a variety of studies oflandscape evolution (see this chapter: Applications:Accelerator Mass Spectrometry in Geophysics andGeochemistry (00079)).

S00905.2 14C Isotopic Tracer Studies

P0290Radiocarbon is an ideal tracer isotope for severalreasons: (1) it has a single production pathway in theatmosphere, (2) it is widely distributed, (3) it is aprimary constituent of all life, (4) it has two stableisotopes (12C and 13C) that can be used to dis-criminate between exchange processes and

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fractionation effects, and (5) humans nearly doubledthe amount of radiocarbon in the atmosphere in thelate 1950s and early 1960s. Radiocarbon is particu-larly well known for its utility in ocean modeling. Itprovides the best estimate available of deep oceanventilation rates over timescales of centuries or longer(<bib51>51). It is also a potent tool in studying theinput of carbon from anthropogenic sources (e.g., seethis chapter: Applications: Environmental Applicationsof Accelerator Mass Spectrometry (00080)), and canbe used to study biologic pathways of carbon as well

(e.g., see this chapter: Applications: BiomedicalApplications of Accelerator Mass Spectrometry(00069)).

P0295The ocean is the largest carbon reservoir on theEarth, and understanding transport and mixingprocesses is a critical element of predictive models forclimate change. In 1971, the Geochemical OceanSections Study (GEOSECS) program was under-taken to determine baseline depth profiles for radio-carbon and other tracers in the Atlantic, Pacific, andIndian Oceans. More than 2200 water samples were

60°S40°S

20°S

20°N40°N 80°W

160°W

160°E

120°E

Hawali1200

800

Depth (m)

400

0

Australia

S.America

N.America

Asia

120°W

−200 −100 0 100

14C(‰)

Longitude

Latitude0°

F0040 Figure 8Three-dimensional representation of WOCE radiocarbon data from the Pacific and Indian Oceans. Adapted withpermission from Povinec, P. P.; Litherland, A. E.; von Reden, K. F. Developments in Radiocarbon Technologies:From the Libby Counter to Compound-Specific AMS Analyses. Radiocarbon 2009, 51, 45–78.

F0035 Figure 7Meteorite in the desert. AMS radiocarbon measurements can be used to estimate the time elapsed since meteorites likethis one fell to Earth.

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collected from 124 stations during a 6-year period.Those 14C results were determined using conventionaltechniques with 200-l samples. The GEOSECS pro-gram allowed scientists to begin to quantify deepwater formation, study transport processes, andestimate residence times for deep ocean water(<bib45>45). In 1990, the World Ocean CirculationExperiment (WOCE) was initiated to update andexpand the radiocarbon results from GEOSECSusing AMS, a relatively new technology at that time.The National Ocean Sciences Accelerator MassSpectrometer (NOSAMS) AMS laboratory was builtfor this purpose, with the goal of measuring radio-carbon in more than 13 000 seawater samples, to becollected over a 7-year period. This effort producedthe largest radiocarbon database in the world, withsufficient resolution to construct maps of oceanicradiocarbon in three dimensions (Fig. 8). The WOCEdata greatly enhanced understanding of deep waterformation, water mixing, and oceanic upwelling(<bib42 bib43 bib44 bib45>42–45), and will serve asa fundamental reference for many years to come.

P0300 The radiocarbon tracer data produced by theGEOSECS and WOCE programs are complementedby surface ocean 14C records from corals. Certaincorals produce annual bands as they grow and pre-serve records of dissolved inorganic radiocarbon intheir skeletons. By counting growth bands, one canestablish the year of growth for a particular coral,and then use AMS to determine the contemporarysurface ocean d14C content. By collecting corals fromnumerous sites within an ocean basin, surface oceand14C can be mapped out through time. Corals canpotentially be used to reconstruct continuous recordsof surface ocean radiocarbon over periods of hun-dreds of years, with annual or subannual resolution.The longest modern coral 14C record studied thus faris a B370-year record from Galapagos. The Gala-pagos radiocarbon record reflects the entrainment ofhigh-latitude Subantarctic Mode Water into theEquatorial Undercurrent, a process that has clima-tological implications. The Galapagos study docu-ments the influence of El Ni~no events on sea surfaceradiocarbon, with a periodicity of 3–7 years; andidentifies two interesting climatic periods – a 10-yearperiod in the 1600s and a 16-year period in the 1800s,both coincident with vigorous volcanic activity(<bib52>52).

S0095 6. Conclusions

P0305 Since its inception in the late 1970s, AMS hasbrought about a great revolution in radiocarbonmeasurements by drastically reducing target sizerequirements and sample measurement times. Newdevelopments in both applications and instrumenta-tion continue to expand the usefulness of the AMSradiocarbon method, which has no peer technology.

Recent trends in AMS have seen the development ofsmaller and more affordable machines that shouldmake this technology increasingly available aroundthe world. The range of radiocarbon dating andtracer applications discussed here reflect the greatdiversity of topics ultimately concerned with carbonin nature. Only a few of these can be mentioned here,and the reader is encouraged to follow the referencesgiven in the following text to further explore theAMS radiocarbon field.

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G. S. Burr AU1and A. J. T. JullNSF – Arizona AMS Laboratory, Tucson, AZ, USA

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